emptnest

I am in my second season of HPDE, still running on Street tires - Kumho Ecsta 4x, size 275-40-17 all around. The tires have lasted longer than I ever expected, but with over 20,000 street miles, and now 6 track days, they are getting close to replacement.

My rears are wearing more quickly than my fronts, and most of the rear wear is in the center of the tire, feathering to better tread at the corners. Most of this wear has resulted from my track days. I try to watch my tire pressures, bleeding off excess pressure at the end of each session as the day and the track warm up. I usually run 35 psi all around, and that seems to give me optimum traction. But based on the center wear, should I consider running a lower pressure in the rear?

Another clue is that the car ( or I) have an occasional tendency to oversteer. I am comfortable with that, and my instructors have been impressed with my ability to correct it. So, I would like to think that my center wear results from putting power down, and that a wider rear tire would address both the center wear AND make the car a little less likely to oversteer. Or should I stay square and try to accomplish the same with lower rear tire pressure?

Up front, wear is less noticeable with the drivers side outer edge showing the most. I think I can get 2 or 3 more days out of the tires, but with the above in mind:

Should I keep my best tires up front for optimum steering and braking? In which case I plan to move front right to front left, and front left to front right, to even the wear on the corners.

Or should I move both rear to front, and vice versa, to even out the center wear? The rears have good corners so that would address the drivers front wear, but I'm concerned that the smoother center tread (if moved to the front) might compromise braking.

Tips will be appreciated.

mikeCsix

Sounds like your pressures are too high contributing to your oversteer problem. My 12 GS is very well balanced and I can control over/under steer with the throttle, in other words I control slip angle such that all tires are slipping as I carve through the turns.

You are finding out you need equal traction at all 4 corners and you should replace all 4 tires.

Instead of focusing on tire pressure, use tire pressure to keep the tires from rolling too far on the shoulders. This will help maximize your contact patch with the pavement and maximize your available traction.

My goodyear Supercar tires seemed to work well with pressures kept at 30 psi. I would start the day at 26psi for the right front and 27psi for the others. After the first session, I would reset the the tire pressures to 30psi all around and continue to do so throughout the day which usually means I was bleeding off pressure all day as the day warmed up.

I Michelin Pilot SS's now, they seem to like 35psi so I start with 29lf and 30 for the rest. Same drill, check pressures at the end of the first session and bleed off pressure as the day progresses keeping pressures at 35. I will try to go a bit lower this year if lower pressures does not cause excessive shoulder roll.

My tires wear pretty evenly too.

Another method is checking tire temperatures with an IR gauge. This method will uncover alignment deficiencies too. If your alignment isn't working for your driving style and track, tires will heat unevenly across the tread.

I run the pfadt Performance alignment parameters, track use with street tires. This is a good guide to get you in the ball park for your tires/style/track conditions with enough traction to allow you to focus on your driving skills and doesn't wear your tires when street driving.

emptnest

Thanks Mike. That's good info. I will be ordering new tires this week, but will run these old ones at least one more weekend.

I should have clarified my car is a stock 93 C4 LT1 w/ 6 speed manual.

Any one with suggestions on optimizing overall performance with the remaining treadlife ?

Bill32

First, wearing the center of the tires does sound like it's a pressure thing.

However,

The oversteer in a 92-93 is most likely one of 2 things: 1. shocks 2. driver input.

How old are your shocks? They could easily be contributing to the oversteer.

35 psi is a good, cold starting point but just dropping the hot pressure back to 35 isn't going to get you where you need to be.

Cold pressures are just a starting point, the only thing that will be accurate is tire temps. And taking them with an IR gun is very inaccurate . And they need to be taken immediately coming off the track without much of a cool down lap with a probe pyrometer.
You should adjust the pressure according to the temps, nothing else.

BTW, I'm an instructor (18 years) driving a 92, square setup, cheapo Hankook's, crap (but new) shocks - KYB. Never have an oversteer problem.

emptnest

Shocks are original FX3, set on performance setting.

Suspension is stock Z07.

Alignment is -1.5 camber all around (front is maxed there) with zero toe. This alignment was set only 2track days ago. It was stock prior to that.

Bill32

Miles on the shocks?

What can cause oversteer is lack of rebound (won't hold the rear end planted). Original shocks on any C4 are basically shot unless you have a C4 with 5,000 miles on it.
And you still may be able fo feel a difference with the FX settings but...

Might not be totally the shocks, might be driving style, might be tire pressures or might be worn out tires since you said they are close to needing replaced.

Really, the only thing to do is put on new tires and learn how to set them up with temps (and keep records).

The front alignment really doesn't come into play as far as trying to cure what's happening here. It's possible (but highly unlikely) that the rear bump steer is off and the rear tires are towing out in bump. This will cause snap oversteer. But it would be very unusual.

emptnest

As far as I can tell, the shocks are original, so that means 22 years and 42,000 miles. You obviously know your stuff, so I will accept that they are shot, but without this conversation I would never know it - the ride is firm, it settles well after a bump with zero bounce, and the FX3 system still works.

The oversteer is not a frequent issue. It might happen only once per day, and usually early. So it's probably my driving more than anything else. I sometimes carry too much speed into a corner, causing me to still be on the brake when I should be accelerating. Once it happens, I better respect the corner, and the rest of the day is "slow in, fast out" and no more oversteer.

For now, I will prioritize my corrections as driver input immediately, tire pressure immediately, new tires after the next event, and shocks as needed based on the results of these other corrections.

Solofast

A stock C4 Z07 properly set up should have about 2-4 psi less in the rear than in the front. I ran them for over 10 years and still have the old setup sheets. Since you don't have offset bushings in the front you'll wear the outside edges of the fronts. On the street the insides will wear more and on the track you'll even it up.

Are you the original owner of the car? Have the bars been changed or dinked with, in particular the rear bar?

Overall 35 psi hot seems high but I don't have experience with your tires. In my time in C4's I found different tires made the car handle differently. A C4 Z07 should be set up to just slightly push at the limit. Sometimes on some tires and on some surfaces I would stiffen the front bar, sometimes I'd soften it, but this was on Koni shocks and on the stock shocks it may be a bit different.

If you want to tame the oversteer (and you should because oversteer is slow) put a 1/16 of an inch washer between the rear bar mount and the frame. This will reduce the compression on the rubber and slightly soften the bar. Conversely you can loosen the front bar bolts, jack open the bar mount with a big screwdriver, and put in a big fender washer between the rubber and the mount and then tighten it back up. The bar bushings on a C4 are soft and by changing the compression on the rubber you can quickly and for no money fine tune the setup. Don't do both, just do one end. I used to also put a tie wrap on the big fender washer and if I didn't like it, I could loosen the clamping nuts and grab it with a pair of pliers and yank it out, even between autocross runs.

mikeCsix

The age of your tires has a lot to do with their traction. On the track, tires "heat cycle" rapidly. They see temperatures they would never see during normal driving, the elevated temps drives the oil out of the tire making them harder and less "grippy" for lack of a better term. Your early-on oversteer issue is most likely not only driving style but also cold tires. Traction improves with heat in the tire, from all that you read about the GY "run-craps" on this forum, from my experience they are not bad tires when properly inflated and up to temperature.

A friend of mine had a C-4 GS and did very well with it. I think the suspension GM developed in the C-4 forms the base for C-5 and 6 so you should be at a great starting point.

I disagree with Bill regarding tire pressures, I think 35psi cold is too high to start with, you will be over 40 after the first session, especially the right front on a counter clockwise track (lots of left turns) and you are wearing out the center of the tire faster than the edges. I did this with the Michelin PSS's and found good traction for the first few laps but traction decreased during the next session and my overpressure warnings came up on the DIC.

Driving on overpressured tires isn't all bad though, it will teach you respect for the throttle much like driving in the rain does. The negative part of grippier tires is the added traction masks mistakes so you won't learn as much until the tire breaks away.

Bill32

Sounds like a good plan.

The oversteer is probably just a "seat time" thing.

Your shocks are probably OK for now, just not as good as they were new.

When you put on the new tires, keep in mind that it's the hot pressures that you should note. When you feel that the tires are gripping like you want, check the hot pressure then let them cool down in between sessions. Take that "cold" pressure and compare it to your 35 psi base. If the track temp hasn't gone up much, don't bleed them down. If the track temp goes up, you may want to bleed them a bit. You will be bleeding air from the cool morning sessions but may or may not want to bleed air in the afternoons.
Tire management is all about hot pressures and tire temps.

Have Fun

Bill32

That's fine.

35 is the "Door Label" pressure which is where I tell intermediate drivers (even C5 & 6) to start out especially in the mornings.

As you said, different tire makes, different pressures.
But it's the hot pressures that count. A track temp of 75 in the morning may need 35, if it's 100+, may need 32 to start out.
It's a big learning curve for drivers.

And it really helps to keep records though most HPDE drivers don't.

Solofast

Another thing with respect to oversteer.

Your car is now over 20 years old. Have you checked the breakaway torque on the differential? The clutch packs in the dif are a wear item. By the time I had 70k miles on my last C4 the clutches were slipping pretty badly.

With a tighter differential your car will push more and when it is worn you it will be loose.

I suspect that you've just worn out the differential clutches and they need to be replaced by now.

Remember that 90% of car prep is maintenance, and if you want the car to work properly you've got to do the dog work and keep the car maintained.

The good news is that aside from clutch wear the Dana 44 is pretty much bulletproof.

I'd also recommend that if you put new clutch packs in the diff you use the factory lubricants. I rebuilt my 44 and we put in a big name brand "super" differential lube and the clutch packs wore out in 3 weekends of autocrossing. Differential clutches need some lubrication, but too much lets them slip and once they slip the wear like crazy. Use the factory lube and one tiny bottle of the factory differential modifier and it will work and last for a long time. I know what folks say about how much better lubricants have gotten, and that is true, but this is a special case where you don't want it to slip too much and there is a fine line between getting it right and it not working at all and it is expensive to go through one of these things. That was my experience, just passing it along.

emptnest

Yes it seems that I may have been running with tire pressures a little too high, at least in the rear. I will try different front and rear and see how that goes.

Thanks for the set up tips for the sway bars. My set up is currently all stock.

emptnest

Yes - I have stayed on these old street tires with skill building in mind. I have resisted the temptation to go to R comps, even with my pending purchase.

I am at the stage now (after six track days) that my situational awareness is good ( no more tunnel vision), and I can feel how the tire performance changes as they heat up and as track conditions change. Not saying I have it all figured out, just that I can tell as handling characteristics begin to change and I am now developing a basic feel for how to respond. The excellent coaching that all have provided here will certainly help my progress. Many thanks!

emptnest

Great tip - simple but effective. Will do. Thanks.

emptnest

Wow - that's a whole new set of variables that I didn't have a clue about. I'm not aware of any problems with the differential; it seems to be ok. I will have to check into the breakaway torque.

I am all about good maintenance. I religiously replace all fluids, and thought ( as many do) that new and better fluids can only provide better service. I never considered that the "better" fluids might be too slick.

As discussed above, my occasional oversteer is probably due primarily to my driving style which is still a work in progress, and then compounded by old tires with high tire pressure. As I better control those factors, I will remain aware to look at the differential ( and the shocks) for further improvement.

icntdrv55

OK, not trying to hijack anyone's thread here, and I apologize in advance for asking this question, as it's probably been addressed ad naseum/ad infinitum on the forum in general. I actually did a quick run-thru of the posts and didn't see anything that covered this question directly, so without further ado….

For auto-x events, it seems the accepted wisdom is to increase tire pressure from normal daily drive levels to +5-10 psi. I understand the reason behind this especially for the typical 60-, 50-, and even 40- profile tire. However, is this still recommended for the ultra low-profile (30-, 35-) tires found on some car models? It seems to me that the lower the profile ratio, the less sidewall deflection would occur, especially on cars equipped with RFs with very stiff sidewalls. The recommended inflation pressure on my car is 30 psi F&R. I could see increasing that to 35 psi, but anything higher seems like overkill that could even hurt tire grip due to smaller contact patch. Am I over thinking this? Thanks!

Xian

Nope, you're on the right track. If we were talking about an all-season 195/60 14 on a rolly commuter sedan then bumping it ~10psi would be a good idea. For performance/low-profile tires on a performance car, you'll need less pressure increase to prevent rollover.

I'd try bumping your pressures slightly, watch how they're wearing, and plan to tweak from there. FWIW, I'm in the high 20's and low 30's running street tires in AS.

Solofast

For every setup and tire combo there is a correct pressure. If you're running a stock alignment, then you will need more pressure to keep the tires from rolling over and destroying themselves. If you have more negative camber you need less. With Hoosiers and a boat load of negative camber I only run 28/25 on a C5Z. Every setup requires a different pressure. If you aren't using a tire pyrometer and have a bunch of negative camber you need to mark your tires with some tire chalk and keep an eye on the amount of rollover. Too much rollover and you'll ruin the tires. Mark the tires, make a run and then look at the rollover and keep the tire from going beyond the proper working tread.