slvr993

I'm just trying to get my head around my suspension and setup. My car is a 1995 993 with the ERP rear links and JRZ s. My guess is camber would become more negative as the car is lowered what about toe??

Thanks

Jeff

C.J. Ichiban

you should always try and do a comprehensive alignment, not just raising or lowering the car for effect. toe is independently set from all the other alignment settings if you have fully adjustable links, generally it's adjusted while the car is up in the air and set down on scales with each small adjustment to check the cross-weight balance. toe can come out of alignment pretty easily if you spin out at the track or just in general after a certain amount of laps. even on 997GT3s which are much newer technology, the rear toe links generally need to be upgraded because of how easily toe gets out of whack.

you can set the ride height to whatever you want, but eventually you run out of travel, and that can eventually has a negative impact on weight transfer. there is such a thing as too low.


Negative camber affects tire wear to the extent that each tire brand has it's own optimal range for track use. some work better at -1.5 to -2 and some work better at -2 to -3. it all depends on the tire. street tires typically get shredded pretty fast if the camber is anything over 2, leading to cording on the inner edges. toe has a large effect to tire wear also, generally the more extreme toe is set, the more twitchy the car feels entering the corner.

ride height and relationship to spring/shock stiffness is a factor to consider...by distorting the time that you have before the shocks go through a complete cycle, you can have a car that "reacts" too quick to certain inputs, or dives too hard forward on the brakes and actually causes worse braking zone performance, to the extreme makes the rear twitchy.

in summary- the goal is to create a car that only changes directions exactly when and how you want it to, but if you go too far low on ride height, you risk deflecting the suspension past the point of utility.

slvr993

CJ - Thanks for the response. This is a trailered / track only 993 which had a very comprehensive alignment this past winter when the new suspension went in. As it sits though (at slightly higher than RS ride height) I think theres room to improve cornering a little more by lowering the CG. I'm just having trouble getting my head around all the variables that change with the ride height. I'm still looking for a really good diagram so I can better vsiualize what happens as the suspension works.

Thanks again

Jeff

bobt993

Jeff, as you lower the car you will need bumpsteer in the front and a tilt kit in the rear. I cannot recall whether you already did this. I am finding a full 3deg of camber front and rear works well in NB form. With the widebody Clubsports flares I increased my front camber a little.

CWhaley

cj gave a real nice explaination of what you need to consider when lowering a car as well bobt993's advise with setting up the car.

Yet, If I understand your question, You want to know what happens when you lower the car. When you look at the Diagram of all components consider the pivot points as points of rotation. As you lower the car each of these points get pulled (moved) as its geometric axis relocates. Thus, caster can move slightly, Camber will pull inward (negative) and finally your toe will pull towards a more negative setting.
While with small amounts of drop may not move these settings much... they will be pulled around. This is what bobt993 is refeering to with Bumpsteer. As the supspension travels thru is movment the toe/Caster can shift dependent on the length of travel in relation to its axis point.
The best thing to do is find the ride hieght the suits your driving style make the changes and have a complete alignment afterwards (that includes corner balancing if your shop can do it).
Hope this helps.

Bill Verburg

Unless class rules prohibit it, I agree that if you are at RS height(or higher) the car needs to come down.

If you get it to a good track height such as used on the Cups then you will want the RS uprights in front for bump steer, in back the tilt kits are more for the higher hp cars, we've been using flat solid side mounts w/ excellent results.

when the height is changed you will definitely want to realign the car. probably want to check the corner balance too.

the camber settings you want will depend on the tires you run

bobt993

Bill, when I lowered my car to RSR spec last year it required very heavy springs and complete removal and reinstallation of the suspension. Chris Cervelli had suggested marking the initial collar positions and doing a rough measurement to a reference point on each corner. After dropping the car in my garage I headed over to Larry's for a full realignment and corner balance. Larry scratched his head muttering under the car as he moved the dolly around to each location. He thought I already had it aligned elsewhere because we ended up adjusting toe on one side a 32nd and about 10 lbs on corner weight.

So yes to realignment, check carefully where you started and if your lucky you may have little to correct. BTW, get the kinematic set correctly in the rear and get the proper gauge (Cervelli has them I believe) for accuracy. If you have full ERP links this will be easy. Without them the factory range will limit your max kinematic adjustment.

Bill Verburg

w/ monoballs on the rear A-arm there is no KT

bobt993

Bill, I think he has ERP adjustable links.