While modern suspension systems work extremely well, they depend on complicated geometry to keep the wheels aligned, and the slightest misalignment results in poor directional control, rapid tire wear, and a host of other problems. It is no longer enough to have the wheels aligned only when you replace the tires: modern cars demand regular alignment checks to ensure long tire life, proper handling, and good fuel economy. But what is wheel alignment exactly and how is it performed? Read on and we will explain the issues.
What is Wheel Alignment?
If cars did not have suspension systems, the ideal set-up would be to have the wheels mounted perfectly perpendicular to the road surface and pointing straight ahead. However, cars do have suspension systems, and they consist of lots of components that all move and pivot in relation to each other, which change the position of each wheel in relation to the longitudinal axis of the car, as well as to each other.
Add to this the fact that almost all moving suspension components are mounted on flexible rubber mountings that wear out over time, which greatly affects the amount of wheel movement during both straight line tracking and cornering. Two more things to consider are the following:
- Rolling resistance of a tyre tends to push the wheel towards the rear of the car
- Torque on front wheel drive cars tends to pull the wheels forward.
Both of these motions exert large forces on the rubber bushings in the suspension, which deform to accommodate, or absorb these forces, but this results in the driving wheels being forced towards each other, which in turn, changes their positions in relation to the longitudinal axis of the car. Think of it this way: draw an imaginary line through each wheel along the length of the car, and the lines would extend forward to meet at some point in front of the car.
Now imagine applying driving torque to the front wheels, and being pulled forward, the two lines would move toward each other, since the deformation of the rubber bushings in the suspension are allowing the wheels to change their positions relative to each other. However, since the car is now moving, the rolling resistance pushes the wheels backward, which has the effect of separating the two imaginary lines, but only up to a point, since the force exerted by torque is larger than the rearward force caused by rolling resistance.
The Wheel Alignment Problem
The problem car designers have is to have the most tire contact with the road, with the least amount of rolling resistance, and to achieve this, they design cars in such a way that when they are at rest, the front wheels are either pointing away from each other, or in the case of rear wheel drive cars, pointing slightly towards each other. This is known as either “toe-in”, or, “toe-out”, which is cancelled out by the various forces at play when the car is moving under its own power. In cars on which the alignment is within specification, they tires will be almost perpendicular to the road surface, and pointing straight ahead, with the two imaginary lines extending nearly parallel to point very far in front of the car.
The same applies to the rear wheels; while they are subjected to different forces, the result is the same – the lines extending through each wheel will closely match those extending through the front wheels. On such a car, the wheels can be said to be aligned, and wear will occur evenly on the tires of each axle.
The Effects Of Miss-Alignment
Wheels can be miss-aligned on one axle, but also between axles, which usually happens when the rear axle is moved out of position because of accident damage or severe impacts with potholes and other obstacles. When this happens, the rear of the car will be pushed out of the track set by the front wheels, and while it may sometimes be possible to adjust the rear wheel alignment to compensate for this, in most cases the problem can only be repaired by replacement of the axle, or by extensive repairs by a competent accident repair specialist.
Other common effects of miss-aligned wheels include:
- Rapid, or uneven tire wear:
This happens because either the inside, or outside edges of the tire tread is forced into heavier contact with the road surface than other parts.
- Poor directional control:
Steering precision depends on the wheels being aligned in a certain way, and if this balance is upset, one, or both wheels will exert continuous steering forces on the car, which the other wheel(s) is forced to absorb, which in turn, causes rapid and uneven tire wear on both ties.
- Increased fuel consumption:
The rolling resistance of the wheels is just one force that needs to be overcome to keep a car moving, and with miss-aligned wheels, this resistance is increased in direct proportion to the amount of miss-alignment. Instead of the tire merely rolling along with the car, miss-alignment causes the tire to “rub” on the road surface, which can add as much a 10% to the amount of fuel used over a given time and period. Needless to say, tire “rubbing” can also reduce a tire’s life by up to 50%, and even more in extreme cases.
- Increased wear on components:
Miss-aligned wheels causes added strain on suspension parts such as ball joints, tie-rod ends, steering racks, control arm bushings, and others, since they are forced to absorb the increased forces involved in holding the wheels in place. With correctly aligned wheels, most of the forces involved in keeping the wheels aligned cancel each other out to some degree, which is one of the reasons these components last as long as they do.
There are of course other settings that influence wheel alignment, such as caster and camber, which determine the amount by which the pivot point of the steering moment deviates from the center of the wheel, and the amount by which the wheel deviates from the perpendicular relative to the road surface respectively.
Both of these settings are critical for correct wheel alignment, and serious deviations also lead to increased fuel consumption, rapid and uneven tire wear, added strain to suspension and steering components, and decreased vehicle control, which is why it is vitally important to have the wheel alignment checked, and corrected at least twice a year.
Doing this will not only save you fuel and extend the life of your tires- it will also give you a comfortable ride, and significantly reduce overall maintenance costs.