Our collection of resources on cycling aerodynamics is a great opportunity to look at forces and speed in relation to cycling and think about how these might be measured and how one affects the other. These resources were developed with year 9 and 10 students in mind. However, some of the ideas and activities may be of interest to other levels.

Cycling is a sport that most people are familiar with. Most students understand that biking into a head wind is more difficult. They may also realise that it is a good idea to have their tyres pumped up hard. For students to understand the concept of force and motion, it is important for them to have many experiences in which their ideas are challenged and developed.

## New Zealand research

Our writer, Steve, discovered that scientists at the University of Canterbury use a wind tunnel to test the aerodynamics of elite cyclists. He wanted to capture the work of lecturer Dr Mark Jermy, PhD student Lindsey Underwood and technician Mark Harris as they worked with BikeNZ to help New Zealand’s elite cyclists improve their racing performance.

The videos and articles show wind tunnel testing in action, and we were fortunate that competitive cyclist Geoff Searle was willing to be tested.

For these tests, a cyclist pedals on a bike that has been mounted onto a platform as wind flows over the cyclist. Drag is measured for each change of position or for different bikes and equipment.

Racing cyclists try to minimise the drag due to air resistance and any resistance from their tyres. Students will be able to relate to many of these ideas when they think about their own bike riding.

## Science ideas and concepts

The first interesting idea is about drag. Steve had always thought of drag as a kind of friction so he found it enlightening to discover that there are actually two main causes of drag: skin friction and pressure drag. Skin friction is caused as air particles collide with the surface that is trying to move past it, as well as collisions of air particles with other air particles that have already been slowed down. Pressure drag is caused by a low-pressure zone that develops at the back of an object. Streamlining helps to reduce this low-pressure zone.

As speed increases, so does the opposing force of drag. The forward force developed by a cyclist will increase speed until the drag becomes the same size as the forward force from tyres on the road. This is a kind of terminal velocity situation. Speed will always change until forces become balanced again. By becoming more streamlined, a cyclist is able to reach a higher speed before these forces balance.

This leads to a third interesting idea. We know that wheels are designed to reduce friction. We also know that friction is actually important to provide forward force (traction) to make the bike speed up, otherwise the tyres would just slip. But why is there also a force of friction on the tyres that acts to slow the bike down? This opposing force is called rolling resistance.

An extra idea that needed to be developed was power. Though scientists normally like to think about motion in terms of force, competitive cyclists think in terms of power. This provides an interesting discussion in terms of how force and speed of pedalling is related to power and the final speed that a cyclist will be able to reach.

## Student activities

Activities were developed to highlight the science ideas and concepts, which focus on the force and speed and will help lead to a better understanding of cycling aerodynamics and how forces affect motion.

• The On your bikes activity requires students to analyse graphs of motion for New Zealand champion cyclist Alison Shanks. A worksheet leads students to analyse graphs of speed, force and power.
• In the Individual pursuit graphs activity, students cut out and tape different shapes, attach tiny pieces of cotton thread and use hairdryers to find which shape has the least drag. The shape that keeps airflow attached for longer reduces the low-pressure zone at the back, so it will have least drag.
• Investigating airflow over shapes activity requires students to investigate their own drag and rolling resistance on bikes.
Published 22 February 2012