Race car suspension Class

rick7343

Anyone got any theories on the front vs. rear roll center heights?

I've only found one article with much info... it stated that the front roll center should be 50-66% of the height of the rear or the car will understeer going into the corner and oversteer coming out...

That is exactly what my car is doing, and I've been trying softer and softer RR springs. It is getting better, but still not good.... I have a The rear RC is pretty fixed since we have to run the stockish 4 link in the rear...

If the rear RC is around 22", then my front should be at least 11''.... sounds crazy!

Opinions?

Ranger Mike

my opinion
stk clip heavy car with iron heads should have frt roll center 2.5 inch above ground.
ifin you are pushing goin int and loose coming out...frt springs way too soft...going into the turn.. the weight is coming forward and washing out rt ft tire grip...tire shear...and weight is staying there ...up front...so coming out the car is way too nose heavy and rear tires do not have enought weight to hook up the rear drive tires,,,results- they spin instead of driving off the turn..

rick7343

OK thanx...

But there is less than an inch front shock travel, and softening the outside rear spring is helping, which would seem that the front wants to roll more than the rear...(?)

rick7343

Interesting... I was down in the big city today talking to one of the leading race shops and they said the light hollow Hotchkiss bar can't be preloaded... they are not strong enough...hmmm...

I suspected the bar was twisted when I set the car up last weekend... (crap!) Maybe that's why adjustments didn't have mush affect...

We are stuck with stock mounted bars... Have to find a solid bar now I suppose...

Why can't things be simple anymore...?? lol

chriswh86

Rick - What springs are you running? Getting a large sprint split up front or in the rear could be crutching a bigger issue

"I suspected the bar was twisted when I set the car up last weekend... (crap!) Maybe that's why adjustments didn't have mush affect..."

Sounds like you are setting your car up with the sway bar attached? it shouldnt be while scaling and setting ride heights etc...

rick7343

Lf. 600. Rf. 650
Lr 200 Rr 175. (4 link)
Car has never felt hooked at all untill I increased outside air pressures, always felt greasey lik it was always sliding...tires squealing , etc... just starting to make some progress.

The idea was to run softer front springs and bigger bar (1-3/8" 540lb) to be able to get the front CofG down in the corners, But the shock package was too stiff to let this happen...So I put the springs back in that the race shop that built the shocks advised (current)

Seems the presures the other cars are running are too soft for my car and the tires get greasy, but running too much seems to take a while for the tires to get hot enough to stick really good...

How is optimum tire pressure determined?

I called the manufacturer, they said to start at : rf 36,rr 34,lr 32, lf 30 and drop a couple psi if needed... most top racers in the class say no more than 28 hot in the outside and 16-18 hot on the inside.... at 16 hot on the inside I can feel the lf tire fold over and hop coming out sometimes...

One of the fast cars that passed me the other night looked like he was sliding all over... not hooked either... had more power than me and got the job done though!
we have to run Towel City recap on 8" Hoosier casings... Also have to run alot of extra stagger to make them work..

wonder if we all are missing something?

Ranger Mike

Rick, you are limited to running hard tires in this racing class. The lite spring big ARB idea is not working for this car on this track. Forget about getting Center of Gravity down in the corners...why would you want to do that? What is the purpose? CoG is a starting point but the Roll Center is where everything happens.

Hard tires require RT front weight in order to increase traction and properly turn the car in a corner. They require rear weight in order to hook up and driver off the turn. Springs control amount of weight transfer ,,, Shocks control rate of transfer. Tuning these two are key to making the car handle.

Tire temperatures are indication of how good the actions above are working. You should have no more than ten degree spread over the inside, center and outside tire temp readings.
Adjust tire pressure until you get the 10 degree spread.

I had a driver once who insisted on running 10 psi on Hoosiers when the Hoosier tech said 20 was minimum..car bottomed out..no wonder..but some people just will not listen and want to run the “ hot setup” without understanding what is really going on...

Go back to more conventional spring / ARB set up and work on the right front wheel geometry. Reduce bump steer, reduce camber build, add Ackermann. On rear , eliminate and roll under/over steer to avoid mixed signals...when you get the car to turn going in ..then work on hooking up the rear end...my opinion

rick7343

"Rick, you are limited to running hard tires in this racing class. The lite spring big ARB idea is not working for this car on this track. Forget about getting Center of Gravity down in the corners...why would you want to do that? What is the purpose? CoG is a starting point but the Roll Center is where everything happens.."

Tires are supposed to be 60 duro, they "ball up" like slicks a bit..

Center of gravity down... why are lower cars faster around the corners?

"Hard tires require RT front weight in order to increase traction and properly turn the car in a corner. They require rear weight in order to hook up and driver off the turn. Springs control amount of weight transfer ,,, Shocks control rate of transfer. Tuning these two are key to making the car handle."

How much weight is too much for the right front? I would like to use all the tires....

How much is too much rear weight..local guys are saying 50% rear is too much...

Other teams are saying their roll centers are way left to keep weight on the left front...Mine is almost in the canter @ 3.5"


"Tire temperatures are indication of how good the actions above are working. You should have no more than ten degree spread over the inside, center and outside tire temp readings.
Adjust tire pressure until you get the 10 degree spread."

I made a mistake by listening to others on camber (6 degrees) and tire pressure... less camber and more air works better so far... with 28lbs hot in the RF it just gets greasy.... I will reset with temps... I had eliminated bump... not sure how to accurately measure ackerman... ?

"Go back to more conventional spring / ARB set up and work on the right front wheel geometry. Reduce bump steer, reduce camber build, add Ackermann. On rear , eliminate and roll under/over steer to avoid mixed signals...when you get the car to turn going in ..then work on hooking up the rear end...my opinion "

Bump is almost zero, camber build is less than a degree per inch...

Am going to fix the rear roll steer since the back of the car jumps over a bit when fuel is applied in the corner...

Last race ,it finally started feeling hooked up on the rear, so it's not really that far off... everyone says it looks good... it doesn't feel good yet, and not much feedback thru the steering wheel.. I changed gearing for last race and lost ground, so it looked worse than it was... new, modern custom cam on the way.... engine is old and tired...

Thanx Mike!!

I was really frustrated because chassis adjustments and changes weren't making any differance until I got some laps on the tires and finally added air..

Ranger Mike

[QUOTE=rick7343;4032703

Center of gravity down... why are lower cars faster around the corners?

In general low cars turn better simply because the distance between the CoG and the RC is shorter. But the assumption is the weight tranfered thru the RC is going where it does the most good. Having RC located to the left is not going to plant the Rt ftont tire As EFFECTIVELY as having it plant when the RC id off set to the right side when turning left

rick7343

So would it nbe wrong to assume with a heavy car and only 8" tires that there is already enough weight on the rf tire to plant it?

The fast guys and most successful race shop all tell me their roll centers are left... they just won't tell me how much..lol

They said on some of their bump stop cars (late models)the RC is outside the car to the left to keep the front left down... so they are tying to elimnate body roll....

Both my outside tires get really hot and get greasy after a few laps, so I think I have too much body roll, so the moment arm on the front would need to be shorter to combat the roll, correct?
(I have tried a lot of rear springs to try and match the rear roll to the front since the rear has a short moment arm..)

We only have 2 nights left, and I doubt I will make tomorrow night... It is our anniversary and time is limited since they put me on shiftwork ... (I am not quite married to the car, my wife keeps telling me...)

The car is actually pretty good in the corners right now, I am underpowered compared to those darn crate engines.... It just doesn't really feel "hooked"...

rick7343

Does moving the RC left have something to do with the angle between it and the tire contact patch relative to the RC height?
I think Mitchell wrote that the height of the RC doesn't matter as much as the angle between it and the tire contact patch...

So you could have a higher roll center,but moved to the left, and it would act similar ro having a low roll center?? There must be something or why do it?? It must change the action of the moment arm too by changing the angle of it also...??...

One of the fastest cars here in my class has the same front clip and a pair of 400 springs in it... so it is possible.... Have to say, he is always adjusting his though...

Ranger Mike

400# spring guy it racing a go kart chassis which has just about zero camber change. I suspect rear end goes loose and he dirt tracks it out of the turn..his success I think is due to driver... Anyway

On way to Bahamas until Thursday

rick7343

Nice!!!!

rick7343

I know you don't agree with the light front end, but it seems to be the coming way... (The shocks are fairly too btw...)

Maybe I should have spent more time with the 400's in mine, even slamming on the brakes it wasn't quite using all the suspension on the right front... I had the bar preloaded alot and shouldn't have... To limited on time to test properly..

Eliminating some of the rear steer, and found my rear whell bearings loose too for some reason... they were new 150 laps ago.. (floater)

Have a good time in the sun...! (it wasn't hot and dry enough in the US??)

rick..

TheSymbol

Thank you for this excellent info :)

rick7343

?? guessing you mean the thread in general...

TheSymbol

Yeah.
Was talking about the thread by RangerMike.
:)