adrial

Raising and lowering a the front or rear of the car won't have a significant effect on the weight on wheels, but it will have a significant effect on the distance between the roll center and the CG --- which is the moment arm that the cornering force acts upon which results in body roll. This is the reason that raising/lowering the front or rear of the car changes the handling balance of the car.

Basically what I'm saying is that raising and lowering the front or rear of the car will enable you to further change the handling balance of the car beyond the control that swaybars will give you. So it can be used as a tuning tool, but I think that it shouldn't be necessary with a proper spring/swaybar combination. Obviously adding rake to the car will have some effect on the high speed handling of the car (as aero comes into play...be it avoiding lift or adding downforce).

I don't have the knowledge of the GT3RS suspension geometry to say which direction will give you more grip at that end of the car. But if there are some pictures of the suspension looking from the front or rear, with the suspension links exposed...I will be able to make some judgements.

And now back to your regularly scheduled programming...

Darren

I guess I read evil to mean suddenly unpredictable, so not a balance issue.

adrial

Right -- my response was in reference to other responses regarding the effect of rake on the handling of the car.

I just have a hard time believing that Porsche put a significant amount of bump steer into the rear of the car that would cause poor, unpredictable handling.

Bump steer in the right direction (toe in under suspension droop) could help lift-off oversteer which I think they implemented in the 993...but it can't be too severe otherwise it causes funny handling over the bumps.

Geoffrey

"I just have a hard time believing that Porsche put a significant amount of bump steer into the rear of the car that would cause poor, unpredictable handling."

Do the measurements, then get back to me then...

RedlineMan

Or...

Another way of putting it might be, "Go through the measurements sometime. I think you might be surprised at the change you see."

So, I would not assume that there are settings that reduce/eliminate the geometry change, but that simply put it within a driver-manageable range?

Even though HAVING an adjustment usually REQUIRES it, I too would think that Porsche would have engineered out such anomalies by now.

Geoffrey

John, Exactly...you can adjust the car to minimize the effects of the geometry changes from suspension movement.

adrial

As I mentioned in a previous post, I assume you mean 0.005" of displacement from the front of the tire to the rear of the tire? Is my assumption wrong?

Larry Herman

Cup Car springs do that nicely.




Eliminate unwanted suspension movement, that is.

Bob Rouleau

Adrial you are correct. My last foray with cups had 18 mins of toe in per side. Very odd. I wonder if the rear tires were defective. Felt like toe out but a switch to 19 inch oem tires and the car was benign. The symptoms are certainly strange.

Geoffrey

Adrial, I meant .050", not .005"

NJ-GT

Not quite right. The toe link position will make the wheel toe-in or toe-out under compression. In case of all the 996 and 997, the toe links are located behind the axles. When the car compresses, the wheel changes to toe-out.

The 997 GT3 RS and 997 GT3 use the same suspension geometry, with the exception of the rear control arms. The rear control arms in the RS are two pieces units that allow adding shims to increase camber and wheelbase, because as you add shims, the rear wheels move towards the back. The same thing happens if you add shims at the front control arms, the wheel moves toward the front, and this increases caster.

I've those same two pieces control arms, with just a little bit of shims.

There are several ways to reduce these toe changes at the rear axle.

1) Stiffer springs
2) Monoballs and solid bushings instead of rubber
3) Cup car toe links

I've done the three of them and the car becomes easier to drive fast. The 997 GT3 is heavier, I haven't confirmed if it is softer sprung but it feels softer, so it should suffer the same toe problems when driven hard on sticky tires.

Driving at 8/10th with MPSC might not be a problem. But as you get to driver harder (or faster) on sticky tires like the R6 or medium/soft slicks, the car will compress more, and you will find those limitations in the suspension.

Nordschleife

Larry

Anybody driving a Cup car on the Nürburgring gets rid of the Cup Springs and Shocks.

R+C

Bob Rouleau

Robin - I assume that's because of he bumps?

Guys if you have ben following the story do note that the car was fine driven hard on 18 in R6 Hoosiers. A switch to cups with no other changes and I had a ton of oversteer. With 18 mins of toe in per side out back, I don't think I was suffering from bump steer toe out. I am also confident that the grip level of the R6 was not very different from the cup. That's why I am so puzzled.

Best,

Nordschleife

Bob

The standard springs on the Cup are designed for F1 style racetracks - the 'real world' is different. The cars that run in the VLN are raised, and given heavily revised shocks and less firm springs.

In my experience, there are a lot of less than perfect race surfaces in North America, I'd be on the phone to Manthey. All they guys who do the suspension don't want to say 'exactly' what they have done, and may even remove certain identifying marks. 'You are buying our product, not our knowedge', sort of thing.

It is very noticable that they are capable of getting great performance out of fairly inexpensive Bilsteins and Konis. But without their years of R&D, I'd look at double digressive set ups, as done by Paul Laqmbert at Stasis in the States.

Remember, with the 996, the RS had softer suspension than the standard GT3 because the Nürburgring was faster that way. Where I drive, the firm setting on the 997 is completely pointless and is slower, perhaps it would be different if I were lapping at Hockenheimring or Leipzig.

Incidentally, I have noticed that there is a tendency to fit the auxilliary shock canisters inside the car cabin. I am not keen on this because it makes it too easy to fiddle with the suspension set up AND loose track of what changes have been made.

R+C

Greg Fishman

Is not the reason to do this to keep the canisters at a more constant temp and keep it away from high temps of the engine compartment.