Race car suspension Class

[Ranger Mike]

good to hear from you again..
since you are turning left and right ,set the car up neutral with 50-50% left to right and maybe 52% rear weight bias.
The longer the trail arms you have the less dynamic change you will have on the rear end.
Locate the top link at center of the track width. Make sure you check bump steer and Ackermann up front. Center the RC. Get good camber curve.
Wheel scales are your best bet when building the car from ground up as you are doing. Engineer the thing to be easy to work on too.
Make sure no rear roll steer in the rear end.

Get on any SCCA forums for your class car and see what the hot dogs are doing on road course set ups.

build it wide and light then add in the ballast to make weight for class you are running.

 

[drobbie]

To follow up with John LNZ's question. When you speak of migration within one inch, I assume you are speaking about 1 inch left or right and not up or down.
From my studies RC movement in a straightline up and down is the optimum. Am I correct?

 

[Ranger Mike]

The 1 inch rule of thumb is strictly a ball park rule I made up. I do not have the time to tweak the race car to be perfect nor the dollars to buy/fabricate the goodies needed to make it perfect.

RC movement to the left or right will add downforce or take away down force on the right front tire. One inch migration will not hurt us and depending upon which way it goes may even help.The formula car we run has 2 inch RC height centerline and dropped to .9”, ran great.Assume a 2800 pound late model, 112” wheelbase, 66” track width, Center of Gravity (CG) = center of gravity and is 13” height.

If we have the Roll Center located in the centerline of the car, 66” track width, RC is 33” from each tire centerline.This 2800 pound car has 56% left side weight. Subtract the unsprung weight and we get 2100 pounds sprung weight. I am assuming things are pretty symmetrical so we will use 56% of this unsprung weight or 1,175#.If the RC was centered it would be located 33” from each front tire centerline and theoretically, we have ½ of the 1,175# or 587# left side sprung weight pivoting on the roll center and going to the right side slamming on the tire. (If the RC is located to the right by 3” we now have a lever between the RC to the tire centerline 3” longer on the left side and 3” shorter on the right side.

36/66= 55% so in effect we have increased lever by 5%. Now we have 55% of the left side inertia force thru the RC planting the right front tire.

55% is 646# so you get 60 # additional down force.Now add an inch of offset to the right 37/66= 56% and this adds 12 pounds. The point is we need to use body roll to create down force (not applicable on BBSS set ups). If we get too aggressive with the RC offset we start to lift the left front tire.So far this is all speculation. Let’s plug in some real numbers.

Weight Transfer due to body roll

First of all, by now, you know I hate the term “weight transfer” but since I poached the illustration off of http://www.autozine.org I will use it since they did a great job explaining it.In this example Weight is both sprung and unsprung weight.Lateral displacement of the CG is d. In this case, d = CG height in inches x sin of roll angle. Sin of 3° degrees is .052, sine of 5 ° is .088 sine of 10° is .173 and CG height is 13”If this racecar rolls over 3° we have .052 x 13 = 0.676Weight transfer to the outside wheel is (Weight x (RC location + d)) / track width2800# x (33” + 0.676) or 2800 x 33.676 = 94,29394,293 / 66” = 1,429# on outside wheelsCouple of things about this drill, folks. The Roll Center is assumed to be on the ground. Note the fact that we use the Sine of the angle of roll to find the additional length of the lever arm and add it to the RC to tire centerline lever length.Compare this to the Race car with centered roll center and no ° roll.2800 x 33 /66 = 1400 – conclusion, 3° roll gets you 30 pound down force, big deal (actually we should do the math right and figure exact RC location to get the true picture but hang with me here..)When we offset the RC to the right 3 inch and calculate

2800 x 36 / 66 = 1527 poundsAdd in 3° body roll and we get 1,555 poundsAdd 5° roll and we get 1,575#

Just for kicks that super dirt late model with 4 inch offset and 10 ° roll
2800 x (37 + (13 x .173)) = 2800 x (37 + 2.49) = 2800 x 39.49= 1,675#
What this tells us is the Roll Center location is important. One inch movement won’t hurt too much.

 

[drobbie]

Great explanation and example. I can now use this to understand what happens with my car. Thanks for the awesome info.

 

[Ranger Mike]

thank you Drobbie, all you all can figure it out as i got a lot of it from books and web sites. I just cut some corners and brought it to you a little quicker.

 

[Ognib]

Hello Ranger Mike. I found this forum a couple of weeks back & was excited to discover this thread.

I'm an old goat now, 70 yrs, and have been working on & "building" my own cars since the age of 14.
My favorite rides have always been the corner cars, which up to now have been limited to tuning stock suspension pieces, as much as possible, with bars, springs, adjustable shocks, tires, etc.

Decided to push it to the next level & am in the design stage of a formula SAE style open wheel car like the college kids build.
I've been welding & working with steel all of my life, so feel confident in my abilities with the cage, engine placement etc.

I've been studying Smiths book, "tune to win", & noticed that in many of his suspension illistrations they are shown with IC outside of the tires.
Your suggestion that this point should be about an inch inside the lower ball joint has left me somewhat confused over the differences in approach to the situation.
Was hoping you would expound on this a bit further & help a noobie wrap his head around the design & layout of a basic, workable first time scratch built suspension.

Many thanks.

 

[Ranger Mike]

Thanks for the kind words and welcome ..nice having a long term wrench chime in!

Steve Smith is excellent in his explanation of Instant Center (IC) width on page 19 of his great book, Paved Track Stock Car Technology.

His advice on IC width states his recommendations presented for Roll Center Height and IC width take into account all considerations on body roll and right tire contact on corner entry and mid turn.

His book is focused specifically on Left turn round track racing.Carroll Smiths excellent book is for the left and right turn chassis design and build and is correct as well.

IC widths controls how the RC acts in body roll. Wider IC means less camber gain. Narrow IC width means radical migration both on location and height. This effects right front tire load and we want to add down force on the rt ft tire to make it grip.

You do not want to bias the right front on a road course as this can make things slightly interesting for the driver.

On road course you want wide IC to minimize RC movement and camber gain.It depends on which pig you want to roast!

 

[Ognib]

Thanks for the clarification. I went back & re-read some stuff & there were a couple of things I had not retained.

Going to do a 2 dimensional model out of poster paper & play with it a bit to help me visualize how this all works.
With the small size of the formula sae cars, I can build it 1:1 for good perspective.

 

[Signguy]

RM,
I've been lurking on these pages for at least 5 years, learning and applying some common sense and mathematics that I learn wherever possible. Thanks for taking the time to share.

We as a group have been asphalt racing for 30 years, with some good success and several periods of no racing at all due to more important life issues. Currently, my group has been part time racing for the last several years with a super late model chassis under several rules packages on 1/2 mile and 1/4 mile tracks (14-18 degree banking). We had little success in tuning the car to have speed or just better tuneability. The engine made good horsepower but has iron-heads with a heavy block. It was Ford package with a Dart block, N35X heads, dry-sump and was nearly 35lbs heavier than a comparable steel-head Chevy. More than that against aluminum-headed engines. At the lower weight rules, we couldn't get more than 49.5% rear and at higher weight requirements we couldn't develop the comparative speed we thought we needed. I raised/moved the RC and tinkered with many other adjustments with no real relative speed improvements. We have subsequently developed a lighter weight engine package, having blamed some of our ills on the engine mass. Our first visit to the track with the new engine was very encouraging.

My question relates to cornering performance as it relates to engine mass. I happened upon thread about Formula Ford racing where the discussion centered on the aluminum vs. steel head cornering performance. It was stated there that on a road course the lap times would be measurably different just from the mass difference of the heads (assuming equal HP). Using some math and Performance Trends software I can see some statistical backup for my conclusion, but not necessarily the entire performance gap. What is your experience with trying to compete with heavier engine package, all other things being equal? I understand geometry and rear-steer gimmicks can't fix mass problems, but I'm just trying to make sure I'm not missing something before re-configuring a nice engine over handling concerns.

Thanks again!
---Signguy.

 

[Ranger Mike]

og, you can get a good idea with the cardborad cut outs but...circle track analyzer is $175 and a must. period. you would not want to try an engine rebuild without a good torque wrench would you?
the software is a must.

 

[Ranger Mike]

signgy welcome...we ran class a few sized up from formula ford. There a big brew ha ha about running an aluminum head. Lower swing mass, lower CG bigger hp (not). Like i predicted, the HP was down when forced to run the same compression ratio as the iron head. All the pros went back to the iron and we kept our. My main thing is the head gasket issue on the al head. We won many many division championships wit ha formula car that was constantly 20 pounds over weight.
do not take this the wrong way but..have you changed drivers? has another racer hot lapped the car to see if maybe it is technique?

 

[Signguy]

Well, the driver is very experienced. We also had 2 other drivers practice the car with the same conclusion. I measured, calculated, squared, plotted everything and never got a glimmer of hope that we have been headed in a positive direction. This winter we cut the car up and put it back together with a new rear clip and an aluminum-headed engine. Now we are in the speed neighborhood with some tuneability, but we are wondering about the direction for the second engine. For the classes we run it's almost as economical to buy a crate/sealed engine than it is to build up a set of full race aluminum heads comparable to N351's.

My conclusions ranged from "can't run against 2700lb cars at 2750 and only 49.5% rear weight", or "N351 heads/dart block are heavier than a stock engine", to must be something stressed or bound on the chassis.

I vaguely remember a Gary Balough story that had him running a brand new concept car (Dillon maybe?) that had a composite tub as a stressed member. Supposedly they couldn't get the car to perform until they cut it loose from one of it's mounting points. I know that we want to control motion with our suspension components, however, and that's why I'm doing the post-mortem.

I read from your answer something that reinforced a tip I got from a spec-Miata racer. 10 or 15 extra HP in factory spec class that features 150hp max... is hard to beat in the corners.

A related question, in my PT software, I set up my points off a marked chassis centerline. When we talk about the RC location, and your conclusion that ~3" right is a good starting point, are you referencing a geometric center of tread, or 3" from the lateral CG location? Right now I'm 3" Left of geometric, 2" right of CG. I'm setting up test configuration moving it further to the right. This assumes I am fairly accurate on my CG approximation. My apologies if you went over that earlier, I couldn't find it.

signgy welcome...we ran class a few sized up from formula ford. There a big brew ha ha about running an aluminum head. Lower swing mass, lower CG bigger hp (not). Like i predicted, the HP was down when forced to run the same compression ratio as the iron head. All the pros went back to the iron and we kept our. My main thing is the head gasket issue on the al head. We won many many division championships wit ha formula car that was constantly 20 pounds over weight.
do not take this the wrong way but..have you changed drivers? has another racer hot lapped the car to see if maybe it is technique?

 

[Ognib]

No, I would not do an engine without a good torque wrench...you have convinced me of the need. However, when reading their web site, I see circle track mentioned but nothing about road course or autocross type cars. am I missing something? My build needs to turn equally well in both directions.

 

[velocity_boy]

references _ Paved Track Stock Car Technology by Steve Smith
Tune to Win by Carroll Smith
Software - Suspension Analyzer by Performance Trends
In order to understand the complexity of a Formula Cars suspension, a basic knowledge of the stock car suspension should first be mastered. When designing a (front) suspension , geometry layout is critical. spindle choice and dimensions, kingpin and steering inclination, wheel offset, frame height, car track width, camber change curve, static roll center height and location and roll axis location are major factors.
The first critical thing to do is to establish the roll center height and lateral location. The roll center is established by fixed points and angles of the A-arms. These pivot points and angles also establish the camber gain and bump steer.

I have used Suspension Analyzer for years on Super late Model stock cars as well as formula cars and it is as critical as a tire pyrometer and stop watch, in my opinion..Saves tons of time figuring the roll center and will show the roll center movement as the suspension moves through its travel.

You say you worked with F1 cars?

Then you're aware no doubt that their suspension is even more complex than you mentioned?

Did you know it is also computerized? And involved telemetry? Where it can be tweaked during the race remotely by the drivers crew? According to what comments he may make about the cars handling during the race? And that, like tires and aerodynamics, as well as braking system, the suspension and other chasis components can be altered before each race, depending upon the track and weather conditions. Also due to the particular strategy the race team will employ during the race.

Did you know that the downforce of an F1 car is such that it can pull manhole covers up and send them flying? Or that the cars at full speed could actually drive upside down?

Yeah...you could say I'm into racing a bit! LOL. F1 and motorcycle, mostly.

NASCAR bores me stiff.

 

[Ranger Mike]

Og, suspension analyzer should have all the capabilities but is true 3 deep so you must measure mount location height , offset and front to rear position. This is a must for road course cars.

Sign - on road course i always use the vehicle center line as everything must be symmetrical. On round track racing, i use the vehicle center line since the engine will be off set a least one inch if not more and the wheels will be whacky non symmetrical off sets. Dirt track racers swap out wheels with a lot of offset changes as the track changes. Same with paved track.

I use a center line calculated from bottom ball joint centers. I paint a center line at he bottom frame, top frame rail and anywhere else i can access.Signguy - if you find the RC is to the left of center as you indicated..you may have located one problem.
Has the car ever hit the wall?
Have you ever checked the rear end ( assume 9" Ford) to see if it was bent?
This is one very often over looked area that will really screw you up. You have to remove the entire rear end from the frame, leave the wheels on both sides. You need to scribe center lines on both tires.
Measure the center to center distance.
Flip the rear end 180 degrees and measure.
do the same at 90 degrees so you have checked the rear end at each 90 degree position.
Many times you tape measure the rear end thinking it is ok but it does not show up until you actually remove it and check it.

 

[Signguy]

Signguy - if you find the RC is to the left of center as you indicated..you may have located one problem.
Has the car ever hit the wall?​

The QC has been checked regularly. It has aluminum tubes, cambered snouts and a bridge so checking it is a routine maintenance item. We'll run again soon so I'll update you on performance. Next testing item will be running a smaller KPI/SAI spindle on the RF. Seems that it's commonplace now, but I've never tested it "back to back".

 

[Ognib]

The formula SAE cars as built by the college competitors are smaller...usually 75 inch or so wheelbase with 50ish inch track width.
This is the general format I've chosen to use as reference...a smaller build is less expensive than a larger car...I'm retired, you know, & on a bit of a tighter budget than in my younger days.
But I've got to build something...I'd go nuts without it.
I'm doing some preliminary drawings on the project & the question has come to mind as to the actual length of the control arm links.
Is their a ratio for determining this relative to wheelbase & track width. I've looked for this but so far haven't found anything.

 

[Ranger Mike]

Og...are you asking about front or rear? are you using 3 link rear suspension?
Are there any governing rules on this area?
in general, the best geometry you can have is equal length and parallel links. The next best is unequal length parallel links and least desirable but most common is the unequal and non parallel set up. How much room you got to hang mounts and provide proper travel and clearance for coil overs will dictate what you end up with. This is why the chassis software is critical.
Dwarf cars and Legend cars have 73" wheel base and about the same 50ish track width. They use 3 link and run paved and dirt. i would research those for some start info as well. Good info on anti squat and instant centers.

 

[Ognib]

I'm in chapter 4 of "Tune to win", suspension geometry. It seems that the double wishbone 4 link is where it's at. Both front & rear. With a wider base.
I'm still sorting through what he has to say about the different combinations & configurations.
What he does say that has stuck with me is, "I believe it's a lot more important to get the roll center locations & movements happy with each other & with the mass centroid axis than it is to get the camber curves perfect - which we can't to anyway."

There are no governing rules to deal with. This is entirely for my own enjoyment & learning.
The only person I'm interested in competing with is myself.

At this point, the chassis & suspension are a blank sheet of paper so I can design as much or as little space for mounts as needed.
I'm not planning on wings or body work on the car...just a bare bones tube frame/cage with steering, suspension, brakes & power train.

 

[Ranger Mike]

PegasusAutoRacing.com
save a ton of time and searching if you are custom fabbing everything.

 

[Vintageracer13]

I just wanted to report in. I had my first race May 13. I had cooling system woes so I didn't finish well but the improvement in the way the car handled was incredible. It felt very stable and didn't want to step out or push. Thank to Ranger Mike and the rest of you that have helped me understand the concepts discussed here. Next race is June 11 and I'm hoping for a good finish!

 

[Ranger Mike]

thats some good news..i am a tad busy this weekend as it is THE race weekend in the USA..

 

[Vintageracer13]

thats some good news..i am a tad busy this weekend as it is THE race weekend in the USA..

I am crewing a Little 500 car Saturday so I am super excited!

 

[Ranger Mike]

Whatever you are racing, the set up in the garage is critical. You have to do everything you can think of to eliminate variables and arrive at the consistent set up.

One major thing is to use that measuring tape for set up only. Paint a red stripe on it and store it in your section of the tool box reserved for crew chief. You know, to drawer with the pyrometer, caster amber gage, Rolaids.

Tape measures can vary up to ¼” over 3 feet…junk. Terrible. You can buy a certified tape measure form the manufacturer for about $100. I bought a 8 foot steel scale from a machine shop going out of business a while back. Had it certified by a gage repair shop along with a set of feeler gages. Bottom line is whatever you use, use one tape only and set it aside.Temperature will cause you problems too. Try to maintain it within a few degrees.

The linear coefficient of thermal expansion for steel is 10.8x 10^--6/degree Kelvin at 20 degrees Celsius (room temp). (Degrees Kelvin are identical to degrees Celsius except the reference level is absolute zero, or −273.15° Celsius, which is immaterial to this calculation. So the thermal expansion coefficient for steel is: 0.00000645in/in/deg °F

A 100' steel tape would be 1200 inches, and it would expand/contract 0.00774 inches per °F.If we have a temperature swing of 40 degree F (cold winter garage of 50 degrees F to hot summer day of 90 degrees F temperature change), a 100 foot steel tape will expand 100X12X40X(5/9)X10.8X10^-6, or 0.288 inches.A 50 °F temperature change would cause a change of 0.387 inch. This may not seem like bunch but it adds up to 1/32” at ten feet just by temperature.Rambling on, I cannot over stress the absolute requirement of a flat surface for car set up. If your garage floor has a drain in it, your floor is NOT flat. It slopes toward the drain. So you can not use it for set up. Find a flat surface to set up and scale the car.

Set aside a complete set of tires and wheels. Paint them red. Mark each wheel with your race air pressure and mark which corner it goes to. Use them to set up car every time. Make sure to air them up. This gives the proper stagger /wedge you want. Use a stagger tape measure as described above. Hoosier makes a good one.

Mark the frame in back of each front wheel and in front of each rear wheel. I use a paint pencil from NAPA. These are your bench marks for ride height. Cut 4 pieces of 2 x 2 tubing to it under these marks. Paint a mark on each set up riser and store them with the red tires. I have two sets, one is for set up one is for the pits. Really saves time if you get wadded up in the heat race and have to quickly re-establish the set up.

I bought a set of tractor weights from a farm implement dealer to replicate the drivers weight. Was a lot easier than using 40 pound bags of salt. Was way cleaner than using bags of grain! BTW, the joke was the tractor weigths had higher IQ than my drive.

Don’t forget to top off fuel tank when scaling. Unhook all the ARB (sway bars).
Don't forget cardboard box for Trophy.

 

[drobbie]

Great Post Ranger Mike, I can't tell you how many times I see teams make small mistakes on t consistency, especially with stagger tapes. Those things stretch easily, or even worse I see one guy with one tape measure on each tire and their yelling across at each other the numbers.

I like the tractor weight reference to IQ, for us it was the use of cinder and deck blocks because we could simulate the driver's position together (also having a 140 lbs driver means we didn't need many) See pic and yes there was a joke about who was better the driver or the concrete blocks.

Thanks for your continued words of wisdom.

 

[Ranger Mike]

good one droobie...plus dummy weights don't talk back, screw up or drink all your beer...lot of merit in um!

 

[smithdl4]

Ranger Mike, I've been playing with the Perf Trends software. Was interested to know which direction the roll center ought to move on a road racing car. It appears to be easy to get it to move toward the outside wheel, but very difficult to get. It to go toward the inside of the turn. Seems that the further toward the inside of the turn the better. Is that correct?

 

[red0u812]

Great read Ranger Mike. We currently run a legend car on 1/4 mile medium bank asphalt track. Wondering if you had any input as far as spring selection and cross weight on these cars. Also we have a second one on the jig , what would you recommend for the roll center placement. We only have a little room on the inner pivot points to move things around. Thanks

 

[Ranger Mike]

Roll center is that point where the race car body rolls, pivots, tilts, acts etc.… because of cornering force. When racing a left turn oval track we bias everything to help in a left turn. Not so in road racing that has left and right turns. It is most helpful to have the RC located in the cars centerline ( no offset). To offset the RC in this type racing would have a sweet race car turning left but an ugly hand full any time we entered a right hand turn.In the left turn paved round track car set up we offset the roll center 3 inch to the right side to add down force on the right front tire. This adds grip or traction to that tire. Dirt track set up add 4 inch offset to the right side for even more down force. This goes for dwarf and legend cars as well.
Redou8 welcome... what is total car weight? what % left side weight? what is % rear weight? what size ARB? Do you know your motion rate of the A-arms and what is coil over mount angle?

 

[smithdl4]

Understand that statically for road racing RC should be in center but you don' race statically. Everything is moving with the suspension as the car dives, rolls, and turns. Your saying that keeping the RC in the center as the car moves around is optimum? Thanks in advance. Your class has been helpful and inspiring. Please keep it up.

 

[red0u812]

Thanks Mike, here is what we have. The motion ratio is .84, Coilover angle 20.5, there are no sway bars allowed. Car is fast but always loose in.

wheel weights Lf 318 RF 289
Lr 375 RR 346

springs Lf 275 RF 300
Lr 185 RR 200

 

[Ranger Mike]

thank you smithd
yes, try to keep RC migration around center point as much as possible. This takes a lot of time but will pay off as you won't load either side excessively thru body roll.

 

[Ranger Mike]

RedOu
springs seem way to stiff and loose in proves this. good starting pint is
LF 250 Rt ft 275
Lr 150 RR 125
this is for 1,175 # car with driver, am not sure if your scale weight includes driver but the above springs will be softer, may cause a push. We want it to push as this is the other end of the spring rate limit. Now we can start to add stiffer spring to optimize. If this change makes the car neutral we are at that optimum spring rate. time to look at the dampers (shocks).

 

[red0u812]

Thanks Mike for the info. Yes those scale numbers are with the driver. Our race track is basically a big skid pad (no real straightaway) .The car is always hanging on the rr. That was our reasoning on the heavy rr, but I'm excited to give your numbers a shot. If it does push should we go up on the rear rate's.
Thanks again

 

[Ranger Mike]

red, you are correct. If it pushes you are getting too much traction so stiffen the rear springs