【正文】 e specs to see what the allowable difference is. Toe Out on Turns When you steer a car through a turn, the outside front wheel has to navigate a wider arc then the inside wheel. For this reason, the inside front wheel must steer at a sharper angle than the outside wheel. Toeout on turns is measured by the turning angle gauges (turn plates) that are a part of every wheel alignment machine. The readings are either directly on the turn plate or they are measured electronically and displayed on the screen. Wheel alignment specifications will usually provide the measurements for toeout on turns. They will give an angle for the inside wheel and the outside wheel such as 20186。s centerline by putting instruments on all four wheels. Steering Axis Inclination (SAI) SAI is the measurement in degrees of the steering pivot line when viewed from the front of the vehicle. This angle, when added to the camber to form the included angle causes the vehicle to lift slightly when you turn the wheel away from a straight ahead position. This action uses the weight of the vehicle to cause the steering wheel to return to the center when you let go of it after making a turn. Because of this, if the SAI is different from side to side, it will cause a pull at very slow speeds. Most alignment machines have a way to measure SAI。s tread. If the camber is too far negative, for instance, then the tire will wear on the inside of the tread. Camber wear pattern If the camber is different from side to side it can cause a pulling problem. The vehicle will pull to the side with the more positive camber. On many frontwheeldrive vehicles, camber is not adjustable. If the camber is out on these cars, it indicates that something is worn or bent, possibly from an accident and must be repaired or replaced. Caster When you turn the steering wheel, the front wheels respond by turning on a pivot attached to the suspension system. Caster is the angle of this steering pivot, measured in degrees, when viewed from the side of the vehicle. If the top of the pivot is leaning toward the rear of the car, then the caster is positive, if it is leaning toward the front, it is negative. If the caster is out of adjustment, it can cause problems in straight line tracking. If the caster is different from side to side, the vehicle will pull to the side with the less positive caster. If the caster is equal but too negative, the steering will be light and the vehicle will wander and be difficult to keep in a straight line. If the caster is equal but too positive, the steering will be heavy and the steering wheel may kick when you hit a bump. Caster has little affect on tire wear. The best way to visualize caster is to picture a shopping cart caster. The pivot of this type of caster, while not at an angle, intersects the ground ahead of the wheel contact patch. When the wheel is behind the pivot at the point where it contacts the ground, it is in positive caster. Picture yourself trying to push the cart and keep the wheel ahead of the pivot. The wheel will continually try to turn from straight ahead. That is what happens when a car has the caster set too far negative. Like camber, on many frontwheeldrive vehicles, caster is not adjustable. If the caster is out on these cars, it indicates that something is worn or bent, possibly from an accident, and must be repaired or replaced. Toein The toe measurement is the difference in the distance between the front of the tires and the back of the tires. It is measured in fractions of an inch in the US and is usually set close to zero which means that the wheels are parallel with each other. Toein means that the fronts of the tires are closer to each other than the rears. Toeout is just the opposite. An incorrect toein will cause rapid tire wear to both tires equally. This type of tire wear is called a sawtooth wear pattern as shown in this illustration. If the sharp edges of the tread sections are pointing to the center of the car, then there is too much toein. If they are pointed to the outside of the car then there is too much toeout. Toe is always adjustable on the front wheels and on some cars, is also adjustable for the rear wheels. Four Wheel Alignments There are two main types of 4wheel alignments. In each case, the technician will place an instrument on all four wheels. In the first type the rear toe and