EricVonHa

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.

RX7 KLR

More toe in for the rear. Don't get hung up on numbers, just keep going in until the rear feels right.

gcrouse

You're right about one thing- the rear has waaaaay too much roll stiffness relative to the front. Curious why you'd put such a stiff spring on the rear only? You won't get that thing close to balanced F/R until you get an appropriate spring up front or put the stock rear back on. Then report back and we can go from there...

MSR

http://forums.corvetteforum.com/zerothread?id=358308

EricVonHa

I shoulda mentioned earlier that the rear spring rate is 525lbs.

I'm considering moving to a front spring that is in the low 400's...

What do you think? Thanks again for all replies!

ifeeltheneedforspeed

I agree, the stiff rear spring may be your problem. Do you have your stock rear spring you can change to and see if the handling effect is improved?

EricVonHa

That's the goal for tonight! Replace the current spring with the stock unit! Then it's off for some testing!

If Gary from VB&P reads this or whoever monitors this forum from this vendor... promises are promises. If you say you're going to call back, please do. The current sport spring rate is 525... the un-returned call was to determine in fact, that a spring rate in the low 400's for the front was actually the proper rate! Ask Gary for my number, he's got it. Thanks.

rbeckham

I know you say your rear is loose, and have adjusted pressures accordingly. However, before changing tires I would suggest lowering pressures front and rear, with particularly a lower pressure in the rear.

Hoosier's rec's are extreme for track, in my experience, and certainly not suited for lower speed events. Youn need to let the tires move a little.

EricVonHa

Anyone know what a "stock" Z07 or Z51 spring rate was for the front of the C4's?

ET

I agree with rbeckham.I don't understand your tire pressures at all.I'm running 315\35's rear,275\40's front,Hoosier A3S03's.
Front 36-37 PSI
Rear 34-35 PSI

Dr. Evil

Heres my setup.
Front
1.2neg camber
1/4 total toe out
5 deg castor

Rear
1.0 deg neg camber
1/4 total toe in


It makes the car a bit unstable on grooved roads but turn in is fantastic. I think you're close on your alignment specs but the Hoosiers love alot of negative camber. They like 2 or even 2.25 degrees negative camber. I use the R3SO3's 275/40/17S.
As for the spring rates there are two camps on this issue where as C4s are concerned. I am in the softer rear spring camp. That is, I like the softer spring in the rear to help plant the tires/power coming off the turns. I lowered my car in the rear as well and it was a bit loose and wiggly in the rear in some tight slaloms so I lowered it a bit more at the track and it helped quite a bit. Food for thought. Try keeping the rear lowered, with good shocks, stock/soft rear spring and play with your tire pressures.


[Modified by red90sixspeed, 8:31 PM 8/6/2002]

h rocks

Eric, you have to dump that rear spring, and that huge front bar. If you are committed to stiffening up the suspension, try locating an old set of "Challenge"/Z51 or Z07 springs from ~'89 thru '91. (the difference in the option designation, is dependent to the model year...same spring rates) Also, you may want to consider running the 30/26mm swaybar combination. The rear needs to have ~ a total of 3/32" to 1/4" toe in, to calm down the back end. This is just a start, and a more goes into achieving good balance.

EricVonHa

Everyone, thanks for the replies. At this time on order from VB&P is a front spring that is rated at 1112lbs. All of the feedback here in this forum and in speaking extensively with VB&P, it was determined that all things being considered with the current setup, that the heavier front spring is a much needed change. VB&P actually recommends a springs in the 1200/600 range for smooth surface lots. Of course, this is with the proper Bilsteins, sway bars and alignment specs very similar to my current combination. My car doesn't exhibit any detrimental nose dive on hard braking, so I opted for a slightly more liberal front spring (that, and the existing rear is a little softer)

Earlier on in the thread, I mentioned that I was going to put the original rear spring as a band-aid fix, remember? Well, it was trashed as soon as I installed the new one! and for a good reason! After trying to locate it for 5 minutes in the garage loft, I realized that I didn't hang onto it because the wheels, upon occasion would scrape the wheel wells. Good enough reason to chuck it! It was a useless sag of a spring...

So, for the event this past weekend, I dialed in some more rear toe and also toed out the fronts a hair more. Band aids at best! The car became a hair more stable before the point of no return, but still had a serious unpredicatable oversteer condition that exhibits itself on a high speed transition. You'll be glad to know that I have never ever been one of the guys to take out the timing lights and that I still managed a 1st in my class yesterday. :cheers: I typically do finish fairly well in PAX and usually win my class... imagine what this thing is gonna be like when I get it back in the rhythm!? Can't wait! :)

So, in summary, the fix for now is FRONT 1112 and REAR 525.

More feedback to come... Thanks everyone!

gcrouse

You should be able to make things work with that new spring; those rates are much better balanced F to R.

FYI I checked my VBP springs and I'm running 1380 F and 590 R; car is very balanced and predictable.