Giving me good vibrations

By Simon Hargreaves - 15/07/2015

Bikes make lots of vibrations, some good and some bad. Here's why...

Anyone with fingers knows bikes vibrate, sometimes strongly enough to make picking your nose impossible. But how we perceive vibration depends on its type, pattern, frequency and cause.

Bikes are subject to different types of vibration. A bumpy road creates random forced vibration at medium to high frequency (around 5-35 Hz depending on vehicle speed). It's perceived as unpleasant, as is head buffeting caused by wind turbulence. Engineers and aerodynamicists try to minimise these bad vibrations.

But we usually talk about engine vibration. There are many sources: apart from the reciprocating mass of the piston and the crank's rotational mass, there's also combustion vibration, various rocking couples, vibration generated by cams and other mechanical friction, by changing gas pressure, and the component drag of oil.

But the vibes that matter come from whirling lumps of metal. There are good and bad vibrations, and their magnitude and type is related to engine design and configuration.

Broadly, there are two 'types' of engine vibration caused by reciprocating masses: primary and secondary. Primary vibration occurs at engine speed, secondary at twice engine speed. Primary vibration is caused by a piston assembly accelerating up and down on a crank, creating an inertia force equal to its mass times its acceleration. With no opposing force, it's unbalanced. To balance it and reduce vibration, singles (or 360° parallel twins, with pistons moving as one) use a balance shaft or shafts, or a vestigial piston, or a weighted, crank-mounted disc. Each counter-balances by providing opposition to the inertial force.

In many multicylinder engines, primary balance is achieved by using pistons reciprocating alternately. So 90° V-twins, 180° parallel twins, flat twins, 120° inline triples, 90° V4s and inline fours and sixes have primary balance. But anyone who's ridden any of them will know none are vibe-free.

This is either or partly because of uneven firing intervals (e.g. a 90° V-twin) or because balancing a reciprocating piston with an alternate piston along a crank generates a rocking couple (flat and parallel twins, triples, V4s). It's these two types of low-frequency vibration we generally like; they're character (a BMW boxer twin, with its rocking from side to side, the off-beat thump of a Ducati). Although they too can be countered; Triumph and Yamaha triples are balanced but need balancer shafts to counter an extreme rocking couple that would otherwise limit engine performance.

But secondary vibration, caused by the uneven acceleration of pistons through their stroke, is unpleasant (it's worse in inline fours because pistons move in pairs). It's usually a high frequency vibration through the bars at cruising speed – if an inline four is turning at 6000rpm in top, the secondary vibes will be in the 200Hz region, which is exactly the right frequency for 'white finger' – the tingling numbness in fingers after a long ride at a steady speed. Within packaging constraints there's little can be done (multiple balance shafts, fitting infinite-length con-rods, or using a Wankel rotary), which is why manufacturers try to damp vibes on inline four tourers with heavy bar-end weights and massive footrest assemblies.

It doesn't always work. As anyone with fingers knows.

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