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The Rear Sub-frame and DIY Alignment 2010/03/09

Posted by Michael in my IS300.

The rear sub-frame of the IS300 seems one of the weak points in the vehicle design. It is stamped from fairly thin and soft steel and is apparently easily susceptible to damage. Excessive inner tire wear is a common complaint noted by IS300 owners, both front and rear. I had experience with excessive wear at the rear wheels.

Performing an inspection revealed that the toe control arm attach points on the rear-subframe were deformed. Toe is adjusted using a cam washer which is normally trapped by a formed lip. The pictures show the damage to this lip…

Similar deformation was evident at the lower control arm attach point for both left and right sides which contributed to maladjustment in the camber. This picture shows a normal lower control arm. Again, adjustment is by a cam washer trapped by a rolled lip. Circled in yellow (picture below) is the lip that was deformed on both my lower control arms.

The cause of this damage is unclear; I had a shop perform an alignment on the vehicle once and they seemed to struggle… it took them almost 3 hours. By that time I’m sure they were as anxious to finish as I was. In frustration they may have torqued down the suspension bolts and crushed the sub-frame lips. Or perhaps this damage occurred during normal wear-and-tear while driving.

I fixed both problems myself.

For the rear-toe I installed turnbuckle toe-control arms similar to these…

The attachment at the sub-frame has a ridged detail that bites into the mating surface. Torquing this down to about 80 ft-lbs locks it in place. Toe is then adjusted by rotating the body of the link and securing the lock nuts when the correct amount of toe is attained.

For the lower control arm damage I used a wooden board and blunt force to hammer the tab back into shape until the suspension cam washer was again held securely in place.

The next challenge was to check and adjust the alignment.

The front suspension didn’t seem to exhibit any signs of damage similar to the rear (that might have come from the alignment shop or otherwise), and the alignment report showed alignment numbers exactly where I wanted them: ~-1deg camber, < 1/32″ total toe-in. Thus I was able to use the front wheels as reference to align the rears. Here is the procedure I developed and followed…

  1. Find a flat level surface to work on (luckily, my garage floor meets this requirement).
  2. Find a flat board large enough to cover the face of the wheel/tire.  It should contact the highest edge of the wheel/tire all the way around (compromises can be made if you don’t have a suitably sized board).
  3. Toe: Place the board against the face of the wheel and mark the outside edge at the front and rear of the board.  This is half of the toe measurement.  Repeat on the other side of the vehicle and measure the distance between the front marks and the rear marks.  The difference between these measurements is an expression of toe.  Convert it to an angle or extrapolate to the edge of the tire tread to obtain actual toe.
  4. Camber: With the board still flat against the wheel/tire, measure the vertical angle of the board relative to the level surface you’re working on.  I used a combination square and trigonometry to calculate the angle.  This is a measure of camber angle.  Note that you might need to account for tire bulge at the bottom due to vehicle weight and tire inflation.
  5. Thrust Angle: Make sure the steering angle of the front wheels is at zero.  Run a string from the centre of the front wheel through the centre of the rear wheel.  I tied the string to one of the spokes on the front wheel.  Ensure the string is in contact with the front wheel.  Hold the string tight and against the edge of the rear wheel.  The angle of the wheel relative to the string is a measure of the thrust angle.  Note that one-side alone is not the thrust angle.  You must consider both sides, and also toe to assess the thrust angle (consider the front wheels might have a different width than the rear wheels).
  6. With all measurements taken calculate the current condition and the adjustments necessary.  The next step requires elevating the vehicle, adjusting the suspension, and repeating the measurements above.  This is an iterative process.  Set the camber first, and the toe and thrust angle second.  With the turnbuckle toe-control arms I have, correcting toe/thrust angle can be made exact by using the thread pitch to calculate number of turns required to correct for the measured toe angle.


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