Caster angle for track car?

[slow car]

Hi im running a 911 Sc track car and am experiencing under-steer in the tighter corners.
I am running 240 A7 slicks front and 275 R7 Rears.
currently running 6 degrees caster.
interested in what other people are running as caster in a light bar car.LB 2090 wet.
Thanks

[3rd_gear_Ted]

Some 255's will cure the understeer.
240 is too narrow IMHO

[Cajundaddy]

Unlikely castor is the problem. Use tires, sways, and corner balance to get the car more neutral in a steady state arc.

[Gianni at Pelican Parts]

Definitely want to try to get some wider rubber up front if you can fit it.

Envious of how lightweight your chassis is.

[john@poolcovers]

Thanks for the feedback,
i will try wider.
The car has a lot of down force!

[stownsen914]

I'd be adjusting swaybars before buying new tires. Loosen the front or stiffen the rear to dial out understeer. Realize that it's typical for a 911 to understeer in slower turns. You'll need to watch out for oversteer in higher speed turns after you make the adjustment. You can look into your aero to help dial out high speed oversteer, in case you wind up with it.

[Walt Fricke]

My thought is you may have too much downforce at the rear (or some other aero imbalance). I ran 225/15s front, 275/15s rear (forget the profile, but low) and it handled well. Based on experience from the top guy in that mostly stock class, I went to 240s in the rear, as that did away with the clearance issues on a stock fender- rubbing the spring plate hardware. Can't say I noticed an under/over steer difference. But nothing like your aero. Plus maybe your engine is a tad more powerful than a US SC?

Experiment with lowering your wing angle - you have those difusers going for you back there as well. Or sways, if in fact your high speed aero is just right - I forgot about that for a moment.

I use the A front, R rear approach too, and it does pretty much even out the F/R wear, though my fronts do cord before the rears - maybe due to camber.

I do run max caster up front. I don't think it has anything much to do directly with grip, but what it does is increase the effective negative camber cornering without increasing it in a straight line, as in threshold braking. I use tire wear as my indicator of best camber, and since my fronts wear out on the outside, I don't have enough and can't get more absent surgery on the shock tower, which rules don't allow.

[pgeorgeson]

What camber are you running up front?

[Cajundaddy]

Wow, that is a lot of down force. That should serve you well in high speed corners but low speed 2nd gear stuff the aero down force fades and we are more dependent on mechanical grip. Use tire, camber, and sway bar to get your low speed corner balance right.

[Evan Fullerton]

If the car is very stiff or has old or not grippy tires, you can actually induce under steer with too much caster and steering angle.

My 993RSR clone would do it when running old tires. At a certain steering angle for a given grip level, you would introduce terminal Understeer. Because of the low grip, the chassis would not roll enough to take advantage of the tire angle as caster adds camber with steering angle and you would basically roll your contact patch away to almost nothing.

I’m not saying this is what’s happening as it could be a lot of things and most 911s can’t get enough camber and castor and roll stiffness to worry about it but you can have too much caster if you have LOTS of static camber and have minimal body roll and old tires.

[slow car]

Hi Thanks for all the feed back.
I am currently running 6 degrees caster.
The tires are fresh and have even probe temps.
The car is extremely stiff with minimal body roll.
I have in car sway bar adjustment and will start to dial the balance out along with reducing the rear aero. Some in car footage below.
https://www.youtube.com/watch?v=mC_fjWoikC8

[stownsen914]

Not to disagree with advice from above, but I'd leave the aero alone for now. It should not affect handling in low speed turns.

[Cory M]

I'm running 7.2 degrees of caster, but I don't think caster is your problem. How much front camber are you running? Reducing camber in the front can give you a better contact patch. You could try wider tires, plenty of room in those flares, but 245/275 is a good balanced set up. You say you are running A7 in the front and R7 in the rear, maybe you're overheating the front tires. A7 is an autocross compound that won't take the heat as well as an R7. If you want more detailed answers you should provide information like your other alignment settings, and where the understeer is occurring (entry, middle, exit, high speed, low speed, etc). Your aero looks fairly standard for a gt class club racer, other than the diffuser and slightly longer front splitter.

[Walt Fricke]

Hoosier A7s aren't slicks. They, like the R7s, are a DOT approved street tire. Never mind that they say on them "not for highway use." That's partly because they are easier to puncture in highway use, and partly because DOT standards for tread must be pretty feeble, so these tires won't work in the wet. Slicks are not DOT. Slicks are true race tires.

I can't say I have experienced my front A7s losing grip faster than the rears. My best laps tend to be toward the end of a race or session - probably because I have by then dialed myself in better (or because the car is a bit lighter by then).