Bob Rouleau

Geoffrey - I will see if anyone has a bump steer gauge around here. I assume I will need to track the car to make it work? I have never seen one and I cannot imagine pushing on the rear by hand will show much.

I am curious though, is your comment specific to the 997 GT3 RS? The 996GT3 handles very well with the same tires and alignment. This problem is unique to my new car. I'd like to understand your point better. Excess bump steer on an unmolested Porsche would astound me unless it relates to the idea that Doc2S has advanced. Note I am not trying to dispute your idea. In fact the problem feels like bump steer - the car starts to take a set then seems to lurch and slide. At first I thought it was something loose in the linkage. A switch back to Hoosiers which did not lurch and hop seemed to suggest that the linkage was OK, besides there was no mechanical noise associated with the behavior.

I wil be at the track next week using Hoosiers with the 2.5/2.4 alignment posted earlier. I hope it will work (and it doesn't rain). If it does, my next attempt will be to raise the rear of the car to compensate for the diameter of the 18 inch MPSC. I am not keen on dropping the front since it is low already and a nuisandce on the street.

Meantime I'll get it on a lift and look for evidence of a cracked or broken bushing. Meantime my pal Apex late has some secret sauce setup on his 997 GT3 and is setting lap records at Calabogie. He is on Hoosiers as Cups don't work on his setup. perhaps he will share if I ask nicely

Best,

NVRANUF

Bob, FWIW here's a visual on 305/19" vs 315/18" rear tires...
The dimensional spec on N1 305/30/19 MPSC and MPS2 (shown) tire is identical.
As you've stated, 315/18" have an outside diameter or 25.5" vs the 305/19" diameter of 26.3" which translates into 815 revs per mile on the 18's vs 788 revs per mile on the 19's....
The difference in ride height has to play some significant role in setup, for driving on the track at the limit...

Good luck in getting your car (re-)sorted!!!

Geoffrey

Bob, This problem is not limited to the 997 RS, in fact, your 996GT2 had .0050" of toe out per 25mm of wheel travel on EACH SIDE.

Bob Rouleau

For added info - a discussion with the tire supplier adds the following info:

1) the shoulder of the 18 inch Cup in 315 is more square than the Cup tire in other sizes - notably the 295's I liked on my 6GT3. Does this improve the contact patch or worsen it? One theory suggests that the stiff and square shoulder decreases the contact patch when under load. Certainly the theory fits the symptoms I had - transfer weight and instead of gripping the back would hop and slide like hell.

Another customer using the 315 Cup corded his tires on the outside(!) edges in only 6, 20 minute sessions. He did not have as much camber as I do but apparently this is very unusual as the 295s last much longer even with suboptimal camber. Michelin Motorsport guys are now on the case. I will report anything useful that comes along.

best,

Bob Rouleau

geoffrey - thanks for the info. The odd part is that my Gt2 handled nicely with the same 18 inch 235/315 Cup tires. My setup on the GT2 was as above, i.e. 18 mins static toe in at the back. I am aware of the toe out under load of a 911 but you'd think that 36 mins total would be enough. In any case it was enough on the 6GT2 and the 6GT3 using Cups.

best,

Professor Helmüt Tester

Bump-steer measurement process:

http://www.longacreracing.com/instru.../7900PI_p.html

TD in DC

Bob,

Mike Levitas recently gave a very good suspension tuning seminar to Potomac. One of the issues he pointed out with the 997 is that you can have two cars with exactly the same alignment "specs" but they could handle radically differently depending upon where the eccentric bolts are in the rear carrier and/or ride height changes. These cars are very sensitive to geometry changes caused by changes in the location of the rear eccentric bolts and ride height. This may not have anything to do with your problem, but it "might" explain why your 996 GT3 handled well and your 997 GT3RS handles poorly despite having the same alignment specs. Just a thought.

TD

Darren

To that point, does this car have rear radius arm bushings? It may be totally irrelevant on this suspension design (I don't know how similar it is), on my regular 996 it got very evil with 265/315 MPSC until I replaced the radius arm bushings with solid pucks AND got the TRG toe links. This was probably because I was running too little static toe in. The combination of rear toe changes with ride height and with bushing compliance can be a recipe for an evil handling car, esp if it can go toe out. If it is going toe out you could try increasing toe-in to a larger amount to prove the point.

NJ-GT

Bob,

I felt the rear springs on the RS softer than the ones in the 996 GT3. Running the 315 MPSC with a 20mm smaller diameter will drop the rear another 10mm, so more weight will be sitting on the rear axle at static loads.

Combine that extra weight on the rear axle, and the softer springs, and it should be easier to reach the bump steer limits on the new car. Making the rear sway bars softer won't help to recover grip, because the rear loaded wheel will still move towards toe-out.

The bump steer problem on the 996 and 997 is on the suspension design. The toe links are too far behind the control arms, so when the suspension compresses, the toe link pulls the rear part of the wheel towards the inside, creating toe out. That's why these cars benefit from pretty high static toe-in settings at the rear axle.

5/32" total or 4mm total or 0.25 degrees per side is what I'm using for toe-in at the rear axle, and what the 997 GT3 Cup that won the Speed WCGT last year was running as well. Your static toe-in at the rear looks good.

You can rotate those front spring perches down 15mm, and test the car.

Personally, I think Hoosiers have so much grip that they can mask the car balance if we don't drive them to the limits. I've been driving them under the limits of grip, when I was thinking I was already pushing them on the limits.

I hope you get to figure this out soon.

adrial

I assume you mean 0.005" of toe out over approx 1" of suspension travel?

So we are talking 0.005" of toe out as compared to the static setting of 18 mins or 0.3 degrees of toe in. 0.3 degrees of toe in over a 26" diameter tire is a 0.136" delta from the front of the tire to the rear.

So, 0.005" is hardly significant as compared to the static setting IMO.
The bushings definetely deflect more than 0.005"

The fore/aft position of the toe link has little to do with bump steer.
The real driver is the relative length and rake angle of the suspension links as compared to the toe link when looking perpendicular to the axle line. I'll post more later...gotta run to work.

analogmike

Lots of good info in this thread but if you drop the rear even 6" the weight distribution will change insignificantly. The c of g is the only thing that determines weight distribution front to rear and it moves a fraction of a millimeter when you lower one end of the car.

Darren

That's about what I needed to run on my 996 to keep the rears from going toe-out before I got rid of the bushings. Now I'm running a hair less than 1/16" total rear toe-in and it's very good -- tiny tiny bit of movement on heavy braking, I think 1/16" exactly should be fine.

There is a lot of toe movement obviously, more than 1/8" total rear toe-in to compensate for mostly bushing compliance! Maybe the Hoosier takes a set in a turn and the MPSC for some reason is creating an oscillation in the toe.

I was able to get rid of most of the factory rear toe change with ride height by changing the upper control arms to adjustible units with spherical joints. I guess that would be an issue with racing limitations, but it compensates for the problem by allowing me to adjust the arc of 3 out of the 4 suspension points. I jacked the car through the range and measured the toe change, adjusted until it was minimized.

Bob Rouleau

Guys, thanks for all the helpful hints and suggestions. I will be acting on them shortly. First will be a tug of war with the suspension to see if anything is loose, perhaps cracked bushing(s). The tires are being sent to Michelin Motorsports to see if there is a manufacturing defect - slipping belts for example.

I won't go down the list but I promise to report if the problem gets solved.

Best,

Jarez Mifkin

To answer Larry's question, The 997 GT3 RS's rear suspension is 1.4" wider than the 996 and 997 GT3, the RS also has a wheelbase increase of 0.2"

JM

fatbillybob

I don't have adjustable bars or shocks so all I got is alignment/chassis settings. For me chassis rake is the most significant way for me to change fore/aft grip. I bet the grip coming back will be really noticeable if your dropped the rear 1/2". If it does, then you can fine tune it and further tune with shocks and bars. Also, tire makers may list a tire by the numbers but in reality the true sizes can be different among different makers and even different mold from the same maker. For example 245 640 18 hoosiers I run sometimes are about 3/8" wider than 245 640 18 Pirelli slicks and they rub on my fenders. If diameters aren't true it will throw off your rake.

Bump steer can be an issue too and if you can't measure it can't you just emperically increase rear toe from the 36mins you got now to say 45 mins and see what happens?