Tuner1

I am trying to diagnose a bit of a rear handling quirk on my 2wd 993TT.

Under hard acceleration in 3rd, 4th and 5th gears (1st and 2nd just spin the whole way through and 6th is GO TO JAIL speed), the rear of my car tends to hunt from side to side, like a strong yawing motion. If the road is perfectly flat and smooth and my concentration is 100% it is no big deal, but if the road has a decent crown or if it is rough it can be very unnerving. I have driven plenty of other powerful RWD Porsches, (996 GT2, 997 GT2, CGT, my ex-930 with 600hp, etc) and none of them had this feeling.

In researching various ideas I came across the factory alignment specs for the 993 GT2 track car. This car would have been lighter than mine by maybe 500lbs but both the GT2 and my car have 600hp+, solid rear subframe mounts, stiff or solid link bushings, stiff springs, wide tires, aggressive camber, etc. The glaring difference is that my car is set up with +18' of total toe-in at the rear (IIRC the max factory spec is +20' total) but the GT2 is listed as requiring +30' to +35' total!

I know that most 993'ers follow the RS alignment specs and use around +10' to +15' total in the rear but these cars have half the HP & TQ of my car and the GT2. Perhaps all that power causes so much suspension deflection under WOT that I need to run lots of static toe-in to keep the wheels from toeing out?

My other theory revolves around the Quaife diff in my car, but considering that Quaifes are often used in FWD applications to reduce the side to side "******" that can come from a clutch-type LSDs, I don't think my problem is coming from the diff.

Thoughts? Feedback?

Here are the factory GT2 specs. I know this car uses radial slicks but I have tried three different sizes and four different brands of tires on my car, including R compounds, with no change in the yawing effect.

**Edit - on the 993 Pcar site the 993RS rear spec is listed at +15' +/- 5' per side, so this could be up to +40' total toe in! If these numbers are correct then the +35' for the GT2 doesn't seem very high and my +18' seems way too low!

jscott82

Sounds more like a kinematic toe problem... Sorry i can tbe more specific, never figured out exactly how that works.. but the kinematic toe comies into play in dynamic conditions (accel). What you describe is the exact feel my car exhibits when KT is off.

Bob Rouleau

The 996GT2 track alignment spec I got from Roland Kussmaul called for 17 minutes of toe in per wheel, or 34' total. Guys running the 996 and 997 GT3 are using similar amounts of rear toe in. I once tried 10' per side rear toe on my 997RS and the car was unstable under acceleration and also under hard braking. I find it odd that you are using so little toe-in on a 997GT2.

As far as I know, Kinematic Toe was exclusive to the 993 cars and not applicable to the 996 or 997series. I would check the thrist angle however. One ther thing that can cause symptoms like yours is a difference in tire diameter left to right. Did you replace one tire perhaps? In any case swapping wheels side to side is an easy way to check for this.

Regards,

Tuner1

Thanks Bob. My car is a 993TT and not (unfortunately) a 997 GT2 The spec sheet I posted is supposed to be for a 993 GT2.

My Kinematic adjustment is equal L to R and all the tire sets I have tried have been new and perfect.

It is interesting that you were told +34' total rear toe-in for the 996 GT2, although I can't recall the factory specs. Is this for a track alignment set up?

Bill Verburg

That is the alignment page for '97 993 GT2 Evo

are all of your joints nominal? no looseness or play. My car was very twitchey before I replaced all the suspension and also was twitchey when I ran the back w/ .3 toe, it's fine w/ .1


not sure why the GT2 would want so much rear toe

Bob Rouleau

The 996GT2 track alignment spec I got from Roland Kussmaul

Bill Verburg

Oh and if you have a monoball suspension that eliminates any KT function

Tuner1

Bill, everything in the rear (and the front for that matter!) is either new or perfect. Hard rubber bushings in the A arms, ERP toe links, ERP solid subframe mounts, camber and Kin links are mint, etc. Fresh alignment to my specs (currently -2.1 camber and +18' total toe in).

As to why you can run +10' toe and the GT2 calls for +35'....well my theory is the power difference. Both cars probably work fine during steady-state cornering but when the big power cars get deep into the throttle things start to move around Porsche might be trying to tame the back end by using so much toe in.

Bill Verburg

I could feel the rear steering the car on the highway w/ the larger toe setting.

Lots of toe creates lots of drag too.

You are saying that your car feels hinky w/ 18' total toe in? 9' /side? Do you have the tilt kit? That changes the geometry considerably but I am not sure how as I never looked into it. I used the flat solid sides which just raise the suspension up into the chassis a bit, this reduces rear bump steer.

trophy

Hey Rob, do you have the solid frame mounts or the tilt kit? The GT2's had the tilt kit which reduced squat under accleration, I would suspect there is so much squat (even with stiff strings) that the suspension is compressing so much that toe etc is changing quite substantially.

The toe thing is to do with the slcks, there are different alignment settings for the GT2 depending on the different type of slicks being used. All slicks have very specific requirements for alignment and outside of those specs handling will be effected. Also notice the difference between the tires for camber settings, this is all tire dependant.

Bill Verburg

one other thought, when I set up the rear of mine I used the least KT that could be obtained evenly on both sides

trophy

I know with my car anything more than .3 of toe in the rear it gets twichy... Also to Bills point I know I have never gone for max KT on my car, I away strive for equality on the middle of the Autometric gauge (around 3.5 from memory) I really have not felt any major difference with the KT change, and no where the difference in the handling that small changes in toe make.

I need to think some more about the GT2 toe specs..... i'll get back to you on that one.

Steve Weiner-Rennsport Systems

What were your KT values????

KT is adjustable, regardless whether one has OEM or RS rubber-bushed links, monoballs, or the adjustable ERP ones. Those in fact, are the way to correct bump-steer in the rear axle.

Steven & Bill raise a valid point,.....optimal static rear toe settings depend a lot on tires as the carcass construction & design play a big role as well as driver preferences.

Bill Verburg

How can there be KT when the A arm rear, inner bush is a monoball?

KT relies on that bush being elastic

Tuner1

Thanks for all the replies.

For sure, but for me this would show up as accelerated tire wear but not much reduced speed

Yes, it squirms and yaws from side to side with +18' total toe in. I have the standard spec (NOT tilt) ERP solid rear mounts.

It doesn't squat a ton with 800# rear springs but with all that torque I believe it moves and twist enough parts that the toe settings move.

I would have through that the extra toe in would make the rear quite stable, just less lively and steerable with the throttle So you have tried more than +30' total on the rear?

The MAX Kin value I could acheive without messing up the camber (i.e. no more than -2.25deg) was around "3" on the Autometric scale. The Kin adjustment is equal side to side.

For sure, although I would guess this is likely as much or more applicable to camber than toe FWIW, I have run Pilot Sport Cups, A048Rs, AD08s, P Zero N-spec and Pilot Sport PS2s...always the same feeling

Keep in mind that this squirm/yaw sensation is while going in a straight line or past the apex under WOT, but within the traction cirlce! I.e. it isn't because the car is going sideways or spinning the tires. The car handles very well at turn-in, mid-corner or corner exit, as long as I don't get near WOT! It also tracks perfectly down the highway.

Honestly, I think my "problem" will only surface in high HP cars.

Unless it is the Quaife...