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Silver Porsche 944

Silver Track Car – Rear Suspension Adjustments

So you’ve got your rear suspension done or maybe you just are interested in checking rear alignment.  What type of adjustments are possible with the rear suspension?  Stay tuned to find out.

On the front suspension, we are used to talking about castor, camber, and toe-in.  All 3 can be adjusted at the front.  At the rear, we think in terms of ride height, camber, and toe-in.  Let’s talk about ride height first.

The rear suspension is a “trailing arm” design.  The hubs and wheels are at the end of the trailing arms.  The torsion bars are at the front of the trailing arms.  As the weight is placed on the wheels, it creates a twist on the ends of the torsion bars.  The body settles downward as its weight loads the torsion bar.  When the torsion bars twist enough, the body comes to an equilibrium.  That is your ride height.  Note: Porsche calls the trailing arm a “rear axle strut”, which makes little sense to me but whatever.

DSCN0117In the above picture with the rear suspension out of the car, we see (from left to right) the hub, the swing arm, the rear axle link (in the background), the aluminum “carrier” for the torsion bar (Porsche calls it a bearing flange), and the torsion bar itself.  In the above photo, I have welded a tab on the end of the torsion bar.  The tab has a piece of blue tape on it.  You can see the splined end of the torsion bar, as it is partially removed from the torsion bar tube.

IMG_0103In the above picture, everything is mounted in the car.  I have cut a round hole in the fender to allow me to pull the torsion bar out for adjustment.

For initial adjustment of the ride height, I run the trailing arm downward (the hub lower than the torsion bar) about 5 degrees (measured with my digital level).  I set both sides sloping down the same amount.  Then I insert the torsion bars.  For each amount of downward inclination, there is only one position where the torsion bar will go in.  This is because it has fewer splines on one end then the other.  You have to rotate the torsion bar and test fit it to find the right orientation.  Don’t force it, it should go in pretty easily if properly cleaned and greased.

If you are lucky, when you are done and set the cars weight on its rear wheels the ride height will be good.  A rule of thumb for stock ride height is to get it so the half-shafts are level.

Without the holes in the fender, adjustments require pulling the torsion bar out into the limited space between it’s end and the inside of the fender.  Not impossible but not as easy.  Plus with the tabs, if it is really stuck, I can put a slide hammer through the hole in the fender and jerk on the tab to get it out.  You want to get all the load off the torsion bar by lifting the car back up or you definitely won’t get it out.

Fine tuning of the ride height, by +/- 1/2″ or so, can be done with the large bolt that has an eccentric cam inside of it. Here is a picture of the eccentric bolt.

IMG_0081In the following picture, the bolt with the nut removed in the one you want to turn with a wrench.  This turns the cam and rotates the trailing arm up and down.  You turn the bolt head on the back side of this picture.

IMG_0090The shop manual says you can adjust the rear camber.  If so, I don’t see how, at least to any great degree. One of the big advantages of a trailing arm suspension is that as it compresses, it holds camber changes to a minimum.  And at normal ride height, the camber is what it is.  That said, on my orange track car, as I have lowered the front and followed suit by lowering the rear the same amount, the half-shafts are now running downward to the transaxle and I have about negative 3 degrees of camber. I haven’t checked it but it but I suspect the camber is pretty much nil at stock ride height with the halfshafts horizontal.  So I beg ignorance on how to adjust rear camber.

What can be adjusted at the rear is toe-in (or lack there of).  I think no toe at the rear is just fine, unless you are going for some exotic track setup.  Rear toe is adjusted by moving the overlapping plates of the swing arm backward and forward until the hubs and wheels become square with the car.  Similar to the eccentric bolt for ride height, there is another eccentric bolt that makes this adjustment.  In the picture above, there are 2 small holes and 1 big hole, in a triangle.  The small holes hold lock bolts, which much be loosened.  The big hole holds another eccentric bolt. This one is accessed from the outside.  As you turn it, the hub will move left and right.

You have to have the car on the ground on it’s tires and loaded to check toe-in.  But that’s the bolt you turn to make the adjustment.  The small hole forward of these 3 holds the link for the sway bar.

And that, my friends, is how you can adjust your rear suspension for ride height and alignment purposes.

DSCN0135

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