sumption of objects with completely different speeds and areas. Specifically,
let’s assume that the area ratio is

(Four cyclists can sit shoulder to shoulder in the width of one car.) Let’s
assume the bike is not very well streamlined:

And let’s assume the speed of the bike is 21 km/h (13 miles per hour), so


So a cyclist at 21 km/h consumes about 3% of the energy per kilometre of
a lone car-driver on the motorway – about 2.4 kWh per 100 km.

If you would like a vehicle whose fuel efficiency is 30 times better than
a car’s, it’s simple: ride a bike.

What about rolling resistance?

Some things we’ve completely ignored so far are the energy consumed in
the tyres and bearings of the car, the energy that goes into the noise of
wheels against asphalt, the energy that goes into grinding rubber off the
tyres, and the energy that vehicles put into shaking the ground. Collec-
tively, these forms of energy consumption are called rolling resistance. The
standard model of rolling resistance asserts that the force of rolling resis-
tance is simply proportional to the weight of the vehicle, independent of

wheel Crr
train (steel on steel) 0.002
bicycle tyre 0.005
truck rubber tyres 0.007
car rubber tyres 0.010
Table A.8. The rolling resistance is equal to the weight multiplied by the
coefficient of rolling resistance, Crr. The rolling resistance includes the force
due to wheel flex, friction losses in the wheel bearings, shaking and vibration
of both the roadbed and the vehicle (including energy absorbed by the
vehicle’s shock absorbers), and sliding of the wheels on the road or rail. The
coefficient varies with the quality of the road, with the material the wheel is
made from, and with temperature. The numbers given here assume smooth
roads. [2bhu35]