Spdfrk81

I'm running 295/30R18 on front with 345/30R19 rears starting at 30psi. Only have run them twice put have noticed slower speed tight radius turns, mainly left handers, the front wants to slide or push in the corner. Not sure if the fronts are to wide and causing this or are the rears gripping to much or are the rears just to big? I'm running DRM shocks and Hotchkis Front and Rear sway bars in center hole.

VetteDrmr

You're experiencing the natural understeer in Vettes associated with big splits between front and rear tire widths. 50mm increase in width (almost 2 inches), plus a LSD diff, is going to push, esp. in slow corners.

My C5 has slight understeer with a 275/305 stagger, moderate with 275/315s. Reduce your stagger to reduce your understeer, then when you get it close dial it in with your shocks.

HTH, and have a good one,
Mike

db2xpert

That's the std tire size pairings.

Try adding 2-3 psi to your front tires. If that reduces the understeer,
add another 2, until you get it where you want it. I run up to 6 more in the front... and tyis works for me. Every car and driver is different.

You could also stiffen the rear sway bars if the tire pressure doesn't give you enough.

VetteDrmr

It may be standard, but it's still too much stagger for neutral handling. I've found that messing with sway bars, tire pressures, etc., are fine tuning adjustments, compared to changing tire sizes. To be sure not a cheap fix, but still the most effective way.

Remember that manufacturers set their cars up to understeer for John Q. Public.

Have a good one,
Mike

acrace

OP - does your car understeer upon turn-in, or does it turn-in nicely and then understeer? There are different remedies depending upon exactly what's happening. If you the car starts to push immediately upon turn-in, assuming that you're not overdriving corner entry, tweaking the front toe would help. If the car turns in nicely and then pushes, more negative camber can help.

Tire pressure tuning is definitely something to try, and you can, as one person suggests, either stiffen the rear or soften the front. Since you have the Hotchkiss bars with the holes and have it set in the center hole, you have some tuneability there.

By the way, I doubt that the fronts are too wide, but I've never run MPSCs on my C6 Z06. Only Hoosier A6s, and I run 315s on the front. The MPSCs may behave differently; hopefully someone with experience on those tires will chime in.

db2xpert

Yes, but those sizes maintain the stock stagger that comes on a Z06.

VetteDrmr

What am I missing in your comment? I thought the OP was trying to eliminate his understeer, and getting the tire stagger correct is the easiest and most repeatable way to make that happen.

db2xpert

And, given his setup, the most expensive.
He should be able to get what he wants by adjusting what he has.

I do agree, that reducing the stagger is best, and I have two different Z's, with some stagger, and they are neutral on track because the bars, shocks, and TP are set correctly.

68sixspeed

agree with the above on tire stagger etc, but also keep in mind that those tires want heat in them and are pretty slippery when cold. A6 would be a much better choice.

VetteDrmr

I'm not sure if your Z's are C5s or C6s, but for sure my C5 needs some stagger. Stock suspension has a lot of oversteer in mid-corner and corner exit with a square setup. I've found about a 30mm stagger gives me neutral manners mid-corner, transitioning to a small amount of understeer on corner exit.

BTW, I consider corner entry understeer to be a technique problem best corrected by driver inputs.

Have a good one,
Mike

racebum

i really agree with this.


to the op, if you altered the suspension at all and have more spring or bar in front, that can bring on understeer. if you lowered the car and the rear came down more than the front, that can bring on understeer. you also can play with alignment settings to change what happens adding more neg camber in front and if this is only slow speed autocross you can even use front toe out to get that fast turn in


really really agree on the driver inputs part. if you haven't been to a driving school or had any formal training, grab skip barbers dvd off amazon. it's all of $20 and a great recap of the basics and how to properly setup for corners, get a grasp on tire slip angles and drive the car properly

edwardo99

At autocross speeds you have one other tremendous tuning tool: rake.

Add more rake to increase turn in/loosen the rear and vice versus. You can do this by either lowering the front or raising the rear, depending on where you are now w/r/t ride height.

Solofast

Tuning understeer/oversteer with tire pressures is a really bad idea. Yes you can, over a very narrow range get some change, but you are basically giving up grip at one end to match the low grip that you already have at the other. Bottom line is you lose grip and nobody does it that way anymore.

Get your tires at each end working to maximum grip (optimized pressure and camber relationship) and then tune with bars to get mid corner oversteer/understeer balanced. You always want to figure out how to IMPROVE the grip at the end of the car that isn't working, as opposed to lowering the grip at the other end. At least that's the way you need to look at it if you want to win.

Corner exit and entry are tuned with shocks and toe (toe to help turn in, shocks for entry and exit balance).

Folks 30 years ago used to tune with tire pressures when they couldn't tune anything else, but those days are long gone.

While you can get changes due to rake (or rear ride height) all you are really doing by raising the back end is raising the roll center and essentially adding rear roll stiffness. Tuning is better done with bars than ride height. Get the car at the suspension sweet spot and leave it there, adjust balance with the bars. Just because you can do it that way doesn't mean you should.

Robert R1

Had the same issue with my Z06/Z07 on Michelin PS2 ZP's.

2 things really helped.

Getting pfadt alignment done but adding even more negative camber upfront.

Having a tire service at the track that making adjustments on tire pressures right after a session based on tire temps.

EyeMaster

If the diff is causing the understeer, would adding some slip modifier (making it lock less) help?