Vibrations in the motorcycle

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Vibrations in the motorcycle

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Vibrations in the motorcycle

Vibrations in the motorcycle
Vibrations in the motorcycle

Commuting, flapping, banging the handlebars, shimmy effect, wobble, weave, kick-back – all technical terms of “Insiders”, the outsiders do not reveal anything.

02/25/2003

Commuting, fluttering, banging the handlebars
Shimmy effect, wobble, weave, kick-back – all technical terms of “Insiders”, The outsiders do not want to reveal that their motorcycle is flapping, swinging or having knocked handlebars, or, to put it more simply, that their box is shaking somehow.

Flutter:
This is a relatively harmless form of swinging flutter, also Shimmy effect called: At a speed between 50 and 80 km / h, the wheel speed coincides with the natural frequency of the steering system, and the front wheel, fork and handlebars swing around the steering axis at around ten deflections per second. Harmless because fluttering can only occur when driving straight ahead or at a slight angle and only when the driver takes at least one hand off the handlebars. The fluttering can be stopped at any time by firmly gripping the handlebars. In addition, radial tires and the stiff chassis of modern machines have significantly reduced the tendency to flutter, so that in most cases slight vibrations only set in after a blow on the handlebar, after a bump or after a longitudinal groove. So that a normally good-natured motorcycle does not turn into a flutter, there are a few things that should be considered. For example, an imbalance in the front wheel, incorrect tire pressure or incorrect load can cause unusually strong fluttering.

In order to get both types of vibration under control at the same time, while still guaranteeing good machine handling, the chassis builders have to make compromises. A steep steering head makes handling easier, but worsens the flutter and pendulum behavior. A short after-run reduces the flutter deflection and is positive for handling, but bad for commuting; a short wheelbase improves handling, but the motorcycle commutes earlier. Only a stiff chassis has a consistently positive effect, but this makes the machine susceptible to the third mode of vibration: handlebar slap.


Vibrations in the motorcycle

Commute:
Weave or just Commute is caused by the actually stabilizing gyroscopic forces of the wheels. Above a certain speed, these gyroscopic forces become so high that the slightest deflection caused by a gust of wind, bump in the ground or a longitudinal groove causes the machine to constantly tilt and straighten up again. The steering system with fork and front wheel and the rear part of the motorcycle with frame, motor and rear wheel swing out of phase around the steering axis. At the same time, the machine tilts around the vehicle’s longitudinal and vertical axis. This complex interplay repeats itself between three and four times per second and can only be interrupted by releasing the accelerator; on the other hand, the pendulum swings increase with increasing speed. Extensive pendulum amplitudes can also be caused by an incorrectly set steering damper, by a tense steering head bearing or by cramped gripping on the handlebars, because the movements between the steering system and the rest of the motorcycle are disturbed.
The motorcycle manufacturers try to set the critical speed at which continuous commuting occurs above the respective maximum speed, but certain circumstances can lead to increased commuting even with modern machines (see table). An edgy rear tire that is driven on long stretches of the motorway can be particularly treacherous, as the motorcycle still runs steadily in a straight line at top speed and only suddenly and violently begins to swing when turning into a fast bend – in contrast to fluttering, swinging can also occur in an inclined position.


Vibrations in the motorcycle

Handlebar slap:
Experts speak of Kick-back. It is not a natural oscillation of the motorcycle system like fluttering or swinging and can therefore occur at any speed. The fork, including the front wheel and handlebars, flaps around the steering axis. This effect is triggered when the front tire lifts off and then touches down again at an angle, as happens, for example, when accelerating sharply out of curves or when driving over a bump. Depending on the speed and the touchdown angle deviating from the direction of travel, the front tire suddenly builds up cornering forces, which are noticeable in the handlebars as a light sugar or a powerful blow. The chassis usually calms down very quickly.
There is only an acute risk of falling if several bumps appear one after the other with the same distance and are driven over at a speed at which the natural frequency of the front, unsprung masses coincides with the excitation frequency caused by the bumps. Then the handlebar jumps from steering lock to steering lock with up to 14 deflections per second, and the forces are so high that a firm grip on the handlebars can make the to and fro, but not prevent it.
In order to calm motorcycles prone to kickback, you should stow as much weight as possible in the tank bag to increase the front wheel load, and distribute the rest of the luggage evenly on the left and right in the suitcases so that a slip angle does not build up on the front tire when driving straight ahead. A well-functioning steering damper also reduces handlebar slap.

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