My dilemma is with regard to AutoCross alignments. Let me explain, but, you can skip the middle part if you wish and just go right to the bottom where the current alignment settings are listed.

Anyhow, the ride is a ’93 Vette with a modified suspension. It has the 32mm front & 26mm rear sway bars with the polyurethane bushings and end-links. Special care has been used to make sure that the bushings do not “bind”. Bilstein shocks are on all four corners. VB&P rear “sport” spring has been installed and the front one is the stock unit. Adjustable camber struts have been installed in the rear. The car has been lowered with the standard removal of the wedges and longer rear spring bolts. Trailing arms are polyurethane as are most other mounting points for the rear. The suspension itself is in excellent condition meaning that there are no wheel bearing or ball joint problems.

While AutoXing, I use 275/40 Hoosier A302’s. Front Pressure: 34psi Rear Pressure: 37psi (typical cold).


The issue with the car’s handling is that the rear is verrry loose. By this I mean, if I’m traveling anywhere from 30 to 60mph in 2nd gear (no accel or decel, just cruising) it is easy to bring the back end of the car around into an instant OVERSTEER condition by cranking the wheel one direction. A couple times, I’ve spun a hard 180 degrees before I could counter steer or adjust throttle.

My thoughts that I wanted to run past you guys:

1. I think I need to “loosen” up the rear suspension a bit. A slightly softer Rear spring may be required (though with this request, I’m trying not to spend money, only maximize the current setup)

2. By raising the rear of the car by an inch or so and re-adjusting toe & camber, I think this will lend a little more travel to the rear suspension by unloading the shocks a little and raising the center of gravity just a hair.

3. I’ve noticed that raising the pressure in the rear tires has lessened the oversteer effect.

Here is the current alignment:

What can be done to make it better with regard to keeping the rear planted and not inducing oversteer.? Toe out on the rear?

Front

Toe: -.03 (“in” each side)
Camber: -1.3 each side


Rear

Toe: -.04 (“in” each side)
Camber: -1.3 each side


Thanks very much, E.

 

you definitely have too much rear spring. out of curiosity, how did you come to use such a high rate rear, yet keep the stock front?

i've spent mucho $$$ just in springs before finding a combination that works for me. i ended up with stock Z07 anti-rolls bars, the VBP xtreme front in an 1120# rate, and the VBP rear in a 440# rate.

the 550# rear sport i tried was too much. since the front rate is pretty high, it wasn't nearly as twitchy as you describe. however, the rear grip level seemed artificially low, and it didn't put power down well at all on corner exits. (this was with 26/24 anti-roll bars.)

reverting to the stock rear spring was a big improvement, but with too much understeer. the 440 spring, then the bars together "fixed" the handling.

now it steers well, it keeps rotating through corners without pushing the front end, yet remains stable and tolerates aggression. if i overcook a corner, instead of wildly oversteering while trailbraking, it the whole car slews in with a balanced drift.

not causing your problem, but check the bump travel up front. dunno if '96s are different, but mine was on the bumpstops (with the '96-specific bilstein sports) at ride height, with barely 1.5" lower than stock ride height. this made suspension tuning pretty difficult, since the front end's infinite roll stiffness always dominated. i had to modify a few parts to get my bump travel back.

lastly, try to crank in as much negative camber as possible up front. when you get the spring rate combination closer to correct, you'll be able to use the extra front-end grip from the extra camber.

good luck!

 

IMHO,

1. Too much rear spring, especilly w/stock in front. Try the stock one, you will probably be pleasantly surprised. Many poeple find great success with lighter springs and stiff bars/good shocks. Right now you are disproportionately high in the rear.
2. Set rear toe IN to about 3/32-1/8" IN on each side.
3. Try setting front to ZERO toe for street. Paint a mark on the tie rods and at the track, turn each side about one-half turn OUT. Make sure you are turning both side sOUT. Wrenches are 13 & 22mm. This should give you about 1/8" toe OUT on each side. Be sure to return them to your marked zero point when you're done autocrossing.
4. Get as much negative camber as possible in the front. Rear may be OK, depending on what you can get in front. Rear setting should be about the same (or slightly less) that front with equal size tires all the way around. Check with a Hoosier expert as to how much camber Hoosiers like.
5. I run Kumhos, so am not a sage on Hoosier pressures for a cold starting point. However, I think you will probably gain from starting with rear pressures about 3 pounds LESS than the fronts.

Are the shocks stock FE1 Bilsteins? They're not going to help a lot, especially with your rear spring. You may want to try Koni Sports (Yellows) or similar that allows at-the-track adjustment for rebound.

Have you checked suspension travel since you lowered it? You may be hitting the stops as previously mentioned. Might not be getting much help from the shocks at all.

Best of luck and let us know how the changes you decide to make work out. Paul

 

Guys, I didn't mention this earlier but the rear sport spring is rated at 525lbs. I'm told that the stock FE1 suspension had a spring rate from high 200's to low 300's. I'm thinking on going to a front spring that is close to the 400 mark. What do you think?

I'll try toeing in the rear a little and the shocks give adequate feedback and rebound as this is a very smooth lot that we typically are running in.

Thanks for all your help!

 

eric, please heed my warnings... you need a mega front spring if you're going to keep the sport rear. at the bare minimum, you should get an '89 Z51 front spring.

note that with a spring marked "1125" up front, the wheel rate i measured came to about 300lb/in. that's still not much. i wouldn't mind trying something significantly stiffer, even, if it were an open-track only car. for auto-x and spirited street, the 1125 works very nicely.

 

MSR, thanks much for the feedback. I'm running a 32mm sway bar up front. I think that would warrant a slightly softer spring in comparison to your setup, no?

Not to generate any bad publicity for anyone, but good customer service, is good customer service. VB&P is losing a little respect at this point. I've called 2 times, received amateur advice from a novice the 1st time about how the toe in the rear will fix the problem. the 2nd time the advice that was given was about the stock front spring rates being in the low 300 range. The 2nd call was ended when I broke the guys confidence by asking "are you absolutely sure?".. Obviously the answer was no, because I was told that someone else was going to be asked the same question. No call back as promised either. Oh well, everyone has an off day. Tomorrow it's three strikes and they're out. I'm asking for the guy that actually signs their paychecks tomorrow and/or does the testing/development of their products. I'm only doing this swap one time and that's enough. I'm sure others have had better experiences with them and I realize that this topic is out of the realm of the average street driven car, but again, they are supposed to be the experts. It's their product after all.

So, tonights agenda? I'm putting the stock spring back in the rear and am going to try it out on the "test track". This should settle the rear end down quite substantially, but, I'm worried that it'll make it too soft. We'll see!

:)

 

The conversion factor is 5.9. Hib's suspension chart is in Newtons per mm, VBP uses pounds per inch. The front spring for 93 FE1 is 73.2 N/mm or 430 lb/in (73.2 x 5.9 = 430). The 525 lb/in converts to 89 N/mm which is as stiff a spring the factory ever offered (84 Z51). I gotta believe it's a little harsh ;)

Eric

 

Eric, sounds like you're fairly sure that the stock FE1 spring was in the low 400's... If so, does Hib's suspension chart show what the Z07 or Z51 front spring rates were for the late C4's??

Thanks a bunch! E. ;)

 

The conversion factor is actually 5.71. The later Z51 and Z07s had springs around 115 N/mm (~650 lbs/in) and 90 N/mm (~515 lbs/in).

So you are only running about 420 lbs/in front, and 525 lbs/in rear. That is pretty disproportionate...if you check out Hib's chart, you can get an idea of what rates C4s typically need, front versus rear. The 32mm (vs. 30) is probably nowhere near enough to compensate for the high rear roll stiffness.

HTH,
Jason

 

MSR, Navyvet and everyone else, thanks very much for your responses. On order is a front spring tested to be 1112lbs from VB&P. I had a lengthy discussion with their guru who has his monitor tuned very closely to autocrossers that buy his products and the same overall feedback was given by him. Thanks again and I'll report back after the addition of the new front spring and re-alignment!