In designing new wheel well in the rear we need to take into consideration the passive 4 wheel steering of the 928... anyone know how the rears move, and by how much?

Rick Harrison

 

The rears pull back under braking. The front connection point is spring loaded. The idea being that under hard braking you get a bit of toe-out on the rears which is more stable and helps keep the car straight. How much? Don't know. Could be a good experiment.

This can be eliminated by pinning that joint. Racers do it and like it. Might search for that.

 

I think that also under a hard conering the rear wheel goes toe-in.

Klim

 

I belive the weissach axle does both, a little toe in under cornering and a little toe out under braking...which is a subtle rear wheel steering. I know the effect is exaggerated with wider than stock tires...so some racers use a pin kit to reduce its effects.

 

[QUOTE=GlenL]The rears pull back under braking. The front connection point is spring loaded. The idea being that under hard braking you get a bit of toe-out on the rears which is more stable and helps keep the car straight. How much? Don't know. Could be a good experiment.
QUOTE]

I think you are wrong on this one. I believe it is the opposite, braking give increased toe-in at the rear. Increased toe-out when braking is what happens in lesser cars becuse of flex in suspension bushings

 

Ian928 is right - the suspension is designed to prevent toe out on over run and under braking. So says Karl Ludvigsen and best not to argue with him!

 

The Weissach axle does only one thing - it eliminates lift-throttle oversteer (or braking oversteer to a lesser degree theoretically) under cornering, which has killed so many sportscar drivers. So, if the car goes around a corner, the tendency is normally for the rear wheel on the outside to turn outward (or not turn at all in best-case scenario). Porsche made the axle with a double trailing arm so the front part will pull the wheel inward at the front under force, thereby steering into the turn, eliminating oversteer. You could say it toes-in. But it doesn't move back and it never toes-out.

 

http://www.ukcar.com/features/tech/s...n/weissach.htm

i have seen better but this is ok

 

Incidentally for spirited street driving, and a kick in the pants, and an opportunity to scratch your head a while, try this:

In a deserted parking lot, get up to driving speed as if you were on a multi-lane freeway. Drive straight. Now, with ample saftey room on all sides, jerk the wheel HARD to one side and HARD back, as if you were making an emergency lane change. REMEMBER TO LEAVE ENOUGH ROOM.

The end result is, if you do it right, you aren't where you thought you'd be. Actually, you're almost back where you started. Took me a while to wrap my head around it when it happened the first time.

 

Yep. Ian is right. A bit of googling and the descriptions all say that.

Having held a few of those arms I frankly don't see how what's there accomplishes what the text and pictures describe. I've got two spares. Thinking sawzall...

http://www.ukcar.com/features/tech/s...n/weissach.htm

 

Hmmm... Started thinking: What happens when we add more horsepower and stickier tires to our sharks? Hard acceleration should induce toe-out, not good for traction I guess, and probably not good for stability either. Maybe pinning the suspension could give less wheelspin and better acceleration times?

 

We already have hard acceleration and reasonably wide rears. I don't see how the Weissach could allow toe-out under any circumstances?

 

Just look at the posted link. "Normal cars" toe out when the wheel retards the car and toes in when you accelerate. Our rear suspension does the opposite of what "normal" cars do. But our cars have much softer bushes than normal to achieve the positive effects of the Weissach axle and toes in an out much more. Possibly to a degree that hurts traction. Maybe some of the racers that have pinned their axles have a opinion?

 

In theory yes. Howver the 928 was designed to take up to 6.5-litre (teeny 4.5-litre was never the intent) so I must believe that Porsche designed the Weissach with this firmly in mind. You may be right, racers pin their axles. But afgain the 928 was designed as a high-torque monster, and the Weissach axle was designed to do the opposite of what you're describing. I would be surprised if Porsche made it able to flail in the wind to the extent that it's so soft that mere torque to the wheels would do *worse* than a Chevy.

 

I agree that the design may be more clever than I realise and this is just not happening. But consider that when the 928 was developed, the 911 was much criticised for its tail-happy handling, and I guess even if the engineers at Porsche was not happy to sacrifice traction for safe and predictable handling, the company lawyers were more than happy to...

Also, IF the 928 had received the engine it deserved, and wider/stickiertires Porsche may very well have used harder rubber in the Weissach axle if needed.

Are the GTS suspension bushings exactly the same as on the OB?