In spite of the success, I was disturbed that the amount of change was less than 2/3 of what I'd expected using Brian's table. I measured the brackets involved, did a calculation and found a result closer to what I measured. I then emailed Brian and asked where I went astray and also if my bracket measurements were accurate. He responded that his original data and calculations were lost in a computer crash. He did say that I'd included one more factor than he did; his object was to get in the right ballpark. He further suggested that maybe I should put together a revised table. Well, I jumped right in and measured, calculated, analyzed, etc and came up with results that were way off from the measured results. This was using my normal ready, fire then aim technique. Getting a sign mixed up here and there didn't help either. Did it a few more times and began to understand some interesting symmetries, and more importantly discovered yet another variable. I finally went back and measured the brackets very carefully and then did the calculations one-step at a time. I've described the methods and all calculation steps in the following in the hope that others will check over the work and point out any and all errors. This is really an extension of Brian's work and we are all fortunate that he brought organization to this area.
The next sketch of the right rear suspension arm came from a TRF catalog. I processed it a bit to remove unwanted items and then added the text.
One thing that influences the camber is the angle the trailing arm bushing axis makes with the horizontal. This angle is determined by the brackets and the frame member to which the brackets attach. Three different brackets were made for the TR250/TR6, each identified by the number of notches in the top (1, 2 or 3 notches). The factory changed the standard configuration at commission number CC66571. The following photo shows both configurations for the right side. Brian Lanoway noted that by reversing the inside and outside brackets and by allowing the brackets to be mounted with the notches up or down, a total of 36 combinations are possible. He then measured the position of the bush bolt in each bracket and used that to compute the relative angle of the bush axis. My first step was to determine the bush bolt position as accurately as possible. Actually this took several steps (one forward then two back, etc.) until I drilled a couple holes in a steel plate to hold the bracket for measurement. The holes were drilled as far apart as possible so that there is no slack in the bracket position even before the nuts are tightened. I then clamped a steel block (from the scrap pile) to the plate as shown in the next photo. The block had been machined on the sides and end and the end was square. I aligned the block perpendicular to the bush bolt. I then measured the distance between the end of the block and the bush bolt as shown in the next photo. This measurement was taken in the middle of the bracket and care was taken to keep caliper perpendicular to the end of the block. This measurement was recorded and then the bracket was removed and reversed without disturbing the position of the steel block. A second measurement was then taken with the bracket in the reversed position. Next, the difference in the two measurements was determined (one measurement subtracted from the other). This difference is the amount the bush bolt position moves when the bracket is reversed. If the two measurements are the same, the bush bolt is on the centerline of the bracket. If the difference is one inch for example, then the bush bolt is 1/2 inch above the centerline in one position and 1/2 inch below the centerline in the other position. Note that the specific position of the block relative to the bracket is unimportant as long as it is the same for both measurements on a bracket. The position of the bush bolt (BBP) relative to the centerline for both positions of each bracket is shown in the table below. A positive number means the bush bolt is above the center. The U or D indicates whether the bracket has the notches on the top (U-normal position) or on the bottom (D). For example 3U means the three-notch bracket positioned with the notches pointed up and 1D means the 1 notch bracket positioned with the notches pointed down.
The next photo shows a typical bracket configuration. The distance between the center of the two brackets is ~13.625". The example shows the brackets where the BBP of the right bracket is above the BBP of the left bracket. Since the right bracket is above the left bracket, the bush axis is at an angle with the centerline between the two brackets. The angle (in radians) between the bracket centerline and the bush axis is approximately equal the difference in the bush bolt positions of the two brackets divided by the distance between the center of the brackets (13.635"). The angle can be converted from radians to degrees by multiplying by 57.3. I defined a positive angel as the case where the inner BBP is above the outer BBP. Combining all this, the Bush Angle is approximately: Bush Angle = (57.3/13.625)X(BPP
Each of the brackets has a different bush bolt height. Changing the bush bolt height changes the bush axis height. Changing the bush axis height while keeping the spring the same results in a rotation of the trailing arm and thus affects the car height and rear wheel camber. The next photo shows the dimensions necessary to compute the effects of changing the bush axis height. The spring is located about 10.5 inches from the bush axis and the tire is about 19 inches from the bush axis. The first thing is to get our directions straight. If the bush axis is raised, the front of the trailing arm goes up and the back goes down resulting in raising the car and increasing the positive camber. If you draw a couple sketches you'll see that the car height changes 10.5"/19" or 55% of the change in the bush axis height. For small changes in the bush axis height,
the amount the training arm rotates (in radians) is approximately the change in bush axis height
divided by the distance between the bush axis
and the spring (10.5"). This can be converted to degrees by
multiplying by 57.3 degrees. The effect this has on the wheel camber must
then be multiplied by the sine of 30 degrees or 0.5. I'm going to abbreviate
the change in camber angle due to a change in bush axis height as Camber
Angle Camber Angle (BAH)= (57.3/10.5)X(0.5)X(change in bracket height) Camber Angle (BAH)=(change in bracket height )X (2.7 degrees) The change in bracket height must be in inches since the distance between the bush axis and spring position was in inches. I decided to reference the Camber Angle (BAH) to the bracket centerline. The bush axis height relative to the bracket centerline is merely the average of the bolt position of the two brackets. The results of these calculations are shown in the next table.
As predicted, when the bush axis height is included, there
are many more unique adjustment possibilities. The camber angles
and ride heights in the table are relative and make sense only in the context
of differences between configurations. For example, I started with
the late standard 1U-3U configuration with - 2.82 degrees camber
angle and -0.21" ride height. I then changed the outer bracket
from 1U to 3U to give a 3U-3U configuration of -1.68 degrees camber angle -0.34" ride height. The net effect computed
from the table should have been a increase in camber of 1.14 degrees
and a decrease in ride height of 0.13". The actual results
were a
slight The following is a more useful presentation of the data. Because of the approximations used in the calculations and limited precision of measurement of the various dimensions, I reduced the resulting data to a single decimal point precision.
The small angle formula can be used to compute the change in road height --- it is 8 inches times the change in camber angle in radians or using 57.3 degrees per radian: Ride Height Change (Bush Axis Angle ) = (8"/57.3) X (Camber angle change in degrees) Ride Height Change (Bush Axis Angle ) = (0.14") X (Camber angle change in degrees) The camber angle change used here is that due only to the bush axis change excluding any change due to changing the height of the bush axis. This angle for going from the 1U-3U configuration to 3U-3U is 1.81 degrees. Plugging this into the formula above gives a ride height change of 0.25". The change in ride height due to rotation of the trailing arm around the bush axis was computed to be -0.13". Adding this to the 0.25 increase just computed gives a net ride height increase of +0.12" --- nearly exactly matching the 0.1" increase measured with the precision yardstick. Now that I know what's going on, I'm going to ignore this second order effect.
Camber Angle Change (Spacer)=(Spacer height) X (2.7 degrees) The effect on the ride height is however different; it is magnified by the lever effect of the trailing arm so that the ride height change is: Ride Height Change (Spacer) = (19"/10.5") X (Spacer height) Ride Height Change (Spacer) = (1.8) X (Spacer height) I obtained a set of 0.44" high spacers from TRF. From the calculations above, these should increase the camber by 1.2 degrees and ride height by 0.8". The measured results were an increase in camber of 1.2 degrees on one side and 1.1 degrees on the other. The measured ride height increased by 0.8". This agreement is in part luck since the height readings with the yardstick are to the closest 1/8 inch and the camber measurement accuracy is probably + or - 0.1 degree at best.
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