TECH WATCH

Does rider weight act as bal­last on a bike? Q

Motorcycle News (UK) - - Garage -

When com­men­ta­tors de­scribe a rider as ‘bal­last’, they’re wrong: bal­last is a load added for sta­bil­ity. Bike sta­bil­ity is a func­tion of in­er­tia, gy­ro­scopic ef­fects, and a ‘sel­f­right­ing ef­fect’. But bikes are nei­ther sta­ble nor un­sta­ble. They’re metastable; the faster they go the more sta­ble they get, demon­strated when a racer falls off and the bike sails on, bolt up­right, into a tyre-wall. So no, bikes don’t need bal­last.

The av­er­age bike weighs around 250kg fully fu­elled and ready to go. As soon as you sit on it you add an­other, say, 84kg. That means 30% of your bike’s weight is you. But, hope­fully, it’s not a dead weight – be­cause you’re not bal­last. You’re a dy­namic mass and your 30% con­tri­bu­tion to the to­tal weight has a sig­nif­i­cant in­flu­ence on the way it be­haves when it’s moving.

All ob­jects have a cen­tre of grav­ity (COG). It’s the point where the sum of the sur­round­ing mass is zero. In a bike, it’s just above the en­gine, for­ward of the rider’s knees or there­abouts. Moving the COG al­ters the han­dling char­ac­ter­is­tics.

If the bike’s COG is fixed, the COG of the bike/rider sys­tem isn’t. The hu­man COG is mid-body, just above the belly but­ton. By shift­ing our COG back­wards and for­wards, side-to-side, and even up and down, we can aug­ment (or re­sist) var­i­ous forces. We can slide back in the seat un­der ac­cel­er­a­tion and for­ward un­der brak­ing, in­creas­ing weight over ei­ther wheel and im­prov­ing ac­cel­er­a­tion trac­tion or brak­ing grip.

And, while a bike’s main steer­ing in­put is ob­vi­ously steer­ing – plenty of peo­ple ride with­out moving an inch out of line – shift­ing body­weight to the left or right con­trib­utes to bike con­trol. There are lots of ex­am­ples: ride off-road across an in­cline, you lean into the hill­side to keep the bike more up­right and find more grip. Sim­i­larly, if you’re mid-corner on a track bike, lean­ing your body to the in­side lets the bike be more up­right for a given speed, giv­ing more grip.

These are all well-es­tab­lished rid­ing tech­niques. But they’re a long way from the rider be­ing mere bal­last.

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