Viken

Yes, you need to pack with shims and rotate the strut tops to achieve Cup camber values. For a street car, I'd leave well enough alone. The specified caster is fine and the maximum achievable negative camber with shims is more than all of us need.

ldw

Does anybody have a photograph of a GT3 street car up on the lift with the bolt securing the diagonal at the front removed so one can see the orientation of the center hole and off center hole or 'castor pucks'? I can then explain the purpose of the holes in reference to the cup cars and GT3 street cars, their implications, and adjustments.

macfly

NB, Bob & Viken, after my camber, toe and ride height were set the castor was measured at 9º we didn't / couldn't do anything to alter it (at Johnson's where the work was done.).

We rechecked it several times, with Steve.A as it seemed a bit odd, but there was no way to adjust it, and everything else was perfectly set. They suggest I try it, and see how I get on with it, and it has been superb, no heaviness in the steering, and feels like the car is on rails.

Viken

Can you please describe the method Steve used to measure caster angle? From what I know, he still does his alignments the old fashioned way with strings. I am somewhat suspicious as to whether or not the 9º reading was accurate.

macfly

When he did the castor measurement it was with something that looked like an old style torque wrench which measured the degree at just off full lock in each direction, and the total was the addition of these two measurements.

As i remember they attached a three spoked clamp that sat over the wheel, and the tool measured the castor from a mounting point right over the wheel center. Since I had never seen an alignment done it was all new to me, but I can always call them up and ask what the tools were if you'd like to know more.

They were sufficiently puzzeled by it to use three similar devices to check and recheck it, because it should have been at 8º as expected.

bob_dallas

very odd indeed

ldw

The issue of proper control arm diagonal 'hole' selection is a mystery even to some long time owners of GT3 cup cars and a few teams believe it or not. These holes are not there for "adjustment" at a given camber per se, but rather are there to accommodate the proper geometry/caster when a high camber situation is created. The arms with their respective holes(2 options) are as per diagram in this thread above.

The arm itself is composed of 2 pieces. This is for the allowance of shims to enhance camber which are placed between the 2 portions of the arm and then sandwiched in place. The inner portion is the same on the GT3 as the cup car (99634154195). Please note this includes the flange, rubber bushing and the entire assembly as per part number. The GT3RS race car has a ball joint type bushing with no rubber for comparison. The outer and longer portion of the assembly houses the bushing which in the case of the cup car and GT3 streetcar has 2 rubber bushings which do not completely surround the piece witch house the 2 holes. The part number for the street car for this portion of the arm is 99634112290 and the cup part is 99634112190. The cup car part, although i have not measured it, may be several millimeters longer.

With a single 1mm shim in place and a ride height of 95mm from the factory measuring point, the least camber possible on a cup car is approximately 2.8 degrees. Without further shims, the maximum is approximately 3.8 degrees. A lower ride height will provide for slightly more camber and a taller height-then less camber, but only slight variation.

So, the first thing to recognize is that the street car at 115mm and the race car at 95mm have different camber values and the arm on the street car may be slightly shorter than the arm on the cup but that is not confirmed. A front height of 95mm would be considered VERY high for a GT3 cup car.

The street car is delivered with the diagonal arm in the center hole. Our cup cars have been delivered in the eccentric(offcenter) hole because the cars typically come from the factory with various shims in place depending on the year. In 2000 just to take the most extreme example, my car was delivered with over -5.0 degrees of camber in the front

The best camber setting for both the street car and race car for a balance of stability and ability to turn is 7.5-8.5 degrees of caster. This actually is a relatively high value compared to other makes of vehicles. The street car is delivered in this range by using the center hole. If you move the arm to the eccentric, it will increase the caster, but it will fall out of the range and subsequently the car will seem to steer straight more easily, but may require more effort to turn. More concerning, this will "pinch" the rubber bushing that is in place and may deform it over time.

The following applies to the street GT3 and Cup car:
As more shims are added, since the control arm lengthens(because of additional shims), the diagonal pulls the control arm and hence the entire wheel assembly forward. Eventually it will get to the point where not only is the bushing pinched in this situation also, but the tire will rub the front fender behind the bumper since it is moved so far forward. In this condition, with the arm extended with shims, because of the new geometry and angles, moving the diagonal to the eccentric, will actually move the wheel back toward its more neutral position in the wheel well and move the castor back toward-or into- the specified range again.

So in summary:
1)The car works best when in the caster range specified which is 7.5-8.5.
2)One can increase caster by moving a non shim control arm(or a 1mm shim control arm in the case of the 2004 GT3) to the eccentric position, but it will pinch the bushing and may send the caster out of specification.
3)Although seemingly a paradox, if one keeps the diag in the center hole and adds shims, it will pull the wheel so far forward that by moving to the eccentric in this circumstance will send the geometry back toward the natural state. These relationships are such because each addition of shim not only enhances the camber, but changes the position of the wheel relative to the wheel well, relative to the fender, and the ride height will dynamically change as well. So it is a complicated arrangement that has been very well worked out by porsche.
4) To somehow attempt to "beat the system" is likely to end in frustration or worse.
5)The best way (and most safe)to get more camber is to use shims, after the strut adjustment is maximized. With respect to shims, the threads will likely tolerate close to 7-10mm in the case of my car which is substantial. Some medial control arm pieces have threads which may have longer lengths depending on the pressing procedure when the screws were placed into the housing in production.
6)It is suggested in the race manual that a 1mm shim will provide 4'30'' of camber change. Depending on ride height, it may be more and in my experience it is more than that.
7)Ride height has significant implications on camber. Lower=more.
8)The rear of the street car has no allowance for shim insertion as the control arm is 1 piece. The cup car does use shims and additionally, the bushing in the rear is a joint without rubber.
9)The eccentric position, or more rear-ward hole is actually the more medial or inner hole as opposed to outer hole as referred to earlier in the thread.

Bob Rouleau

Lynn - Wow. Thanks!

ldw

hope it helps as opposed to making a complicated set of mechanical relationships seem even more confusing...!

mds

lynn, looking at the diagram I would have thought that moving the diagonal to the eccentric hole would move the wheel forward, because the eccentric and the diagonal are on opposite sides of the arm. What am I missing?

ldw

Mike
You are correct and the caster will increase slightly in doing so as i said in paragraph 6. The arm will be pulled forward ever so slightly-which pulls the wheel and carrier forward. This very example, reveals the precise engineering that has gone into the design of our cars. The diagram is schematic, but given the angulation of the diag arm, and its relationship to the control arm (Which is less than 90 degrees to the centerline of the car to begin with-control arm to centerline that is), AND the precise angle that the centerlines of the holes form with respect to the diag, as the holes move further from center line of the car with more camber or more shims, then the sweep of the diag (which is fixed at its more forward attachment) will push the main control arm backwards if placed into the eccentric hole, relative to the center position. Understand that the lines of the centers of the 2 holes are NOT on the arc circumference of the diag arm sweep. Yes, without shims, in the delivered configuration, by forcing the diag into the eccentric (which means you have to torque the arm with some force to overcome the rubber bushing resisting this) it will move the main control arm forward and enhance caster. As that arm grows though, and given its relationship to the diag on angle and the relative hole positions, a point comes in the sweep, that this position of eccentric is less caster than the center hole. It is best seen by actually inserting shims and putting the arms through their motions. Remember that the growing shim control arm has a fixed medial pivot point, the diag has a fixed forward pivot, the arm is moving higher or lower depending on shims as well, so allot is going on. The main control arm holes are actually moving in 3 planes as we add shims. Out, medial end slightly down, and forward as it gets forced that way by its connection to the diagonal attached to the sub frame toward the center of the car and forward of the control arm.

If it sounds complicated, it is because the geometry and relationships are. Change 1 of these items, and everything also changes. At least in the front, there are only 3 arms if you include the steering rack tie rod. In the rear, there are 5!! That gets into bump steer geometry which many of the best porsche race engineers still argue about with respect to what set ups are best.

bob_dallas

Thanks Lynn - that makes a lot of sense and is very helpful objective information.

macfly

Thanks Lynn, there is a certainly a lot to digest there.

I now need a little clarification on my set up. Reading your comments I am guessing that my caster has increased to 9º because the shock crown was rotated to give me the -2.5 camber rather than shims being used. It seems that from what you say that my 9º caster is hurting the geometry, and damaging the car. More concerning, this will "pinch" the rubber bushing that is in place and may deform it over time

The car does seem to ride really well, and everything feels very steady and stable. Turn in is a fraction heavier, but it adds to the overall feeling of stability, and I rather like that.

ldw

Macfly
Sounds like you have made several changes, but you are still using the center hole for the diagonal(as delivered position). At -2.5 the rubber bushing should be able to cope with that, but it may be somewhat pinched.
Porsche would clearly feel that 9 degrees is too much as it is out of specification but not by allot. Keep in mind, 8.5 degrees caster is quite a bit compared to other makes of vehicles. Also, it is possible that you may have 8.5 or so, and the measurement at that time you did it by hand (which is just fine) revealed 9 degrees. Caster is a tricky measurement. If you had it checked on a laser type alignment machine, it may read differently. As another data point, my car has 1.1 and 1.5 degrees of camber and has a caster of only 7.5. If I added shims and moved the strut top medially to get 2.5, there is no way my caster would be 9.0. It maight be in the 8.0-8.5 range. I know this from experience in the race car platform. Even when we have experimented with moving control arms around or rotating the bushing on the GT3RS to the point of tire rubbing against the front bumper, I have never seen a value of 9. That is why i am concerned that the top strut relocation has moved the top of the shock and strut in more than just the medial or inner direction.

If the rotation the strut top simply provides camber and does NOT change the angle of inclination of the strut, then you could go to the eccentric and see that you will loose a small amount of caster toward the normal range. I think that since you did not lengthen the arm though, you still may be in the range of the sweep whereby doing so may add MORE caster. I think that the caster may have been added because of a slight change in angle of inclination front-rear. Technically, if the strut top was only located more medially, the caster should not change because the angle from the strut top down to the ground compared to the angle of the strut itself should not have changed.

Obviously it (rotating the top) changes the angle of the strut with respect to left and right, and that is how you were able to get more camber. I suspect this may have changed the angle of inclination slightly in the front rear direction as well.

I can not endorse rotating the strut top since porsche did not offer that as a public option. Of course I am well aware of the technique though. Exchanging left to right accomplishes a similar purpose. An option that some have seemed to treat as something totally different....it is the same.

Perhaps check out your caster on a laser unit. If in spec, then OK. If not, take a look at the rubber bushing and consider its deflection. If appears like it is a problem, then try the eccentric hole.

I might tend to bet that the caster is not quite 9. If it is really nine, i would think your front tire would rub the rear of the bumper and inner fender well in a fashion that is not acceptable.

Also if you determine it is 9 because the inclination of the strut has changed, i would consider moving it back. This would mean that you have altered the geometry of the system in a way that porsche did not intend. When we change caster on the GT3RS race car, it is done by adjustments at the bushing hole locator. The system on that vehicle is totally different with a rotating bushing. This is not the case in the GT3 street or cup. The bushing is fixed with the 2 options based on camber severity so that caster may REMAIN in the specified range.

-2.5 is nowhere near "cup" camber, so i suspect the bushing is happy.

bob_dallas

I think I recall Andrew (Macfly) complaining about tire rub (that I don't have)a few weeks back. Perhaps this explains it. I'm running -2.5 degrees caster and 8 degrees of camber in the front.