Bicycle stability and centre of gravity (or mass)

In the current issue of CTC’s ‘Cycle’ magazine there’s an article by Chris Juden, the Technical Editor. Chris normally knows what he’s talking about. The article compares two electrically assisted bikes. One has the battery higher than the other and he wrote: “The Trek has the battery high up, which makes the bike a little less stable.” I’m not sure this is true.

The familiar balance a broom on one finger trick suggests a bike with a higher centre of gravity should be easier to balance. Tightrope walkers use the same principle with long balancing poles to keep them from falling off. Because their centre of mass swings through a longer arc, they are less likely to tip sideways. So I think an electric bicycle with a higher battery is actually more stable than one with a battery low down.

I haven’t ridden any though, and Chris Juden has. I suppose it depends on what he means by ‘stable’. I can see how a lower centre of gravity bicycle is easier to manoeuvre at slow speed or when pushing it along by the handlebars but if, for example, you want to balance a bike when stopped, it ought to be easier when standing up on the pedals to mimic a broomstick. Penny farthings are presumably stable (never ridden one of those either), and they are much taller than normal bicycles.

This appears to be confirmed on Wikipedia: Bicycle … dynamics.

Center of mass location: A bike is an example of an inverted pendulum. Just as a broomstick is easier to balance than a pencil, a tall bike (with a high center of mass) can be easier to balance when ridden than a low one because its lean rate will be slower. However, a rider can have the opposite impression of a bike when it is stationary. A top-heavy bike can require more effort to keep upright, when stopped in traffic for example, than a bike which is just as tall but with a lower center of mass.

A cyclist moving in a straight line stays upright by steering the bicycle under its centre of gravity. The higher the centre of gravity, the wider the ‘stability zone’ beneath its sway and the easier it is to balance. So more stable I think, but only when moving forward at a certain speed.

If this is true, there must be a speed at which the stabilising effect of a higher centre of gravity kicks in – five miles per hour, perhaps (and a very high speed at which the gyroscopic effect of the wheels takes over). The stability argument is put forward for low rider racks on touring bikes and they are certainly beneficial when pushing and manoeuvring a loaded tourer at very low speed on foot. A set of panniers is much heavier than an electric bicycle battery and it may be that cycle tourists rarely reach a speed where raising the load would improve balance. I imagine low riders on the front relate in some way to easier steering.

According to Socrates, to make a right turn, you turn the handlebars left. I’ll check this next time I cycle.

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