Motorcycle Vibration from Character to Destruction

Vibration on a motorcycle can range from lending a bit of character to the plot, all the way to destroying components – along with any pleasure in using the two-wheeler.

So what about the vibration? In order to know what can be done about it, as well as what can't, let's look at vibration in detail.

We all tend to think of engines as being the primary source of vibration on a motorcycle – they usually are and we'll come to them, but any moving part can generate vibration, and these secondary sources are easiest to minimize. Take a wheel for example – as you'll see from a washing machine anything rotating out of balance will shake around. It's down to the simple laws of mechanics – anything moving in a straight line will keep doing so unless some force makes it do otherwise. All the wheel components are moving in a circle, not a straight line, so there must be radial forces acting on each bit to cause it to move this way. If the wheel is perfectly balanced, then all radial forces cancel out. If it isn't, then there's an unbalanced radial force that has to be reacted by the axle bearings – hence a vibration.

It's not just rotary motion that's important – take exhaust gas exiting an engine. If a slug of gas is accelerating out the back, a reacting force must be bearing on surfaces inside the engine and exhaust system. The other thing about vibration is that its effects on parts of the motorcycle (brackets, bulb filaments etc) and parts of the rider (particularly joints and circulation) depend on the frequency as well as the intensity of the cause. This phenomenon of resonance is related to the natural frequency at which the part would vibrate, and the amount of damping (usually from friction) present. The point about resonance is that it's essentially the storage of energy, and so even if the source is relatively mild in its vibration, the resonating part can build up a far more violent shake.
As far as purely rotating parts go, the road wheels are the only bits big enough to worry about actively balancing. Simple static balancing by attaching lead weights to the spoke or rim is enough on any motorcycle. Balance is achieved when the wheel has not preferred stopping point after being spun by hand.

Engines are entirely different and the conventional piston engine is a bit of a disaster in the balance stakes, especially in single-cylinder form. Apart from all the rapid pressure changes from combustion, the basic geometry of a crank, con-rod and piston induce a rich slew of vibrations that cannot be completely balanced out at source. Bob-weights on the crank can partly balance a piston, and also allow vibration in one direction to be traded for another direction, all by fiddling about with the so-called balance factor – the fraction of piston/top end of con-rod weight balanced by the bob-weights.

Balancer shafts are an interesting idea – bob-weights on a separate engine-speed shaft, timed to cancel out part of the piston vibration. These can make single or parallel twins as smooth mechanically as a four, but they can't cancel out the higher frequency tingles and you can't magic up the firing impulse smoothness of a multi. Let's not condemn the simple, single-cylinder though – all engineering design is a series of compromises, and it has virtues of compactness, simplicity and riding characteristics. When well-sorted, the vibes that do come from it are part of its defining character.

Something like a pitted ball-race will vibrate akin to riding on cobbles but simple slop also allows parts to run out of center and hence become effectively unbalanced. It's then a vicious circle, in that these very forces will further accelerate wear.

There is, however, a silver lining in that paying attention to vibration – and especially changes in it – can often reveal the onset of a problem in time to do something about it. On parts such as helicopter gearboxes where failure would have obvious catastrophic consequences, electronic vibration sensors signature constantly analyzed to warn of trouble. We don't go that far on motorcycles (for now), but it's amazing how your own senses can be so acute.

While a basic single-cylinder engine can't be directly balanced, multi-cylinder configurations deal with it. Sadly, the most popular multi-cylinder – the vertical twin four-stroke – doesn't really, since it's essentially two single-cylinders sharing a common crankpin, though it does have the benefit of more frequent power impulses. Thus making it smoother. Flat twins are good, V-twins better than you might think and 120 degree triples have a magic relationship with mathematics that allows them to cancel out all of the basic piston vibrations. In many ways, they do this better than four-cylinders – but all twins, fours and triples still have some unbalanced forces at higher frequencies than the basic crank speed. These are significantly lower than the basic thump of a single-cylinder or vertical twin, but can still be unpleasant at certain revs.
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