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.
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“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
Fine tuning your road bike position
Time trial bikes and riders tend to hog the aero headlines, but even in the UK, with its huge time trial scene, most riders aren’t on a TT set-up. They’re on a road bike. So how does aerodynamics affect them?
Aerodynamic drag still matters more than you almost certainly think it does. If you ride a road bike on the flat at anything much over 10mph, aerodynamic drag is more than half of what slows you down. By 20mph, it’s almost all of what slows you down. Even a rider attacking on a 10 per cent hill could be expending a fifth of his energy overcoming wind resistance.
Happily, you can get most of the benefit by tweaking your position, because something like four-fifths of the drag is off the rider, not the bike.
For fast riding you can make big gains by dropping your head and shoulders lower over the bars, and getting your back as level as you can manage. The most efficient way to do this is by bending your elbows to around 90° and keeping your forearms parallel to the ground. Old-style TTers used the drops for this, but most current bikes are set up with lower bars, so it’s probably going to be about right if you keep your hands on the hoods.
The downside is that unless you do a lot of training in that position, your arms are going to hurt. The drops, while a little slower because the arms are straighter, might be more sustainable. Whichever you use, keep your elbows as far in as you can.
The next step is to shrug your shoulders in, if you can, and tuck your head down as far as you are able while still being able to see.
Freewheeling downhill, all of the same applies. But you might get a little more by moving your hands to the top of the bars, either side of the stem, and tucking down over them, elbows in. In general, for freewheeling, keep your thighs parallel — that is to say ‘a quarter-to-three’ with the pedals.
You could, of course, try descending on the top tube like Tony Martin, but it’s not a very stable position, and needs to be treated with a lot of caution.
How much might all this save? It does depend on the rider and the ride, but over a moderately hilly 100km ride, it could be five minutes or more over a normal position on the hoods.
You can gain a little more all round by avoiding loose, flapping clothing, not stuffing your pockets with capes and the like, and using an aero helmet.
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