OC_

This is for my buddies 89’ supra. We installed a buddy club “racing spec damper” coil-over setup on his car and we are getting the rear inside tire to lift off the ground during hard cornering. What are the most common reasons for this to happen? It looks like the front is really diving hard and the back come right off the ground! spring rates are 1007lb/in front and 559lb/in rear. Any ideas how to cure this problem? This car does have a full cage installed and the chassis is nice and stiff. He is running wider tires on the rear of the car.

This problem is most noticeable on tight autocross courses.

DocMazda

i am not sure if you have a upper strut tower bar (and if so, is it adjustable?) i hear this could cause your rear tire to lift during tight turns. if so, try loosening that up a bit. other than that, some thing is too stiff, and needs to be loosened.

Carl Byck

It's mostly driving style, turning in too late, and too hard. The second cause is too much bar in the rear, if they are adjustable go to full soft in the rear, right now the sway is lifting the tire. I had this on my 97 Supra, i went to higher spring rates, and less bar to solve it. There are a combination of factors, so change one thing at a time. Carl

DamonB

If it's picking a rear tire up that's bad, makes it tough to put power down with only one driven tire on the ground!

The car is lifting the inside rear tire because there is too much roll stiffness in the rear. This is usually due to too stiff a rear sway bar coupled with too soft of a rear spring. It could also be due to too much compression and/or rebound damping in the rear.

As the chassis rolls during cornering the outside tire compresses it's spring while the inside tire extends its spring into droop. This twists the swaybar but the total amount of roll stiffness the swaybar can contribute is no more than the rate of the springs. As the swaybar is twisted it is overpowering the inside tire's spring and lifting that tire from the ground.

The inside rear tire may also lift because there is too much compression and/or rebound damping in the rear. If too much compression as the car tries to roll the outside shock won't let the chassis heel over very quickly and as the weight transfers anyway the inside rear tire lifts. Alternatively if too much rebound the chassis heels over in roll but the shock won't allow the inside rear tire to droop to the ground quickly enough.

The key is when and for how long the tire lifts. If it's carrying the tire in the air around the corner it's a spring/swaybar problem. If it lifts the tire just for an instant and then settles it back to the ground as the car rotates the rear shocks are too stiff.

RotaryAXer

iTrader: (4)

Is it possible that the shocks are just too short and don't have enough travel? It may be that the shock is fully extended before the car gets to steady state cornering limits and the shock then lifts the wheel as the car continues to roll.

RoadRaceJosh

I was wondering the same thing. Take a picture of the car when it lifts an inside wheel so you have a point of reference. Then jack up the rear of the car and measure where the rear suspension is out of travel. The picture is uses as a scale drawing to see if it's a suspension travel issue.

Porsches like to carry the inside front tire on tight turns as they typically set up the rear suspesion on the soft side to tray and tame oversteer. This goes back to roll couple and front spring rate on the Supra. If you don't have enough roll stiffness on the front you can lift a rear tire.

Travis R

Did you see the front spring rate listed above?

Carl Byck

Well, Damon and I agree, but as always damon was a little more eloquent Having tracked a Supra with almost those same spring rates, I amalmost positive it is too much rear bar, possibly combined with not enough front roll stiffness. The ~1100lb front spring is actually pretty close to optimum for that car, so its probably the bar. I have photos of my 97 Supra lifting the rear inside tire, both at AX , and on the road course(at 80+mph ). If your near the limit, or like I said, turning in too late(trying to make an apex you have missed), look at your bars. You may need to go to one of the importers of obscure JDM parts for the right bar, or if you can buy bars from the manufacturer of the coilovers. The best way to go would be to send the bar out, and have and adjustable, and or lighter bar made. This will run around 200.00, but it's well worth it. Also, is the car corner weighted? what sort of rake are you using? Carl

Carl Byck

I just noticed it's mainly AX, plenty of top Supra guys in SM2 run NO rear bar. Carl

RoadRaceJosh

Yes, I saw the spring rate, but what is the motion ratio? I don't think it's a front roll stiffness problem, but I didn't specifically state that either.

What I really want to know is if the "Buddy Club" dampers have enough rebound travel.

SpeedracerRX7

Ok I am still a bit of a novice on setups and suspension geometrys, but what I was thinking is that if you could turn the dampers up in the front that would level the rear of the car out. It wouldn't squat the front outside tire as much and thus not let the inside rear come off the ground at least as much. Once you are off the brakes the balance of the car shifts back to the rear and can keep it down. If I am totally off base let me know but like I said I am a bit of a novice. I keep trying different things at autocross' and hope to soon do the same at the track.

RoadRaceJosh

It's not a good idea to try and prop up the car with compression damping. You can end up sliding a tire if you hit a bump in a manner similar to hitting the bump stops. It's usually a good idea to first look at the end of the car that isn't working which in this case is the rear.

DamonB

Springs are displacement sensitive, dampers are velocity sensitive. That means that dampers contribute absolutely nothing to the car when the car is in any steady state (accelerating at constant rate, braking at constant rate, turning at constant rate etc). So after the car has made the transition at turn in and settled the dampers are doing nothing. Literally. If the car is carrying the tire in the air as it travels through the corner no shock change will fix it.

My FD is autocrossed in a stock class so I'm stuck with the soft stock springs. I use lots of compression damping in the front end to try and hold the chassis level but that only works for very quick transitions like in a slalom and brings along the problems Road Race Josh mentioned above. Any turn I'm actually going to travel around will see the car lean over the same amount no matter what I have the shocks set at. The stiffer shocks can make the car react quicker since they allow the tire to load more quickly, but they will have no effect on body roll or suspension displacement. The springs and bars are all that do that.

jimlab

Nice crankshaft, RoadRaceJosh. Looks like a Crower billet ultralight...

OC_

wow.. thanks for all the replys!

We as just running the stock sway bars.
the car is not corner weighted yet and we need to find a palce in our neck of the woods that can do that.

he also says the car has a lot of understeer, which i think is crazy since its loading up that one rear tire so much!!!

How bad is it when he lifts the tire? Well, at a tight autoX, i watched him have the tire airborne going all the way around a 180° hairpin.


I think we might start out by diconnecting the rear swaybar just to see what happens. I read in GrassRoots motorsport that this one guy ran his FC with no rear bar (his rates where 550font and 300rear???). I know this is a supra and not an FC but you think its worth a try on the supra?

RoadRaceJosh

Well, I stole that photo. It's an Eagle 440 crank. Can't remember if it's 4.15" stroke or 3.75, but it makes a good avatar for a Chrysler guy.