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Scrub Radius and Alignments

Custom wheels and oversized tires can look good and give a vehicle a unique appearance. But it’s what you can’t see that can become really ugly, especially if the alignment and suspension angles have become altered in the installation process.

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Custom wheels and oversized tires can look good and give a vehicle a unique appearance. But it’s what you can’t see that can become really ugly, especially if the alignment and suspension angles have become altered in the installation process.

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If you come across this problem, it is likely that the offset of the new wheel has moved to accommodate the increase in rim width. The change in the offset moves the mounting flange of the wheel and changes the point where the flange centerline and steering axis inclination meet the ground. A change in offset can alter the loading on the wheel bearings and can lead to a failure. This creates a scrub radius that will affect tire wear and can cause a change in stability.

If there is a change in ride height front to rear on a passenger car, it can affect caster and steering. An increase in wheel and tire diameter can also affect braking in all vehicles.

The steering stabilizer or damper for a truck or SUV can dampen a shimmy or cause a return-to-center condition. A steering damper for a passenger car is usually associated with manual steering on a vehicle with rack-and-pinion steering, and on the power steering linkage of light trucks and SUVs. A steering damper is a shock absorber with a 50/50 valve that applies the same hydraulic resistance in both directions.

The steering damper has to operate in a horizontal position. This means that there has to be a way to prevent aeration and foaming of the fluid in the reservoir where it compensates for fluid displacement as the shaft moves in and out of the damper.

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Short Long Arm

An independent front- or rear-wheel suspension is either a strut suspension or a short long arm (SLA) suspension. The manner in which the components are mounted to the chassis configures the geometry of the suspension. In a strut suspension, the strut takes the place of the upper control arm, shock absorber and steering knuckle. The SLA suspension is made up of a steering knuckle and upper and lower control arm.

When the shock absorber is mounted to the lower control arm and chassis, changes in wheel and tire size and chassis modifications can alter a vehicle’s handling and braking characteristics. Here is a list of conditions that may result: harsh ride; wander; shimmy; steering wheel not returning to center; excessive lean, etc.

A steering stabilizer may dampen some of the conditions, but it won’t correct them. Power steering is both an assist and a damper, and most vehicles have power steering. The rate at which the power steering lines meter the flow of fluid to the piston is the same as if a manual steering damper would meter fluid through its piston. Modifications to the chassis can affect the built-in dampening of the power steering, such that a change in caster can make a power steering system overly sensitive.

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Suspension geometry is what makes vehicle steering controllable. Track, caster, camber and toe are the four factors that control the stability of a vehicle both in a straight line and around corners. Track is the alignment of the wheels to the centerline of the vehicle. The wheels for each axle are the same distance from the centerline and are parallel to it.

Caster is the angle generated by a line through the center of the upper and lower ball joints and a vertical line through the center of the upper ball joint. Caster is positive when the angle is toward the front of the vehicle and negative when it is toward the rear.

Positive caster causes the steering to center when traveling in a straight line and to return to straight after turning a corner. Camber is an angle generated by a line through the center of the wheel and vertical. Camber is positive when the top of the wheel is toward the outside of the vehicle and negative when it’s toward the inside.

Toe is the angle through the center of the wheel and a line parallel to the centerline. Toe-in is when the front of the wheel is angled inward to the center of the vehicle. Toe-out is when the wheel is angled outward and away from the center.

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When vehicles had straight axles, the steering knuckle was attached to the axle with a kingpin. The kingpin is set at an angle that would cause it to run through the center of the tire on the ground. This is called kingpin inclination. The purpose of the angle is to prevent the wheel from sliding sideways when the wheel is turned. Scrubbing is the term given to this sliding action.

Steering axis inclination (SAI) is the same as kingpin inclination. It is a line generated through the center of the upper and lower ball joint to the center of the tire on the ground. It is positive when the angle is inclined inward toward the center of the vehicle and negative when the angle is inclined outward. The distance between where the SLA and the centerline to the wheel intersect the ground is called the scrub radius. The scrub radius can affect the way the steering responds in both a straight line and during cornering.

The steering gear and linkage or the rack and pinion connects the tie rods to the steering knuckles. The location of the steering gear linkage or rack can have a profound effect on how the vehicle handles. The biggest effect is called bump steer. When a vehicle suspension travels over a bump, the movement of the suspension and tie rod is that of an arc. That movement can cause the wheel to turn in or out and disturb the direction of the vehicle – especially in a straight line. A lift kit should help ensure that the position of the tie rods do not change in relation to the suspension location. A change in position can cause a bump-steer condition.

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The best performing suspension and steering systems have a quality that is called neutral steer. Neutral steer can be defined as a system that tracks straight and returns straight with no over- or under-steer after turning. (This can occur if there is no bump steer and minimal chassis roll.) The steering will also have a controlled return to center when the wheel is released. This is where damping is used to slow the movement of the steering linkage to ensure that the wheel returns to center. Every OE system is engineered for safety and neutral steer.

So, how does this information apply to you?

The condition of the suspension and its alignment are key to the safe operation of the vehicle.

If you live in the Northeast or Midwest, the suspension is exposed to more severe conditions. The weather and road conditions can contribute to the deterioration of suspension components and vehicle alignment. When you are replacing brake pads, it is also a good time to take a look at the bushings, ball joints and tie rods. They could be part of the reason you are doing the brake job in the first place.

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