Angle on the Kinematic angle ...
#16
Rennlist Member
Keep in mind that there are two kinematic effects:
(1) change in toe due to deflection of the wheel (e.g. compression or sag, like when cornering); and
(2) change in toe due to the radially soft bushing in the lower a-arm (arm 5 in Bill's picture).
As an example, when you brake in a straight line, the rear suspension droops, and wheels toe out. This would be bad. But the lower a-arm compresses due to the soft bushing, causing toe-in to offset to kinematic toe change.
"Adjusting your KT" deals with Effect #1. Effect #2 (soft bushing) is more of a "dynamic" toe instead of a "kinematic" toe, and you can't adjust it.
(1) change in toe due to deflection of the wheel (e.g. compression or sag, like when cornering); and
(2) change in toe due to the radially soft bushing in the lower a-arm (arm 5 in Bill's picture).
As an example, when you brake in a straight line, the rear suspension droops, and wheels toe out. This would be bad. But the lower a-arm compresses due to the soft bushing, causing toe-in to offset to kinematic toe change.
"Adjusting your KT" deals with Effect #1. Effect #2 (soft bushing) is more of a "dynamic" toe instead of a "kinematic" toe, and you can't adjust it.
#17
I had used the method garth describes in post #1 to set the kinematic on my coupe, the differential between sides was 2.4 degrees and I adjusted it to zero, that is the difference between the angles of arms #2 and #5 was adjusted to 1 degree on both the L and R sides.
but I didn't know if this was actually correct, as garth writes 'if the method has any validity.'
lately I bought a kinematic gauge from gert but it doesn't fit the coupe. today I got a chance to test it out on my cab. the bubble showed around 3.3L vs 4.6R, I adjusted both sides to 3.5 at the same time adjusting camber.
I haven't set the toe yet but I did crawl under and measure the angles of #2 and #5 and compared L to R, it ended up as almost exactly the same differential between arm angles.
so I'm going to say that garth's method has validity and can be used to set the kinematic without spending $300 for the gauge.
but I didn't know if this was actually correct, as garth writes 'if the method has any validity.'
lately I bought a kinematic gauge from gert but it doesn't fit the coupe. today I got a chance to test it out on my cab. the bubble showed around 3.3L vs 4.6R, I adjusted both sides to 3.5 at the same time adjusting camber.
I haven't set the toe yet but I did crawl under and measure the angles of #2 and #5 and compared L to R, it ended up as almost exactly the same differential between arm angles.
so I'm going to say that garth's method has validity and can be used to set the kinematic without spending $300 for the gauge.
#18
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993 Alignment Primer - . . . . . .
Back to the learning curve and the 993 chassis alignment ... Signed, Grasshopper ...
A good read of the alignment features btw the 964 and 993 vehicles .. +1... to Bill Verburg
http://forums.pelicanparts.com/porsc...-analysis.html
A good read of the alignment features btw the 964 and 993 vehicles .. +1... to Bill Verburg
http://forums.pelicanparts.com/porsc...-analysis.html
#19
RL Community Team
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What is WSM, the factory shop manual?
#20
******
The Porsche Motorsports type of 'hook' gauge that mounts on the caliper mounting bolts must measure caster .... and this is used to correlate to the correct kinematic setting: that this actually works makes the kinematic effect seem an awfully lot like 'bump' steer adjustments done on the front ends of track cars. I have gone through the process of setting bump steer with my oval track racer friend ..... and the effect of the K link on geometry changes as loaded height varies looks very similar to 'bump': my head hurts now, trying to sort this out, so I'll be quiet
The Porsche Motorsports type of 'hook' gauge that mounts on the caliper mounting bolts must measure caster .... and this is used to correlate to the correct kinematic setting: that this actually works makes the kinematic effect seem an awfully lot like 'bump' steer adjustments done on the front ends of track cars. I have gone through the process of setting bump steer with my oval track racer friend ..... and the effect of the K link on geometry changes as loaded height varies looks very similar to 'bump': my head hurts now, trying to sort this out, so I'll be quiet
Another way to describe it is w/ the idea of bump steer which is more usually used to describe the front suspension, caster at either end puts more toe change aka bump steer into that end.
The rear is a bit more complicated due to the articulation of the 5 arms and the unique joints in the rear A-arm, where there is a rigid pivoting ball joint on the leading arm and a fairly soft in the lateral direction, enhancing any toe change that the basic geometry provides. It's been my experience that both rear A-arm joints get loose w/ time and use muddying the water so to speak, the best rear for street use is the stiffer RS which damps KT effects a lot, the best for track use is the all mono-ball RSR which has zero play so the only toe changes are those inherent in the geometry used to set the suspension.
At the front the A-arm is set the opposite the leading arm has a softer bush allowing more lateral movement than the trailing arm. Again the can be improved by using RS bushes which tightens the leading leg bush or aftermarket harder bushes on both legs to tighten the front even more, or again the ultimate all mono-ball as used on the RSRs for zero play. The front is also complicated by the actual steering mechanism so you have the additional headache of worrying about the steering arm geometry, here you have 2 basic choices stock wheel carriers and outer steering arms or RS wheel carriers and outer steering arms.