ACEparts_com

My car is at RS ride height with pss9's and I can't help but notice that when the suspension is set to firmest the car drives much flatter without quite so much tram lining etc.

I've read between the lines and understand the RS uprights can cure this trait on lowered cars.

Questions:
As we're merely moving the pivot point in order to have the tie rod more horizontal, is there not another way of achieving this?

If I change to RS uprights, all else being equal will the car sit lower thus requiring another corner balance etc?

Thanks in advance!

chsu74

PM Steve Weiner for details. I understand it as ride height 10mm above RS is OK with stock. Any lower will require RS or GT2 uprights to avoid bump steer.

Basal Skull

The stock uprights have an longer arm with the tie rod attaching to the top of the arm. The RS/GT2/EVO uprights have a shorter arm and the tie rod end attaches below the arm. I believe the shorter arm allows quicker steering in addition to helping with the bump steer geometry. Also since on the RS upright, the tie rod attaches to the bottom of the arm, I believe (please someone correct me if I'm wrong) the attachment point can be adjusted lower (?with spacers) so that they can be adjusted somewhat to improve bump steer; where as for the stock uprights, since the tie rod end attaches above the arm, you can't lower it easily. The tie rods are different too. The shop that installed my RS/GT2 uprights said they spent some time adjusting for the bump steer, so I assume it can be adjusted somewhat based on how it is attached to the arm.

Anytime you mess with suspension components and ride height, I would do a corner balance.

Bill Verburg

correct
left is RS(akaGT2Evo) right is stock 993


stock the steering arm on top.
stock

RS, on the bottom


Mostly wrong, the steering ratio is not appreciably affected, the RS has a slower ratio than the normal 993 because of the steering box gearing. All the shorter arm does is restore steering geometry to a range where bump steer is reduced, essentially getting the tie rod back close to parallel to the ground. Bump steer can never be eliminated because the physical arrangement of the front end parts does not allow that, ideally the A-arm and tierod would have the same outer and inner pivots so that they would move in the same arc w/ suspension travel, in practice the best that can be accomplished is a compromise where the arcs differences are minimized in the range determined by the ride height and suspension stifness.

The stock and RS tierods have different geometry based on the wheel carrier geometry and must only be used w/ it's matching component.
These must always be used together, the inner tierods are freely interchangeable


Top bent outer stock tierod w/ stock rubber connection inner, bottom straight outer RS tie rod w/ GT2 evo/RSR monoball inner, RS inner has a stiffer rubber connection but otherwise is identical to the stock inner

there is also an aftermarket outer w/ adjustable offset joint hat can be used to further alter the tierod geometry
stock RS outer w/ aftermarket adjustable o/s outer

Basal Skull

Cool, great explanations and pics !
I have the adjustable aftermarket tie rod ends, but looking at it I don't really understand how it is used to do adjustments to bump steer since it doesn't look like it can change the position relative to the steering rack.

I also have retrofitted a manual steering rack (to get rid of hydraulics - which I'm liking!) and thought the shorter arm somehow compensated slightly for the slower rack ratio.

ScottMellor

Another important point is the spindles are raised in the gt2 unit, thus lowering the car while retaining your suspension travel.

ronnie993tt

Better than Streather for we mechanically challenged. Thanks for posting pics.

Basal Skull

Didn't notice that before but you can clearly see that in the pics!

ACEparts_com

Thanks all. Now I just need to decide if I should 'upgrade' from pss9 to pss10 while I'm in there?

911PERVY

Alan, this is well worth doing, my turbo has PSS10's and I got rid of alot of bump steer with the GT2 setup, I can now change gear while booting it around a bend which before was touch and go!

Not sure if I would change PSS9 to 10's though

Bill Verburg

I don't believe that to be true because I have never seen a change in ride height just from swapping the wheel carriers. But I'll be the first to admit that I never measured it.
Where the normal line from A to the shock intersects the line determined by the shock mounting holes determines the spindle height. If any one does this swap measure where the normal line intersects the line connecting the centers of shock mounting holes.

Basal Skull

Bill isn't the line you are drawing off by 90 degrees? Isn't the measurement you're after at the 9 o'clock point in your pics? Which looks just from the photos different.

Bill Verburg

I don't believe so, the ride height is determined by the coil over springs on the shock body, so the spindle height wrt the shock is what is being discussed.

The line I drew is the normal(90°) to the shock mounting plane, where that line intersects the line joining the mounting holes determines spindle height, if it is closer to the top hole w/ the RS wheel carriers than w/ the stock wheel carriers then the spindle height has decreased and the car will be lower in front.

As I said I've done several of these and never seen a change in height, but again I wasn't really looking either.

another way to determine w/o measuring is to stand them on the shock mounts w/ the axles aligned and then note the relationship of the mounting holes

ScottMellor

That is what my Tech told me. I do know the ride was 10 times better afterwards.

Bill Verburg

Well yeah of course, the car is a much better driver w/ less bump steer

Juha posted this pic recently which shows the relation ship between the strut bolts and the axle height