The design of the rear suspension of the XR is called SEMI-TRAILING LINK. What this means is that the wheel travels up and down in an arc. This arc is not straight with the vehicle centerline, but is slightly angled. This angle causes the up/down to have toe and camber changes. Toe and camber changes help the car corner. (Old VWs (68-up) were straight with the centerline, so they didn't handle as well.) When the springs sag (or are shorter) the car sits lower than designed, this causes the toe and camber to be some value other than what WE WANT FOR PERFORMANCE.
Alignment Method 1 (thanks to John Blizzard)
Well, I got motivated today. Talked to a friend of mine who had a BMW shop in CA. for many years and asked him what he did for BMW's to correct rear alignment (camber) with short springs. Simple he says, take the bolt out of the inner pivot point on the trailing arm and slot the hole upwards pushing the arm up, when you get it where you want it, tack weld a washer on both sides and you're done.
Did it today at his shop. Works great! Doesn't put too much bind on the bushings and corrects the camber . I like this much better than shims in the hub carrier. Haven't had it to the alignment shop yet but, according to my camber gauge( an old hand held unit from my Formula Ford days) I went from almost 4deg. to 2.5 deg. I can live with that. Atleast I'll get more than 6000miles out of the rear tires now.
Alignment Method 2
The implementation of the design on XRs does not include the usual type of adjustment of toe or camber. What we do is to loosen the wheel bearing carrier from the lower suspension member (trailing link) and put shims in between to change the angle. What were two parallel surfaces (trailing link and bearing carrier) are now at an angle. Since the bolts on the bearing carrier are pretty close together, small shims will make big changes at the tire tread. Note that the bolts have different dimensions top to bottom compared to front to rear. This means that the same shim will affect the tire differently front/rear compared to top/bottom. The trigonometry for the angle changes is in the table below.
The bearing carrier has four bolts. The pattern is roughly level with the ground. To change the angle AND KEEP THE BEARING CARRIER FROM BENDING, ADD SHIMS IN PAIRS. Either tops or fronts. (For those in disbelief, try an experiment with a book and some quarters.) This is VERY IMPORTANT, USE PAIRS.
To measure toe, paint a stripe on the tire tread and while turning the tire and scribe a line in the paint. Use a tape measure to get the difference between front and rear of the tire pair. The front of the tires should be less (0 to 1/8") than the rears. If this is not so, add/delete shims per the table below to the FRONT bolts (same amount both sides of the car).
For camber, either buy a camber gauge or use a 24" level. To use a level, clamp it to the fender with the other end on the ground. Adjust until the level is so. Measure the distance from the top flat part of the rim to the level, then measure the distance from the bottom flat part of the rim and subtract. Note, 1° is .244" on a 14" rim or .261 on a 15" rim and for those lucky people .279" on 16" rims. Use the following equation or chart below to find camber: ( camber° = (sin^-1(distance))/rim size ).
If your tires are not sitting exactly on level ground, it gets one step trickier. You must turn the car around, putting the left rear exactly where the right rear was and the right where the left was. (put tape on the ground to mark where you were) Take camber measurements again. To get the final value, take the two measurements of camber for left and average them, average the rights.
Adding shims (about 10 min) (tools needed, 17mm, 19mm, breaker bar, torque wrench):