I posted also in the C4 tech forum, but maybe some gurus are here at the Autocrossing & Roadracing.
I recently installed 2 Michelin Pilot sport 335/35/YR17 in the rear and the same 2 Michelin Pilot sport 275/40/YR17 on front.
Before this, I was running with a 5 year old set of tires (275/40 ZR 17 on all corners) with a very minimal tread left. Tires were General (a Continental brand).
I used a set of Zr1 wheels. (9.5" front and 11" rear).
Before the swap (needed) the car was handling fine, very responsive glued to the ground. The downside was only that in a sharp corner I was not able to use
additional throttle due to oversteering.
Now it seems I have in the rear 2 flat tires.
It seems I have 2 rear tires with very weak sidewalls. I have the feeling that I start to turn the steering wheel and the car reacts only 1 second later.
Slaloming is a very disaster...
Car is aligned perfectly and a second check confirmed that the car holds alignments specs.
During the turn the car seems better, but when I'm finishing the turn the car need an additional second to go straight.
I'm speaking of touring/city speed!! not autocross speeds! (I haven’t tried higher speeds... I'm afraid..).
Contact patch seems good even with higher than suggested inflation I can't spin rear tires!
I really don't know if it is a sidewall problem or a body roll problem. The only thing I know is that the only change made is the tire swap and a new alignment.
I tried every kind of pressure from 20 psi to 50 psi with the same feeling.
If I can rate the before and the after I can say that with a optimal rate of 10 before was 8 now is 1.
Can't be only a tire problem... It seems I have something disconnected or some mechanical part loose.
During the installation there is some dangerous operation that the tire man can have made? (broke something.. disconnected something...)
Checked the lug nut and they are in torque specs. Sway bar bushing seems ok, and it seems there is no end play on tires ( hub bearings).
I can’t believe to the theory of the smaller rim. Yes I’m using a 11” rim for a tire rated for a 11” to 13” rim. This can be a part of the problem but I think the BIG problem is
On a broken/disassembled/loose mechanical part (good before the swap and bad after the swap), or a defective tire set.
Please help me, where I have to see first, in order to find the reason of this??
Thanks and sorry to have re-posted this subject, but I really need your help.

 

No expert about this, but this is what I think. New tires have much more tread. So the new tread squirms around alot. That is why racers will shave there tires down. You have two choices. Get use to what you have, or shave the tires down. I always go for the first choice. I only get about 4000 miles out of a sent of tires anyway. Hope this helps.
Steve

 

The bigger tires to seem to track more. I have a 275/315 street set of tires and 315 all around set for Autocross. You should feel the tracking with 315s in the front. The other thing is tread. When new you have a lot of tread which will absorb some of your turn to start with. Its that delay you feel in the time it takes to start turning from you input.
You just kinda have to get use to it, start turning a little earlier in anticipation for the delay, or shave you tires.
I did noticed on my Goodyear F1s that after a couple thousand miles and autocrosses they did become more responsive as the tread got worked in. First autocross showed how much it really does flex, had the sides of the tread walls shearing off some.

What are you using for an alignment? Did the guy give you the settings he used?

 

Thanks,
I said to the tire man to follow the vettebrakes "advanced street specs".
It seems I have a bit more body roll...
A stiffer rear sway bar can help?
I have to say tht the tire man said tht my front toe (with old tires) was over 2 OUT. The car was so resposive , but I finished the front tires very fast...
I have to toe OUT a bit this setting on front?
Thanks... I'm checking all bushings....
-Beppe-

 

[QUOTE]
I have to say tht the tire man said tht my front toe (with old tires) was over 2 OUT. The car was so resposive , but I finished the front tires very fast...
I have to toe OUT a bit this setting on front?
[QUOTE]
2 toe? Not sure what that is, sure isn't inches. If you had a lot of toe out and now when to a VetteBrakes Advanced alignment this is why you feel like its not responsive!
Toe out in the front is going to make it "darty" now if you are use to it you can turn like crazy, will feel like its turning the instant you ask it to. You do this for autocross, but it kills tires. Even if you went to 0" toe in the front its going to make a massive difference in feel. It would feel unresponsive to you. The VetteBrakes Advanced really isn't all that aggresive IMHO.
These are the alignment specs I've been using street/autox, I find it a nice middle ground, tire wear is good, yet really helped the overall feel.

Front camber -1.5*, caster whatever you get just try to make it even should be around 5-6*, toe 0" or 1/16" out
Rear camber -1.5*, toe 1/8" in (this gives your rear-end stability in turns and doesn't seem to affect straight line accel too much).

See if you can get a before & after printout from your alignment guy. I happen to get an awesome deal on alignements, as many as I want for $80/yr from TireKingdom. I have "trained" the guys there to use my alignment specs and give me a print out after each time I change mine.

 

The stiffer the sway-bar, the more the car wants to slide on that end. Stiffer front, more understeer. Stiffer back, more oversteer. On my C-5 I have 1/16" toe out in front for better turn in. In the back, I have 1/8th toe in for better exit speed. Some guys in autocross go as much as 1/2" toe in. None of these settings are tire friendly.

 

Really need to know your alignment specs. I'd cast a wary eye toward the toe setting on the rear wheels. If I've got this right (it's been a very long day), toe-in on the rear wheels will make the car rotate much faster, and tend to keep rotating. At least until you get it figured out, I'd set the toe, front and rear, to 0. That'll give you pretty good turn in characteristics on the fronts, and hopefully the rears won't tend towards oversteer so much.

Let us know what the numbers are, maybe something will jump out.

HTH, and have a good one,
Mike

 

Unfortunately I haven’t the specs of my alignment. The guy said he used the vettebrakes advanced street specs.
For sure he said that the “before” alignment specs were so extreme and OUT of at least 2 times of any reasonable alignment he ever seen.
I have to ask if saved the specs used for my car. He has an electronically/dynamic machine to do alignment so I hope he has these specs.
I have to say that my car is NOT in the OVER STEER side. I can’t loose the rear even if I want and even with a lot of throttle in the middle of a sharp turn, but it seems I have additional body roll than before. This is the reason I asked info for a stiffer rear sway bar. I can’t say that car is in the UNDER STEER side, but for sure is not Oversteer. I can’t approach to a turn with the right feeling because when I start to move the steering wheel the car reacts too later and this unhappy feeling is at 90% on the REAR. As I said it seems I have more rear body roll/side wall flex/rear flat tires feeling. It seems I’m using a 20” diameter steering wheel 1950 car. When in the turn the car does not seems is loosing the right ideal lane, does NOT loose the rear, but I continue to have this mushy/spongy sensation on the rear.
I will try to see if the rear toe is in the IN side or the 0 side, then I will go to a 1/8” IN as you suggested.
Then after the results, I will try to see the front toe and I will try to see if with a minimal toe OUT I can feel the car reaction better.
This mushy/spongy feeling on the rear can be the result of the incredible additional traction I added to the rear, and this can make me feel the limitation of the weak frame/spring/stock rubber bushings combo?.
Before the swap of tires (maybe) I was loosing the rear before the frame show their limit..?
(Yes, before It was an OOOOVERsteer car and I used a softer 1996 rear leaf spring to cure it…with a relatively good success). (now I have the same weak 1996 rear spring…)
An help is very appreciated.
Thanks
-Beppe-

 

Sorry, I was headed the wrong direction, but it was a LONG weekend.

Look at it this way: you're used to driving the car with 275s all around (which is what I race on). With this setup I experience oversteer (as you stated), and great turn-in with little or no understeer.

Now you've added 120 mm of tire width to the back, a 22% increase in grip. This will surely cure your oversteer problem, but now I think you're going to have a big understeer problem, and I think that's what you're experiencing now.

Also, with that much tire width, I imagine you'll have the rear trying to respond to every road bump, rut, and discontinuity you hit, which might translate into the car feeling "nervous."

Finally, don't take my word on alignment specs. Heck, I barely know what to do on my C5. You might ask in a different topic what kind of alignment specs you should use for a C4. When in doubt, go conservative. Then, at least you'll get the car back to where you can get some experience with it and then start tweaking.

HTH, and have a good one,
Mike

 

Interesting, as this post gets longer the more info we get from you. At first it appeared all you changed were the tires which was a big leap forward in size and should have been a great handling enhancement but wasn't and had us all scratching are heads.

But now we know there was an alignment done (which is always dubious unless you were there and can verify the settings) and also a rear spring change. I'm going to quess that the tire change brought the car back to something realistic and all the other changes you had made before were to compensate for the tires that were on the car before. Even with the increase in tread depth you should have been better off.

IMHO I'd go back to the original rear spring and a set of reasonable alignment settings ( THAT YOU CAN VERIFY ) the settings mentioned above seem reasonable but I don't have any experience with C4's. There is a golden rule in racing of one change at a time that can be tested otherwise you are going to chase your butt all day long with multiple changes.

 

I think there is NOT a single culprit on this bad handling..
If I sum all of these cause, maybe I will go to the solution.
Culprit can be:
Conservative alignment
Solution I will try to open a bit the front toe (out) 1/8" should be a reasonable amount to make the car with more reaction.
I will go to add some negative camber on front also.
Bushings seems good but 14 years old.
I will start changing the sway bar bushings with poly ones in order to cure the increased body roll.
About body roll and understeer sensation I think I will go with a bigger rear sway bar (now I have a 22mm stock).
New tires with new tread.
Yes, with a 10/32" tread the car CAN't be so responsive as before (no more than 2/32" on old tires).
335 on a 11" rim.
Even if the tire is for a suggested rim of 11 to 13, I think that a larger rim can help the sidewalls to works better. An enlargement to 11.5 or 12 can be a good improvement.
FE1 1996 rear spring.
I switched to this rear weak spring, one year ago to fight the eccessive OVERSTEERING of my car and to replace my stock 90 rear spring (much stiffer) because it was with a visible crack in the middle.
This cured a bit the oversteering, the car was less responsive, (not so responsive as I like but in any case a real good setting).
If I do all these improvement I think I will re-gain agood feeling as I like.
Thanks
Any other additional advice is wellcome.
-Beppe-





[Modified by conv90, 10:31 AM 7/6/2004]

 

Good luck on getting her set up like you like, make sure and let us know what changes work and how well they work.

Have a good one,
Mike

 

Conv90, IMO, there were too many changes made at one time to definitively zero in on a single problem. My guess is that the combination of the fat rear tires on too small a rim will give you a wacky slip angle sensation. Also going to a super soft rear spring isn't doing you much good, and is exacerbating the problem.
Rear toe in will keep the car more settled in turns and upon acceleration exiting a turn. (toe in will not make it oversteer as noted in an earlier post) You will need about a 1/4 to 1/2 degree less of camber in the rear vs. the front with the big rear tires.

 

On the BSP Corvette forum everyone recommends replacing the worn rubber bushings on the differential mount with poly. If you think about it, the entire rear suspension is connected to the chassis at these two points. Any looseness here will result in a vague feeling during transitions and turn-in, and increasing grip will just exacerbate the problem by placing even more force on these bushings resulting in greater deflection. The downside of replacement is increased road noise.

 

I'll throw in a word of caution.
be careful when stiffening the rear in any way, be it bushings, swaybar or springs. take it to a large empty parking lot FIRST! don't try any testing on the street.
I tried a larger rear sway bar and scared the crap outta myself.
another experiment, with stock sway bars and poly bushings in the rear bar (rubber in front sway bar) was a doughnut spinning machine (very fun, but still too loose).
so be careful. a little can go a long way.