Having a good, well-balanced setup is a product of making sure all of the chassis elements are in place. Many times I have run across a car that has all of the right ingredients, such as roll-center design, a good balance of front and rear suspension design, proper weight distribution, and all the rest except one: a rear end that is not properly aligned.
Rear alignment is critical to the success of your setup. It can be the final setup parameter that can take you to the front or keep you in the rear of the field. The problem is that no amount of manipulating the other setup components will overcome a rear alignment problem. While it is probably true that a car out of alignment is more forgiving on dirt, it is important for both dirt and asphalt race cars to have a properly aligned rear end.
Proper Alignment If you've ever followed an old pickup truck that has jumped one too many ditches and bent the rear suspension out of whack, you'll know what an improperly aligned rear end looks like. Remember how it tracks sideways down the road, somewhat like a crab? That is exactly what is happening with your race car if your rear alignment is wrong, and it doesn't take much to really affect your car's performance. The rear axle centerline should be exactly perpendicular to the centerline of the chassis. Also, the right-side tire contact patches should track in line with each other, regardless of the difference in track width of the front and rear tires.
Tuning with Rear Alignment Many racers have tuned the tight or loose condition of their cars by moving the right-rear tire forward or back. There are much better ways to adjust handling than this method. Using a small amount of rear steer as a product of chassis roll is a way to develop more bite, or free up a severely tight car (mostly on dirt tracks) and that is fine. But if a car tracks crooked down the straightaway and into the corner, the negative effect will be felt all the way around the corner.
When taking measurements,...
When taking measurements, you can use solid links in place of the shocks to locate the rear end in exactly the same position that it would be at ride height with the suspension supporting all of the weight. In a big-spring car you would remove the springs also.
How to Measure the Rear End To measure your rear alignment, follow these simple instructions: Using the diagram, follow the numbered steps to set up a line that is 90 degrees to the centerline of the car and lays behind the rear end far enough to allow easy measurement. The car can either be at ride height or up on blocks placed under the tires with all the weight on them. It is much easier to have the car at ride height plus a constant amount in order to be able to crawl under the chassis to take measurements and make marks on the floor.
Position the Car The best way to position the car is to put the car on jackstands or equivalent stable supports so the chassis is far enough off the floor for you to easily get underneath it. Add the ride height plus some constant distance to each corner. Remove the shocks and springs and place solid links where the shocks normally mount, so the rear end is supported in the same relative position that it would be at ride height with the tires on and all weight on them. We want to simulate the position of the rear end at normal ride height just like we will race the car.
If your car is a perimeter chassis (both sides of the frame are symmetrical), first measure between the inside framerails at the front and rear and divide those numbers by two. Measure from one side of the frame at the front and rear using the half measurements and place a mark on the floor at each end of the car at centerline. (I use a piece of 2- or 3-inch-wide masking tape placed on the floor to mark on.) These marks now represent the centerline of the car.
If you are measuring a straight rail (offset) chassis, measure the same distance (say 10 inches) off the front end of the straight rail and the rear end just in front of the rear axle and place marks on the floor to create a line that is parallel to the centerline of the car.
In this example, the rear...
In this example, the rear end is pointed to the right of the centerline of the car. This will help free up a tight car, but will produce a severe loose condition under throttle application. The loss of bite off the corner will hurt the lap times more so than what might be gained from freeing the car up.
Here we see where the rear...
Here we see where the rear end is aligned so that it is pointed to the left of the centerline of the chassis. This makes for a very tight handling car, and in most cases, the driver cannot steer against the tendency for the rear of the car to pass the front on the left side.
Here we see the layout for...
Here we see the layout for the process. Note that the lines formed by the intersecting marks are right triangles. Take your time and measure very accurately. Re-measure if you feel everything is not right once you pull the rear string line across the outer points of the triangles.