Changes in front suspension

[turboteener]

Did Porsche keep the same front spindles for the whole production run or did they alter the suspension geometry somewhere in the mid years? Need to know if all front spindles are the same throughout the years.

[Danglerb]

Changed a few times, 78/79 and 87/91 I think are similar, 80/86 a bit different, and not sure on the GTS (not sure period, but this is how I understand it). Changes were I think related to the brakes and mounting points, not so much geometry.

[Barbapapa]

Originally Posted by turboteener

Did Porsche keep the same front spindles for the whole production run or did they alter the suspension geometry somewhere in the mid years? Need to know if all front spindles are the same throughout the years.

The ball joint points are the same in 78-86, for 87(86.5) on they were changed with the big difference being the lower ball joint being 'inverted'. The control arms also changed for 87 on. The suspension 'geometry' is either early or late.
What do you have planned? If you're doing a custom build you might consider the brake mounting too, later ones are radial mount for the Porsche brembo calipers but very early ones might be easier to fab brake adapter brackets.

[turboteener]

Its a secret, but lets put it this way. Outside of the CGT the 928 is the only Porsche that got a proper dual a-arm front suspension, like all sports car should have. It would be perfect on say a lightweight sports car based on a certain Porsche chassis.

Which later years are we talking about. 87-91? Radial mounts would allow use of the later 996-997 capliers.

[Barbapapa]

There might be some other considerations for custom use. Front or rear steer?

[turboteener]

The rack can go for either one. Since it is a one off custom, I can make it either, Isn't the 928 a front steer? I like rear steer, because it takes the slack out of the system when the car moves forward.

[Barbapapa]

It's rear steer. I hadn't considered the slack as a factor, although heavy braking will push inward at the rack which might be slightly more desirable. I had an inner tie rod collapse inward on a fresh racer on slicks so make sure you get new ones while you're at it.
Rear steer does have some benefits and the outer tie rod isn't stuffed way inside the wheel getting in the way of stuff.

[turboteener]

Plus it helps create toe in under bump thus stabilizing the car.

[Danglerb]

That tube chassis 914 must be insanely fast on a tight course, how about putting some links in your sig to more info/picts etc.

[Barbapapa]

Originally Posted by turboteener

Plus it helps create toe in under bump thus stabilizing the car.

No offence intended but you might want to spend some more time studying suspension design. I enjoy discussing it but there's too much to explain on a messageboard and there are some things that you'll never know until you've read a proper suspension design book or two. There's also some affordable software including one that has a default sample of a Porsche 928 of all cars! IIRC Susprog3D perhaps?

[Danglerb]

http://home.earthlink.net/~whshope/
Good stuff on suspensions.

[turboteener]

Apparently I had that backwards.

[Barbapapa]

Originally Posted by turboteener

Apparently I had that backwards.

No, not backwards but rather not relevant. Sure toe in under bump in not as bad as toe out but both are bad. There is something in the geometry that's important to understand that has nothing to do with where the rack is. It's regarding the plane formed by suspension points and where the tie rod is relative to.

Again, I'm not trying to be rude in saying you need to study books but I've gone through the same thing and I can tell there are some aspects of design you also need to know, it's the best advice I can give and I mean it as good advice. You'll understand once you start off memorizing a Herb Adams book and then move on to more advanced ones.

The subject of suspension design just can't be learned on any website, even the best website is like an iffy Cliff Notes. Beware the fool sitting on his butt copy, pasting and parroting messageboards all day, it's easy to get sucked into that circle jerk but you will waste your time.

[turboteener]

Rack location will determine ackerman. Actually Adams claims the opposite. Toe out under bump will actually create a better driving car. When the wheel toes out under bump the car will tend to lessen the steering angle. A car that toes in under bump could create a condition where the driver has to make a steering adjustment (to lessen steering angle) on a heavily loaded chassis mid turn, thus upsetting the car.

I assume you are referring to caster angle and trail. Or are you referring to scrub radius and KPI. Since tie rod location is a factor of rack location, it is fairly important. Rack location will have a large influence on how the car feels to the driver. It may not necessarily heavily influence the cornering capabilities of the car, but it will affect what the driver can do with the car. Rack location will also affect bumpsteer, so it needs to be taken into account as well. Milliken agrees with me that rack location is fairly important.

Thanks for the input.