There are three aspects of the TR4A, TR5/250 and TR6 Independent Rear Suspension (IRS) that can be adjusted:
The design incorporated a toe-in (wheels point in or out) adjustment capability through the use of shims between the trailing arm brackets and the frame. I leave the toe-in adjustment to the alignment shop where I have it done as part of a 4-wheel alignment. My only contribution is to provide plenty of spare shims for both the front and rear suspension.
Neither ride height nor camber adjustment capability was incorporated into the design so one should not expect the alignment shop to be able to fix such problems without a little assistance. The following describes how I adjusted both the camber and ride height in my garage without the use of any special equipment. I suspect any alignment shop can do the same thing if furnished with the necessary parts.
The concept of camber is illustrated in the following sketch The most common camber misalignment is excess negative camber, where the top of the wheel leans in toward the car.
I can think of five possible causes of excess negative camber:
Frame problems are common for these vehicles, especially those that have been driven in the presence of road salt. Much of the frame is covered with oil from the always-present leaks and not prone to rust in those areas. However, the oil usually doesn’t coat the fame cross members to which the trailing arms mount. If the frame is not solid, it must of course be repaired.
The factory apparently realized there was a camber problem as early as ‘69-‘70 because they made available a spacer to put between the bottom of the springs and the trailing arm and also redesigned the brackets.
Fortunately, the camber can be adjusted by manipulating the configuration of the trailing arm mounting brackets and through the use of the spacers under the springs. As mentioned earlier ,these same adjustments also affect the ride height so the two adjustments must be treated together.
Measuring Camber: The first thing I did is determine if the camber is misaligned and if so, how much and in what direction. (Remember, if it ain’t broke, don’t fix it --- or, don’t treat a well patient cause you might make him sick.) All that is needed to get a rough measure of the camber is a flat surface such as a garage floor, a large square and a ruler, preferably calibrated in tenths. The next photo shows these tools in action. (Notice the clean hands, almost like a surgeon.)
First and most important, all tires must be inflated to equal pressure and the car must be pushed with no driver or passenger straight to the place of measurement. If the car had been turned or worse, the back end jacked up and then put back down, the suspension will not be at equilibrium because the bottom of the tires are restrained from moving in an out by the floor. The same goes for any load on the suspension from driver or passengers.
Next, the square is set on the floor and positioned near the tire with the vertical side of the square aligned with the center of the hub and as nearly vertical as possible. Oh, forgot to mention to remove those wheel trim rings. Next, measure and record the distance between the square and lip at the top of the wheel. Then, without moving the square, measure and record the distance between the square and lip at the bottom of the wheel. Next, subtract the measurement at the top from the measurement at the bottom. If the two measurements are equal, then the camber is zero, great! If the difference is negative (top measurement larger then the bottom measurement) the camber is negative; the top of the wheel is leaning in. A positive difference indicates a positive camber; the wheel is leaning out. I take three sets of measurements for each side. If the computed difference varies more than ten percent on one side I keep taking the measurements until I get consistency. One more measurement that is needed is the diameter of the wheel between the points where the other two measurements were takes. My wheels measured 16 1/8 inches. The subsequent calculations work only if the diameter and the other measurements are in the same units (inches, feet, millimeters, furlongs, etc). I also measure the height of the lip of each fender (both front and back) directly over each wheel at the same time I take these measurements.
Computing: All that is needed now is a little trigonometry to convert the measurements to the camber angle. For those of you that remember more about the young women in your trig class than small angle approximations or worse yet, took a woman’s studies class rather than trig, I’ll help you out:
Camber Angle =(57.3 degrees) X (difference between top& bottom)/(diameter)
This wheel should have a noticeable lean in. If I do the measurements and computations and the results don’t match what I observe (such as wheel leans in and I compute a positive camber or the wheel that leans in the most seems to have a smaller computed negative camber, etc) I consider that:
If you look closely at the sketch you’ll see that the axis of the bushes is not parallel to the axis of the hub that fits in the round cylinder with the studs protruding. The next sketch shows this more clearly. Because of the ~ 30degree angle between the bush axis and wheel axel, as the trailing arm moves up and down the camber changes. For example, when load is added like people getting in, the back of the car goes down and the camber changes in a negation direction. A slight positive camber with no load will give a nearly vertical wheel under moderate load.
If the bushes are more then ten years old I’d suggested replacement before doing anything to adjust the camber. The standard bushing as well as uprated rubber and poly bushes are available. I installed new standard bushes on my '76 about 18 months ago at the same time I replaced the frame rear cross members, that part of the frame to which the trailing arm brackets attach.
Springs: The next thing I looked at was the springs. Recall that I mentioned measuring the height of all four fenders at the same time the camber was measured. There are two things to consider:
New Packing: When measuring the fitted spring length I noticed the rubber packing pieces at each end of the springs were in bad shape so I decided to replace them first. I purchased the uprated poly packing pieces from TRF. The rubber packing pieces are about ~0.25” thick and the new poly packing pieces are ~ 0.29” thick (photo below)
I decided to install the new packing pieces and measure everything again.
The height increased and the camber improved because of the thicker packing. The height became equal. I noticed that part of one of the old packing on the left side was not under the spring, which might explain how the height changed more on the left side.
Try Spacers: As mentioned earlier, the factory made available spacers to put under the springs to to fix the negative camber problem. I decided to try a set of spacers next. The spacers (next photo) raise the springs 0.44” and are placed between the spring and the lower packing piece.
In the accompanying note on Rear Suspension Geometry, the effect of spacers are computed to be:
Camber Angle Change (Spacer)=(Spacer height) X (2.7 degrees)
Ride Height Change (Spacer) = (1.8) X (Spacer height)
The spacer height must be in inches and the resulting ride height
change is in inches. The 0.44” spacer raised the height 0.8” and increased the camber (in a positive direction) by a little over one
degree, matching the calculated values very closely. The wheels looked pretty good with this setup but I
thought the height was too high. I also expected the new poly packing to compress slightly making the camber more negative. I decided to remove the packing to lower the height and work on the brackets instead.
The Brackets: Triumph made three different brackets for the trailing arm as shown in the next photo. The number of notches in the top edges identifies the brackets and is marked on each bracket in the photo. Up to CC61570 a 1- notch bracket was used on the inside and a 2-notch bracket on the outside. Beginning with CC61571 a 3-notch bracket was used on the inside and a 1-notch bracket on the outside.
Since there are 36 possible trailing arm bracket combinations with the one notch, two notch and three notch brackets - either in the 'up' or 'down' positions, I measured the bracket and trailing arm geometry, applied some trigonometry, and then created a table to determine the range of camber adjustment possible using the 36 combinations.
I've had several discussions with Brian and as a result revised his
original table to account for several additional variables.
(That exercise is documented in the accompanying note on Rear Suspension
Geometry.) The resulting
table is shown below. The camber angle and ride
heights are relative between bracket configurations. Also note
that the negative camber angle DECREASES as you go down the table. By the way, 3U means positioning the bracket with the three notches up, etc.
The 1U-3U configuration has a relative camber of -2.8 degrees. Moving down the table then next entry with the 3U inner bracket is the 2D-3U configuration that has -2.3 degrees relative camber. That is an increase of 0.5 degrees-- not enough. I needed between 1.3 and 1.6 degrees to get to zero camber (the spacer had been removed). The next entry down with the 3U inner bracket is the 3U-3U configuration that has -1.7 degrees relative camber. This is an increase of 1.1 degree. I would have liked a little more increase, but the 3U-3U configuration is the combination with the greatest positive camber with the 3U inner bracket. So I tried that. The results were: