Regardless which direction you go the first thing you need to establish is that the rear axle is square to the motor plate and the rear end in the center of the chassis (or offset as much as the motor plate). There are a number of ways to accomplish this. Here is the procedure we use.
1. Check the motor plate for flatness with a good straight edge, install the motor plate with light bolt tension. Be sure to check that the plate sits flat on the frame tubes. File off any high spots in the paint. Do not flex the plate while tightening the bolts.
2. With the bare chassis on stands, pull a length of waxed dental floss from the middle of the front chassis tube, through the hole in the motor plate, to the middle of the rear chassis tube.
3. Using a nice readable tape measure, at the front, measure from the line to the side frame, left and right, and center the line within a sixteenth.
4. Perform the same procedure at the rear. Now double check both ends. When satisfied , mark the line location on the tubes with the edge of a triangular file.
5. Use a long sheet rock t-square, check the squareness of the motor plate with the centerline of the chassis. The edge of the square should be within a sixteenth of the line at the end of the square. Project the square both forward and rearward. Be careful when setting the square on the motor plate as not to disturb the line to achieve the most accuracy. Patience counts. Shim the motor plate carefully to correct any out of squareness.
6. I prefer to have the chassis on the floor on ride height blocks, but you can perform this procedure on stands. Install the rear end, torque tube, and ball housing (cover not required). Assemble the rear suspension completely (less shocks and springs), connect the jake ladder or panhard bar (rear) and all the links so the rear is completely located and has only one degree of freedom (up and down). Do not take any shortcuts here by omitting any link, bolt etc as it will severely affect the accuracy of the setup. Set the rear at the correct distance from the motor plate. Set the rear at ride height.
7. With a very straight bar clamped to the motor plate, measure back to the
rear axle at the widest point, left and right. Be sure to measure to similar diameters on the rear axle. Adjust the links to achieve parallelism between the motor plate and the rear axle (to zero). Now check to see if the ball pops in and out of the housing with absolutely no friction. Adjust the rear left to right with the panhard bar rod end to center the ball. Unfortunately this will change the parallelism of the rear considerably, so perform the entire operation several times until the rear is parallel and the ball is centered. Close enough is not good enough. Spend the time required to get the rear perfect and the ball perfect. There will most likely be a small error if you check the centerline of the rear between the frame rails. A little clearance here and there between the driveline boltholes accounts for this. The most important measurements are rear axle parallelism and ball location.
Now for grins, bump the rear up one inch or so and check the measurements back to the axle again. This simulates the car under acceleration. Ideally the rear will remain perfectly square, but unfortunately the geometry of the single link rear is not that good. Now move the right side up and the left side down an inch or so. This simulates chassis roll in the corners. There definitely will be some parallelism change but hopefully the right side will be a bit shorter than the left with the ball still centered.
8. With the rear back at ride height now install the front end completely and check the location distance to the motor plate and square the front axle.
I hope that these guide lines will help a few of you to get to victory lane or at least have a better handling chassis. And who knows….maybe beat up on a few of the high tech teams along the way.
Sometimes it’s the little things that make a big difference. Good Luck!!