Chassis - Caster on two-wheelers
The many explanations of caster show how easily less precise follow up can lead to incorrect conclusions. What is the usual argument? Of course, the steering axle of the front wheel hits the road somewhere and the
distance from this point to the center of the tire contact surface is the caster. Let's leave it at that for now.
| The point where the swivel axis or steering axis penetrates the ground is also
called the track point. |
And thee it seems logical that the greater this distance, the higher the restoring forces. Is that really the case? To understand, you sometimes have to exaggerate. We do this here with the imprint of the famous Easy Rider
motorcycle from the 1969 film of the same name. First of all, you can see that the steering axle has nothing to do with the two spring-damper units of the telescopic fork, but is strictly based on the bolt for the handlebars.
But you probably already knew that. And you probably also knew that the caster has to be picked-up between the two vertical lines. Now, turn the handlebars 90°, which was probably not possible on the original machine.
Now you can see from the two horizontal lines how much the steering head and with it the entire front part of the motorcycle is lowered.
However, the tire width has been ignored in the image at the top. Here you can see what an important role it plays. You have to subtract this measure between the two horizontal lines from the lowering determined above.
However, if the front tires had the same dimensions as the rear, the difference at the top would be significantly smaller. You can see how many factors it depends on.
Now you could of course argue that this is a very rare motorcycle and that even today's choppers don't have such an extreme steering angle. You're right. But take a look at the front of the bike below. Would you consider it to
be somewhat representative? Then you can also see the lowering at 90° steering angle.
You can measure again this on almost any bike. A normal folding ruler will give you already significant values. Whenever you turn 90°, the front structure sinks. Why do we insist on this? Because this contradicts the
argument that the longer the caster, the greater the restoring forces for the steering.
And then the caster is also shown as proportional to the steering head angle. You can apparently specify the caster in degrees (steering head angle) or as the distance described above. No, you can't. In the pictures above
and below, the steering head angle is the same, but the caster as a length measure is different.
However, this would mean that weight on the front steering head would have to cause the handlebars to steer either to one side or the other. Riding hands-free would therefore be impossible. We must therefore add that the
lowering of the steering head or stem is extremely slight for normal steering angles, so it only becomes noticeable when the steering angle is strong.
It is best to add the caster offset to the steering head angle. This is clearly visible in the picture below. It can be found, for example, on a somewhat heavier, mobile hydraulic car jack, where the rollers are always aligned in
such a way that they follow the tracking point when pulled.
What is the effect of the caster offset, which increases always from top to bottom in the three pictures of the front end of the bike? If you again pay attention to the point of contact of the front wheel with the ground, the lowering
of the front end at a 90° steering angle becomes greater as the offset increases.
You can learn how important the caster offset is when you ride a two-wheeler with a particularly large wheel. It doesn't even have to be a circus bike where you can turn the steering while driving. No, the steering head angle
that is maintained with too much offset makes normal driving almost impossible. You simply can't find the middle anymore. Passers-by think you're drunk.
| With this bike you won't be able to get a single meter without using your hands . . . |
If one were to formulate a regularity, the caster offset and thus the caster would have to become smaller as the steering head angle increases. It prevents one from entering the area of greater lowering even during normal
steering maneuvers. What we have found here is exactly the opposite of most publications on caster, not just for two-wheelers.
Now you know why in the data of a motorcycle, for example, the steering head angle is always given in degrees and the caster in millimeters. But low caster is not the only factor responsible for steadfast straight-line travel.
Added to this is the driving speed and thus the gyroscopic forces of the front wheel, as well as the contact area of the tire, which follows the tracking point when the front wheel is not driving (tire caster).
And if you still doubt the above representation, here are official data from BMW:
| Model | Steering head angle | Caster |
| BMW R18 | 57,3° | 150 mm |
| BMW R nine T | 63,4° | 105 mm |
| The nonsense increases if one distinguishes between positive and negative caster. The latter does
not exist in practice. |
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