This article is part of a section of the RcTek site devoted to radio controlled model car handling. As car handling is an extremely complex subject, it will be quite some time before it is finished.
This article explains about the effects that caster angles have on the handling on the wheels of your model car.
This article assumes you have either read the article on Caster Angle Basics, or are familiar with what is meant by these terms.
The pink arrow in the drawings below denotes the direction of travel of the model car.
Caster is basically used to add to or detract from the directional stability of your model car. How the caster is set depends on the the method of drive that the car has, front wheel drive cars usually have very different caster settings than a rear wheel drive car, for example.
The whole basis of caster can be better understood by studying the image of the furniture/shopping trolley caster to the right. As there is horizontal difference (A) between the centres of the pivot points (marked by the red dots) and the top is also free to swivel (rotate) around, the wheel will take the path of least resistance and trail behind the direction of travel (supermarket trolleys seem to be excepted from this rule of course!).
The trailing effect that is caused by the horizontal difference between the centres of the top and bottom pivot points is the most important aspect to consider because, as stated above, front and rear wheel drive cars are ordinarily configured with different caster angles. More on this below.
In the images below the green patch represents the contact patch area of the tyres.
Neutral caster has the upper and lower pivot points aligned vertically. The forces that bear down on the car and the wheel have only a single vertical point of contact which is at the mercy of any external forces that may act upon it. Any car set with neutral caster will be an unstable one.
Negative caster, which is sometimes referred to as leading caster, has the upper pivot points positioned in front of the lower pivot points. Because of the horizontal offset between these two pivots the forces that bear down on the car are transmitted with a forward bias. Depending on the driving configuration of the car, this either adds stability or makes it un-drivable.
Positive caster, which is sometimes referred to as trailing caster, has the upper pivot points positioned in behind of the lower pivot points. Because of the horizontal offset between these two pivots the forces that bear down on the car are transmitted with a rearward bias. Depending on the driving configuration of the car, this either adds stability or makes it un-drivable.
With negative and positive caster the horizontal offset between the pivot points of the wheels effectively gives two contact points, and the one with the greater force upon it creates a self centering effect that acts to keep the wheels pointing in the forward direction.
The reason the same caster setting can generate such a difference is that front wheel drive and rear wheel drive cars generate their driving forces in different ways. Although front wheel drive cars are not popular (and sometimes banned) in radio controlled model car racing, we still feel that there is a need for them to be included.
The red arrows in the drawings below represent the direction of the driving forces on the model car and the pink arrows denote the direction of travel of the model car.
The front wheels on a rear wheel drive car are propelled forward by the thrust created from the driving wheels (the rear wheels push the front ones). You will see why positive (trailing) caster adds stability, due to the pushing force, if you study the diagrams to either side.
Front wheel drive cars are not pushed by the rear wheels, they drag the rest of the car behind them. The driving torque provided by the front wheels mean that negative (leading) caster leads to a stabilising effect due to it pushing down on the front part of the tyre.
The majority of four wheel drive remote controlled cars do not use permanent four wheel drive, they use one-way bearings to disengage the drive to the front wheels. As such, they are effectively rear wheel drive and benefit from positive caster. They are not as dependant on it as full time rear wheel drive cars though, as the drive to the front wheels adds stability, as well as increased tractive power.
Increasing caster angles will only add stability to a certain point where the wheels will start to oscillate (shimmy). This is easily demonstrated by pushing a (good) shopping trolley as a reasonably fast speed. There are a couple of reasons why this happens;
The self centering effect of the wheels is not a free quirk of nature, it relies on an imbalance of forces being placed on the wheels. More caster angle generates more imbalance and there comes a time where this imbalance is too great.
Also, the greater the caster angle the greater the horizontal offset between the pivot points on the car. This will then elongate the contact patch and the loadings too much on the front or rear of the tyre and lead to sensitivity in the tyre to minor imperfections in the track surface.