Race car suspension Class

[rick7343]

wow... the RC sure move around a lot when the car dives and rolls...

It seems to want go to the left side... is this bad? Compromises as usual...

My suspension actually measures very close to the demo street stock...

So, since I can only move upper mounts, should the roll center be calculate with the suspension at rest, or while down and rolled?

Starting to think that drawing the lines on the floor might be less time consuming...lol

 

[Ranger Mike]

flash puppy you may have a very twitchy car when your done...that said..try to keep the links as long as possible..note IC locations in previous posts..maybe think about a leaf spring like to old T buckets used.. keep the coil over angles between 15 and 20 degrees if you run those ..how about torsion bar rear suspension?
good for low profile mounting if you have room...

 

[Ranger Mike]

rick..excellent point..you nailed it..In my haste to post a reply, see post 309, i did not do a good job following thru..I assumed the post i did on the RC location was for a car that you could tweak to keep the RC close to its static position..the migrating RC has always been a problem..but..in my opinion. if you can keep it some where to the right side and reasonably low during first inch or two of travel..you will put enuff load on the rt ft tire to turn the car...

...the set up you have, if it is like the attached, is pretty good for a stock class racer..about as good as you can get on the front end restrictions the rules dictate..
RC ends up about 2.1 from ground and 5 inch offset to right in 2.8 inch dive...not too bad

you are far more better off with this set up than the guy who starts out with it centered and high and it moves to the left when in dive

If ignorance is bliss, why are so many people unhappy..same this racers

 

[Ranger Mike]

caster - inclination of the steering axis from the vertical in the longitudinal plane ( side view). Positive caster means the top ball joint is trailing the bottom ball joint..i.e. top BJ is further to the rear of the car than the bottom BJ. Think of a bicycle front fork or that broke down beat up shopping cart I always seem to get at Wal-Mart..The effect is that the wheel self centers with positive caster. Negative caster is just the opposite. Caster does several things. It provides positive feed back to the drive, self centers the car, changes the amount of weight transferred during cornering, builds the drivers biceps. When car with Positive caster turns left, it lifts the left ft. tire, dips the rt. ft tire. The steering axis multiplies the effect of Positive caster and the amount of lift / droop. The greater the inclination the more Positive caster will change the cornering height of the car as the wheel is steered. You can plot this on a curve from 0 to 20 degree and see the lift if you have a set of turn plates. When the car is turning left, the result is jacking weight into the left front tire, and right rear tire, taking wedge ( cross weight) off the right ft tire and left rear tire. The more Positive caster the more loss of cross weight. Anti dive discussed above adds to this effect.

Caster split- In round track racing we run a caster split. The right side has more caster than the left side. Typical setting is +1 Left side caster, + 3 right side caster. If set up correctly, the car will steer real easy down the straight and darn near turn by itself through the turn. if you run power steering then you can add another degree on the right side Positive caster. Run too much caster and your driver will develop muscles quick. Most drivers are happy with 2 to 3 degree split, any more than that, things get tuff to turn.

 

[TRH299]

Just bought an old street stock GM metric chassis race car from across the state. It has tubular uppers front control arms that are mounted to custom mounts. it also has the tall Impala spindles. The local 3/8 mile dirt oval requires stock upper/lower control arms with stock type front shocks mounted through the center of the springs. Hidden weight jacks are allowed as are custom upper arm inner mount locations.

I picked up some junkyard upper arms and am trying to figure out the best place to mount them. I found that if I swap the right/left side stock control arms, it looks like I can then get the pivot axis to be parallel to the lower control arms.

My goal is to have as low as possible upper control arm pivot axis that is parallel to the lower control arm pivot axis and then weld the mounts on positioned to give me the proper inital camber/caster with no anti/pro dive.

Has anyone heard of doing this right/left control arm swap to a metric chassis where rules permit?

 

[dano844]

Mike I have been following this thread with dedicated interest for awhile and I am now hoping for a little advice. I am the process of building a road course GT car from a 2005 superspeedway cup chassis. The car will be using 13" wide GT slicks on 16x12 rims. I am wondering at what height a good RC design should be located on the front along with a good scrub radius (I have Perf Trends CT Analyzer). The chassis will be set pretty low(4" clear at static RH and 2"- at dynamic) with a low static V8 engine location for a static CG height of around 16" or so if you consider the cam tunnel the CG. I am using the as-built lower pivot locations with fabbed lower A-arms that are extended horizinstally 2.5" from standard cup stuff and going full coil over. I have plenty of options as to how high/low to locate the dynamic RC as I am re-fabricating at this time. I am wondering where you think setting the dynamic RC would be best applicable to get the best mid corner speed. The rear is truck arm with a panhard bar. Wanted to go watts link but just could not get it done this year maybe next year.

I also read the post above about track width front to rear. At what point does a wider front track width become an issue. I will be at 80" front and can set the rear at whatever I would like, to again maximize mid corner speed. How much offset is too much or where does the difference become too much of an issue.

Thanks
Dan

 

[Ranger Mike]

Dano..welcome..you have the most important tool in your tool box...the chassis software..good show...I recommend you use it to mount upper links as parallel to the lowers as possible and get best camber curve. Front RC will be lower than the rear and try to go as low as you can and keep it centered through full dive. No offset since you are going left and right on the course.
I did some research on Track width.. will work on scrub radius latter but think about the following.

See post # 255 above regarding TRACK WIDTH (TW)

The Ford GT had 57" front and 56" rear TW and won Le Mans..Henry Ford spent millions on this so why reinvent the wheel..use the resources the big auto makers figured out and move on to other things you can improve on..

Here is a cut from a SAE paper that at least is on of its own.

"When selecting the track width, the front and rear track widths do not necessarily have to be the same. For example, track width is typically wider in the front for a rear wheel drive race car. This design concept is used to increase rear traction during corner exit by reducing the amount of body roll resisted by the rear tires relative to the front tires [4]. Based on the corner speeds and horsepower to weight ratio of FSAE cars, this concept should be considered by the designer". Take a car with 50/50 weight distribution, equal roll stiffness each end, same tires etc in a steady state corner (no wonder vehicle dynamics is confusing, we regard continual acceleration as steady state)
As the rear track is decreased, the load difference between inside and outside rear tires is increased for the same couple. The maximum lateral force capacity of a tire pair is achieved when the tires are equally loaded. So, when the rear track is decreased, the lateral force capacity of the rear tire pair is decreased, and the result will be more Overseer.
The track width is normally measured to the centerline of the tire. If the front tires are narrower than the rears, but at the same time you are exploiting the maximum width available - then the front track will be wider than the rear (but not necessarily the overall width of the front).

The more track width you can have the better you can take corners for a given tire size because the load transfer from the inside to the outside is less, so the tires are sharing the work better.

Even with the likelihood that track width is driven by the regulations, a wider front track can be useful. As a car brakes and enters a corner (assuming trail braking by the driver) - then the load is transferred from the inside rear to the outside front (diagonally) - a wider front track will help the car on corner entry.Going through my database of front/rear track differences in sports cars a new fact has come to the fore. When I compared F/R track in cars released in the last 4 years the ratio has changed from that previously noted. It is now 3:1 wider at the front.

Wider at front:
Noble m15 89mm wider at front
Ferrari F430 53mm wider at the front
Lamb Gallardo 30mm wider at the front
Porsche Cayman 42mm wider at the rear

Bottom line - wider front track ( compared to equal ft to rear track width) means the body will roll LESS because you have a longer distance between wheel center line to take the same amount of load transfer. Less roll means more " stability ", less push ( under steer). For road course set up keep front TW at least 1 inch wider or as a minimum, equal to rear TW
.
See post # 255 above

 

[dano844]

Mike thank you for the reply, in regards to the Dyn. RC should I go below the ground level or keep it above? and if so how much below/above would be a reasonable location? If upper/lower links are close to parrellel wouldn't that put the ICs way outside of the track width? Is this a good thing?

As for the track width offset (65"F - 63.5"R) I was thinking of using 1.5" difference F-R. Do you think this is too much offset? Or would this have any negative effects on handling and if so what?

Thanks again
Dan

 

[Ranger Mike]

One other aspect of a low RC is you have minimal track width change during body roll and thus tire scrub is minimized during cornering. Try to keep the RC at least .5" above ground.

I think the 1.5 inch track width is right on...

Regarding the IC and scrub radius...i am working on this post ...but if it falls between 3.5 and 6 inch you should be ok..more on this later

 

[Ranger Mike]

If we draw a line through the top ball joint , bottom ball joint to the ground we have the Kingpin Inclination Angle, also known as Steering Axis Inclination. Typically this angle is between 5 and 10 degrees and 7 to 9 degrees being best compromise for Stock cars, big sedans running 12:1 ( most common) 10:1 or the very quick 8:1 which both require power steering. 6 to 8 degrees for open wheel road course cars running 16:1 steering ( two turns lock to lock).
The scrub radius is the distance in front view between the Steering Axis Inclination and the center of the contact patch where both would theoretically touch the road.

The point at which the steering axis line contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability. If you have not already guessed, the easiest way to change scrub is by changing your offset with either new wheels, or hub centric wheel spacers. Scrub radius should be kept at a minimum but not Zero. Zero scrub radius (center point steering) means the car will react too quickly and steering is " darty". The car will steer very easily and give no feedback to the driver. It is like driving your car on ice. No feel.

If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a locked differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

Any bump or cornering force that is applied to the tire can exert a twisting force on the steering that is proportional to the length of the scrub radius. Increasing SAI helps to self center the steering. But if too much SAI causes lateral scrub of the tires in the turn and really heats them up. Lot of wear.

When in a left turn, Scrub lengthens the right side wheel base and tends to loosen the car. When you counter steer, it shortens the wheelbase and adds understeer. This is a driver friendly situation as it has a very stabilizing effect to the cars handling in driver feed back. The driver needs feedback in a turn because it tells him how heavy the tires are loading and when they are on the edge of traction.

Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly typically performs well with a lot of SAI and caster, a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.

As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. Additionally, if you plan on doing some homework on, and modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero.

Our open wheel formula car runs about 2.5 inch scrub radius. Late model stock cars will run 10 to 12 inch wide tires and 4.5 to 5 inch scrub. The left front will have 1 to 1.5 inch more scrub due to positive camber used on that side. As far as I can recall scrub should be between 3.625 and 6 inches with right front having 5 inch.. being " ideal' for most fast late model cars.

Mechanical Trail - SEE POST # 319 ABOVE - Also know as or caster offset , is the horizontal distance from where the steering axis inclination intersects the ground to where the front wheel touches the ground. The measurement is considered positive if the front wheel ground contact point is behind (towards the rear of the car) the steering axis intersection with the ground. More trail will give the feel of " returnability " of the steering. Less will give the feel of the steering being " floaty". .

 

[dano844]

Mike thank you again for all of the information. I will try to stick with around the 5" scrub radius at both sides of the car. I will be measuring and mocking this weekend to get the front settled. I will let you know if I need anymore advice. Again thank you for your time, it is greatly appreciated.
Thanks
Dan

 

[Ranger Mike]

we had some discussion about Anti Squat..I found this when I was digging through my note..food for thought

 

[Ranger Mike]

I had a question on changing rear end mount brackets..here is one case..it is an example of fabricated mounting bracket for Panhard or J bar..make them beefy and lot if support..car manufacturers spend lots of time and money to design the proper mounts and I don't like to deviate from them unless it is for max performance and can be justified..but you can do it..

 

[smokin butts]

measuring centerline

Do you measure the centerline of your car at

1 between the Aframe mounts.
2 between the tire contact patches.
3 between the moment center where the car rolls which would depend on left side weight .

 

[Ranger Mike]

smokin..which ever method you use..you have to use it every time. The tire patch method is ok but if you ever go to offset wheels or spacers..you have all readings off form that point. The recommended method is from A-arm mounts..I go off the lower A-arm mounts, drop a plumb line from each to the poster board, find the center line and project it back to the rear of the car with a long piece of aluminum and find my center line from the frame rails at the rear. The drive line will be close to center but I found that components are usually offset a bit. I use a 3 foot carpenters square to project the center line up to the car frame at several points and PAINT a line on the frame, chassis etc...so I don't have to repeat this drill.

 

[1meyra]

Starting out with a new year traveling from track to track with a super late model offset chassis 2750lb with 10inch tires was wondering what spindle height and degree would be a good start some tracks are flat and others are banked up to 18 degree or should we changing for each race.

 

[Ranger Mike]

1meyra welcome
I assume you run paved tracks. Spindle height most common on super lates is 7.75 inch. It has lower Roll center which means less camber change through spring compression. Spindle kingpin inclination can be between 5 and 10 degrees. Five years ago 8 1/2 degrees was about the max..now 10 degree is the current hot set up. You can even get 12 degrees now.. Sweet Mfg. even makes a spindle where you cans wap out slugs to change offset by 2 degrees..trick for 2012 in the late model community. I would not change out spindle between tracks. Go with shocks and springs / ARB (sway bar).
these are only starting points and each chassis may differ..

Flat tracks 0 to 6 degree banking you must increase rt. ft. spring rate to control body roll. If you run 350# spring on the rt. ft. on 3/8 to 1/2 mile 10 degree track, swap a 400# spring into run 1/4 mile flat track. You may need a slightly stiffer rt. rear spring to balance out the car. Higher bank tracks ( 0 to 18 degrees) turn the corning force into more down force than lateral. This means you have to increase the spring rate to counter this added down force. Make the lt. ft. and rt. ft. springs 25 to 50 # stiffer and lt. and rt. rear springs 15 to 25 # stiffer.
Run about 1/8 inch toe out on the low bank tracks and on fast 1/2 mile use 1/16 " to keep down the scrub. On tight 1/4 tracks run 1/8 to 3/16 toe out.

 

[smokin butts]

When straight line braking , such as in road racing whatstops wheel hop or rear wheel lock up without dialing all front brake in. is there any rear measurements that can be adjusted to stop so much weight transfer to the front when braking , or be able to make the whole car lower during braking

SMOKIN

 

[Ranger Mike]

i am in germany this week..will reply when i get back,,,thanks for the interest

rm

 

[Ranger Mike]

smokin..
some notes on wheel hop

you said When straight line braking , such as in road racing whatstops wheel hop or rear wheel lock up without dialing all front brake in.


Rear wheel hop is basically the suspension gets into a resonant situation and doesn't dampen itself out.

Under acceleration, the rear suspension compresses. Due to inertia, the wheel continues traveling upward. This unloads the tire, reducing the acceleration force. The spring then pushes back down on the wheel, and it regains traction, forcing the wheel back up.


Play and movement in suspension arms can exaggerate the problem.

The best way to fix it depends on the vehicle. Vehicles suspension can be designed with "anti-squat" capabilities, but at the expense of ride quality. 4 link cars with solid rear axles can use "no hop" bars to change the suspension geometry.

Angle of upper link on rear end will effect wheel hop. A large amount of anti squat ( down hill angel of top link) will cause rear end lightness and rear wheel hop under braking. The 3rd link shock will effect it as well.
Shocks, shocks shocks..check the shocks...stiffer shocks will help.


Leaf spring cars can use traction bars to eliminate it.

is there any rear measurements that can be adjusted to stop so much weight transfer to the front when braking , or be able to make the whole car lower during braking

Stiffer ARB ( sway bar). Softer rear spring package. Need to keep more weight on rear end.

Also, test the car with a light fuel load because the weight difference (sometimes in the neighborhood of 150 pounds) can affect wheel hop as well. Wheel hop is less likely on production-based sports cars with an independent rear suspension because the center- section of the axle is hung by the chassis, but it can still occur especially if an unskilled driver is punching the brakes and shifting the gears

 

[smokin butts]

when working with a roll center program !

1 what dive and roll numbers is a good base to start with?

2 if your roll center migrates on roll ! how much is too much and how much is not enough

3 where should instant centers be on dive ? how close together should they be

 

[Ranger Mike]

We are going to have to deal with migrating roll centers when we have restrictive rules like stock mount points , metric this and that etc..depending on how you like to set up the car, I would look at 2 inch dive maximum, 1.5 degree roll...I do not personally look at instant centers as such, since i use the software program, the Roll center and location are what I look at and only to see impact on the camber curve. We know the RC will be going all over the place but if we can keep it going to the pavement, and staying around the 3 to 4 inch offset during the first inch of travel ( 1.5" would be ideal) this is about as good a set up as you can get on the tires they make you run on. I have seen RC offset up to 5 inch to the right and still planted the right front well though I believe 5" is too much leverage ( on asphalt).

 

[Kozy]

Does lateral RC position actually matter? I have read some convincing arguments that it does not, only the height...

 

[darraten10]

Hi sorry to change things up I have just found your forum and up to page 5 and found it to be totally interesting and learned alot. I am from a micro sprint dirt background and have a question. Previously with castor and things you guys have said i think if the car rolls to the right(left hand turn) the car will loosen through the turn me i have always found it to tighten increase inside rear drive through weight transfer. From reading i believe we run too much castor which will be reduced and too much roll center so with the rolling to the right this would depend on spring weight on whether it was a push not so much the movement due to the amount of weight transfer as a softer spring would allow it to get off quicker and transfer less weight. Our car is like a lot of dirt cars panard front jacobs ladder rear. How does a jacobs ladder rear and panard rear change different to ride height changes. Cheers Danny

 

[Ranger Mike]

Kozy
if you are running circle track you absolutely should offset the roll center. see page 16 post # 215..bottom line is you have to plant the right front tire with enough transfering weight to turn the car..on dirt the boys run 4 inch offset on asphalt 3 inch. I think it is the greatest close hold secret of round track racing...darraten10

welcome..

good to see new racer!
you have many questions rolled into on..how much caster are you running and where is the Roll Center? See post on page 16 for rear panhard changes and why..I will dig up notes on jacobs ladder..

 

[Kozy]

Ah, roundy roundy cars, that explains it. I know nothing about that.

 

[mender]

is there any rear measurements that can be adjusted to stop so much weight transfer to the front when braking , or be able to make the whole car lower during braking

Stiffer ARB ( sway bar). Softer rear spring package. Need to keep more weight on rear end.

Mike, a stiffer sway bar won't affect longitudinal weight transfer, and a softer rear spring will allow more rear end lift than a stiff spring. Weight transfer caused by chassis/suspension movement is the only component of the total weight transfer that can be altered with adjustments; the weight transfer due to decel force is determined by the C of G height and is built into the car.

Dialing in more anti-squat will tend to pull the rear of the car down during braking but that usually unloads the tires and can also result in wheel hop.

 

[Ranger Mike]

Mender, i may have not understood your post ref: sway bar...I always thought the ARB was 1/3 of the proper handling package ( see post 19 on page two above)..
I am with you on the anti squat dial in..but after looking back thru this forum, we never discussed Anti Roll Bar ( sway bars ) and will work on post to do this.

 

[mender]

Mike, I was answering in the context of:
"When straight line braking , such as in road racing ..."

When there is no cornering involved the ARB just rotates in the bushings so it has no effect.

 

[Ranger Mike]

Mender..you are correct...the ARB would have little to no effect on straight line braking..which I never think about unless the car is coming into the pits...excellent advice!
thanks

 

[rick7343]

I have to say, I never knew there were so many things to consider when building a oval track car... lol

It seemed so much easier 15 years ago when I last raced and we weren't supposed to move anything...

So I'm wondering the consequences of less than ideal scrub radius?

To widen the stance of the car (chevelle) the previous owner was using 8" wheels with 1-1/2" back spacing... I have yet to run the numbers... hmmm...

What should I be looking for in scrub radius with 8" wheels? What if I can't get it witout sacrificing track width?

Thanx!

 

[Ranger Mike]

Hey Mender..i am working on ARB post but I been rebuilding the old NAA Ford tractor. Dandelions don't wait on anyone!

Hey Rick...glad to hear from you..I think if you review post 264 and 255 on page 17 about track width...and then post 325 on page 21 about scrub radius..good info..personally scrub radius is drivers choice..
depending on feel if car is too darty or grabby..

me being a non driver and selfish crew chief interested in WINS..screw the driver feel and feed back and make it as wide as you can get away with...

 

[rick7343]

So scrub radius has no adverse affect on grip That's good to know...

 

[rick7343]

So how about the affect of weight placement?

Since the car has a stockish 4 link in the rear it has a high rear roll center... so would placing the weight higher in the rear of the car help the rear end roll?

We always used to keep the weight as low as allowed, but now I'm wondering...

 

[Ranger Mike]

Yes you pick up more roll when weight is placed high on the chassis..but..also you get all the ill effects too..see post 272 on page 17..we discussed fuel cell mounting...

on left turn circle track cars i always remember an old guys axiom..low, left and light..build it as low as you can with as much low left side weight as possible and as light as the rules will let you...guess i better research polar moment for post after i finish the ARB post...