Soloontario

I am a Time-Attack guy and can tell you the Michelin Pilot Sport Cup 2 really likes some heat to work, much like an R comp although not really quite as fast. I doubt it would make a good autocross tire.

For autocross, get the Bridgestones.

NTMD8R

Richie, they have been bad over all settings.
One thing... I bought 265/35/19 and 285/35/20 R888s and mounted them to the OEM
rims last February (2015).
I had ordered wider wheels, and it took 6 months for them to arrive.
So we drove it for the first 5 or 6 events last year.
It handled OK, but still came out... which was when we first noticed it.

Then in July (2015) the wheels came, and I bought 295/30/19 and 315/30/20 R888s
and mounted them on the new wheels.

That's when all hell started happening.
That's when I starting playing with sway bars and alignments.
Nothing solved the problem.
Nothing even addressed the problem... pretty consistent no matter what I did.

So 2 weeks ago, we re-mounted the OEM wheels with the narrower tires.
It is better, but only marginally.

It MUST be the 19/20 R888s.

And I have bought a set of OEM 18/19 wheels, and Bridgestone RE71s
are on their way.

Sheeeeeshhhh... I have 3 sets of wheels, and 4 sets of tires, and I haven't had the
car 2 years yet.

conemark

Since it seems like you solved your debate, I just wanted to add that anecdotally, the Rival S is not as good (some would even say a bit dicey) in the wet compared to the RE71R, but similarly the Rival S puts power down better when it is dry.

RichieRichZ06

So you do have an alignment shop that can check and confirm rear caster on the car? Most shops have no clue it's even adjustable and how to check it with the digital gauge. I would start there if you have not specifically confirmed the settings are right.

NTMD8R

Richie,
yes, I took it to my GM dealer to check it.
It was -1.0 and -1.9, so they set it to zero.

Even though I am getting new (smaller) wheels and different (RE71) tires,
I still would like to know why the back end comes out under acceleration out of
a corner.
I hope these new wheels/tires fixes this.
Otherwise I am still back at square 1.

MSR

I kind of feel like my C7Z51 isn't a stellar handling machine. I've had lots of autocross experience in many different cars, this one is pretty easily the most difficult to drive. It doesn't tolerate aggression. Like you, I feel like it needs to be mostly straight before it will take significant throttle. Worse, once it comes out it is difficult to control, such that I need to let off rather than drive through it with throttle control and countersteer. Also, for the first time in probably 20 years, I lost control of the car and went off into the grass!


I've been "working around it" by changing my driving style. I plan to try a larger front bar. After all, almost all of the nationally competitive C5s and C6s have larger front bars. (This is by no means exclusive to Corvettes, actually. Generally, autocross setups tend to go faster when they can tolerate aggression and be stable, perhaps mostly because so few of us are capable of national caliber smoothness while maintaining friction circle.) I see now, though, that one of things you tried was the oem front bar with no rear bar. If that didn't fix it for you, then I'm at a loss.

The last change I made was to increase rear positive caster, in the hopes the dynamic toe-in will help. This will be tested at the next event. Next change after that will be a Hotchkis front bar. I will let you know if I see a significant improvement.

FWIW, I have 255/285 RE71Rs on 18x8.5/19x10 wheels. I'm at -2.8 camber up front with 1/32" toe-out, and -2.1 camber rear with 1/8" toe-in. I bumped rear caster to +0.5 degrees, from about -0.5, as yet untested in competition.

-michael

NTMD8R

Michael,
wow, thanks for the information.
So I'm not the only one.
Yes, I am aggressive.
Using the R888 tires, here's what I have tried...

OEM (Magride) Front sway bar, OEM (magride) Rear sway bar
LG Front sway bar, LG Rear sway bar
LG Front sway bar, OEM (magride) Rear sway bar
OEM (Magride) Front sway bar, OEM (Z51) Rear sway bar
OEM (Magride) Front sway bar, no Rear sway bar

Now I am back to
OEM (Magride) Front sway bar, OEM (magride) Rear sway bar

I didn't detect much difference through all these settings.
Notice now my alignment is
-2.9 camber, 1/4" toe out
-1.9 camber, 1/4" toe in
We ran 2 events last Saturday/Sunday.... again nothing spectacular
Another guy drove it on Sunday (he has a C7 Z06/Z07) and is a VERY good driver....
again... "the back end keeps coming out".

Since I've driven this aggressive style in Autocross for 35 years, it might be a little
difficult to change much. I am aggressive, but generally smooth.

Worst case scenario... I sell the car, and go back to a C6.

sccaGT1racer

I wonder what your spring rates are and how they differ from the c6? Ive read through your post and it looks like you have tried all the obvious fixes.

NTMD8R

racer,
Yeah, several of us have asked what the C7 spring rates are.
We can get NO answers from anyone.
We do know that Z06 and Z07 springs are stiffer, but we have no idea of the numbers.

mountainbiker2

Try more toe in at the rear. Like 1/2"

jking32

Could it be the Magnetic Ride Control causing the issue? Is this happening on cars without it?

NTMD8R

mountainbiker,
wow... I already have 1/4" on both wheels.
I am concerned about going that far (1/2") for just driving on the highway to the site.

jking,
good question.
I have no info from anyone else about this problem, and whether or not they have magride.

mountainbiker2

Total 1/2" Just turn it in at the track, back out when you go home. Not hard at all. That's what we did for many races.

el es tu

What about Pirelli Trofeo Rs? They are a sub 100 treadwear street legal R compound and supposed to be faster than the sport cups...

blackozvet

Keep in mind that bigger rim sizes equals lower sidewalls,
what sort of sidewalls do these tires have, and how stiff are the sidewalls ?

low stiff sidewalls are not conducive to good handling.

I recently had a conversation with someone I know who is quite knowledgeable on suspensions about going from 17" to 18" wheels to get better tire selection, he told me that my car WILL be slower on bigger wheels, because of the loss of sidewall, it will make the car break away easier and with little warning.

If your wondering about the effect of wheel size and sidewalls, keep in mind that F1 cars use 13" wheels with big sidewalls.

mattastick

If anything, I would argue the opposite of that point. Less sidewall height (generally) means higher cornering stiffness (there are some assumptions here that the sidewall is of the same construction in both diameters, etc. but in general, I think the point is valid). This means the tire (and thus, the car) react faster to slip angle than a softer sidewall (lower cornering stiffness) setup. This may make the car "peakier" to drive, but it increases the amount of available grip, and the grip is available sooner, relative to steering angle. The same can be said for stuffing a ton of tire on a narrow wheel. It works better if the tire has a stiff sidewall.

Also, F1 cars use 13" wheels because of the rules require it. It's the same reason that they do a lot of things.

NTMD8R

More good information.
Especially the opposing views on sidewall height.

I welcome more thoughts/insight on that topic.

blackozvet

some evidence in relation to the sidewall theory, a friend of mine who runs in a different class at hillclimb to me using this theory changed from 17" wheels with brand name tires (forget the brand) was finishing 2nd or 3 rd most meets, he changed to 15" wheels with r888 (no other changes to the car) and went 1.5 seconds faster, won his class and reset his class track record. A well setup car on this track would be lucky to gain 3 - 5 tenths on new tires.
The guy I know who was explaining about sidewalls also said that higher sidewalls also help the castor work better on a turning car.

As far as F1 goes, they recently pushed for F1 to go 18" saying that this would reflect the average wheel size on production cars, the F1 engineers are all against it, they have stated that they would have to re engineer the whole chassis and suspension of the cars and there would be no advantage, the cars could actually be slower and harder to drive. (although that might make it more interesting to watch !)

With wheel sizes going up (and people on here complaining that the c7 isnt handling as good as it should) it would appear to be a case of styling OVER function ?

MSR

I'm in the camp that believes taller sidewalls are "better." I do know a few people who are faster on sharper, peakier tires, but these are people of national caliber. And, they may be faster in spite of, rather than due to. The overall equation is quite complex.

However, I do not believe that is anywhere near the dominant factor in this case. My C7 is on 18/19s. I raced a C6 (a few years ago) on 18/19s and it was pretty easy to drive. It didn't put power down like a C4, but it wasn't tricky in the least. (I won SS in it.) An additional point, I perceive my car feels more stable and easier to drive on the stock Michelins (19/20s) than on the Bridgestones.

One of the dominant factors may really be driving style. My C4 had more power, but would soak up all manner of violent inputs. It thrived on them. And it hooked like crazy. The C7 feels amazing at high speeds where inputs happen slowly, and there are distinct phases of cornering. That it feels so good like that makes its behavior in autocross especially frustrating. ("Its behavior" most likely = How it reponds to my driving.)

If that's all it is, then I think this is a good thing. It will force me to relearn my driving, maybe stop left-foot braking, and figure out how to really push the limits on corner entry and smooth exits. I'll become a better driver. But if it's some handling anomaly that is artificially limiting, then it's actually hurting my driving by training me to underdrive, in a sense.

The real problem is that I don't know which of those two things it is. It occurs to me that I should just enlist someone who's talent and opinion I trust to codrive with me. I could potentially eliminate a lot of frustration, time, and money in tuning the car if they end up just telling me to deal with it because the car is good/fast the way it is.

-michael

NTMD8R

Michael,
I've had a national champion (Glenn Hernandez) drive my car last August...
he said "there is something wrong with this car... the back end keeps stepping out".
I've had 2 local champions drive it...
One said "this car is EVIL".
The other said the same as Glenn.
The last one has a C7 Z06/Z07... he goes around the course like on rails.

Another guy with a C7 Z51 magride A8 car with 18/19s and RE71s
puts 1.5 - 2 seconds on me on a 50 second course.
I am NOT that bad a driver.

I am going to try 18/19 with RE71s... wheels are in, still awaiting tires.
Hopefully next weekend.