 Chassis - Motorcycle 1
The chassis of single- and two-track vehicles differ fundamentally. This becomes already clear when you measure the wheelbase. For example, while it is relatively fixed for four-wheelers, for two-wheelers it depends very
much on the load, as can be clearly seen in the picture above. Do you see the wheelbase shrinking even when it is occupied by just one person?
Although a two-wheeler has a significantly lower payload, it is still a lot compared to its own weight. Already if this is fully utilized, you have to assume that the chassis conditions are different. This situation is taken into
account in its design.
The main cause of the enormous changes in the wheelbase in the picture above is the steering axis. This is because it is created at the distance or along the center line of the suspension/damping, while the swing arm is
responsible for the change of the rear wheel forwards or backwards. The inclination of the front steering axis to the horizontal or vertical is called the 'steering head angle'.
Apart from the wheelbase, hardly any other value within the chassis of a two-wheeler has as much influence on the driving characteristics as the steering head angle. This also simplifies the physical relationships in
contrast to a four-wheeler, which has to deal with the roll, pitch and yaw axle. Here, too, caster occurs when the steering axis hits the ground before the middle of the contact area.
There is no such thing as rolling, and yawing is limited. Only pitching is an important issue, because it is very pronounced on many two-wheelers. But let's stay with the steering head angle. The larger it is, the more the
two-wheeler is inclined to continue driving straight ahead. Riding hands-free, for example, would not be possible without this angle or a certain amount of caster.
A longer wheelbase does the rest. Both are linked to a larger turning circle and an increase in the required steering angle. But not only is the initiation of a turn completely different on a two-wheeler, namely through a short
steering angle to the outside, but many more aggregates also play a role in the driving behavior.
When mounting the engine, attention is usually only paid to dampening vibrations if they cause particular problems. The possible stiffening of the frame usually takes priority here. The effort required to design a steerable
front wheel is also greater, classical for example with two connected, pivoting fork bridges and also two so-called stanchions.
At the rear, it is easier with one or two-armed swing arms. Here, the wheel guidance and suspension/damping are separated from each other. There are two or one of them at the rear, but almost always a combination of
them. With the one-armed swing arm below, of course only one central spring/damping element makes sense.
The term 'telescope' emphasizes the ability to move within the standpipes. If you look at these in relation to the spring-loaded sliding tubes, it quickly becomes clear that the forces required vary depending on the driving
situation. The more the (transverse) load affects each of the two tubes, the more difficult it is to move them against each other.
With telescopic forks, this can be felt when driving, as the suspension hardens or there is suddenly more damping. The components required for this are well protected and integrated into the tubes. The stanchion or
sliding tube often runs parallel to the steering axis, so that it can also be gripped here.
The compression behavior can be observed well in vehicles for rallies in difficult terrain. Here the usable stroke of such telescopic forks is particularly large. It's a good thing that things are quite rough here, because
otherwise you would clearly feel the increased friction between the two tubes, especially when the sliding tube is extended far.
It is therefore important to pay attention to the response of the suspension and damping of telescopic axles. A lower unsprung mass of the sliding tube can help a little, while the standpipe is made of steel to be more
resilient. A larger distance between the two sliding bushings between the standpipe and sliding tube also helps to prevent excessive wear under load. The axle for the front wheel is often mounted in front of the telescopic
tubes and these are extended downwards.
A standpipe on the outside and a sliding tube on the inside are better suited to absorbing the forces described. But these, called 'upside-down' forks, are also more complex. The brake and the mudguard are always
attached to the unsprung part of the telescopic fork. If, as with telescopic forks, there is suspension/damping on the left and right, they must of course be manufactured exactly the same.
Here you can see how there can only be one such element at the front. The picture above shows the Telelever system from BMW. The steering axis goes through the ball joints of the telescopic fork at the top and a
triangular longitudinal control arm in the middle at the bottom. This triangular control arm is acted upon by a central, relatively short spring strut, which is mounted pivotable on the top of the motorcycle frame.
There are no longer any dampers or springs in the actual telescopic fork, so there is a lot of travel and distance between the sliding pieces. Because the compression no longer only occurs by pushing the telescopic tubes
together, but also by the evasive movement of the upper ball joint attached to the frame.
The steering head angle measured to the horizontal becomes smaller, the steering axis flatter, the caster when braking increases, which provides more stability. In addition, the effect of ABS is accompanied by less
pitching vibrations. Ultimately, the entire construction is heavier, but the unsprung masses are smaller.
The hardening of the suspension and damping caused by the rather unpredictable bending of the tubes due to increased friction when diving is now eliminated. Without these additional effects, for example, the
suspension can be made to respond more harshly. The heavily dimensioned, relatively low-positioned longitudinal control arm absorbs road shocks better and ensures greater stability.
The Duolever system has taken things up a notch. It really reminds you of the front suspension of a larger car, with two wishbones, only arranged lengthways rather than crossways. The only difference is that the tie rods
have a lot more space there than here, which changes their appearance considerably. Because they also have to overcome the spring deflection in this tight space.
As with a four-wheeler, the upper wishbone is shorter than the lower one, which in turn determines the steering head angle. What has to be turned when steering now has less mass. Otherwise, the advantages and
disadvantages of Telelever remain: stability due to the lower longitudinal arm, less dive when braking, complex construction.
What a telescopic fork cannot do, or can only do to a limited extent, is to temporarily harden the suspension when braking in order to reduce the diving effect. In cars, this is also called the 'anti-dive' mechanism.
This was also the case here, for example, in heavily motorized S-Classes, to mitigate a diving effect on the rear axle during powerful acceleration.
The picture shows an additional swing arm, which in this case is called a pushed swing arm. If you now imagine a brake caliper attached directly to this swing arm, then when braking, it is pulled along by the brake disc in
the direction of rotation of the wheel. This causes the swing arm to rotate in such a way that it at least partially compensates for the compression at the front.
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