Vehicles do not always travel in a straight line and wheel alignment must also be correct when turning. Stopping premature tire wear is not as simple as a wheel alignment. Dynamic wheel alignment must be considered when diagnosing tire wear.
Technicians adjust wheel alignment with the tires pointing straight ahead. This is how it has always been done. Usually this is acceptable. Another factor exists in dynamic alignment. Toe-on-turn is a measure of the relationship of one wheel to the other, in a turn. Not addressing this important factor leaves much to chance.
When a vehicle turns a curve, the wheels travel in different circles. The width of the vehicle is added to the radius of the inside circle. This means the wheel, in the direction of the turn, must rotate more than the outside. When this does not occur, the tires will wear and handling may be less than optimum.
The steering-arm accomplishes toe-on-turns. These arms attach the outer tie rod to the steering-knuckles. When rotated, this arm causes the inside wheel to turn faster than the outside. They accomplish a different turn rate because of their precise shape.
The easiest method of checking toe-on-turns is with a graduated turn plate. Bearings allow this plate to turn freely with vehicle weight. A scale marked in degrees measures how far each wheel turns. After adjusting the static-toe and centering the steering, the technician sets both plates to zero. They then lift and lower the vehicle onto the plates. Jouncing the suspension assures it is neutral.
Rotating the inside wheel to 20 degrees should turn the outside wheel to 18 degrees. Checking in both directions will reveal any problem. Measurements more than one degree from these, suggests something wrong. A bent steering arm is the normal problem, but finding which one is not always easy. Several simple checks identify the culprit.
Make a comparative measurement, from the tie rod stud to the face of the brake rotor. A noticeable difference in the measurement, from side to side, shows a problem exists. This check reveals a bent steering arm, but does not tell which. Identifying the bent arm takes a few more steps.
Bending a steering arm will change the toe on that wheel. The wheel with the damaged arm will toe in or out, depending on how the arm is malformed. Centering the steering will reveal the bad arm, if the other wheel alignment is the same as before the damage. With the steering wheel straight ahead, the undamaged wheel will point as it should. The wheel with the bent arm is out of line with the others.
Often someone has attempted to set wheel alignment, before checking for the problem. The number of threads remaining on the tie rod is another indication of which arm is bent. With the steering wheel at the center, adjusting the wheels to a straight position should leave the adjustment close to the middle. Too many or few threads remaining means, there is a problem.
Static-toe can probably be set with a bent steering arm. Attempting this will result in problems. The bent arm will change dynamic-wheel alignment and affect handling. Quality alignment technicians diagnose and correct the problem, before preceding.
A bent arm is not usually apparent to the eye. In the photo above, the lower arm shows damage. Without the red line showing the misalignment, it is difficult to see. Replacing this arm solved a persistent tire wear problem for the owner.
Not every technician checks toe-on-turns. Training is necessary to diagnose such problems. A high-quality wheel-alignment shop, eliminates persistent tire wear by checking dynamic-alignment, and static.