emptnest

I am novice HPDE driver. Just did my second day on track, at Pittsburgh. Car is stock 1993 Vette with ZF6, Z07 suspension, J55 brakes, running Carbotech XP 10 front, XP 8 rear, on Kumho Excsta 275-40-17's square.

I ran 3 sessions of 20 minutes each, and my times improved noticeably with each session. But one turn in particular caused me problems. It is a 180 right hand turn, between two straights, with a vertical climb. So, approach to the curve is fast, with most speed being scrubbed off prior to turn in.

My problem was under steer, which was compounded by the Traction Control then kicking in, which prevented optimum acceleration for track out. Rather than slowing down, I hunted for a better line which I never really found.

I think I made several mistakes. First, I think my tires were over inflated. I started the day at 35 psi front, 33 rear. I suspect that this was the source of my under steer. Second mistake, I didn't check my tire pressures after each session.

So despite my mistakes, my times improved, meaning that my approach speed into this problem area coninued to increase, further exacerbating my difficulties there. My third mistake was that I tried to carry that increased speed into the turn. In hindsight, and ignoring the tire pressure issue, I suspect that I could have turned a better lap time if I had slowed for the curve so that I didn't activate ( and fight) the traction control.

So that's my self assessment. Critiques as to whether I have reviewed this and learned from it correctly will be appreciated. What else should I have done differently?

93Rubie

What kind of alignment do you have? How is the front sway bar setup?

Understeer isn't a huge problem for me at higher speeds. C4's do struggle at low speeds with high steering angles but not at higher speeds. At least not in my experience.

Was this a "push" or a "plow?"

I would turn the ASR off, its WAY too invasive on C4's for any kind of performance driving.

I assume you had an instructor?



Honestly, I think you want a bit of understeer, oversteer tends to cause more issues.

strano@stranoparts.com

So I assume you are talking about turn 10?

First, I agree that if you any clue what-so-ever, turn the ASR off. It's only going to get in the way.

Second, is the car got any sort of alignment done? A lot of C4's I see with positive camber which certainly doesn't help.

Third, yeah the pressures could well be too high as they will climb a lot from heat you might have ended up over 40 hot and that likely didn't help.

The corner is increasing radius and you are driving into the hill. If you have understeer there, you are either just way, way too fast into the corner, or carrying way too much brake when you turn in. Or the car is messed up is some pretty spectacular way.

emptnest

Yes this was turn 10 at Pittsburgh, and the car was plowing.

Alignment is bone stock, for street. Ditto for the Z07 sway bars.

Yes, I had an instructor, and was instructed to leave the traction control on.

My instructor told me to quit fighting the TC, but not how. I didn't figure out till afterward that slowing down was my most effective option. That's obvious now, but not so much then.

I carried no brake into the turn. I was religious about straight line braking only. But, as I understand it, the traction control was probably applying rear brakes when it was active during my under steer.

I ran Summit Point 2 weeks ago and did not encounter these problems there. I'm not sure, but I think lower tire pressure at SP was the difference. I added 3 psi prior to Pittsburgh.

Thanks for the feedback.

blackozvet

Do you have any video ?

Traction control doesnt stop understeer,

most people understeer into hairpins because they are simply carrying too much speed into the apex for that particular corner,

there's an old saying "fast in, slow out or slow in, fast out" that applies to hairpins,
but as most experienced drivers know, that understeer can be controlled ,

to control understeer exiting the apex, you need to "feed the wheel back out", meaning you have to straighten the steering wheel as much as possible, inexperienced drivers tend to turn the wheel more into the direction they want to go, this exacerbates the understeer,

if for example your going into a right hander, and the car begins to understeer as your exiting the apex, you need to keep the wheels as near as straight as practical, if you feed more right steering in, the wheels turn more to the right and begin to slide because of the increased slip angle .

it is something you have to practice (somehow) and it feels unnatural at first !

does that make sense ?

Hat_Trick_Hokie

Not sure about setup advice on C4's, but below is a link to a lap I ran their earlier this year for reference of the turn in question. Turn 10 starts at 0:54 in the video.

Bill Dearborn

As somebody else mentioned Traction Control doesn't do anything to correct understeer. If you were getting interference from traction control you were either hitting some bumps in the turn that caused sudden changes in one of the rear wheel speeds or you were hitting the throttle hard to get the back end to break loose to counteract the understeer. TC will prevent using oversteer to correct an understeer issue. I don't know about all C4s but my 86 was a car that understeered like hell. You need to get your entry speed down so you can get the turn in initiated without scrubbing the tires sideways. Instead of trying to power out of the turn right away it might be correct to drive toward the apex without understeer, lift off the throttle slightly at the apex to get some trailing throttle oversteer thus yawing the car in the direction you want to go and then roll onto the throttle.

Bill

Bill32

Bill Dearborn's post is right on.

I've run my 92 on track for 6 years (but only taking students out, 18 years as an instructor).

I run 35 psi cold with Hankook street tires.

Your instructor may not have been familiar with C4 traction control, it's worthless, I won't even run it on the street.

The TC is not sophisticated to apply the rear brakes during understeer unless the rear tire are breaking loose (as Bill said).

Turn it off, learn to control the understeer with steering input and throttle application.

35 MAY be a bit high for Kuhmo's but the only correct way to be sure is to take tire temperatures.

Blackoz is also correct. But without being in the right with you in that particular turn/situation, it's extremely hard to pin down what was happening.

Slow In, fast Out - yep
Understeer can be corrected - yep

Try some of the things these guys mentioned, have fun.

emptnest

To all those who responded above, thanks for the advice. I seriously appreciate that you have taken the time to help me. I can't wait to get back out there and apply the suggestions that you have given.

Slow in, and fast out. Got it.

Bill Dearborn

Just want to point out that using trailing throttle oversteer to get around a corner could take some practice but the C4 should make it easier. I used to come into the apex of Turn 7 at the Glen with my 86 then roll onto the throttle aiming for trackout. If there were cars on the track in front of me I could do a quick lift off the throttle and roll right back onto it while in 3rd gear. That would make the back end kick out and line the car right up to pass the cars on the right. I never was able to get the guts to do that imy C5s or my C6Z because they had so much more trailing throttle over steer.

Bill

emptnest

Bill - I am particularly interested in learning this technique. I experienced minor over steer several times in Turn 1 at Summit Point. It was mild and easily managed. And even at that time, I saw the advantage that you point out.

That is part of why Turn 10 at Pittsburgh was so perplexing to me. I expected that the climb through the turn would make oversteer less likely (well I think I got that right), and even if I did have some, a little is good. So, I attacked the turn aggressively. I was not prepared for understeer, and did not adjust appropriately.

Thanks again for your advice.

SouthernSon

Does the '93 have AH? I was under the impression that the vehicle only has the anti-slip control (or something on that order) that simply kills the throttle when the rear breaks lose. No?

emptnest

Not active handling. Just simple traction control as you supposed. My post above (where i supposed that the TC was compensating for under steer) was mistaken. Bill D is right; I tried to induce oversteer to escape the under steer, but the TC wouldn't allow it. I was trying to apply more throttle.

If only I had known that I could have initiated some over steer with LESS throttle. Yeah, I'm filing that one away.....

I also compounded my under steer by mistakenly steering even more in my intended path, as Blackozvet described.

Short summary, I screwed the pooch in that turn. Lessons learned.

Slow in, fast out.

strano@stranoparts.com

Uh, well a better setup car would also help. Like having some negative camber in front, which will give you more grip. Also I'm sure you are seeing more wear on the outsides of the fronts which again is helped with camber.

Do not overlook the alignment.

emptnest

Yes sir, I absolutely agree. The car is currently doing double duty on both street and track. Plus, I presently drive it to / from the track. So I plan to keep the street alignment for the rest of this track season (only two events remaining)

But Next season, I plan to use it almost exclusively at the track, and to trailer it. So, part of my winter work will be a good track alignment.

Bill32

Yes, it only kills the throttle.

It can actually be dangerous on the track.

About the only thing it's good for is if you hydroplane on a wet highway.

SouthernSon

You are right about the danger! My '96 almost caught me by surprise while jumping a slight curb when entering a highway in front of a Kenworth. ALWAYS, without fail, turn off the ALS.

93Rubie

As someone who drives to and from events on the same tires he races ...err...competes on. You CAN run a full performance alignment and the tires wear just fine. As long as you keep the front toe near zero.

The wear from auto-x/track WITH negative camber keeps the tires wearing VERY even. The street miles do NOT hurt the tires as long as you are also using them at the track/auto-x,etc....

I have yet to have a set of tires wear funny on me. Your car without an alignment has to understeering pretty good. You'll be amazed what a good one will do to the car. Even with the limited negative camber you can get the front of a STOCK C4.

If it is setup right, it should push a bit, but NEVER plow.
FYI, just yank all the shims from the front upper control arms and set the toe to zero up front. Unless you change arms/bushings that is all she wrote with how much negative camber you get. It can vary from C4 to C4. I got lucky...
RF=-1.0 LF=-1.5.

emptnest

Ok, so go for max negative camber, zero toe. What about caster? I guess max camber (zero shims) sets caster to zero - right?

What about rear alignment??

Does the weight of the car have to be off of the tires to make front or rear adjustments?



BTW - my C4 is a 40th anniversary Ruby red too.

93Rubie

Caster will be dictated by the camber. These cars are camber limited in the front. Yank all the shims. What you got is what you got. They really need the camber more than the caster. Mine still have L=5.3 and R=6 degrees of caster with all the shims yanked out.

Unless you start replacing the upper arms for adjustable ones or use offset bushings in the lower arms. Which you can do if you want/need more camber/caster down the road.

Camber/Caster adjustments up front, you need to raise the front end to remove the shims from the studs that hold the upper arm.
Toe can be adjusted weight on the wheels.

Rear=I run my camber around negative 1.5. Toe-in of about .10 on EACH side. Again adjusted weight on the wheels. Watch when you loosen the cam eccentric bolts on the lower strut rods out back. It will want to rotate, hold it with a wrench and loosen the nut.

That being said, that is for auto-x. You might want something else for track use. I would definitely do the front end as I described. You cannot get too much camber on the front of a STOCK C4.
You can try the rear settings I have and go from there.