How To Measure Rear Alignment
Get Your Rear In Line
Having a balanced setup is not only a product of all the chassis components in place but working in harmony. All too often a car that has all great products of a good roll-center design, front and rear suspension design, good weight distribution, and chassis strength, can still be hurt by the rear end was not being aligned properly.
Rear alignment is critical to racing on asphalt tracks, the rear out of alignment of a dirt race car is more forgiving on dirt tracks, So you guys running on asphalt pay strict attention to this article. Dirt racers play with rear end alignments and dream of ways to make their cars turn better, but always when setting up your race car place your rear in its proper alignment before tweeking for race day. Remember that moving the axle centerline left or right in the chassis is only a bandage for a more severe problem.
Proper Alignment
Look at the definition of Alignment first. Have you ever been driving down the road and get behind a really old truck, or even an suv and see the rear end tracking to the right as the truck drives straight? This is from the rear end being out of alignment. The rear of the truck wants to wander to one side or the other, and it usually tracks opposite of the front end.
Fine Tuning with Rear Alignment
Many a racer have tuned at the track with a tight or loose condition 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 to enhance the effect of chassis roll is a way to develop more bite, or free up a severely tight car. To much of this kind of adjustment can be harmful to how the car handles through the entire corner, Entry, mid, and exit may appear to the driver as something majorly wrong with the setup of the car, and it can be caused from the right rear tire being too far forward or back, use only small adjustments and always reset the rear if it doesn't work before you try another change to something else on the chassis.
How to Measure the Rear End
Follow the directions below with your car setting up high enough on blocks so you can get underneath it, make sure and this is important to have your car all set as if you were at the race track, air pressure in tires, all the fuel weights in car, make sure that the car is in race trim.
Position the Car
The best way to position the car is to put the car on blocks high enough so that you to get underneath it. Add the ride height plus some constant distance ie Block hight, to each corner. Remove the shocks and springs and place dummy links where the shocks normally mount, doing this will keep the chassis at ride height with the tires on and all weight on them. We want to keep our ride height just like we will race the car.
First step
If your car is a perimeter chassis (Symetrical) both sides, 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 the front and rear of the car. These marks now represent the centerline of the car.
If you are measuring a straight rail known as an "Offset" chassis, measure the same distance off the front of the straight rail and the rear rail just in front of the rear end and place marks on the floor in front and in back of the car to create a line that is parallel to the centerline of the car.
You will then have two points at each end of the car to use to develop a line that is at a right angle to the centerline of the car. Using chalk line or stretch a string over each of the centerline mark at each end of the car to form a line. Make the line long enough to extend past the rear of the car. If you use a string, you can hold the string in place with some sort of weight to hold the string tight.
Second Step
Make a cross mark on the “centerline” somewhere behind the rear of the car.
Third Step
Measure up the line toward the front of the car 96 inches (8 feet) and make another mark on the line. Use Tape or chalk to make your marks, when using masking tape use a marker and use enough tape to make one foot increments to 9 feet.
Forth Step
From the line you made in Step 3, measure two distances, 10 feet from the front point and 6 feet from the rear point, This forms an intersection on each side of the car. These two points will be used to construct a line that is exactly at a right angle to the centerline of the car.
Fith Step
Stretch a string or pop a chalk line over these two outer intersection points. The line that crosses the “centerline” should be directly over the rear 8-foot point on the “centerline.” If it is not, recheck all of your measurements and re-establish the intersecting points that form the perpendicular line.
The triangles formed on the ground (if you haven’t recognized it yet) are 3-4-5 right triangles whose sides have been doubled. In geometry, coincidentally, if a triangle has sides that measure 3 units, 4 units, and 5 units on each side, it is a right triangle. We just doubled the numbers, and we still have a right triangle. This is the easiest way to layout a perpendicular line.
Sixth Step
Drop a plumb line down off both sides of the rear axle caps and mark the point on the floor. Be sure to use the same position on each side of the rear hubs. The hubs themselves are machined and are best to use. Don’t use the back or front of the axle tube. You will not get a precise measurement..
Using these measurements will show you were the rear is located. Adjust the length of the trailing arms to square the rear end. You may have to adjust both sides to make it square, remember that even though you get the same measurement for both sides, the rear may be out of square.
Lateral Rear End Location
Make sure the rear end is perpendicular to the centerline of the car, also make sure that it is positioned correctly between the rear framerails. Do not allow the rear end to sit skewed in reference to the frame rails, this can hurt the performance of your suspension, and cause major binding.
One of the handling adjustment methods that is passed down for generations involves moving the rear end to one side or the other to change the handling balance. This is an unreliable and temporary fix that should not be made if all of the other chassis components are set up properly. Remember that the manufacturer of the chassis will have the axle center line set to how they prefer the chassis to move.
Alignment of the Right-Side Tire Contact Patches
Rear wheels should be positioned so that the two right-side tire contact patches line up or be equal. This is because in the turns most of the weight is on the right-side tires and the car will want to track inline with the right-side tires and the point of tracking will be where the tires contact the racing surface. It is important to know how the right-rear tire tracks in relation to the right-front tire.
Measure from the “centerline” you have created to the center of the right-front and left-rear tires. Make sure the front tire/wheel assembly is positioned so that it is correct in relationship to the car’s chassis as it would be at ride height, like you did with the rear end. Adjust the length of the panhard/J-bar to align the right-rear tire to meet the right fronts centerline.
This process should take you about an hour to complete.. With little effort to make sure your car will track correctly and have conciderable performance. Repeat this entire process at least once a month, or when you change your wheel offsets, Make sure you check it after greeting the wall or being involved in a crash. Once you are sure that your car is aligned, you can then seriously perform enhanced chassis setups.
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