Basic Principals of Wheel Geometry

Wheel geometry is the topic on wheel position and their behavior in relation to the activity of the vehicle and it's movement.  This means that the wheels need to move in a certain way as the wheels move, and therefore should be in an appropriate position when stationary.  If any of these are incorrect or out of line, the vehicle will not handle and operate at it's full potential, and will also 'scrub' the tires out.  There are three main measurements in accordance to wheel alignments:

1. Caster
2. Camber
3. Toe

There is also the trapezium angle of the front wheels as the vehicle turns.  These wheels have to be paralell to the 'turning circle' of the vehicle.  This means that when a vehicle is turning, the inner wheel must be in a greater angle than the outer wheel.  This is because in the turning circle, there is actually a circle within the circle, the outer circle representing the outer wheel, and the inner circle representing the inner wheel.  Here is a photo I took of a speaker, it has an inner and outer speaker.  We can see that the inner circle is much sharper.

Here is a drawing of the basic principal of the 'turning circle - trapezium'.

The S.A.I.  (steering axle inclination) is the center of pivot of the turning wheel.  The S.A.I. will almost always be the point of the center of the strut.  This means that the strut is the center that turns around when the wheels are angling.  The further the wheel is to the S.A.I., the less stable it is.  This means that when people fit wheel spacers to their vehicles without using a wider wheel, it becomes less stable.  The reason is because the further it is,  The more distance the wheel actually travels from left to right.

Now back to the three adjustable components for wheel alignment - starting with CASTOR.  The reason why people can ride a bike straight with no hands on the handles is because the front fork of the bike is on an angle.  The greater this angle is from a vertical line,  the more castor effect it has on handling.  Basically if you were to mow a lawn with a lawn mower,  The handles are far away from the actual lawn mower.  If the handles were right above the lawn mower, it would be very easy to turn, but very hard to go straight.  Another principal of the castor effect is that the wheels will actually 'follow' the direction of the movement naturally.  Here is a picture of a robot I made when I was younger,  It has a castor wheel on the rear and 'follows' the direction of the drive wheels, whether one is powering more than the other.  (eg. right wheel powering more will turn the robot to the left).  The castor wheel is only there to stabilize the robot. 

Note: this is actually the wrong way,  the axle is supposed to be behind the S.A.I. , represented by the red line.

Notice the angle of this component in relation to a 90 degree angle.  This is the castor effect which allows the wheel to 'follow' the direction of the robot.

If the wheel was further away from the SAI (red line), then the wheel would sway more from left to right during changing turning.

Here is another example of the castor effect.  This is a castor wheel which is found mostly on office chairs and furniture.  These wheels have a pivot point which is in front of the wheel, which means they will naturally turn which ever way I turn my office chair or piece of furniture.

CAMBER is an important alignment measurement - It is depended by how much the tires 'scrub', which means how evenly they contact the road.  If uneven contact, one side of the tire will wear out faster than the other.  This is also unsafe as it has minimal contact to the road surface.  Camber can be adjusted in many ways depending on the design of the vehicle, but in most cases will involve moving the lower control arm in and out.   Here are a few pictures of custom cambering:

Here is a tyre after 30 minutes of driving from brand new.   The inner side (left) is far more worn than the outer side.

 Here is the vehicle itself.  This is sitting at 1 degree negative.  Although it does sound like a lot, especially to wear out the tyre as much as it did, it is also lowered on after market springs, which are a lot harder than stock springs.  This means that the tyres are not pushed down as much as the stock springs, and the weight distribution is not behaving as well.  The tires are also stretched, meaning a smaller size has been fitted to a much wider size rim.  This means the tire must be inflated to a much higher than appropriate setting - around 10-15psi more than manufacturer's recommendation.

This I actually cannot explain - I took this photo at a drift racing event and this car had just come to the pits (notice the melted tires).  The outer is actually worn much more than the inner.  Drifters use very custom wheel alignment settings to their driving style.

Here is a project car - a photo also taken at the drift event.  This vehicle is being built for drift racing and has maximum cambering.  That is a huge negative camber setting.

TOEING another part of wheel geometry. (It is adjustable in all cars, because it is dependent on the tire rods)  It is how straight forward the wheels point when the vehicle is stationary and steered straight.  If this is out of line,  it also affects the steering trapezium.  If there is too much toe-out,  the outer wheel will have less angle, and the inner wheel will have more angle.  Each individual wheel can have it's own toe setting when it's out of line, not necessarily both wheels have to be pointing in or out to be toeing.