Porsche964FP

-964 man points

Roll bars are exceptional - on rails. Mind you, I installed them at the same time as the KWVs, so difficult to tell since the suspension upgrade would have done wonders also.

True bump steer is always going to be an issue. I'm interested in your drop link solution as opposed to 993RS uprights/ hubs.

alexjc4

The difference betweem the c2 and c4 is of course we have the addition of equivalent of a VW Golf/Rabbit driving the front wheels. So my instinct is to mix in some Fwd settings. From memory front wheel drive cars will use toe out to improve turn in and reduce understeer. Otherwise same, lots of camber and caster. Without power steering FWD setups tend to dial down the caster so you aren't having to lift the front of the car with your arms.

The 993 uprights seem to be the most common solution for bump steer. Why is it you dont like that solution?

sml

kos11-12 ... dont waste your time debating caster. with your new wheels, you'll need to go max caster to stop the tyre rubbing on the corner of the guard at the 9 o'clock position (as the max caster will pull the wheel laterally towards the front of the car).

anto1150

IMHO, there's no difference in caster and camber between a C2 and a C4. You can work on the toe, bars and rake to reduce a C4's tendency to understeer.
Compared to a C2, you'll need a thicker rear bar, some more rake (about 5 mm?) and less front toe (but I wound never go in negative land because you loose grip under hard braking and high speed stalility).

kos11-12

Very interesting point, isn't that some FWD cars have the steering rack at the front and height, but also the wheel carrier arm is at the front,
Those may have a toe In when suspension compress ....?! Maybe that's why they dial some toe out...?! But yes that may work...!
Wonder why I have + 0,12 degrees individual toe at the front.

The 93 RS wheel hub is a buitiful pieces of engineering,
A few peopol reported curing the bump steer issue by raising the front (raising the steering rack) lowering the point where the outer tie rod is bolted has the same effect, but there is a few thing to consider, it will probably work better on a 18 wheel as there is more space between the carrier arm and the edge of the rim, to be able to play with spacers I think using a spherical bearing increases a lot the feel of the steering, I will try to post a drawing of the part I am think making. I imagine the actual 64 outer tie rod with a spherical bearing pressed in ....

kos11-12

Yes for sure the caster as to be max ,

Anto,
a bigger bar at the back has also been mention by a few peopol who race the C4...
probably toe , what do you mean for the rack about 5mm ?

anto1150

The height difference between rear and front.
Compared to the tipical Euro RS heights, on a C4, I'd also try to rise the rear end a little (say 5 mm).

kos11-12

Ha yes of course,
that's what I have 5mm lower than RS at the front !
I need to remeasure it all ; it was done a few years ago and have the alignement done again...

actualy thanks to Garreth a rennlister here who race a C4 in US

anto1150

Rather than lowering the front below Euro RS specs, which will lead to increased bump-steer, I'd rise the tail...
Just my personal opinion.

kos11-12

yes good point, I will go through everything soon, a shop in London did it for me a few years ago...

Anto
by the way I live in Cannes not far from Italy... is there any good track days organised, I think Imola is the closes ...

Porsche964FP

Out of curiosity which London shop?

kos11-12

GT ONE, Chertsey Surrey,

anto1150

Kos,

I think Mugello is #1, and it's also not far from you! (closer than Imola).
Here's a link for you: http://www.promoracing.it/?zona=calendarioauto

kos11-12

Thanks for the link ,
Yes that's an option as well 50 km difference from Cannes...

boxsey911

Yes, 21 was fitted to the rears of early C2s. My guess is that Porsche reduced it to 20 later to perhaps reduce the potential for oversteer. My '90' has the 21 and I really like the way it feels..a little bit oversteer without being too aggressive.

+1 to the suggestions for max caster (mine's at 4 30 which is close the upper limit). It weights up the steering, reduces the 'light front end feel' and improves stability at speed.