Forces and speed

New Zealand’s elite cyclists spend time in the wind tunnel at Canterbury University to find ways of reducing aerodynamic drag (air resistance). At high speeds, this drag is the main force that opposes motion. Reducing aerodynamic drag is a way to go faster without having to put in any extra effort.

Speed changes until forces become balanced

A force is anything that pushes or pulls on something else.

When biking on a level road, your forward force comes from pushing and pulling on the pedals to make the back tyre push backwards against the road.

The two main forces that oppose your motion are aerodynamic drag (air resistance) and rolling resistance of the tyres against the road caused as the tyre is compressed.

When the forward forces are bigger than the opposing forces, you speed up (accelerate).

As you go faster, the force of air resistance pushing back on you increases. Eventually, the forces become balanced (the forward forces are the same size as the opposing forces). Once the forces become balanced, your speed stays the same.

The speed you reach as the forces become balanced is called the terminal velocity (It is like when a skydiver jumps out of a plane – speed will keep increasing until forces become balanced.)

Two ways to cycle faster

• Push harder on the pedals to increase the force pushing you forwards. Your speed will increase until the opposing forces increase to balance your larger forward force.
• Reduce aerodynamic drag. With less drag at a certain speed, you will be able to reach a higher speed before the opposing forces balance out the forward force.

Some ways to reduce aerodynamic drag

The rider on a bike causes about 70% of the drag, so finding the body position and equipment that works best for each rider is important.

• Tight and smooth clothing – body suits are designed to minimise the friction due to air resistance. Some body suit designs also use a kind of dimpled golf ball effect to try to reduce the drag caused as air flows around the arms and legs.
• Armrests on handlebars (aerobars) – allow the rider to get into a lower position and reduce frontal area so that there is less air being pushed against.
• Aero helmet – allows air to flow more smoothly over the head and back. (These helmets are used mostly in track or individual time trial events. Most helmets for road cycling still have gaps for air to pass through to cool the cyclist).
• Adjusting seat and handlebar height or extension to reduce frontal area and make the back flatter.
• Frame and wheel design – carbon fibre construction on modern bikes allows designers to try different shapes to reduce drag. For example, teardrop shapes that are rounded at the front with a narrow taper at the back have much less drag than square shapes.
• Smooth tyres – less air resistance caused from the tread pushing on the air (tyre design and pressure is also important to reduce rolling resistance).

Typical drag and rolling resistance values

As speed increases, rolling resistance of the tyres stays nearly the same, but the aerodynamic drag increases a lot. At high speeds, 80–90% of a cyclist’s effort can go into fighting aerodynamic drag. Rolling resistance of the tyres and friction in the bearings and chain are small by comparison.

If you can improve your aerodynamics, you will be able to reach a faster speed before the opposing forces balance your forward pushing force.