I've recently started shifting the balance of my car forward and I've come upon a problem.

On corner exit I am having the RPMs rise 200-300rpm from tire spin before returning to the normal tire slip. There is no accompanying bang / noises, nor is the car becoming unsettled during this.
I believe general wisdom here would be:
1. Add front bar
2. Pull rear spring rate out

Problem with that is that I've done the opposite of both to get the car more neutral at / over the limit and I like how the car works otherwise. Anyone have suggestions on what to do here, or is it time to pony up for a proper diff?
General setup is:
Penske DAs
850lb/700lb springs
Karcepts front bar on 3/3 for holes
Custom rear bar similar to Karcepts, full soft
255 square RC1 on 10s

Car is being run in TT4 for contingency reasons. Lap times are local TT4 podium and TT4 lap record ballpark / quicker than local TT5 records, so reasonably quick.

 

It's not springs, it's the rear bar pulling the inside rear wheel up. Only real solution (short of a clutch-type diff) is a softer rear bar. Try the OE AP2 bar to start, you can get them cheap.

 

have you been dialing up rear rebound to the point its jacking down?
are you allowed a clutch diff?

 

Have you tried adding front bar, aside from theorizing? You could go 3/4 or 4/4 for a really minor step up in rate and possibly get the inside rear tire to sit on the ground more. There might not even be much of a negative effect for front balance.

What rear rebound do you set the car at? If running too stiff the tire might not be returning fast enough.

I'm not too far off what you're running in terms of rates, 900/700, Karcepts 3/3f but I run a little more rear bar to get it to rotate - 4/4r (188lbs/in). Still on the OE Torsen.

 

I have not tried adding front bar back in, I think it is definitely an idea. Rear rebound is in the middle of the adjustment range, I suppose I could try pulling some out, as you and ncrx suggest.

I have not, but diff is open... other than the $2500 price tag.


I am on a much lighter than stock rear bar and it is on full soft.




I do appreciate the suggestions, it looks like I will try pulling some rear rebound out then adding some front bar back in.

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the only other thing i can think of is a lot of front rebound bias and the rate of weight transfer rearwards is too slow

 

The problem is the inside rear being unloaded, mainly in a short transient condition.

If you reduce traction in the outside rear tire, the car will need to add traction to the inside rear tire to balance it. That would involve less rear roll stiffness or more front roll stiffness. Since it is transient, it may be possible to do that with the shocks.

Lowering the CG will reduce the amount of weight transfer increasing the weight on the inside rear tire. However, this also changes the geometry so rebalancing is often needed.

Since the problem is transient, it may be able to be addressed with the shocks. This would be a low-speed adjustment on the shock.

If the problem is in faster corners, an aero adjustment may fix it.

As mentioned, a Salsbury or other non-Torsen rear should also work if the rules and budget allow. As the tires get stickier (there are S2000s on 315/30 A7s) the Salsbury rear may be the only solution since the car would generate enough lateral force to unload both inside wheels.

Update: race tracks have different turns, road courses usually run clockwise in the US, and all the turns aren't of equal importance. I remember reading about tuning a Spec Miata where the car had a wedge screwed into the scale balance (like an oval track car) because it benefited the car on the most important turns. Mark Donohue also said it is what they did on the Penske Trans-Am cars for Lime Rock, which only has one left-hand turn. That could also be part of a solution.

 

Farb
rear shock control
ride height change
rake change slightly more rearward based

All setting I would tinker with at your next outing.

Is your rear toe where you want it? Does this wheel spin happen when turning both directions?

 

Why would increasing the rake help?

 

I'm saying he should reduce the rake by reducing rear ride height, not increase it. My initially suggestion was poorly worded, sorry. Lower CG, less lateral weight transfer.

As a note I don't know how close OP is to bump stops currently which would cause this change to have a greater effect in handling potentially for the worst if he's engaging his stops after a rake decrease change.