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Caster and What Does It Mean

Caster and what does it mean where the SuperPro catalogue mentions notes like the following ;

  • 2 x Offset bushing for caster adjust
  • 1 x Standard bushing & 1 x Offset bushing kit
  • Standard replacement kit

Figure 1

I have studied this angle and its effects on driving and handling in a vehicle over the years and a quick search on Wikipedia gives us a definition and the words from the definition - car racers sometimes adjust caster angle to optimize handling characteristics in particular driving situations. I think this pretty well sums up my reason for an interest in this angle.

Caster angle is the angular displacement from the vertical axis of the suspension of a steered wheel in a car, bicycle or other vehicle, measured in the longitudinal direction. It is the angle between the pivot line (in a car - an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical. Car racers sometimes adjust caster angle to optimize their car's handling characteristics in particular driving situations.

In Figure 1, is the caster angle, red line is the pivot line, grey area is the tire. A positive caster angle is shown with the front of the vehicle being to the left. (Wikipedia)

SuperPro origins were born out of vehicles that were not a pleasure to drive due to caster differences. These were commonly known as a pull to the left cars. Manufacturers tend to build in preset alignment specifications and in many vehicles caster adjustment was simply not offered on an original vehicle suspension system.

When wheel-aligning a car, I always went to great lengths to correct the caster on a vehicle where possible, however this was not possible that often. As a young wheel alignment mechanic, I knew motor sport offered all sorts of suspension angle adjustment so why not offer some to my customers. So, in 1987 the first SuperPro bushing were born, tested and sucessfully starting the large range we now have available.

What does an incorrect caster angle mean;

To all motorists;

  • Unpleasant handling, vehicle veers to one side of the road unless continually corrected by the driver.
  • Tyre wear, is dramatically increased due to steering correction, means extra costs.
  • In correct angles can give heavy or light steering.

To enthusiast motorists;

  • optimizing this angle gives improved handling and tyre grip.

To the wheel alignment technician;

  • an angle that can be adjusted to assist many handling options however the most obvious is;
  • to create a caster offset to negate the steering pull caused by the camber of the road.

The SuperPro bushings offer the technician three options for wheel aligning.

Standard replacement kit;   

This kit contains two replacement SuperPro bushings, designed with a minimum of noise vibration harshness. SuperPro design innovation helps to get a part with a long service life while delivering improved and positive handling and control.

1 x Standard bushing & 1 x Offset bushing kit;

A kit with one replacement bushing and one eccentric bushing, both with SuperPro design innovation. This allows the alignment technician to increase caster on one side of a vehicle and keep the other side standard.

2 x Offset bushing kit for caster adjust; 

This kit offers the alignment technician extra adjustment to change caster on both sides of the vehicle. Or, allowing the enthusiast driver to have an increase in caster on both sides of the vehicle for better performance on the track or for club motorsport work.

Today I delivered an X6 BMW to its owner, normal problem on arrival to our workshop, pulls left and having uneven tyre wear. A SuperPro caster correction kit was fitted and the car was in my different car to drive zone. The steering had feel, the vehicle tracked straight and I know this customer will get a long tyre service life.