 Motorcycles
In the chapter 'caster and kingpin' the two-wheeler has already been mentioned. The yet defined sizes caster and kingpin result in: The greater, the straighter the running stability. In theory, this would be even
better at higher speeds, but also the opponents of stability would grow, for example, the influence of wind and the forces by bumps in the road.
Of course, stability when straight-running and willingly cornering exclude each other. You guessed already that these two characteristic values of a motorcycle must be adapted to the intended use off-road.
Their change e.g. case of the constructive modification of a travel machine into one for off-road, however, can be imagined as simpler than lowering of the center of gravity.
Its distance from the ground could even increase with enlargement of the spring travel. Speaking of spring travels, these of course also complicate an accurate measurement of the two values. So, you have to
determine fairly exactly the loading condition. At the low curb weight of a motorcycle, especially in relation to the payload, of course this has much more influence on the driving dynamics than e.g. in a car.
Let's turn to the steering, but first in an unusual way. If we consider the above mentioned influences such as crosswind, we have to look at them in ratio to the forces at the gyro. As such, act mainly the
wheels, with the more mass far away from the axle of rotation, the stronger.
Sometimes can be added also a transversely arranged crankshaft, but however, we ignore it here. A gyro, for example, the front wheel, has a considerable inertia. If you hold a rotating wheel left and right at the
axle and tries to move the axle to the left to the front or on the right to the back, you suddenly notice an unexpected tilting movement of the entire rotating wheel to the right.
In practice this means that steering a two-wheeler while driving is automatically connected with a tilting movement in the targeted direction. Amazingly, this also applies the other way round. If a two-wheeler
e.g., is hit by a lateral gust of wind, it tilts slightly to the right, but would perform a steering movement to the right without the help of the driver and thereby prevent overturning.
The gyroscopic forces have a stabilizing effect. Although there may be slight overreactions, which are perceived as swerve outwardly, but they are quickly reduced and driving straight ahead is guaranteed. If the
driver in motorcycle GP highly tear their arms during or after crossing the finish line, then the machine is stabilized not only by the steering geometry, but also by gyroscopic forces.
By the way, at the bicycle such gyroscopic forces are usually not possible, because it is simply too slow. Here, the steering geometry must ensure as already described for straight-line stability. Should be
here an error by a front wheel, forcibly moved to the rear, such a bicycle is not to direct. Artists use such wheels in circus.
Until now we have still not intervened actively in the steering. If we do this now, in the two-wheeler arises a peculiar phenomenon. Did you know that a two-wheeler actually can only bring in a curve by the
handlebars first moved against the direction of the targeted curve, i.e. at a right turn at first shortly to the left?
For a two-wheeler one moves never fully upright around the corner, as it is the case perhaps with a multilane vehicle. Thereto belongs a certain inclination and the can be reached just by the short steer angle in
the opposite direction. As not only with two-wheeled vehicles, many actions happen unconsciously when driving, but perhaps you can pay attention to it once.
Overall, a two-wheeler is in principle much less stable than, for example, a four- wheeler. By here is not meant the possibility of overturning, but rather, certain possible instabilities in the so important
connection between the front and rear wheel. Unusual example: a construction machine with two rigid axles, steerable only by a pivot point in the center. The steering axle is at the two-wheeler substantially
behind and not next to the front wheel as case of the four wheel.
Of course, the pivot point is still displaced relatively far forward, but separates after all the much larger mass at the rear of the at least not insignificant smaller mass of the entire front suspension. The frame
takes of possible double loops at the bottom up to beams under the seat bench a relatively large space in order to end then into the relatively small area of the steering head.
It is clear that here is a kind of weak spot, available for influences such as flutter at relatively low, vibrations at higher speeds and kickback of handlebars in cornering. There are efforts to get this problem under
control, for example, by a stiffer frame or snubbers.
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