Positioning the Rack

I built a jig that simulates the rack in mid position. The jig is c-clamped to the bottom of the frame rails right behind the lower crossmember.

I started with some large angle iron and found out that I had to add some spacers at the end to drop the jig low enough to clear the oil pan. Then there was interference between the jig and the lower control arm. And finally, I got tired of trying to slip in 3 blocks of wood on each side to lower the jig so I welded in some more angle iron at 22 deg angle so that the tie rods would run straighter.


The jig that is under constant modification!

I originally used a laser to align the front wheels to the rear wheels as a first step. I've documented the procedure on the steering info page. I know use the simpler string method (see previous link). I checked for 0 toe-in using my pathetic toe-in checker made out of an old broom stick handle and two c-clamps. The right c-clamp has the t-handle fixed in place so it can't slide back and forth. The left hand side c-clamp allows the t-handle to slide. I measure the length of the t-handle for toe-in. I used some rubber hose and a wood screw to get around the tire and to the rim. The measurements are at the front of the wheel's rim. I've since made a more sturdy toe-in checker out of the original steering shaft and rebar.

right jig  left jig

Here's the low buck toe-in jig after many evolutions with many more yet to come...

To position the rack, you have to make a lot of measurements and then graph them. The resulting graph will indicate where the rack or tie rods should move to. You graph toe-in/toe-out versus body position. This means that you start as low as the car will go and raise it in 1" increments while measuring the toe-in. An important note, is that the springs are removed from the car so that the wheels are free to move up and down throughout their travel.


Here's a yardstick held in a drill press vice for measuring height and a jack for raising the body

Some terms that are confusing are bump and rebound. Bump is when a wheel goes over a bump like a speedbump. Under normal driving conditions, the wheel rises up in comparison to the frame. To simulate this, the opposite is done, the frame is lowered in relationship to the wheel.

The term rebound under normal driving conditions describes when a wheel drops into a pothole. The wheel lowers in comparision to the frame. To simulate this, the opposite is done, the frame is raised in relationship to the wheel. The gist of all this is that the lowest point measured on the frame is actually the highest point that the wheel will be. This is indicated on the following graphs with 8" at the top and 14" on the bottom.

The lowest point that I drop the frame was 8", this was as low as my jack could go. The ideal ride height is 11" so I figured that I would measure +/-3" from 11" which works out to 8" to 14". I found that starting at 14" and lowering the frame is much better than starting at 8" and raising the body. The suspension is so stiff without springs to push the control arms down, that I would raise the wheels of the ground at 12" and get false readings. Starting at 14", I made sure that the tires were flat on the ground by pushing them down.

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Copyright April, 2011 Eugene Blanchard