By now, we have a basic understanding of how to maintain our car tires in a good condition. Let us now look at some technical details.
This page is devoted to understanding steering geometry and the role it plays in uniform tire thread wear, an extended tire life span, comfort and safety in the handling of the vehicle. It is vital for all vehicle owners to understand the setup of the 4 important front wheel steering angles, why they are there and why it is important to initiate checks during a scheduled wheel alignment exercise.
- Camber-negative/positive-looking at the vehicle from a front view, it is an angle of tilt of the wheel relative to the vertical of the vehicle, negative or positive. As in the illustration, the top of the tire is tilting away from the center line, an example of positive camber. This is a steering geometry design feature employed on most street vehicles to give stability when the vehicle is moving in a straight line. Alternatively, if the top of the tire is tilting inwards from the center line,it is referred to as having negative camber. Negative camber is usually employed on performance vehicles for the purpose of better cornering ability during high speeds. Too much of positive or negative camber is not desirable as it is dangerous for vehicle control causing uneven tire wear.
- Toe-in/out– looking at the front tires from a top view, it is an angle that determines how much of the leading edge of the front tires are turned in /out from a straight ahead position. Most street vehicles are designed with toe-in while race or track vehicles are designed with toe-out. The purpose of toe-in together with positive camber is to ensure that all the four wheels of the vehicle roll with true rolling motion even when suspension movements are encountered. It gives the vehicle a good cornering entry ability and it promotes uniform tire wear. A combination of negative camber and toe-out promotes enhanced cornering ability usually employed on race/rally vehicles where uniform tire wear is not an important criteria.
- Caster-negative/positive-a side view of the vehicle front wheels reveal that the upper suspension pivot point is tilted backward as in the case of positive caster and it is tilted forward in the case of negative caster. Positive caster is usually used on street vehicles which require a balanced steering effort basically for good direction control, high speed stability and cornering effectiveness. However, with positive caster setup, the effort required to turn the steering wheel is higher as compared to effort needed for a negative caster setup. This is where the role of power steering comes into play. Negative caster setup is usually employed on track /race vehicles which requires less steering effort for going around corners with considerably less straight line stability resulting in high speed drifting.
- King Pin Inclination(KPI)/Steering Axis Inclination(SAI)-this is an angle created by drawing a line through the upper and lower suspension pivot points of the steering axis in relation to a vertical line as viewed from the front of the vehicle. KPI creates a scrub radius on the road surface which in turn causes the tire to turn away from the center position. To compensate this effect, toe-in is applied, together, straight line stability is obtained.This steering angle is necessary for lateral weight stabilization to obtain straight line and cornering stability. Incorrect angles will lead to instability and difficulty in keeping the vehicle in a straight line. Assuming there is difference in the angles between left and right , the driver will experience the vehicle pulling to one side.
After much research and development, manufacturers have built in these steering geometry settings which are most suitable for the vehicle. As owners, it is our responsibility to ensure that such checks are carried out according to the service schedule or whenever a tire replacement is necessary. Ensure that specifications are followed for best results. Your tire retail outlet with a good alignment machine will be able to check and advise the accuracy of these settings.
In present day vehicles, due to the complexity of the setting of these steering angles, manufacturers have built-in or set these angles at the factory and no adjustment is possible for KPI/SAI and caster angle. Only toe-in/out and camber angle settings are adjustable. The toe-in/out and camber angle are often upset during normal operation especially so if the vehicle is operating on rough and uneven road conditions, example, roads with pot holes.Periodic inspection and adjustments are necessary to ensure proper wheel alignment to obtain uniform tire wear.
KPI/SAI and caster angle usually do not change drastically unless due to wear and tear of pivot mounting parts or the vehicle is involved in a major accident causing body and chassis misalignment. This then involves major front end collision repair with replacement of sub-chassis frame.
When buying a second hand vehicle, it is so important for us to ensure that the vehicle was not involved in any major road accidents. Shoddy repairs are sometimes carried out and vehicles are sold to unassuming customers who will then end up carrying the burden of endless wheel alignment problems and premature tire wear. If possible, a complete check and verification of these important angles by a competent tire outlet must be done before we even commit on buying the vehicle in question.