Squaring a race car is key to proper race car set up. Pointing all 4 wheels in the same direction allows for all of your adjustments to perform in a consistent and predictable fashion.
Let's spend a minute getting on the same page in reference to squaring a race car. For the sake of clarity this article targets Late Model style cars, but the recommendations apply to many race car types. For the purposes of this article we are assuming that the housing is perfectly straight and your frame is in factory condition.
If you identify your goal before starting it helps in the end result. Common knowledge states that the rear end must be square. However, the question at hand is square to what? Referring to the drawing below will help keep things clear as you read the recommendations.
To have a rear end installed correctly we must square the housing to something. But square to what? The common answer is that we need to be square to the frame rails. If your frame rails are perfectly straight then a good result can be gained from utilizing the frame rails as a reference point.
The reality is that frame rails are not straight. Race car frame rails are made from mild steel that is simply pulled from a rack. The steel is not that straight to begin with and welding helps to distort it further. My opinion is that the frame rails are hard to rely on consistently.
Really our goal is to "square" the rear end housing to the front end versus the frame rails. The steel is along for the ride. The front suspension pivot points are the true reference points. By embracing this concept the frame can have some normal bends and bows and you can still be assured a square rear end. My recommendations involve squaring to the front pivot points as this will provide the most consistency.
In addition, we want the rear end housing to be located correctly from side to side in the car. For best performance lining up the right side tire seems to be best. To set the rear end location correctly the front end adjustments must be set up correctly and be in race ready condition.
The right side tires carry the highest forces so keeping the right side tires in line with each other creates the most stability and maximizes left side weight. Lining up the right side tires is my recommendation.
However, your car builder may support a different left to right location so you need to insure that you follow car builder's recommendations so that the needs of the rear suspension links are met. For this article we will assume that the car builder laid out the suspension with the right side tires lined up. You will want to pay special attention to this detail as it is a higher priority to have the trailing arm brackets and top link hardware to be located in the proper location. For example, you want the top link to be perpendicular to the rear end housing and parallel to the car. Trailing arms must also be running forward at the correct car builder design angle. These design elements must be considered so be sure to consult your car builder about his design parameters for the Left/Right location for the rear end housing.
Ok, now that we are on the same page, let's review: The front end is set and is race ready. The car is set at ride height by using solid links in place of the shocks or it is sitting up on stands at race ready ride height with air pressure and stagger set properly. Your rear end housing has been checked and it is not bent and it is ready for installation. We also need to insure that the panhard bar is set to the correct height and that the trailing arm and top link angles have been set properly.