The resistance experienced by a moving vehicle (a person on a bike being a particular type of vehicle) has components as follows:
- a resistance proportional to weight, usually from bearings and tyre compression;
- a resistance proportional to speed, usually from wheel-ground friction and dynamic tyre deformation;
- a resistance proportional to the square of the net speed at which it is passing through the air.
To understand the actual resistance experienced by any particular vehicle, it is necessary to know/discover the value of the first, and the coefficients for each of the second and third. Despite ferreting around a bit, I’ve never been able to find typical coefficients for people on bikes, but I bet that they’re either in the book mentioned above, or well known to pro cycling bods.
For most vehicles, the output engine/motor power is known, and some cyclists think they know their sustainable output power (I have my doubts about whether most really do). That power can expressed as (force x distance)/time, allowing the force available at any given speed to be calculated.
This allows two curves of force vs speed, one for the vehicle output power, and one for the vehicle resistance, to be plotted against the same axes. The speed at which the two curves cross is known as “balancing speed” and is the speed beyond which the vehicle cannot be accelerated, as illustrated here on the back of an old envelope:
Best ways to make the vehicle go faster?
- increase the output power;
- decrease the coefficient applicable to the third component, the fancy name for which is ‘streamlining’, and for cycling involves picking cyclists with a small frontal area, narrow people, and making them crouch down, and wear silly hats, and making the bike ‘aero’;
- decrease the coefficient applicable to the second component by making wheel and surface as smooth as possible. This is why trains have such low resistance, and in bike terms means a dead smooth road and ‘tread-less’ tyres as hard as rocks;
- reduce weight, because of its impact on the first component;
- reducing bearing friction.
Because most cycling isn’t on super-smooth surfaces, there is an extra loss to consider, in terms of energy consumed in wobbling bits of cyclist around and flinging the bike and cyclist in the air as the wheels go over bumps, however tiny, so optimising the second component gets interesting, and takes us to the complex debates about tyre sizes and pressures.
Did I get that right?
