As pro teams and bike firms invest ever more in wind-cheating kit, the appeal of ‘going aero’ entices us all. Marc Abbott shows you how to take your first aerodynamic steps
Whether we’re taking part in a Thursday night club 10-mile time trial, riding with our mates or contesting a Grand Tour, all of us can benefit from the time-saving pursuit of aerodynamic perfection. But can aero gains be made without an enormous monetary outlay?
We spoke to Kevin Quan, lead engineer of the Cervélo P3, who is now director of engineering for carbon wheel firm Knight Composites. The quickest way to get more aero is to sort out your position on the bike.
“In general, the drag is most affected by the riding position,” says Quan.
“A rider sitting upright with arms straight has a CDA — drag coefficient x frontal area — of around 4.3. He or she can reduce this by around 20 per cent to 3.4 by simply crouching into a racing position, and using the drops of the handlebars.”
- Get low on the bike
- Keep clothing skintight
- Invest in an aero frame
- Pay attention to your wheels
By extension, it will come as no surprise to learn that buying an aero bike will improve your efficiency, as Quan explains: “If you’re using an aero bike and wheels, you can reduce the CDA by a further 10 per cent — five per cent for the frame, another five per cent for the wheels.
“Also, an aero bike set-up such as this will ‘sail’ and reduce drag further when the wind blows from the side. This is something that can’t be achieved by body position alone.”
Cheat the wind
Doesn’t having a slab-sided bike that acts as a sail make it more unstable in high winds?
“Teardrop-shaped rims are the most aero, as far as our research shows, because they take advantage of lift in sidewinds,” says Quan.
“Much like the sail of a boat in sidewinds, a bicycle can actually tack into the wind. The rim and frame is the sail and the tyres are the keel. The deeper the rim, the larger the sail, the more thrust — drag reduction — is available.”
The one thing we can all do to cheat wind better, as Quan advises, is stick to skintight clothing: “With regard to clothing, it’s definitely a case of the tighter the better. If a rider chooses to wear baggy clothing, they can say bye-bye to the five per cent drag reduction from their aero frame and/or wheels.
“If you want to be more aero, it’s best to stick with the Lycra.”
More side-force is experienced from slab-sided, aero tubes and wheels, but for most racers this is a trade-off they’ll gladly accept. Modern America’s Cup boats use solid sails that look and behave very much like aero bike down tubes do.
The drag coefficient is multiplied by the frontal area (in this case in feet-squared) to give a CDA figure. The crouched rider obviously has less area.
The drag coefficient (Cd) of an upright bike rider is 1.0. The Cd of a crouched rider is 0.88. A modern car’s drag coefficient is around 0.4.
The drag force equation can be simplified to F = Velocity x Cd x A (or force of drag = velocity squared x coefficient of drag x frontal area). This drag force is typically expressed in terms of grams.
When a bike is tested in a wind tunnel, the total drag of a bicycle and rider is typically 3kg.
A rider can decrease the frontal area (by crouching down) or decrease the coefficient of drag (using aero shaped things), or they can do both.
This article first appeared in the May 21 issue of Cycling Weekly