Race car suspension Class

[bigbolt52]

Here is a video of our last race, our car got toe knocked out to 3/4" on the first lap. http://youtu.be/8kNNFME3F8k

 

[Ranger Mike]

offset the frt roll center toward the right side tire...yes..drop the roll center as well..i held off on this until you can tell me if the ride height can be lowered by an inch. we have the right side springs too soft. the car is pan caking the right side springs. This makes for a loose car going in. the tire temps are indicators and the right front should be the hottest tire on the car. detroit muscle cars had the left shock mounted in front of the axle and the right shock in back of the axle tube to stave off axle wrap under acceleration.

note in the video the driver is sawing the steering wheel in the turns. the rear end wants to come around..looooose...Bigbolt if you can not change the Roll Center in time...change the springs out and get tire temperatures right so you have similar temps across the tire face on the right front..and the other three. concentrate on this and worry about front to rear temps after you have close temps on inside, middle and outside of each tire. make good notes..

 

[bigbolt52]

I can do everything but get temp in the LF, geometry just won't let it hang low enough. and I can't lower the car or stuff will hit the ground.

 

[bigbolt52]

Roll center on the front is no problem and I can drop the panhard bar 1" in the rear.

 

[Ranger Mike]

Once you get stiffer rt ft spring it will throw some weight onto the left front tire

 

[bigbolt52]

I hope its enough to get some temp. I'll update after this weekend, I have the roll center down to 4" in the front and to the right 2", with a ride height of 4.75", I'm dropping the back down to 5" ride height and dropping the roll center over 1" in the rear also, with the spring changes recommended, thanks for your help.

 

[Ranger Mike]

Me too.. Got huge race next weekend
Oil cooler has leak
Transaxle layshaft pinion bearing is toast
Broke thrust washer
And replacement bearing has no flange face
Got to get on the lathe and tool up a spacer
And it's 88 degrees in the garage
Ya got to love racing

 

[autodoctor911]

If you ever need to go lower with the rear RC than possible with the Panhard or Watts can achieve there is also the Mumford Link:
The RC is the imaginary point where the two long links would intersect.

 

[bigbolt52]

Well it worked awesome on Friday night in hotlaps then got rained out, Saturday night car was junk, horrible. Took a huge swing on something I wanted to try for a long time and moved to both coilovers in front of the axle, wow! My son started coming through the pack like gang busters and then on a restart the leaders started playing brake check games, a car got turned sideways right in front of my son (15 yrs old) and bang, no more car. We are done with this class, unpredictable cars, and terrible drivers we have had enough, going into the late model class.
On a positive note the front end was awesome thanks to your help but now its a mangled mess, but my son is ok, I do appreciate the help.

Here is the accident https://www.youtube.com/watch?v=7Yb4uHdPM9E&list=UUsdXuwezocDERdqB0LdvvuQ

 

[Ranger Mike]

anytime you can walk away it is good..glad he is not hurt...that is the important thing..
thanks for the kind words...and good racing in the late model class

 

[mikey531]

We are racing a 2550 LB asphalt modified. What are the advantages and disadvantages of having the left rear spring in front of the rear end and the right rear spring behind the rear end? We also have a decel/acel push/pull bar on the center of the rear end. Approximately 600 lbs of torque from the engine.

 

[mikey531]

welcome...nice to meet a fellow racer..short answer is spring mount location can help a little bit on spring wrap under acceleration..torque link is to dial in hook up under acceleration as well..long answer is both questions are addresses in above post Race car suspension Class
by Ranger Mike

rm

Thanks I have read the posts and the big problem we have seems to be traction off the turns. The third link is supposed to increase bite off but they have tried it with staggered springs. Here is the info.
http://www.rightfootperformance.net/...4121065422.pdf
We have the accel set at 14 degrees down and the decel at 5 degrees up I think. I will have to double check the angles.

Thanks for the info above!

 

[mikey531]

The track is low banked 1/4 mile, we have been trying to get more bite off the corner by changing the angle on the accel bar. I think we are going to move the left rear spring behind the axle and try that. Unless anyone has another idea?

 

[Ranger Mike]

Usually a Loose Off condition is caused by too little Stagger. Another cause is too soft a right rear spring.
Typically 3rd links are used to tune the chassis. You have to have it handling properly before monkeying with the fine tune settings.
my main concern is the mounting location of the 3rd link. It should be mounted at the center of weight of the car. i.e. if car has 60 inch track width and 53% left side weight then
60 x .53= 31.8 inch and 3rd link should be mounted 31.8 inch to the left of the RIGHT TIRE CENTERLINE. Is it?

Next question is what springs are you running and how much stagger?
Is the 1/4 mile track tight radius or wide sweeping radius. Is track a paperclip or more of an oval?
any idea of shock absorber rates you are running?
what is % rear weight, left side and cross weight?
The good news is that it sounds like you have mastered the corner entry and now have Phase Three problems coming off. You are getting close.

Detroit muscle cars used to have the rear shocks mounted in back of the differential on the Rt Rear and in front of the diff. on the Lt rear to fend off axle wrap under acceleration.
In my opinion all coil overs and sliders should be mounted in back of the differential. This gives you the longest spring base (maximum distance between spring mounting points over the wheelbase to deal with sprung weight). Screwing around with one in back and one in front is asking for trouble. When you mount the left side spring in front of the diff. you have shortened up the left side distance by 5 or 6 inches. That adds up to 5% diff.

Mikey and bigbolt..not directed to you..just in general...
Rant to follow – Why in the heck do racers mess around with tricks and gimmicks to make a car handle when the car is not even close to the basic setup! Happens every time I'm in the pits...current trends, fads about super low tire pressures , trick locker rear ends..concealed sway bad loaders, tire soaking tricks..arrrrrr!

Square up the car. On a “ flat “ garage floor level up the race car frame. At your four bench marks make sure it is same distance from the floor. Take out the frt spring and disconnect the ARB (sway bar). ALL A-Arm /control arm frame mount points had better be parallel to the floor. if not..fix them. (may not be possible if you run metric frame but make sure the left and right side are mirror image of each other. The ARB should be parallel to the floor. Move to the rear end. Make sure it has not been bent. check it for rear steer. Square it up. Make sure the mount points are the same and make sure it moves parallel thru bump and rebound. Put 5 degree down in the 3rd link and put it back together.
rear end. Get the left side, rear and cross weight right. get the caster camber, toe and bump steer correct. get the spring rates in the ball park. Stagger should be close to what everyone else is running. ( if you want to win make sure the roll center is where it should be on the front and rear). Take it to tune and test and use a pyrometer. don’t forget the cardboard box!

 

[mikey531]

RM with the spring in front of the rear axle the distance between the front mounting point and the rear mounting point shortened the spring base about 16 to 18 inches. You said 5 to 6 inches would be 5%. 5% of what?

Thanks
mike

 

[Ranger Mike]

Mikey..on super late model if you swap coil over mount it would change 5%. on your car it is even more dramatic. Keep them both on the rear of diff. for max spread

 

[mikey531]

We ran the car last night and it was much better. Last weeks feature was canceled so they gave us a 50 lap race and we couldn't buy tires and I messed up and bought a tire that I guess was old as it was 10 to 15 points higher that the others after we had it mounted and practiced, so we were stuck with that tire. We put it on the right rear but were still able to get the car good and it responded to changes we made at the track. So we are going to leave the spring behind and work on getting it to come out better. It goes in good and turns in the middle but just couldn't get it to bite coming off. The car was just bit tight after practice so we left it alone for the feature. We are all done for this year but would like to try something more to get faster. We can still practice for awhile. Any suggestions? You said on a late model it would change 5% I don't know what would change the 5% ? Thanks again for your help.

 

[Ranger Mike]

Usually loose off the turn is due to Stagger. Since I do not know the stagger you have tried I am flying blind here. The other cause is too soft a right rear spring. From what you said, you bolted on a right rear tire that was 15 points harder than a new one...right? Since racing is all about tires..this could add to loose condition off the turn. Especially on asphalt.
One last time on the 5% thing. On a late model car swapping a coil over to the front on one side would reduce the mount point to mount point distance by 5%. I do not like this as you have less ability to manage the sprung weight by 5% versus keeping the distance at maximum possible.
A lot of racing is managing weight transfer with the tools available.In this case it is maximum lever you have to control it. When you mount coil over in front of the differential you set up a trapezoid. Now you no longer have square set up. If anything you have softened up the left rear spring by shortening the lever over which it acts.
Of more concern is the location of the top mount points. The rear coil overs should be mounted at or below the Center of Gravity. Ideally below the CG. This will help traction under acceleration. Now if the rear coil overs are mounted higher than the CG, when you accelerate you lose leverage. The G force vector acting on the CG is the same BUT leverage has been reduced and weight that COULD HAVE been dumped to the rear springs has been reduced.

Besides that it is a band aid. The real problem is stagger or a soft right rear spring or other basic problem.

 

[mikey531]

We may have found another problem. We checked the rear end with our toe gauge, we scribed a line and then front to rear it is toed out a strong 1/8" and top to bottom a strong 1/8" is this enough to affect handling?

 

[Ranger Mike]

A bent differential can cause all kinds of frustrating handling problems. I assume you used a good tire wheel set to measure? here is a good link to assist. http://www.circletrack.com/techarticles/rear_end_alignment_measurement_tech/

the center line generated by the rear wheel should be parallel with the vehicle center line or you are inducing an understeer/oversteer condition. The other huge problem is bearing failure and leaking seal. How did it get bent? usually the driver does it by whacking the wall. So for reliability sake fix it.

 

[Damien Mulvey]

Hi ranger mike...i am writing too u from Ireland.we run a 700kg space frame car on a flat quater mile oval with Hoosier h12 tyres..we have a arm suspension on the front and 3 link on the rear with a panhard rod.just wondering about front roll centre and should it travel towards the wall or the infield when in dive and roll or should it travel very little...many thanks Damien

 

[Ranger Mike]

welcome Damien,
The Roll Center is that point where the suspension linkages act on during cornering, acceleration, braking...any type of weight transfer. The more consistent you can make it , the better. So as little migration as possible is what I try to maintain. We have to realize that the height and location will move in the real world. A minimum movement (especially during the first inch of dive) is desired. If you can keep it with in a 1 inch location from ride height over total dive you will have a good set up. Our formula car drops about 1 inch in dive but laterally is stable. I assume that you are turning right when racing so offset it to the left to help load the left front tire.

 

[Damien Mulvey]

Yes we are turning right but for the last ten years our cars would have started with the roll centre 3 inches to the wall and it would slide across towards the in field which I'm starting to think now was the wrong way as we have a push rite in the middle of the Corner that no one can solve.. What would ur thoughts be on this if u don't mind thanks

 

[Ranger Mike]

phase two handling is when all the initial nose diving has taken place and you need to hook up the rear tires. If the stagger is not enough the car will push. Increase stagger.
The Front Roll center has done its job and the left front tire was properly loaded. The weight coming forward has been kicked back. But if the left front spring is too stiff or the left rear spring is too soft the tires will not hook up. Before swapping out the springs try raising the rear roll center to help the car turn.

 

[Philthy]

Is there a desirable relationship between king pin inclination and caster on a circuit race car. I am racing a transam type car with 310mm wide front slicks. I seem to be getting a lot of RF inside tyre wear at tracks that have mainly right hand turns and still some outside tyre wear despite having static -4 degrees camber. I am also thinking front instant center may be too long?

 

[Ranger Mike]

Welcome Philthy and thanks for the great question- 4 degrees neg static camber sound like a lot. Our purpose built left hand turn cars only run 3.5 ° negative camber on High bank tracks, eve less on flat tracks. Perfect tire temperatures reflect perfect static amber settings. Can you check tire temps after just exiting a turn at the race course?
Typically, on our left hand turn cars , if you have 3 to 3.5° camber , you want the outside tire temp to be 10 to 15 ° cooler on the right front tire.
Don’t try to get the tire temp even across the tire as this will mean a push condition (under steer as the tire will have too much positive camber and no make contact with the track for max adhesion. Don’t forget, with huge static camber if you have a lot of straights and few turns the inside of the tire will be in constant contact and the outside of the tire will be loafing along not carrying any substantial load.

a few other things to check-
Bump steer- you may be adding a lot of toe in / toe out if you do not know what it is. I keep bump steer under 0.030” per inch
Camber curve over 3 inch suspension travel – on a flat track you want 4.25° negative camber over 3 inch of bump travel as a starting point.Caster should be even so the car will steer neutral.
King pin inclination and scrub radius - please read post #325 on page 17 of this same post.
What is you Ackermann? see post #636

 

[Philthy]

Welcome Philthy and thanks for the great question- 4 degrees neg static camber sound like a lot. Our purpose built left hand turn cars only run 3.5 ° negative camber on High bank tracks, eve less on flat tracks. Perfect tire temperatures reflect perfect static amber settings. Can you check tire temps after just exiting a turn at the race course?
Typically, on our left hand turn cars , if you have 3 to 3.5° camber , you want the outside tire temp to be 10 to 15 ° cooler on the right front tire.
Don’t try to get the tire temp even across the tire as this will mean a push condition (under steer as the tire will have too much positive camber and no make contact with the track for max adhesion. Don’t forget, with huge static camber if you have a lot of straights and few turns the inside of the tire will be in constant contact and the outside of the tire will be loafing along not carrying any substantial load.

a few other things to check-
Bump steer- you may be adding a lot of toe in / toe out if you do not know what it is. I keep bump steer under 0.030” per inch
Camber curve over 3 inch suspension travel – on a flat track you want 4.25° negative camber over 3 inch of bump travel as a starting point.


Caster should be even so the car will steer neutral.
King pin inclination and scrub radius - please read post #325 on page 17 of this same post.
What is you Ackermann? see post #636

Bump steer is good. Unfortunately virtually no ackerman (nearly 8 times wheel base) due to front steer with brake rotor in the way.
I will read post 325, thanks.
The camber gain is less than your suggestion and I think this is the root of the problem. That is an easy fix with changing the top control arm angle on either the chassis or the upright, or both.

 

[Ranger Mike]

Bump steer is good. Unfortunately virtually no ackerman (nearly 8 times wheel base) due to front steer with brake rotor in the way.
I will read post 325, thanks.
The camber gain is less than your suggestion and I think this is the root of the problem. That is an easy fix with changing the top control arm angle on either the chassis or the upright, or both.

Akermann will help it turn and lack of it will definitely impact on tire wear. lack of camber build will also contribute..good job on identifying he problems..

 

[Ranger Mike]

Camber build and toe in vs toe out
I just returned from a race weekend at the local sports car course. Had an old friend racing a corvette. He had 10 inch tires on the front. The left front was really wearing from the inside edge to the middle and the portion from center to outside was like new. Looked like a bad camber build situation. No way would I try to correct this at the track unless I had a lot of time. You have to measure the spindle at ride height, take out the spring and shock, chart the camber over 3 inch suspension travel and see what gain you have. You can do this without the computer software but the program helps.

One question arose regarding toe in vs. toe out. On round track where you are turning left only, you can get by running toe-out as this will assist the outside tire to follow the different radius. Add to this the fact that most round track cars run caster split ( like 3.5 degree rt ft caster and 1.5 lft ft caster) the normal risk of running toe out being too darty is minimized.
On road course you run toe into take advantage of the self correcting effect and is more dynamically stable if you hit bump or get a gust of wind.

 

[Ranger Mike]

A lot of racers in the very restricted stock class ask me what the hot set up is for a particular class. My answer is – How high is the sky?
Racers are running Chevelle spindles, Impala spindles, C10 truck spindles Mopar ball joints , the combination of possibilities are all over the place.
When you are limited on components that you can use the options a re limited. Compromise is the rule. When the rules restrict you I am a firm believer in using the suspension software to find out where the roll center is. Next find out where it goes in roll and dive. Next figure out what camber curve you have. Find out what bump steer you have.
You will go insane trying to do this without the software. Just swapping in the “ hot set up components “ and not knowing why is a sure fire path to last place.
Example- A Chevy Camaro has a stock roll center about 1/2 inch above the track. That’s why they called it the “ hugger” when introduced in 1967. One hot setup is to RAISE the roll center to improve the camber build. Don’t forget..racing is all about tire contact patch.
Why is the front roll center located to the left of center line? Two answers. 1. You may have so much aero down force that you need to unload some down force on the right front tire at turn entry. The other reason is because of the mix of components you have installed, the roll center will migrate to the right of center line location on roll and dive.
A lot of the time you will need a lot of left front static camber since you can not use custom A-Arms and change mount locations due to the rules.
Racers use Mopar ball joints do move the ball joint up in the bottom A-arm to give a little more travel before the lower a-arm hits the frame.
In a restricted class, most of this work is accomplished by changing the angle (mounting location) and length of the upper a-arms (and a taller spindle). It would also be good to have the inner lower control arm pivot level with the pivot point on the lower ball joint. Don’t be afraid to use the software and run some different scenarios with spindles and upper a-arm locations.

You might get close by running a Camaro spindle on the right and the taller truck one on the left. If your ride height has the lower control arms level (pivot to pivot level) this might get you in the ballpark with roll centers without moving the upper a-arm mounts. But we can prove it on the software. Using a taller truck spindle on the LF and an Camaro spindle the RF you might get the roll center in a favorable location for dirt, but you would have to prove the results on the software. Again you are mixing and matching and when you have a non symmetrical situation things get real squirrelly unless you got the software.
You might also have to lengthen the LF upper a-arm a little which could be against the rules but who measures A-arms anyway??

 

[bketner]

Great read,i have included a pic of a front end that we run for 1/4 scale,after reading everything I see 2major problems
1) the front a arms are parallel and does not allow adjustment of role center
2) when we put positive camber in left front,it puts left front king pin angle at a positive killing the scrub radius

 

[bketner]

Thank you ranger Mike and all involved for this post,this is the stuff that you can figure out over time,but at the same time the kind of info and experience that people take to the grave with them.
Thanks in advance
Bret

 

[Ranger Mike]

Ha...good one..Bret...there is a song that says " the good, they die Young.." so I'll be around for quite a while!

 

[bketner]

Ha, I like it,any info or suggestions on this suspension Mike would greatly be appreciated?

 

[bketner]

I planned on drafting the front and rear suspensions out on the drawing board, what computer software would you recommend to do some work on this type off car?