Lukes07Vette

hello everyone,

I just finished my first autocross event and my C6Z was giving me some issues with understeering. My instructor even commented on the understeer being there while he drive the car. Is this a normal problem with the C6Z's? Or do i need to do something to the car to make it a little me autocross friendly?

Hib Halverson

What kind of tires?
What tire pressures?
What alignment specs?

flyby763

It's pretty normal if you're on stock size tires. Alignment would be the easiest way to help, add a little tor out in front to help turn in. Check your tire pressures too.

Lukes07Vette

dunlop DIREZZA DZ101 (front)

car has factory alignment.

tire pressures were all at 30psi (3 different people checked them at the track)

pkincy

Here you go. Just make sure what you change is legal in the class your would like to run. I think Stock class still allows one bar to be non oem (front or rear).

AzDave47

Different rear tire brand/model? Mis-matched tires can contribute to car imbalance depending on tread compound and tire construction differences.

Other comments are on point as well. If alignment changes are permitted, more negative front camber should help front grip (the recommended toe-out above will improve turn-in and balance). If larger tires are allowed, closing the stagger front/rear will also help.

My Z06 understeered quite a bit with -0.7* front camber, but was quite a bit better at -1.7*.

Suns_PSD

Add in camber and use the throttle to correct.

I have a Z51 that runs Z06 sized R888s (F 285s & R 345s) with max camber (F -2.1 & R -1.4 I think was the most we could get) and it's a beast at the track. But the nature of these cars is that they want to push at first, then oversteer on exit under throttle. I use the throttle to easily correct the understeer then I just baby the throttle to prevent the impending exit oversteer.

Have fun!

Lukes07Vette

the only issues I am going to run into are:

1. I cant lower the car any lower, it wont get into the drive way.
2. I drive it daily, so I a cant put track tires on.
3. Im not sure how much alignment alteration I can do while maintaining DD use.

AzDave47

Per my question above (post #6), are you running different brand/model tires in the back than in the front?

Lukes07Vette

Yes. I am running Proxies in the rear. Dunlop up front

BLW BY

It all starts with the right tire. Different tire brands require different alignment specs. Different drivers require different alignment specs. Different types of driving require different alignment specs. (ie. DD, road course or auto x).

So everyone doesn't get their panties in a bunch I won't post my alignment specs.
I will tell you 90% of my driving is on So. Cal. streets. I get about 18,000 miles or a year out of a set of street tires and never change my alignment specs. from a road course to an auto x back to the street. I do put 100 tread wear tires on to do track events so I don't shred my street tires.
Figure out the first 3 variables and then you can start on what alignment spec. is right for you.

R

AzDave47

Matching up tires would be the first start. If that's not in your budget right now, just play around with tire pressures to take the understeer out.

Budget's always come into play. tires mounted for the street, another set for the track, brake pads/rotors/cooling, it goes on.

Adjustable sway bars can really change the front back balance, but it all depends on the rules for the event(s) you will run.

Suns_PSD

To get the car balanced doesn't require track tires. You just don't want to have track tires in the rear and street tires up front as that will exasperate your issues.
Add camber. The tires don't wear much faster and it will handle better everywhere including on the street.

Bill Dearborn

Also using the throttle to correct understeer means lifting off the throttle, if you get on the throttle you lift weight off the front and make it under steer more. Sure you might be able to tromp it to get power oversteer but you can't control the car as well doing that. The cars have pretty good trailing throttle oversteer and you can get the back end to come around quite nicely just by lifting your foot off the throttle until the nose of the car is close to being aimed in the direction you want to go and then reapplying throttle to exit. When you lift the torque reaction of the engine transfers weight to the front wheels which increases their grip while decreasing rear wheel grip. Since engine compression is being used to slow the car some of the rear tire grip budget is used to do that slowing thus further reducing rear cornering grip. When done correctly the driver can make the back end come around quite nicely. The other thing you need to be sure you do is not to overcook corner entry. That is where most understeer is encountered and the best place to keep it from happening. Remember slow in equals fast out.

Bill

Lukes07Vette

how much camber should i go with? Front and rear? keeping in mind I dont want to eat up my tires.

PS. I am aware that I can only do so much with to my car if i want to keep it usable for daily use. I just want to know what I can do to make my autocross days a little more enjoyable. I am just out there to have fun, not super serious about it yet.

thanks for all the advise so far. It has been a real big help.

AzDave47

See this Phadt setup link:

http://www.pfadtracing.com/blog/wp-c...-9.12.2011.pdf

You want at least Street/Track performance, which isn't even as aggressive as the factory settings. If you are serious about autocross competition, consider the next setting, street performance track/"race tires".

STANG KILLA SS

a square tire setup will solve your problem.

sway bars, springs, camber etc all help a little but, but you still have more traction and more available friction circle in the rear, this will always cause under steer as the front will loose traction first while the rear is still grabbing.

ive done all the above suggestions, and the square tire setup made the single biggest difference. night and day. freed the car way up and turn in response and grip is phenomenal.

AzDave47

... and what "square" tire sizes would you propose for a C6 Z06 that comes with 275/325 tires to begin with (recognizing the stock 325's can't take all the power anyway? 295's may fit on the front 9 1/2" rims and maybe as small as 305's would fit on the 10" wide rear rims.

It certainly appears that the OP is bargain hunting as he does not have the same brand of tire front/rear so I don't think a $2-4K set of new wheels and another $2K for tires is in the cards in the near term.

Suns_PSD

You can and should run around -1.5 to -2 in the front and about 1 degree more (cause negative numbers), as in -.5 to -1.5 in the rear.

If you zip around a few turns here and there on the street, and you do some autocrosses, your tires will likely last longer with more camber because you won't kill the outside of the tread. And the car will feel much more secure everywhere.

Camber doesn't hurt the tires nearly as badly as toe does.
I'd recommend buying one of those lifetime alignments for something like $179 if they have good alignment guys so you can try many different settings and if you have a real track day, you can dial in max camber and then back it back out the next day.

Hib Halverson

What brand of tires are on the rear?