lordpantsington

So the suggested spec I've found is for -3° 50' (3.833°) in the rear. What thickness of shims is used to get this number? Is there any adjusting to be done at the top of the top strut mount to get there?

I ask because I've been building up this:

and am currently working on adjusting things so the virtual alignment is sound. Toe is set. Current shim is 14mm total. Current camber is -1.2° ish. Something is still hinky. :/

Thanks!

claykos

There is an eccentric bolt at the inner lca pickup which is used as well as shims. The shock mount has no effect- its not a strut. Some cars have adjustable upper links which can be used as well. Factory uppers are fixed length.

lordpantsington

Thanks

Does anyone happen to know the length of a factory upper from center to center of mount hole?
The current measurements are set to the spec of a 2009 RSR (261 and 255mm F&R). With a 37mm difference in rear ride height, I can see that perhaps shorter uppers are required. Especially required, if there are less than 14mm of shim in the LCA.

OlsenMotorsports

You need to set the dogbones to the factory lengths that the cup car came with. In the later years of RSR we are a lot wider on track width then the old cup cars. (how to adjust them is quite tricky as all three links including tuning fork have to be adjusted at same time to keep wheelbase and rear castor in symmetry) Also like Claykos said if you still have the eccentric there is some adjustment there.

If not there are 4 different variations of the inner knuckle on the lower control arms. The longest one is 20mm longer than stock for when we run max camber. I believe the measurements are 10, 14 and 20mm longer than factory on the Knuckle. Word of advice? The studs on the knuckle WILL loosen up and break at 14mm of shim (we learned that the hard way) and there should never be more than 10-12mm of shim in it. That is one of the reasons that PM came out with the longer knuckles.

1mm camber shim is equal to .25 degrees of camber on 2009 and up rsr and .2 degrees in earlier years (because the shocks are longer in early years.)

Hope that this helps

lordpantsington

Tremendously helpful!

I feel silly, I've completely forgotten about the track width difference. I suppose that also means the trailing arm length is going to change.

That is great information on the knuckle sizes. I ran across a few photos of the extended length knuckles and could never figure out what model they come from.

As of right now there is no physical car. I've built this up from whatever info I can scrape together, and photographic reference. This is all being done virtually to examine how it all works, so there is no danger for me with my 14mm shim, but it is a good warning to have down if someone happens across the thread and for some reason decides to stuff 14mm shims in. IIRC I decided on 14 because of some GT3CC regulation I've come across. On quick glance through my data collection the max as set by regulation: 13mm and 5mm, in GB and AUS respectively. Very interesting to now know the reason why that regulation exists.

So this leaves me with good news, bad news. Good news is the lengths of my model are set parametrically, so easy to adjust. Bad news is the data I have is not valid for this application. At least I know for certain it cannot be used. Back to researching. If anyone has lengths of uppers, trailing arm, and a known camber measurement for a certain shim size, for a 996 GT3 or Cup I'd be very happy to hear from you.

Much appreciated,
Andrew

OlsenMotorsports

No worries.... theres a lot of speed available in setup i that suspension geometry... the key to speed in a GT3 is all about wheel base, diff, and rear castor... everything else is just fine tuning but thats where the speed is

lordpantsington

Still going...
Had to drop the uppers to about 230mm to get the camber in the ballpark.

I find it very interesting that you mention rear castor. The only discussion about castor I've come across is always in relation to the front. The alignment data I have doesn't even bother to measure it. If I may ask, what sort of rear castor should I be expecting?



Also what I'm seeing is loss of static toe-in during damper extension, and gain of static toe in for damper compression @ 10.7cm rear ride height. Does that seem generally accurate for these cars?

As always thanks for input

OlsenMotorsports

1.25 +- measured on the caliper studs is a good castor for that suspension. You can achieve that through the dogbones and wheelbase. Tricky but you can get there. Rear castor is not generally discussed..... A lot of people say it doesn't exist

Depending on what track you are at generally bump steer should be 1 mm over 10mm of compression or droop either way. That can be adjusted also

Glad to help!

GT2rainge

That is very interesting to hear you talk about rear castor, I've done lots of searches on this and have not found any information about it.
Would you mind sharing how it is measured and what the goal is in getting the adjustment right?
Thanks in advance
Neil