Does rider weight act as ballast on a bike? Q
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.