Sustainable Energy - Without the Hot Air

http://www.ck12.org Chapter 4. Technical Chapters

Then

energy-per-distance of bike

energy-per-distance of car

=

(

cbiked

ccard

Abike

Acar

)(

vbike

vcar

) 2

=

(

3

4

)

×

(

1

5

) 2

=

3

100

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. Collectively, these forms of energy consumption are calledrolling
resistance. The standard model of rolling resistance asserts that the force of rolling resistance is simply proportional
to the weight of the vehicle, independent of the speed. The constant of proportionality is called the coefficient of
rolling resistance,Crr. Table gives some typical values.

TABLE4.5:

wheel Crr

train (steel on steel) 0.002

bicycle tyre 0.005

truck rubber tyres 0.007

car rubber tyres 0.010

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]