Wheel Alignment

Aligning a vehicle's front wheels is the job of balancing the steering angles with the physical forces being exerted. The steering angles are; caster, camber, toe-in, steering axis inclination, and toe-out on turns. The physical forces are gravity, momentum, friction, and centrifugal force. Since so many factors are involved in front wheel alignment, it is also called front end alignment, steering alignment, steering balance, or steering geometry. Alignment is more than just adjusting the angularity of the front wheels. With steadily increasing production of front wheel drive vehicles with independent rear suspension, four wheel alignment is often required. For ideal wheel alignment, certain conditions would have to be met. Both front tires will be the same brand, size, and type. Each will have the same degree of tread wear, and be inflated with the same pressure. Each wheel is properly and equally adjusted for angularity, each tire will maintain the same area of tread contact on a smooth road surface. Obviously, it is impossible to maintain all these requirements. The steering control rods are used to adjust toe-in and toe-out. The upper and/or lower control arms are adjusted to affect the camber angle. Caster is usually not adjustable. With all the weight balance factors to be checked out and corrected, it is obvious that wheel alignment is more than just an adjustment of the steering angles. The whole theory of wheel alignment revolves around balanced weight distribution on the wheels and proper tire tread contact with the road surface while the vehicle is in motion.

Weight Balance on The Springs

The springs control the up-and-down motion of the car and so, the height of the car above the road. If one or more of the springs is collapsed or broken, it causes an unbalanced distribution of weight. This unbalance creates a lopsided appearance, puts an added strain on the related parts, and changes the angularity of the front wheels. This condition may also occur when the load is distributed unequally. In fact, anything that changes the ratio of weight on the springs will have a definite bearing on the alignment angles and on the area of tire-to-road contact. This can wear tires unevenly, making it necessary to replace them prematurely.

Wheel (Slave) Cylinder

The wheel cylinder, also called the brake cylinder or slave cylinder, is a cylinder in which movable piston(s) convert hydraulic fluid pressure into mechanical force. The piston(s) within the cylinder move the brake shoes or pads against the braking surface of the drum or rotor. There is one cylinder (or more in some systems) for each wheel. The cylinder is usually made up of a single-bore cylinder casting, an internal compression spring, two pistons, two rubber cups or seals, two rubber boots to prevent entry of dirt and water, and a bleeder screw (valve). In drum type brakes, the wheel cylinder is fitted with push rods that extend from the outer side of each piston through a rubber boot, where they bear against the toe end of each brake shoe. In disc brakes, the wheel cylinder is part of the caliper. As the brake pedal is depressed, it moves pistons within the master cylinder, forcing hydraulic brake fluid through the brake lines and into cylinders at each wheel. The fluid under pressure causes the wheel cylinders' pistons to move, which forces the shoes or pads against the brake drums or rotors. Two-way pressure is applied when the wheel cylinder is activated. Brake fluid enters the center of the cylinder, forcing the pistons apart. Pushrods at the piston ends then apply equal pressure to the brake shoes. A return spring pulls the pistons together when pressure is released.

Wheel Balance and Unbalance

It is important to check to see that the wheel and tire assemblies are in balance before aligning the vehicle. "Static" balance is the equal distribution of weight around the wheel and tire assembly. "Dynamic" balance is the equal distribution of weight on each side of the vertical centerline of the wheel and tire assembly. "Unbalance" (or imbalance) exists when the weight is distributed unequally around the horizontal axis of the wheel and tire assembly. Unbalance can exist in the tire, wheel, brake drum or rotor, or even in the hub. It also occurs in any combination of these components. Unbalance can be detected with the aid of special equipment, which usually indicates the proper location for wheel weights to restore the proper balance. Even with regular maintenance, however, uneven tire wear can result from drivers' habits, such as side wear from excessive cornering speeds. To counteract uneven wear that leads to unbalance, the tire industry recommends that tires should be rotated every six to eight thousand miles.

Wheelbase Balance

Another important factor when locating axles and spindles for wheel balance is the wheelbase measurement. Wheelbase is the distance between the center of the front wheel and center of the rear wheel. This distance (left front to left rear, right front to right rear with wheels in the straight ahead position) must be exactly the same on each side for proper weight balance. Correct wheelbase contributes to the ability of the car's rear wheel to follow directly in line with the front wheels (called "track"). Shorter wheelbases allow sharper turning radiuses, and longer wheelbases give a smoother ride and increased stability.

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