Race car suspension Class

[Ranger Mike]

light is right

We all know the saying “ opinions are like A------s, every one has one”

Let us look at the above post and put some numbers to it.
Given the race car has a ultra stock restriction..we assume the tires are hard, tall and narrow.
If we assume the engine is stock as well and may put out maximum of 150 horsepower
we need to look at the power to weight ratio.
150 / 2300 = .065
150/2450 = .061
looking at the attached table we see that the lighter car will make 3 mph more speed over 1/4 mile.

Lets look at the weight transferred ( see post 19 on page 2) -
2300 x .35 = 805 pounds transferred to the front and right side
2450 x .35 = 857 pounds transferred to the front and right side
remember that right front tire has a pretty small footprint to begin with and we don’t need an
additional 30 to 40 pounds overloading it. Don’t forget that additional left rear weight is going right over to the right front going into a turn.
The McPherson strut situation has bad camber build to begin with.

To summarize, for the same horsepower, light is faster, light is easier on the spec tires, light means slightly more acceleration off the turns.

 

[Ranger Mike]

Polar Moment

Many private messages on this weight question-
Let us look at weight (mass) and how it effect a race car-

UNSPRUNG Weight -Everyone parrots “ Unstrung weight is bad”..but why? Because we can not control it. This is why we go with light weight wheels, tires, turn shock absorbers upside down. Sprung weight must be placed where we can have maximum control of it. Ballast is the biggest problem so we minimize its height and mount it at the polar moment to minimize its effect during cornering.

Sprung weight – Weight we can “ control” as far as that thinking goes. During this discussion we focus on Ballast or weight we need to meat the governing rules of the race track for minimum weight and percent left side weight and percent rear weight. So where do we place this ballast weight?

Simple moment of inertia is used to estimate resistance to rotation.. It is analog to inertial mass, but for rotation instead of linear displacement. Moment of inertia: Kg*m2

The area moment of inertia or second moment of inertia, is used to estimate resistance to bending. Think of a beam when you want to know how much will it distort under load. It’s a measure of the resistance of a section of a solid to perpendicular loads. Area moment of inertia: m4

Polar Moment of inertia is a quantity you use to estimate resistance to angular torsion. It’s a measure of how much resistance to twisting around an axis has a section.
Polar moment of inertia: m4

Polar Moment is the center of all forces in a race car. This is the Point about which the car pivots during weight transfer ( cornering). This point will move the least amount during this action. Indy cars have low polar moment of inertia. Sportsman Saturday Night special have HIGH Polar Moment of Inertia. Look at the attached illustration. Indy car has Low Polar Moment of Inertia as all the Mass is concentrated as close to the CG as possible. The door slammer has a V 8 mounted up high ( usually with minimum height requirement), has a big old battery mounted up high and a fuel cell mounted past the axel ( outside the wheel base). High Polar Moment does not have to do with height. High means it takes a lot of force to change the DIRECTION of Mass. Think of a 50 pound fly wheel vs. a 10 pound flywheel. Once the Mass rotates ( as going into a corner) it wants to keep rotating. Think of a bowling ball and a volleyball . The volleyball is easy to get rolling and easy to change directions..not so with the bowling ball.

So why is it important to know about the Polar Moment? We want to build the car with the desired percent of left side weight, front to rear weight. Only when we achieve this do we want to add weight to meet the minimum weight requirement when we scale. We want to add this Mass or weight at the Polar Moment as this is the point where it will move “ the least” when cornering.

Think of the old teeter totter at the local school. If two people of equal weight sit on it at the same position from the middle pivot, no teetering or movement. The center pivot is the polar moment. Now bounce up and down and the teeter totter moves but notice the center pivot has the “ least movement” relative to the rest of the board.

So if I go get the neighbors fat kid and put him on the right side of the teeter totter opposite me, the teeter totter will tilt in my favor. I have to scoot toward the center pivot point to “ equal up” the weight distribution. In fact, an observer looking at this mess from the side would see my weight was shifted 10% bias toward the center pivot point. If we were to measure the distance between the seat positions ( think track width) and measure the center pivot point from my seat position we would see a 60% left side weight bias.
If we then try the same drill but at 90 degrees ( think wheel base ) and the center pivot point – we have front to rear weight bias.

All polar moment is the intersection of these two points AT CENTER OF GRAVITY HEIGHT.

By the way, most door slammers run 18 inch CG Height and super late model cars with dry sump oil pans and aluminum heads get as low as 16” CG Height. There is a method you can use to measure exact CG on your car with wheel scales and a floor jack,

You can run up to 70% left side weight on asphalt cars but 55% is max on dirt, and 52% rear is good for asphalt and 55% max rear wt. on dirt..
see good article on this at

http://content.yudu.com/Library/A1vgq2/RacecarEngineeringFe/resources/33.htm

 

[Ranger Mike]

Center of Gravity Height explained

Ok I been getting a lot of PM about finding Center of Gravity.

If you do not have access to a set of wheel scales you can rough estimate the CG by measuring the distance form the pavement to the center line of the cam shaft on the typical V8 door slammer or V6 for that matter..even OHC 4 cylinder will get you close.

Use your local Landmark or Farmer s Coop grain elevator scale. Measure your wheel base. Mark the mid point of the wheel base on your car with a piece of masking tape. Drive on the scale so the front tires are on the scale and the tape is aligned over the entrance edge of the scale. Record the weight. Drive off the scales to the point the masking tape is over the exit edge of the scales and only the rear tires are on the scales. Take a reading. You now have the Front vs. Rear weight. Simply divide the front weight by the total to get % front weight. Then multiply the wheel base by this percentage.
Say we have 3000 pound door slammer with 104” wheel base. we scale it as noted above and get 1590 front weight, 1410 rear weight. 1590 / 3000 = .53 or 53%.
104 inches x .53 = 55.12 inch so the COG is located 55 and 1/8 inch forward of the REAR axle.

This will give you COG in one axis or 1 dimension...front to rear.



If you have wheel scales you will be able to measure this a lot more accurately but the same math is involved.
With the wheel scales you can calculate the COG in two dimensions. We get Front to Rear as noted above and we can calculate left to right % by substitution Track width for wheel base.
Same 3000 # car that has 60 inch track width. Our wheel scales show 55% left side weight. This means COG is .55 x 60 = 33 inch to the left or 3 inch toward the left side from the center point of the track width and 55 1/8 inch forward of the rear axel. Better but his does not give us the true COG which is a 3 dimensional point.

To measure the COG most accurately, we need to prepare the car as race ready..tires properly inflated, full fuel load, driver ( or substitute weight of driver PROPERLY DISTRIBUTED). Don’t just throw in a few sacks corn that total the drivers weight, you need to replicate the weight of torso , helmet and legs as close as possible.
Note: You must replace the shock absorbers ( dampers) with solid links to replicate the race ready ride height. These solid bars will permit the car to be raised without collapsing as the shock would do under load.

the following is from Longacre who make a fine series of wheel scales. http://www.longacreracing.com/articles/art.asp?ARTID=22

To find the 3-D COG height we need to use a little trig. Specifically, we are using the Law of Tangents, and the Pythagorean Theorem. We use the wheelbase in place of the Hypotenuse and we will use 10 inches for the short leg of the right triangle since we intend to raise the car 10 inches.
a2 + b2 = c2



Center of Gravity Height Formula

COH = WB x FWc
TW x Tan q

Center of Gravity Height Formula

Definition of Variables

CGH - Center of Gravity Height
WB - Wheelbase (inches)
TW - Total weight
FW1 - Front weight LEVEL
FW2 - Front weight RAISED
FWc - FW2 - FW1 (change in weights)
HT - Height raised (inches)
Adj - Adjacent side (see below)
Tan q - Tangent of angle (see below)
CLF - Left Front tire circumference
CRF - Right Front tire circumference
C - (CLF + CRF) / 2 (average circumference)
r - Axle Height


The center of gravity height is found using the rules of trigonometry and right triangles. Specifically, we are using the Law of Tangents, and the Pythagorean Theorem. The following diagrams are greatly exaggerated for illustration purposes.
Tan q = opposite / adjacent

Tan q = HT / Adj


Pythagorean Theorem


So, in our exercise, when we raise the car 10" we are creating a right triangle with the following properties:
Hypotenuse = Wheelbase = c
Opposite = Height = b
Adjacent = a
C = 2 p r ( r is axle height of 10 inches)
Therefore using the Pythagorean Theorem:

Adj = square root of (WB2-HT2)


Once we know the value of the adjacent side of our triangle we solve for the tangent of q using:
Tan q = HT / Adj

Ok, now that we know the tangent of the angle we can calculate the center of gravity height based on our weight measurements using the following formula:

COH = WB x FWc
TW x Tan q
WB is the wheelbase
FWc is the change in front wheel weights
TW is the total weight
Tan q is the tangent calculated above
This calculates the Center of Gravity Height from the axle height.

To find the CGH from the ground, you must add your axle height to the above calculation. You can measure your axle height or calculate it using the average of your two front tire sizes and the formula for the circumference of a circle.

C = 2 p r ( r is axle height of 10 inches)

C is the average circumference found by adding the LF and RF sizes and dividing by 2.
p approximately equals 3.1416
r is your axle height
For example: Your LF is 85.5" and your RF is 87". Your average circumference is (85.5 + 87) / 2 = 86.25". Your axle height is (86.25 / 2) / 3.1416 = 13.727". Add this number to the CGH to find the center of gravity height in relation to the ground.



Now you have your true 3-D COG.

 

[Ranger Mike]

I apologize for many " errors' in the above formulas used in calculating 3D COG...I wrote the post in Windows messanger because of spell check..when i copied and pasted it some symbols translated to what ever software is used on this forum and the letters changed to th edefault format...i recommend you click on

http://www.longacreracing.com/articles/art.asp?ARTID=22

for details

 

[Rahul jeta]

although all your methods was on the mark,but since i am learner,i do have difficulty in understanding this second point.
2. draw the ground line ,vehicle center line and center of the left and right tire contact patches. Determine where the outer lower control arm ball joints (BJ) are located by bolting the upper and lower control arms to the spindle and bolting the spindle on the wheel to be used...some round track cars have different wheel offsets so be careful. mark these BJ centers on the drawing.

my question is..how will we determine the outer lower ball joints..?
i want to design a suspensiom system,and i wann know where to start from?
and how will we locate the pivot joints and how will we determine the lenth and angle of the arm..?
I am using double wishbone

 

[Ranger Mike]

starting chassis from scratch

Welcome and great questions...
Please note on Page on page 6 of this post..shows typical ball joint. To do a good job you can contact the manufacturers (Moog) being one), and have them send you diagram of exact pivot point. Another alternative is to find and old ball joint of same type you intend to use and cut it to determine the center of the ball stud.
You have a huge question that has many possible answers.
Will your suspension be:
Front wheel drive, front engine
rear wheel drive rear engine
rear wheel drive front engine
front wheel drive rear engine...??
What will vehicle weight be?
What is wheel base and wheel track width?
Where are the heavy components like fuel cell, transmission, differential, battery, driver going to be located?
What size tires will be used and what are wheel specs?
What is desired ride height to be?
Do we have a roll cage ? is the vehicle a hard top of convertible?

The short answer is once you figure out where the 4 tires will be you can start to connect them to the chassis and locate the engine, transmission etc..
finally, after the heavy bits mounting locations are finalized we work backwards to tweak the wishbones and mount points to get the roll centers we want.
very short answer...

 

[Rahul jeta]

Thanks for the reply..:)
Currently I am working on a hybrid trike,it doesn't have any engine.
its a tadpole having rear wheel drive.the track width is 45" and the wheelbase is 90.

 

[latemodel18]

Big Bar Soft Spring Setup

Ranger Mike I have been going thru this whole forum trying to learn about this kind of setup and I am enjoying it very much. I was hoping you might be able to look at some of my numbers and setup and be able to give me a few pointers. We are running a template body tubular chassis on a hosier 970r 9" tire on an 8" rim, Our springs are 175 across the front and Lr-200, RR-350 with a medium rubber and 1.375 sway bar, panhard bar is 8.875 left side axle and 10.125 right side chassis. 2736lbs without driver, left side is 59.9- cross 54.4 before preloading the bar and rear 51.2 wheel weights are lf-673, rf-689, lr 918, rr-456, trailing arms are at 3 degrees uphill on the left and 1 degree on the right, top link is centered and a lot of downhill angle,{sorry forgot to write that down before I left the shop} and 2-7/8 stagger with a Detroit locker on a medium banked 3/8 mile asphalt. At the track last weekend we ended up starting off with 6 rounds of bar in the car to try even out the nose but as testing continued the driver said he was tight thru the corner and loose off so we bumped the RR up to a 375 with medium rubber took 1-1/2 out of the bar and put 1 round of cross in evenly and he liked the car much better but still was not able to pull it to the bottom at the apex when he needed to, still a little snug. I might be wrong but I think I could be crutching the car with to much bar and maybe need to up the rr more. I done some measurements today and plugged them into my roll center program and I tried to attach it for you to see but no luck. It shows RCH-1.7 and RCL-2.9 at static and 2" dive it moves to RCH-4 and RCL-7. Not real sure where the roll center should be with this setup as I am only into this 3 weeks as a first timer working on race car setups. I am sure I am still off on understanding all this but what I have learned has pretty much been from reading on this site. Thanks again and really enjoy it.
Rod

 

[Ranger Mike]

Thanks you fro the kind words and welcome. You have a good grasp of what is going on with the set up.

I had to re-read the part about springs...350 right rear...whoa...
just from what you tell me I think you have really twisted the car to make it work half way..
When you preload the ARB you take away the purpose of the bar...go as light as you can on front springs to keep the nose as low as possible until you hit the corner where the ARB will contribute to roll control. When you preload the bar you are really stiffening up the total front spring rate. I much prefer going up on the front spring rate and keeping the bar neutral.

no way would I run that much preload on the ARB and have my rear springs stiffer than the fronts.

you have a good initial set up regarding weight percentages..almost ideal in fact.
without knowing the A-Arm motion rates and the like a ball park setup on coil overs
350 # on lf and rf, 225 on lr, rr, and 220# ARB

My main question is the Detroit locker situation. Is the driver using a lot of trail braking on turn entry?
If he is, the locker outside ratchet will not sense the power off and will stay locked up, the differential will not work and the car will push.
Did you remove the holdout ring on each side to provide instant lock up of the cogs on the driven assembly.
Sounds like you are running about the same stagger as the folks running a spool and this is not letting the differential do what it is supposed to do.
can you calculate the ARB spring rate.

Please confirm the Roll Center static location.. it is to the left in static...correct?
Also where does the RC end up on dive..to the left?
Looks like you have classic push going in loose off caused by RC being too much located to the left , not providing enough leverage to stick the right front going in...the current twisted chassis setup is a result of band-aids applied at the track to fix the car...

 

[latemodel18]

yes Mike the RC is 2.9 to the left at static and goes to 7 left at 2" of dive and the RC height is 1.7 at static and then goes to 4
Thanks
Rod

 

[Ranger Mike]

If you are running a race track that could use the big bar soft spring set (BBSS) up...like half mile or longer track...then there would be some justification of having the Roll Center on the left side. The BBSS setup uses aero down force to plant the right front tire in a turn. You run super soft springs to lower the nose and seal off the sides to create downforce. With this set up if you have too much force on the right frtont tire you need to take some of it away by moving the RC to the left until you get the proper amount..tire temps and handling, lap times..etc...will tell you this.
On short tracks this is not a good idea as your top speed just is not there. So on short tracks you maintain the RC offset to the right as is discussed in previous pages on this post.

Because the BBSS set up is the current fad, everyone is going to it without understanding why it is used and where it is used. So copying a set up used by the hot dogs on a long track , and trying to make it work on a short track will have the following results.

Aero will not be enough to download the right front tire so instead of turning at corner entry it will push. Since you have not transferred enough rear weight and cross weight to the right front tire because the RC is biased to the left, the left front tire have a lot of front end weight staying there thru the middle of the corner. Since the left front spring is so weak it is not able to transfer weight to the right rear tire on corner exit. The car is loose off.
Add to this factor the Detroit locker variable and you end up chasing your tail at the track.
The plus side is the fact you got the weights where they should be so the basic package is there..

 

[Ranger Mike]

heat cycle of the tire

Like I have said many times , it is all about tire, Tires, TIRES. The driver that makes them last longest and has something left at the end of the race should win..all other things being equal. So where do you find an edge? The days of tire softening should be gone..but I suspect a lot of you are doping the tires with softeners. I did it too and there are a lot of tricks doing this and it is dangerous as you can make them gumballs and deteriorate to the point the tire disintegrates..this discussion will not go into tire softening,

Heat cycle – commonly known as scuffs, scrubs, a heat cycled tire has been brought up to race temperature then permitted to cool. You want the rubber bonds to be stretched and heated then cool down and permitted to re-attach to each other in a controlled manner. Specifically, tire heat cycling means a new race tire is brought up to temperature, removed from the car and the valve stem core is removed, the tire is stored in a cool place , laid flat with not other tires stacked on it, and is given a minimum of 24 hours to relax and relink the bonds between the rubber molecules. Putting race tires through an easy initial heat cycle and then not running them for a minimum of 24 hours allows the rubber bonds to relink in a more uniform manner than they were originally manufactured. Heat cycling actually makes the tread compounds more consistent in strength and more resistant to losing their strength the next time they are used. It increases the life of the tire and you increase the number of total l heat cycles the tire has before the grip falls off.

You can do this at the track on tune and test day. Heat cycling can be done by installing a new set of tires on the car and running 10 to 15 minutes on the racetrack. You have to run easy laps, and build up speed as the session continues. Run slow lap times 5-10 seconds off your normal pace. Do not cause heat shock by spiking the tire temperatures by spinning, sliding or locking the tires. Drivers attempting to heat cycle tires in the morning for use a few hours later in the afternoon will not experience any benefits from the morning attempt at heat cycling. Heat cycling tires on Saturday means not using them again until the same time on Sunday.

One weird thing I ran into that caused me to build a tire heat cycle machine was an incident regarding tire stagger. The drill on tune and test day was to air the tires with nitrogen ( more on this later) measure the stagger ( outside diameter of each tire with a small tape measure deigned for this. The tries were hot lapped and when the car came in we immediately jack up t he car and remeasured the stagger. In one case the stagger was SMALLER than when the tire went out? How the heck did this happen..the danr thing shrank! It took a few days and many telephone calls but I found the answer. Seem that when the tire is manufactured, it is inflated after the last press mold operation. The post manufacturing inflator was operated by some college kid working over the summer and the tire was over inflated and permitted to cool and took a set that measured considerably higher diameter than normal so when he tire was heated up again, it snapped back to the proper nominal diameter. Well this got me to thinking of a better way to heat cycle the tires so I built a heat cycle machine. I’m out of beer so got to run..more on this tomorrow.

 

[Ranger Mike]

Tire heat cycle machine

Tire heat cycle machine
The problem with scrubbing in the tires is heat shock of spinning them coming off the corner. This is much more controlled method to run in the tires..takes about 20 minutes per tire, monitor the temps with an infra red pyrometer while it is running.

Just about everything on this machine came from McMaster - Carr , a huge mail order mill supply house in Cleveland. I got the motor from Grainger because they were local and i had to experiment to find the right H.P and pulley diameters...but air cylinder, pillow blocks, control valve, air regulator, and rollers, (from roller conveyor), are from McMaster - Carr, as did the later linear bearing for the tire truer.
The frame is 2 x 2 inch mild steel that is hinged to pivot up to load the inflated tire. The downward pressure is applied by large air cylinder hooked to air compressor. i have a directional valve to accutate the cylinder, up to remove tire , down to load tire slightly then i kick in the ac motor for rotation..once the tire is up to speed, i can crank in more downward pressure with the air regulator.This down force varies with front vs. rear tire..too much pressure makes the tire wobble so you have to read the surface tire temp via infra red pyrometer to get maximum heat over whole width and not load the tire to ridiculous load that can cause wobble. Most tires are pretty consistent so the down force is pretty much the same in spite of compounds.

i added a tire truing attachment to make the tires round to .010 inch after mounting. I also check run out of mounted tires and have found one out of 4 tires mounted by local tire guy or Hoosier / Goodyear tire trailer folks will be off as much as .060 inch. To fix this you simply break the bead and re-seating the tire and most come in around 0.020 inch when properly mounted...btw..there is a small witness line running around the border of the tire/wheel and you can take a set of dial calipers and measure this as well...( the above 0.060 inch run out correlated exactly with the witness line offset.

There are many other things we do when heat cycling the tires..logging thermal growth, change of stagger etc.,,,it does take a few dollars to build it ( $ 1200 in 2001 dollars) but i did this when we had a fat race budget running 4 state super late model series so the only expense i had recently was a v belt...
the tire shaver/ run out gage portion cost about $ 500. The end results are we get twice the tire life from a set of tires..without them falling off the tire grip scale...

 

[Ranger Mike]

Aero Drag and Aero Downforce

Ok now that we have a pretty good idea about how the suspension works, let us look at one of the least considered and most misunderstood things that happens at a race car at speed- Aerodynamic Down force and Aerodynamic Drag.

A lot of this info is straight out of Chassis Engineering by Herb Adams..recommended reading for this class.
Waddell Wilson ( famous crew chief and engine builder ..won 22 Nascar races I think) once said “ Anytime the car is moving, your moving air” or something like that..he won Daytona 3 times so I will listen to him. Anytime the car is moving , it is literally pushing thru a very heavy curtain of air. Imagine a curtain that weights 14.7 pounds per square inch. If you think about it , at sea level , there is a 2000 pound column of air pressing on your body area that would smash small car. That 7 foot tall curtain covering that 6 foot wide bay window in the front room has 840 square inches of area. A similar wall of air would weight 11,760 pounds and it is only an inch thick per or simplistic calculations. That is a lot of weight to push thru. Now consider that when a car is moving the air pressure measured on the top of the car is going to be different than the bottom of the car. It is never equal in the real world. to be so would mean we have a perfectly symmetrical ( same on top and bottom) shaped car. If there is more LESS pressure on top of the car than the bottom of the car we have LIFT. This is not good. We want more air pressure on the top of the car to develop down force. The amount of down force we have is measured in Lbs. One more amazing thing- Down force changes with the speed of the car. In engineering terms the change in magnitude of down force is in proportion to the speed squared. Ifin we got 50 pounds of down force at 30 mph we got 200 pounds down force at 60 MPH. Now we know how down force is created. We increase the air pressure on top and recue it underneath the car..simple!

There are many formulas used to calculate down force. All the big dollar teams use wind tunnels that have wheel weight scales and gauges to measure Drag, wind velocity et al.
But..this is not necessarily a good thing. The wind tunnel is designed to move air into and around a stationary vehicle. But you don’t race in 185 mph wind, you race at 120 mph through relatively still air. It's a different set of dynamics between the two conditions. This is not exact science and all you can hope for is seeing relative improvement not exact data. Air moving through a wind tunnel has a significant amount of energy whereas still air on a racetrack or on the road has none. One pound of air displaces about 13.07 cubic feet of volume at sea level. If one pound of air is traveling 75 mph in a wind tunnel, it would have 110 pounds of inertia. There is approximately 20 pounds of air contained in the volume of the race car. That equates to 2,200 pounds of total inertia. Each molecule of air has a lot of force trying to keep it going in the flow direction. It will take a lot of force to change its direction and once you do change its direction, it will carry a lot of force trying to keep it going in the new direction. Compress that high-energy air between the car and the walls of the wind tunnel and you introduce more variables for which you can account. There are a number of things you can do to figure out Aero on your car that can yield many advantages...cheap...you can do this yourself. More on this in later posts.

Why bother with Aero stuff??
Please re-read post # 19 on page 2 of this Thread.
Let’s look at a 3000 pound door slammer with 50% front and 50% left side weight. If you know the tire performance curve from the
manufacturer charts weight (vertical load in static pound) vs. Traction (lateral load in lbs.) you can calculate the Cornering efficiency.
What do you do if you do not have the tire performance curve?

Option 1. Take a Swag and guess. Typical Corvette corners at .84gs, road race sedan like Tran Am - 1.15 Gs,
2800 pound Super Late Model door slammer on 10 inch slicks set up to turn left only -1.30 Gs.

Option 2. Measure your Cornering Force.

Cornering Force in Gs = F = ( m*v2 ) / R
the above formula did not translate well when I pasted it, it should read mas times velocity squared divided by R

from our cone killing days in SCCA Autocross..skid pad testing ,,go to parking lot, airport,,what ever, set up circle 210 feet in diameter ( this is a 1/4 mile circumference flat track),
drive around the circle as fast as you can without spinning out..
G = 1.225 x R / T squared
R= Radius of the turn in feet
T = Time in seconds to complete a 360 degree turn

We did this and came up with a lap time of 15.65 seconds. This when squared is 15.652 = 245. Working backwards we have 1.225 x 210 / 245 = 1.05 Gs
If we multiply the vehicle weight by the G force of 1.05 we get 3150, close to the figure we calculated using the tire performance chart. See chart 14-2

Lets look at what happens when we increase the Cornering Forge to 1.15 Gs.

1.15 = 1.225 x 210 / T squared or 1.15 = 257.25 / T2 or 257.25 / 1.15 = T squared which when square root is found = 14.95 lap time.
We will cover more on adding aero down force in later posts but let us look at the other Aero happening – Aerodynamic Drag.

Carroll Smith said a lot about this in Tune To Win (a must for serious racers). It takes horsepower to move a race car and the les s HP you use to take care of Aero Drag the more you have to out accelerate the other guy. The formula for Aero Drag in this discussion is - Fd = Drag in Lbs. = Drag Coefficient (Cd) x Frontal Area (surface area in feet squared) x Velocity in MPH squared x / 391 which does tell us the pounds of drag but we really need to know the horsepower required to overcome the drag so Drag HP = Cd x Frontal Area x Velocity squared
A typical grocery getter you see at the supermarket needs 20 HP to over come aero drag at 40 MPH but 160 HP to run 80 MPH. Our Formula car runs 140 MPH on a 150 HP engine so we have a lot smaller frontal area and better Cd. See attached chart and I am out of beer..to be continued.

good link to see trick aero stuff on a real race car ( without fenders)
http://insideracingtechnology.com/usgpbar.htm

http://www.circletrack.com/chassistech/ctrp_0609_short_track_aero_drag/viewall.htmlhttp://www.circletrack.com/ultimate...ace_cars_explained/viewall.html#ixzz2WMvQWMrw

 

[Ranger Mike]

Bernoulli and Coanda

Bernoulli’s Theorem
The main formula used in the Aerodynamics department is Bernoulli’s Theorem. This can get confusing so I will attempt to simply it to the basics. It involves Pressure, Velocity of air in this case and Area of the surface that the Air is moving around. The Theorem states that the total amount of Pressure times the velocity times the Area one side of an object MUST total up to the same number around the other side of the object.
P1 x V1 x A1 = P2 x V2 x A2
Let’s look at the attachment showing flow around a round tube. Obviously, when the tube is stationary, we have equal pressure and velocity (none since it is not moving and air is still) and the Surface area is equal. Even if we hung the tube out of the window of the grocery getter at 30 MPH we still have equal numbers. Note the major turbulence and partial vacuum caused by the layers exiting the tube. This is Aero Drag and will be discussed more in later posts. Next, we look at a piece of strut used on formula car control arms. It is of the tear drop design that permits the air to re-attach to itself after flowing around the major width, thus greatly reducing DRAG (note the Cd is 10 times less that the round tube). Again we have equal numbers on both sides of the equation.

When we look at the cross section of a Formula Car front or rear wing designed for down force, see attached Jpeg, things get a little trickier. The fat teardrop on the right side is the leading edge. Air hits the wing generating a small degree of lift which immediately drops of on the top of the wing. Air rushes under the bottom of the wing and reconnects wit hair separated by the wing a few inches past the trailing edge ( left side of the sketch). Note a vacuum and turbulence just after the trailing edge. Now see photo attached. I measured three wings and all three had about an inch more length on the bottom side than the top. The beauty of this theorem is that both sides of the formula MUST equal out to the same number. We have air passing over two different surface areas. One is an inch longer than the other. The only things we can mess with to equal out the numbers are pressure and speed since the Area of each side is fixed. The air is going to have to travel faster on the bottom side to make up the difference ….and we have the same amount of “pressure” being applied to a surface that is an inch longer. So what happens when you have a given force applied over a larger area vs. being applied over a smaller area? In this case we have lower pressure on the bottom side than the top side of the wing. Down force!

Lesson here is this- On the top of wing or the race car we want lower speed ( relative to the bottom air speed) and high pressure and we want higher speed ( relative to the top of the wing/car ) and low pressure on the bottom side to create down force.

Coanda Effect
Henri Coanda built and flew the first jet powered air craft in 1910. Use that one at the next triva b.s. session! A moving stream of air in contact with a curved surface will tend to follow that curved surface rather than going in a straight line. If you take a small piece of paper 3” x 5” and hold it by the narrow end with both hands so the long end dangles and the paper forms a curve, take a deep breath and blow over the curve , the paper will LIFT..magic!

There is a real good article at www.formula1-dictionary.net and
http://www.terrycolon.com/1features/ber.html

Now there is a whole lot of stuff we could discuss here like attack angle, Aspect ratio, lift to drag ratio, pressure vector etc.. but this is a race car suspension class and not a tutorial for aerospace design and build so I gladly leave this to folks much more qualified than I to take over. What I will say is that these basic principles do apply to the weekend door slammer running only 70 MPH at the local bull ring so stick with me, racers.

 

[Ranger Mike]

Front end down force

My college friend Bugsy, had a 350 cid Chevy Camaro. It was hopped up and the front end got very light at 120 MPH. This was because the front end generated 300 pounds of LIFT at 100 MPH. We didn’t know about this at the time and are very lucky that no cows were out where new were making a top end run that night..the sins of youth. When you generate lift that is 10 % of your total body weight, you have problems.
Let us review what aero is all about. Back to our grocery getter, stick your hand out the wind at speed. Keep the palm flat ( parallel to the ground). This is air flow flowing over and under your car. Now rotate your palm 90 degrees so your palm is perpendicular to the ground and the wind forces your hand backwards as the full impact of the air mass is hitting your palm. This is Drag. When you rotate the palm 45 degrees so the thumb it pointed up you experience lift. Rotating the palm so the thumb is 45 degrees down you experience down force.
Palm at 90 degrees, your hand has lots of resistance. The air pressure pushes it back. This is drag. Measured in pounds, total drag s a combination of the vehicle's frontal area, dynamic pressure (air density and velocity), and a shape factor that defines how slippery it is. The shape factor is a dimensionless numerical expression called the coefficient of drag (Cd). Think of Cd as an efficiency factor, like horsepower per cubic inch (hp/ci) is for engines.

Here is the real deal. Aero forces go up with the square of the speed, meaning that when you double the vehicle speed, you increase the total drag resistance by four times. That is bad but even worse is the fact that horsepower requirements to reach a given speed go up with the cube of velocity. At 200 mph it takes eight times as much power to push the car through the air as it does at 100 mph. We will address drag later on.

Just about all door slammers have natural positive lift. Older cars with blunt front ends generated a whole lot of lift. My 55 Chevy was very light at 100 MPH because air got underneath and pitched the nose up. Even modern, fuel-efficient, jelly bean cars have lift. Positive lift is bad. So if lift is bad, why don’t the automobile manufacturers remedy lift. To counter lift we have to add down force and the cheapest best way to do this is to add an air dam ( spoiler) to the front end. This adds cost and even worse, adds drag. Drag sucks fuel economy and now the fuel mileage dictated by the government is not met and the car is not legal. The car manufactures walk a tight rope and balance the fuel mileage with front end lift and since it is illegal to drive over 70 MPH in the US ( Texas may have higher speeds..don’t mess wif Texas) lift over 70 MPH is not in the mix. Down force cancels lift but increases drag. So what do you do?

I think it was Fredrick Taylor who said “Form follows Function”. Ifin you are running the 1/4 mile drag strip ( 1000 feet now a days, I guess) less drag is the priority. Ifin you plan on turning left you better put some down force in the car. Our Formula Car driver likes down force. Short track racers are just getting into the aero aspect and can benefit by improving the Aero on their car. If you race a pure stock class car made in the past 20 years, where the rules say no body modification, you probably will not be able to reduce drag much. Even if you took off the windshield wipers, antenna, and side mirrors and sculpted the door handles and taped off the grille, you would not improved things that much because the automobile company spent millions making the car slippery to meet the fuel mileage standards. In the case of a Sportsman class rear wheel drive V8 engine, you can add things that can also even reduce aero drag.

Three areas you can make major improvements to aero are the lower front nose, sides and top rear of the race car. We will cover the front end here and the rest later as I am almost out of beer.

Front nose- You can attain a small amount of aero by adding an Inch of rake to the ride height. Make the rocker panel an inch lower at the front than the rear portion. Less air wil go under the car .This will cause a low pressure area and we have down force. Adding a front spoiler will cause more down force. A Spoiler is really an air dam. You are basically stalling out the air in front of the air dam and what ever small amount of air that gets under the air dam is moving faster than the stalled air and we have a pressure differential. This is increased when we have a splitter. .

Front end splitters on a race car produce aerodynamic down force by creating difference in the air pressure on upper and lower side of the splitter when the car moves.
It is attached to the bottom of the front bumper and stays parallel to the ground.
Splitter is about 2x longer than the ground clearance to be effective. So if the bumper is 3 inches off the ground, the splitter needs to stick out 6 inches. They are dangerous, and illegal on street cars for obvious reasons. See link below and look up splitter for more info..gotta run..no more beer

http://www.formula1-dictionary.net/

 

[Ranger Mike]

side skirts and rear spoilers

If you have an air dam ( spoiler) mounted on the nose correctly, you now have low pressure under the car. We want to keep it there. One technique is to mount side skirts to the rocker panels. The design of the car plays a vital role in the effectiveness of skirts. Ideally, one wants to obtain lower pressure under the car by either speeding up the flow or simply removing the air altogether. The latter is usually not done due to the sanctioning bodies rule on body components. . Skirts are used to block flow from entering or leaving the sides with the purpose to retain low pressure by sealing to the ground or aid flow speed by minimizing flow through the sides. . Lots of people think the skirts are there to keep the under car air from spilling out, but always remember that air flows from high pressure to low pressure. If you are generating down force from under the car then side skirts are a must.
The effectiveness of the skirts depends primarily on how close to the ground the lower edge can be maintained. That edge should be less than a .8 inch from the ground, otherwise the skirts' effectiveness diminishes rapidly as the gap increases. in 2012 NASCAR issued a technical bulletin to teams outlining the changes, right-side skirts now must have a minimum clearance of 4.5 inches, with a maximum clearance of 5 inches. Left-side skirts must have a minimum clearance of 5 inches with a maximum clearance of 5.5 inches. but remember we are talking about 200 mph speeds here and suspensions that pancake down with the tremendous down force generated by the aero stuff we are discussing.
There is another method to block high pressure air from spilling under the car at speed. You can stick vortex generators on the front bumper and a wall of spiraling air will block out the high pressure air from seeping under the car. Not for everyone but it is a proven technique.Rear Spoiler - A rear spoiler is a device attached to the car's upper rear surface (usually the trunk lid) with no gap between it and the bodywork. If it has a gap,it's considered a wing, which is far more complex. As its name implies, a spoiler's purpose is to "spoil" the fast, smooth,low-drag airflow coming off the roof. By sticking up into the airflow, a spoiler causes the airflow to detach and separate, reducing its velocity and creating a pressure rise that decreases the rear lift tendencies. Although a rear spoiler primarily adds down force, in some situations it can also decrease drag, depending on the spoiler's height, angle, and length of extension off the deck. The more vertical the spoiler's angle,the more the down force at the price of increased drag. Exact results vary per vehicle and can only be determined by cut-and-try testing, but some studies suggest that the most down force is achieved with a spoiler height that's about 8 percent of the car's wheelbase. That means about 8-9 inches for a 106-inch wheelbase. For any drag decrease, the spoiler height will usually have to be less than 1 inch, but up to 2 inches there is usually no drag penalty. Down force that they generate has been shown to increase with increasing angle (measured from the horizontal plane). A 60° rear spoiler causes a change of about −0.20 in the lift coefficient CL. Adding a rear spoiler helps delay the air separation of the air stream coming off the roof. When the air stream finally hits the non channeled air past the rear of the car we have turbulence and a dead zone that causes drag. The more we can delay the time the air finally detaches, the less aero drag we have. Look at the back of a semi tractor trailer next time you are on the highway. It is usually pretty dirty because the dead zone keeps all the yucky dirty water and crap swirling at the rear of the trailer instead of carrying it away. Studies have been made where vortex generators were glued to the trailers near the rear edge and mileage improved 3 to 5%, lights remained cleaner longer and the trailer handing was dramatically improved as it did not whip lash as bad. See Airtab pic...I may buy sum..

 

[Ranger Mike]

Diffuser

Diffuser – the non tech answer. Ok think of a carburetor, more specifically the venturi portion. We all know that air coming into the throttle bore is accelerated as it moves thru t he venturi area. It is being forced to move thru a smaller ID and has to speed up (Bernoulli's law). A diffuser replicates this principle in that it scavenges high speed air rushing in under the car body ( the first suction point) , accelerates it until it hits the diffuser, then slows it down and eases it back to normal air speed thus creating down force and reducing aero drag. As air flows in from the front and sides of the flat bottom, its follows the flat floor until it reaches the diffuser. At this point we have a maximum suction peak. The air hits the expansion chamber, is slowed down until it reaches the same speed as the slower free stream of air passing around the outside of the car. It does this by providing an area for the air to expand and slow down. It reduces drag by not causing flow separation of the air stream at tunnel exit. It provides a degree of wake infill ( helps fill in the area behind the race car ) thus reducing drag.
This whole air flow thing got me to thinking if there was a way to merge the two streams together. I did not know it at the time but it had already been done. Next post - diffuser pumps.
The aft part of a car underbody is where a diffuser is usually located. It works by accelerating the velocity of the airflow underneath the car. The pressure under the car is affected by the diffuser so that it can expand back to ambient in the diffuser, as the car moves through the air. Since the pressure below the car is lower than on the side and above the car, down force is produced if implemented correctly.
The faster you go, the more down force you generated. Think of fast moving air ( being generated by the diffuser) and then slowed down at the rear creating a vacuum effect, sucking the flat body to the ground.

If we dump the exhaust header into the diffuser expansion chamber portion we can help extract the air from the rear of the car more effectively .The hot exhaust gasses produced effectively energies the airflow, ( heats up the air stream - think expanding) helping to raise the low pressure air .This fast moving air flow returning back to the ambient atmospheric pressure at the exit of the diffuser, reducing drag levels. Hot exhaust gases also aid in expansion, again aiding in the airflow speed transition between fast moving underbody air and slow moving ambient air. Resulting in higher vacuum effect, more down force and reduced drag.

FYI- a true flat bottomed car (one without a diffuser) will produce down force in and of itself when run in rake. Essentially the entire flat bottom becomes one large diffuser. It too has two suction peaks, one upon entrance, the second at the trailing edge of the flat under tray. A diffuser acts to enhance this underside suction, it acts like a pump, encouraging better flow under the car. One thing to note is that the rear wing interacts with the diffuser "driving" it. The proximity of the low pressure side of the rear wing encourages better flow through for the underbody.

I attached a photo of a diffuser. It is flipped bottom side up so you can see the strakes and get an idea of how the air is channelized and permitted to expand. I posted a photo of the old formula car and we are doing some tuft tests. We taped tufts of yarn onto the diffuser to see what kind of flow we had. We took two Sears shop vacuum cleaners and place the hoses above and below the diffuser. These babies tout the fact they are 175 MPH shop vacs and wear ear plugs ifin you are planning on extensive testing. We wanted to see exactly how the two air streams were interacting. I can only upload 3 photos at a time for each post so may take a couple of posts to cover this. My point is that you can get a pretty good idea of air flow in your garage without a lot of fancy equipment.
As you can see by the photo. we have air flowing out of the diffuser expansion tunnel and we have air flow coming off the top of the diffuser and interacting with the bottom portion of the bottom wing. We initially thought these two streams were acting independently. Further testing found these two streams were combining to a degree. We had to find an empirical way to measure the flow. We took a light weight piece of aluminum and made a hinge with racers tape. We used the tried and true dial indicator and now had a method to put a number on the whole activity. The plan was to measure the dial indicator with the bottom vac only, then the top vac only then with both running at the same time.
Top vac on - 0 reading - the air stream was flowing over the top of the piece

Bottom vac on - .300”

Both vacuums on 0.200”

I assume the air dropped because it attached itself to the top vac flow and was being pulled over the top of the aluminum piece. This was confirmed when we placed a taller post in the air stream and more tufts were flowing straight horizontal at a higher level than when only the top vac was running. Conclusion- air is like taffy and will combine and increase flow. We did a lot of flow testing at the race track with WD 40 and we were able to find dead zones. Good flow meant a straight oil stream. We noted these areas and did extensive tuft tests on the dead zones to improve the flow.

 

[Ranger Mike]

Diffuser pump

Let us look at the above post # 575 specifically the attached illustration 50b. Note that after air passes over/ around an object and is not channeled to slowly transition back to ambient atmosphere as the tear drop design does in 50c, the two air flows will counter rotate in mini vortexes immediately behind the object causing drag. This is the situation when air comes in under the car at the front is accelerated and the decelerated and comes off the roof of the diffuser. Air flowing over the top of the diffuser comes off the bottom of the diffuser and we have this condition as noted in attached 50b.
At the same time we have air flowing over and under the bottom wing designed like 50c and has 10 times less Cd ( drag).

So if we took a mini wing designed like 50c and mounted it so it would pull air flow from the diffuser and attach it to the much better air flow from the rear bottom wing ( see 81 car pic and note the little wing thingie bolted on the end of the diffuser) ..we in theory would have a net gain of flow and a reduction in drag. This is what the diffuser pump does. It took a while to tuft test and find the best location and mounting angle. See the 2 x 4 with many holes drilled at different heights. We had to play with fore and aft spacing then attack angle. The results proved that the air flow was moved up and the flow was increased as our little dial force gage indicates. I have many more photos of the testing but only can upload three at a time. You get the idea. You can do a lot of things with two vacuum cleaners and some yarn.

 

[Ranger Mike]

cheap telemetry

Couple of things to tie up aero discussion. I bought this little 3 PSI gage from McMaster Carr. You can get a good idea of pressure on car roof and at bottom of front air dam by plumbing this gage to these locations. Do not run vinyl tubing unless you can tape it down so it will not flop or vibrate when you are driving. You need a stretch of pavement so you can get speed up to around 50 to 60 MPH and be able to read the gage. Most race cars do not have speedometers so note your RPM on tachometer in high gear. It doesn't matter what the speed is as long as you can maintain it so you get a decent reading at the given RPM. You want to find out the low pressure on the bottom and the high pressure on top. Then you can make aero changes accordingly and re-measure the pressure difference to find out if you are improving it.

The little fluid force gage and actuator rod was used to measure front A-arm movement on track during tune and test. I had to mount a VCR to read the thing. Mounted the gage next to the tach so we could see where we were in the turn. Played the video back in slow motion. Hey..it was not precise but got what we were looking for.

 

[Timaladd]

Mike, I race a crate latemodel on 1/4 to 3/8 dirt ovals. I see a lot of my competition using a 12" rear spoiler. I can get all the traction I need coming off. Am I missing the boat by using a 6"?

 

[Ranger Mike]

Timaladd welcome. Usually we run taller rear spoilers to put some down force in the car at the end of the chute, not coming off the turn. If you suffer too much over steer ( too loose) going into the turn you may want to add some height on the rear spoiler. I think the size track you race is too short to get true aero advantage and too much spoiler may add way too much drag and hurt you. A lot of the tall wing thing is money see monkey do. So if you are hooking up ok all the way around the track, sounds like you are dialed in.

 

[British Menac]

Hi All,

I do not wish to highjack any present discussion here but I do have a quick question I would like to ask the more experienced ones amongst us here ...
For a start, I do apologize for the generalistic nature of the question but I'm really looking for a general feel of relationships in dampers.

My question is this ...
Whats the current ratio of damping force in compression compared to rebound?
I know this may lead to some discussion and may even warent a separate thread so again I aplogize if that's the case.
I know current trends, especially on road courses are for very stiff compression damping force with high gas pressures to further support the outer wheels/ tire in cornering but in comparison to rebound ... how much stiffer?

I'm particularly interested in touring car, formula cars ... on-road cars really.
Although. Does this ratio go more to 1 to 1 in off-road?

I have heard ratio's for on-road touring and super 8's as high as 8 or even 9 to 1 in compression to rebound. Compression being the stiffer/ stronger force ... is this true?

Thanks for any input.

Anthony

 

[Ranger Mike]

Welcome Anthony...good question. I have only addresses this on post 217 page 13 briefly and is bares discussion. If memory serve me..dubious at my age..a typical passenger automobile uses single tube dampers ( shock absorbers to those in Dixie) . As the name implies this is a single tube mounted to the suspension component(usually lower A-Arm or control arm. It is filled with oil and a piston forces oil through an orifice as it moves up or down. The end of the piston rod is attached to the automobile body of chassis. There is equal force required to compress the shock and retract the shock ( rebound action). This is known as 50/50 valving so that the typical grocery getter can handle the normal bumps and pot holes the car will encounter.
For strictly drag racing application the front shocks of a rear wheel drag car will be 90/10 that is 90 percent " up" force ( takes a lot to compress the shock) to keep the car nose in the up position to give maximum weight transfer to the rear wheels. On some dirt race cars they run a left rear tie down shock that is biased to maximize the compression and make it very difficult to pull the piston back to neutral position. Please permit me to dig out my notes and I will get back with you in a day or two..at track testing now..

 

[British Menac]

Thanks Mike.
I really would appreciate your knoledge and input on this...

Anthony

 

[Ranger Mike]

I had a long chat with Jim Stimola of SRP Engineering. He has years of rebuilding Indy car dampers, Nascar Shocks and is in my opinion the go to guy on shocks. Short answer on ration of compression to rebound..how high his the sky? There are just too many variables to make a blanket statement. And since shocks are one final step in fine tuning a chassis, the secret specific combination of compression and rebound for that chassis on that day given that particular tracks condition is a very close hold item.
Single tube shocks have one setting for compression and one setting for rebound. Rebuildable gas shocks like Penske have infinite setting on compression and rebound AND have a nitrogen reservoir that can be variable as well. The combination settings are darn near INFINITE! Add to this the fact that shocks have their own motion rate that may differ from t he springs mounting angle and thus a different motion rate and the permutations go up. So is it possible to have 9 to 1 ration on compression...sure is. Is this the hot set up for all touring sedans..no way. All I can say is we swear by Penske shocks and fine tuning is done every time we go to the track. To get to this point you must have the spring rate and ARB spot on and a gas shock can be pumped up or reduced to one spring rate setting ( add or reduce total spring rate by one increment). Hope this helps.

 

[British Menac]

I see. Well thank you Mike for putting yourself out to talk to Mr. Stimola of SRP.
I understand about these guys wanting to keep specific settings to themselves and I believe we all in racing do the fine tuning at the track...
What I was looking for was a more generalistic slant on compression-to-rebound ratio's.

The old way and unfortunately some still set their dampers this way was to have compression very soft and tune with rebound resulting in rebound damping becoming way too stiff.
More recently I know its been found that a stiffer compression relative to rebound gave better results in keeping the contact patch of the tire in good contact with the surface.
Gas pressures of 1/4 to a 1/3 of corner weight have also helped in supporting the outer tire in cornering for better road holding.

What I was wanting to try and find out was to what extent this ratio had changed. Specific settings do not interest me. If a certain team like to run their dampers on 7 low velocity comp' and 4 on rebound with 250 gas pressure does not interest me.

What I was looking for is in general terms how much this ratio has changed ... it would seam that teams and certain people think this might give too much info' away. Well that's too bad.

Thank you again Mike for asking and your work in this field and the effort you put into this thread is fantastic.

Anthony

 

[Ranger Mike]

Anthony..thank you and if you like..send me a pm with specs on your car..weight, motion rate, spring / arb configuration..tires etc and I will send on toe Jim..may get more insite..

and better yet, if you want to post for all to see..would be good learning experience for those who follow this forum.
we call can learn..thats what its about..

 

[Ranger Mike]

weight transfer when cornering

My original post on page 2 post 19 was from an old chassis set up class I took years ago. It never did go into detail on how we find the weight transferred during cornering. Was a rough estimate of metric chassis and spec tires etc..I did a lot of research after it was apparent that the figures were not as accurate as they could have been. But racing is about constant adjustment so here we go..

The purpose of this post is to find out two things.
1. The total amount of weight that is being transferred during cornering
2. 2. The amount of weight transferred to the front end during cornering.
It is pretty easy to find number 1 but a real bear to get to number two, Without going into too much detail, the former is straight math and the latter requires major calculations diagonal weight transferred because inertia and momentum in a turn.

We have a Spec Late Model stock car .Weight is 3111 pounds, 66”track, 103” wheel base. The race track rules state minimum crankshaft to pavement height is 10.5 inch. We have coil over shocks and small block Chevy with aluminum heads. Just how much weight is being transferred under cornering conditions?

Specs- total weight of vehicle 3111# and we scaled the car and found these Wheel weights -
LF= 777, RF= 747, LR= 1027, RR = 560

First thing to do is calculate the height of the Center of Gravity. We are not going to be super accurate in this drill as we want to find out a ball park figure of correct springs to use to get us in the game. When we measure the engine block we find the cam shaft is 4.5 inch above the crankshaft. So 10.5 inch minimum crank height plus 4.5 inch = 15 inch CG Height ( CGh).

Next we have to figure the circular acceleration of the track we intend to race.
From our cone killing days in SCCA Autocross. Skid pad testing ,,go to parking lot, airport, whatever, set up a circle 200 to 300 feet in diameter, drive around the circle as fast as you can without spinning out.
Alternative- go measure the local race track. Measure the width of the infield. I use an old Bushnell Yardage Pro range finder used in golfing to measure the distance; this will give you the diameter of the turn. Divide this by 2 to find the radius. Next hot lap session, use a stop watch and time the car from when it enters the turn to when it exists the turn.
G force = 1.225 x R / T squared
R= Radius of the turn in feet
T = Time in seconds to complete a 360 degree turn
1.225 is a handy conversion factor
typical Corvette corners at .84gs and road race sedan like Tran Am 1.15 Gs. These were measured using skid pad testing. My guess is Late model purpose built stock car with 57% left side weight – 1.15 Gs1. The total amount of weight that is being transferred during cornering

Tw= Gs x car weight x CGh / track width
1.15Gs x 3111 x 15 / 66 = 813 total pounds transferred during cornering.
How good is this number? I looked at several recommended spring packages from various chassis books, chassis manufacturers, my chassis set up notes.

2800 to 3200 pound purpose built late model on 3/8 mile track 10 degree banking, recommended springs 350# LF, 350# RF, 220# ARB.

Initial impression is that we have 920 pounds of spring up front to counter the 498 pounds coming forward. Looking at it more in detail we have a coil over spring configuration mounted at 15 degrees. Looking at the attached illustration that 350 pound spring is really acting like a 191 pound spring. So we really have 191 + 191 + 220 (arb) or 602 pounds up front to counter the weight transfer.

The recommended coil over spring package in the rear was 225# and when you correct for the mount angle and the motion rate you have 94#. So by adding both rear wheel rates we get 94 + 94 + 602 = 790 pounds of resistance.
We calculated 813# above.

Weight Forward = Gs x car weight x CGh / Wheel base
WF = 1.15 x 3111 x 15 / 103 or 521 pounds transfer to the front of the car.
But we have a total front wheel rate of 602#..what gives??

We need to add the diagonal weight shifting to the right front tire when cornering.
We need to revisit the old high school trig book.
We have the 103 inch wheel base and the 66 inch track width. We have two sides of a Right triangle. The hypotenuse is 122 inch. ( ifin you can not figure out how to do this..go away).
We can find the angle of the hypotenuse intersecting the long leg at the right front tire location which is 33 degrees. Looking up the Degrees table the Sine of 33 degrees is .544

Diag wt = (Gs x Right Rear wheel weight x CGht / hypotenuse ) x sine A ( angle @ RF)
(1.15 x 1027 x 15”/ 122) x .544 = 79 pounds
79 + 521 = 600 pounds and our total front wheel rate of 602#..

Is this a perfect formula,,No. It is close enuff to get in the ball park on the spring package.

 

[smithdl4]

Ranger Mike;
Thank you for this classroom undertaking. It has been extremely enlightening and, as if you read my mind, you went back and explained how the springs were calculated. I went through what was done early and had lots of questions. This answered those so well that I just went through your calculations for computing front springs using my road racing car. I did the calculations using 1.15Gs. The car weights 2897 lbs, with , 709 RF, 738 LR and 746 RR. The track is 64" both front and rear with a wheelbase of 96". The front bar rate is 355 lbs/degree and the rear is 271 lbs/degree. Springs are 700 lb front and 200 lb rear. The shocks on both ends are mounted at 9 degrees. The motion ratio in front is 0.765. The rear motion ratio is 1. Both the shock and bar are attached at the same place on both ends. In computing the cross weight since the car was a road race car, I averaged the rear weights and used that to figure the transfer. When I computed I arrived at a rate of 1006 lbs of resistance for the front and 594 lbs of weight transfer. If these calculations are good is the car too stiff in your opinion? Final question, do I do about the same thing to compute rear transfer only subtracting cross weight from the rear instead of adding? Appreciate your thoughts and thanks again for your work here.

 

[Ranger Mike]

Well Thanks for the nice words, Smith. At first glance you have way to stiff springs and ARB. This is confirmed when i ran the numbers. A 2897 pound car with 64” track , 96” wheel base and I figured a Center of Gravity height of 16 inches. I came up with 832# total transfer. When you look at weight transfer coming forward I came up with 555 #.
1.15Gs x 2897# car x 16 “ CGht / 96” Wheel base = 555#

add to this the calculation of diagonal weight transferred. Our triangle is 64/96 = Tan A which is the angle of intersection at the right front contact patch. This is 34 degrees and the Sine of 34 degrees is .56 so when new figure t he hypotenuse of our triangle – which is the square root of ( 64 x 64) + (96 x 96) = 115 inch

diagonal weight transfer is the left rear wheel weight x Gs x CGht / 115 x sine A= 66#
we need to have spring rate of 555# + 66# = 621# up front.
You have 700# front springs. If we find the true wheel rate we get your motion rate of .765 x spring rate = 535#

You have a 355# front ARB + 535# + 535# = 1425# front end to counter 621# coming forward..a little stiff I think.

out back we have 211 pounds being slung around. Total weight transferred under 1.15Gs is 832# – 621# coming forward = 211#
we have a rear ARB of 271# plus 200# rr spring..again a little stiff.
One more huge wrinkle. The above diagonal weight calc is a little heavy because your have a REAR ARB. I would not worry too much about the impact of this until we get to where we are fine tuning the set up.
This is a formula for ball park figures on the springs and ARB. Intuitively, my gut tells me the ARBs and the springs are way to stiff.
I do not know the method you calculated the front motion rate or is you are running coil overs or traditional A-Arm and coil springs? Also we should find true CG height..
But still looks too stiff any way you look at it.

 

[smithdl4]

Ranger Mike;
Thanks for the affirmation on the car being too stiff. Wouldn't the MR be applied to the bar also since it is not being applied directly to the outer ball joint pivot point? Using what you did in your reply, how would you select springs and bar(s) for that example if you had to start from there? Is there a general guide for how much spring versus how much bar to use? This is kind of like asking a magician how he does his tricks. I appreciate your time and patience. Thanks again, Dave

 

[Ranger Mike]

i am in Detroit , I do not have my notes but if I remember correctly, the ARB is figured by diameter, length and length of the arm to get one rating. I forget the exact formula but I think you have to multiply the modulus of elasticity of the steel ( 500,000) in there some where. As far as the springs vs ARB,,,I wrote a lot about the BBSS big bar soft spring set up. If I remember the figures this morning when I did the calc, you need around 300 #springs and 200# bar..but..that did not take into account the rear ARB. In rear maybe 150# spring and 100 # bar..i am not sure about your rear motion rate of 1...have to see the dimensions,,Don't forget the more gas you burn off he less weight is coming to the front and the more the car will push. The closer you can get the ARB to the same rate as the springs the better handling it will be.

 

[Kozy]

Ranger Mike;
Thank you for this classroom undertaking. It has been extremely enlightening and, as if you read my mind, you went back and explained how the springs were calculated. I went through what was done early and had lots of questions. This answered those so well that I just went through your calculations for computing front springs using my road racing car. I did the calculations using 1.15Gs. The car weights 2897 lbs, with , 709 RF, 738 LR and 746 RR. The track is 64" both front and rear with a wheelbase of 96". The front bar rate is 355 lbs/degree and the rear is 271 lbs/degree. Springs are 700 lb front and 200 lb rear. The shocks on both ends are mounted at 9 degrees. The motion ratio in front is 0.765. The rear motion ratio is 1. Both the shock and bar are attached at the same place on both ends. In computing the cross weight since the car was a road race car, I averaged the rear weights and used that to figure the transfer. When I computed I arrived at a rate of 1006 lbs of resistance for the front and 594 lbs of weight transfer. If these calculations are good is the car too stiff in your opinion? Final question, do I do about the same thing to compute rear transfer only subtracting cross weight from the rear instead of adding? Appreciate your thoughts and thanks again for your work here.

For a pure lateral acceleration analysis have a look at this tool. You'll need to shift the defaults around quite a bit, but I think you have enough information to make use of it.

http://blackartracing.zxq.net/Load Transfer 2.php

 

[Kozy]

Well Thanks for the nice words, Smith. At first glance you have way to stiff springs and ARB...

Might need to clash swords with you again on these...

When you look at weight transfer coming forward I came up with 555 #.
1.15Gs x 2897# car x 16 “ CGht / 96” Wheel base = 555#

I am still not quite understanding your concept of diagonal weight transfer if it is being applied to pure lateral acceleration.

Why is this weight/load coming forwards? At 1.15g we assume pure lateral acceleration, and under any combined lateral and longitudinal acceleration both values must reduce to stay within a circle relating to that 1.15G maximum (the friction circle). IOW, you cannot be generating 1.15g in the Y direction AND 1.15g in the X direction at the same time, so the load on the outside front as calculated here is going to be more than the car will ever see in reality.