Garth S

I have done my best to measure and set the kinematic toe using a digital level/protractor ..... so far, so good

Firstly, why would anyone do such an inane thing? The short version of a long and ugly story is that the only set of Porsche kinematic gauges within 1600 kms were ineptly applied to my car on the second visit to have this service performed - at a center 100+ km distant: the LR was done more or less correctly, while the RR was horifically off due to the gauge on link 5 ( rear half of the LCR) being allowed to touch the rim during the adjustment phase .... but what do I know ??....
The result was a car that was a ***** up disaster to drive home - so locally, we improvised on a Hunter DSP400 to correct the damage on RR camber and toe ( and front toe -out! ) .... and set the kinematic angle by sighting against a triangulation copied from the LR ... via a cardboard cut out For shade tree methodology, that's pushing it - even for me!

A better solution was needed: looking at the WSM alignment chapter, it is apparent that the kinematic toe adjustment is achieved by setting a differential angle between the back beam of the 'A' arm/LCR ( link 5), and the rear toe arm ( link 2): that is regulated by cranking on the kinematic (link 4) eccentric. This is all detailed in WSM 44 - 13 to 15. Despite all the verbage on bubble/scale units, it is clear that it is the differential angle between link 2 to 5 is set to be ~ equal side to side - BUT the absolute angles of these two links are very likely different side to side, due to factors like corner weighting , absolute suspension height , camber angle, ...

What was needed was a way to measure these angles on what I believed to be the correctly set LR corner, and transfer that to the RR, once the toe and camber were tuned in.
The tool of the hour was pilfered from my friend, the stock car racer .... who is also a source for load cells & computer for corner balancing: a digital level, shown in the pic below as applied to the toe link.

Using a felt tip marker to set reference points, the LR provided an angle of 8.4 deg on #5 and 4.2 deg on #2 for a differential of 4.2 deg: this is at a LR camber of -1.4 deg and toe of 0.20 deg, suspension height of 125mm .... so my best guess is that the kinematic angle is 4.2 deg.
The RR was then cranked to ~ this differential after resetting toe and camber to match the left corner: a "kinematic angle" of 4.1 deg evolved fron the link 5 vs 2 angles of 7.9 and 3.8 deg respectively.

Result - well, not bad as it appears to be a good match side to side, if the method has any validity. As far as absolute - well, my only plausible target was 4.2 deg differential: I can not advise that to be correct, for I have no other reference point.

Perhaps, if anyone has a digital level available ..... and wants to go through the hassle of measuring these angles on a loaded suspension, perhaps we could validate the kinematic angle.

Anyway, if I've missed the obvious, any advice appreciated.

race911

Gotta love ingenuity! Kind of like a trackside repair on a Saturday night.

tj90

Good job garth. Will be reading the replies. I love these kind of posts!

AX993

Garth,
How did your car drive/handle after your adjustments ?

TheOtherEric

I bet that this kinematic angle of 4.2 deg will vary based on ride height of the particular car. Hopefully someone else will report numbers.

One other point: my impression from the Manuals is that you want to minimize this K angle difference; i.e. the Manual specs a MAX of 1.5 scale units, which I assume means you want 0, but no more than 1.5 bubble units. Then again, it's not certain that 0 bubble units = 0 degrees. Not sure what to think of that.

Garth S

The car seems to be fine - tracks well on the road, etc. But to be fair, there is still frost here in the AM's, and the back roads still wear ridges of sand from deicing; therefore, no heroics behind the wheel to really push the car.

On a side note, the pinned inner tie rod ends work well ....

Basically, all I can say is that the "K" angle has been made ~ equal side to side for a specific ride height: what bothers me is that I have no way of telling whether the LR side selected as reference is truly correct unless I can get my hands on a gauge set ..... and I'm not planning to sink another 3 bills into the hands of the dufus at the dealership who has already twice fubared my alignment.


Eric - I also believe that the magic differential angle is dependent on suspension height .... as the suspension is lowered, one can see increasing angulation of links 2 and 5, and the delta ("K" angle) increase. That presumably dictates the curvature of the bubble gauge tubes used, such that when the two gauges read within 1.5 divisions, the magic differential angle has been achieved - whatever it may be.
However, I am quite certain that 0 bubble units does not equate to a differential of 0 degrees: it is some finite #, likely between 2 and 6 degrees .....

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

race911

Let me throw this out there, and up front I'll say I have NOT TOUCHED rear alignment on either of my 993s. (I do have extensive experience on all 911s from short wheelbasers to 964s, both on and off alignment racks.) The C4S is bone stock 4X4 height, and the yellow car is slammed to the point of being absurd (but damn does it work!). So.........I always thought if I measured both of these cars likely set at their operating extremes, I could SOMEHOW come up with a compromise measurement that splits the difference, and is correct for the typically lowered car.

Or should I shut up and go sit in the back of the room?

Garth S

Sounds reasonable to me .... unfortunately, this region does not have any 993s running around - certainally none that can be shanghaied for a measurement.

TheOtherEric

That's exactly right. The Autometrics / Motorsports "hook" gauge does measure rear caster. That means nothing to me, but that's what it does.

And yes, kinematic toe is the same thing as bump steer: the toe changes due to suspension deflection.

I'm going to use this info to figure out how to use the Autometrics gauge. Seems to me that for a lowered car, you probably need to "max out" the kinematic adjuster, then fine tune just one to get equal readings side-to-side. But I need to figure out (1) if that's right, and (2) how to "max out" the KT bolts.

Bill Verburg

the 5 control arms are in 2 horizontal levels

the upper level consists of 2 relatively stiffly bushed arms that ensure good stiffness of camber under lateral loads by arm3(aka the camber arm) and arm 4(aka kinematic) which provides wheel guidance in the direction of caster angle. Arm 4 is one of the 2 horizontal control arms available which are specified as being bushed stiffer for the RS.

On the lower control arm plane we have the A- arm(arm 1 & 5) and strut arm 2(aka toe).


The confusing part is that arm(the rear arm of the A, arm5)has the particularly flexible bush responsible for Kinematic toe. the A-arm is the other arm specificly bushed for the RS(again stiffer than the standard arm), note that the bushes are designed to be elastic in a limited direction.

you are correct that KT is essentially using bump steer to a desired result.

the extension of all 5 arms meets at a point that defines the axis of revolution of the wheel, when the inner bush on arm 5 yields it changes the intersection point and thus the axis of wheel revolution and thus toe

AX993

Bill, so when I replace my stock lower A-arms (arm 1 & 5 above) with the RS ones (with the stiffer bush), I'm guessing I also need to replace arm 4 (Kinematic arm) with the RS ones to match ?

Another thought; It's really hard to visualize how all those arms work together in action. Wish Porsche
(or someone) had a animated video of how it all worked. Guess I'll have to spend some time studying
the drawing you provided (thanks for providing it !!)

Bill Verburg

There is a little more of an explanation here in post #20

AX993

Bill,
Thanks for providing all that info !!
I need to spend some more time studying and thinking over those diagrams, but I think I have a better idea of how it works now.
Also, Looks like I need to order the RS spec Kinematic arms before I install the RS lower A-arms.
Thanks again,
Glenn

Reynard38

Outside rear wheel toes in under load making the car less tail-happy during trailing throttle. At least that the way I understand it.

Brett B

Bill, thanks for all the great info.