In the past, it was common...
In the past, it was common to string a car by making marks on the floor. This is a good method for checking rearend square and can be used to line up the right side tires. It is a little slower to do than the raised string method but can be more accurate due to the process of dropping points down to the floor with a plumb bob and then making direct measurements to the rearend. Neither string method will show you Ackermann amounts.
The laser system is one that attaches to the wheel hubs (models for both Wide 5 and 5-on-5 hubs) and can be rotated 360 degrees, and leveled both horizontally and vertically. The laser uses targets at each end of the car. For the analog (string) method, the nylon string we used was from the local hardware store.
We will go through the process using both methods in order to show you how to perform each step.
Step 1 - Check both the front wheels and the rear wheels for runout. This means as the wheel rotates, the outer edge of the tire will wobble slightly. You must compensate for this slight distortion by finding the extreme high spot at a point equal in height to the hub height. Use a jackstand to hold the tape steady and rotate the tire, noting the distance from the stand. Upon locating the high point, mark it with an arrow and then rotate the tire (front or rear) so the arrow is at the top pointing straight up.
Now, check the toe at the rearend. Use toe plates for the analog method. Even small amounts of toe are not acceptable. Be careful how each person holds the plates. Do the measurement several times to ensure accuracy.
If using the laser system, thoroughly clean the surface of the hubs and make sure there are no protruding threads from the bolt holes. It is recommended you go over the hub surface with a flat file to eliminate any bulges or edges of metal that would cause the laser to be improperly aligned. Attach the laser to the rear hubs.
Set four targets, two on each side, at equal distances forward and rearward from the vertical center of the rear hubs. Center the laser beam on each target and measure the distance between the centers at the front and also at the rear. If these widths are not the same, the rearend is toed and must be straightened before continuing.
We used a highly accurate...
We used a highly accurate coordinate geometry software program to calculate the exact amount of Ackermann-or steering angle difference-between the two front tires needed for quarter- and half-mile racetracks. We then translated the angles into inches of toe. For a half-mile track, we would need only about 1/64 inch (or 0.03 degrees) of additional toe so the tire would follow the needed track due to the difference in radius between the front wheels. For a "tight" quarter-mile track, we need slightly more toe gain, or about 1/16 inch (or 0.11 degrees) of additional toe. These numbers represent fractions of degrees of Ackermann, not whole degrees. Remember, each 1 degree of Ackermann is 1/2 inch of toe.
Step 2 - Once the rearend has been toed straight ahead, shine the right rear (RR) laser so that you can measure the offset to a straight framerail at the front and rear or two places offset from the centerline of the chassis. Most car builders will align at least one framerail parallel to the intended centerline of the chassis. It may be located on the right side or at the weight box on the left side next to the driver. Adjust the rearend so that each measurement to the frame is equal. This means the rearend is perpendicular to the framerail and hopefully the intended centerline of the chassis and the installed body.
This measurement can also be done with a string by using a square to come down to the floor, then measuring over to the frame or centerline of the chassis to align the string with the framerail. You can measure over to the tire to check the rearend alignment in relation to the framerail. Equal measurements to the tire front and rear means the rearend is perpendicular to the chassis centerline.
Step 3 - Center the front steering rack. This is done by turning the steering wheel lock to lock, then back half the number of turns from full lock in either direction. Once midrack (or midbox with drag-link steering) is found, lock the steering column with two vise-grip-type pliers. With the lasers attached to the front hubs, level the front lasers (vertically) and reposition the targets so that they are equal distance from the center of the front hubs. We used 14 feet as a convenient distance that would clear the rear of the car, while not extending too far to the front if the space were limited in the garage. Remove the front lasers from the holders and align the centers of the four targets with the beams from the rear lasers.
Reattach the front lasers and rotate the right front laser so that it shines on each target. Note where it is positioned in relation to the centerline. If the point where the beam strikes each right side target is not an equal distance from the target center as well as on the same side, adjust the length of the right side tie rod to align the right front wheel parallel to the rear wheel. The points do not need to be exactly on the target centerline, only equal distance from the centerlines.
With the string, simply measure to each side of the tire with the tape and make both measurements equal by adjusting the tie-rod length.
This handy tool produced by...
This handy tool produced by FinishLine makes adjusting rearend toe fast and easy. The tie rods-one on each side-can be widened or narrowed, which moves each wheel in or out. Rearend toe adjustments can be made at the racetrack in minutes.
After leveling the laser (vertically),...
After leveling the laser (vertically), carefully place the laser targets equal distances from the wheel hub. Position each target left to right so that the beam is exactly on the centerline.
The laser beam is a pin point...
The laser beam is a pin point surrounded by a halo of red light. The targets are black with lines etched into the aluminum. When the laser beam strikes the thin etched line, it will shine brightly so you can easily see when the beam is centered.