Air is thin at walking speed. But it seems to get thicker as a cyclist accelerates. Trying to maintain a speed in excess of 40 kph becomes a battle against nature. And speeds above 50 kph are strictly for world champions. There's a name for the air resistance which builds according to the square of speed – it's called drag.
And it is measured by a number, the drag coefficient. The figure for a big square-fronted bus is 0.8. A good road bike plus rider generates more drag at 0.88 – but with aero bars it beats the bus, at 0.70. The figure for a sphere is 0.47, a flying bird 0.40 and a streamlined Tesla automobile 0. 24. But the natural champion is the dolphin, with an underwater wetted area drag coefficient of 0.0036.
There are three factors underlying the dolphin's low drag: shape, smoothness and frontal area. Bike engineers focus on the same aspects in their search for a cycle which will slice through the air like a bullet. Unfortunately, human beings are not bullet-shaped, which means that the rider generates up to 80% of the drag. In striving to cut drag down to size, a rider who competes against the clock may imitate the dolphin – with a streamlined helmet, smooth skinsuit, and a position on the bike that presents a smaller frontal area to the wind.
Let's assume your favorite race is a 40-km time trial. You can shave up to three minutes off your time by using a longer stem and bull horn bars. You will present a less frontal area with flatter arms, and the airflow will speed up over your lowered back. Caution – we're not talking comfort here, so get used to your new TT position. Aero clip-on bars with cups to support your forearms would be the next logical step. Frontal area is narrowed so much with your elbows close together, that the bars could take up to two minutes off your personal best for your favorite 40 km course. You'll soon get used to steering with your body most of the time, using the bull horns only for powering away from the start line, plus tricky technical corners and climbs
Okay, so now the front end of the cockpit resembles a caribou with antlers, but you have a new personal best to boast about. We have the optimum shape, and now we need to think about surface. A good streamlined helmet plus a skin suit could bring you home two or three minutes earlier. In the fifties, racing cyclists wore wrinkly woolen jerseys, with pockets everywhere and spare tubulars slung across their chests. That sounds like a recipe for the drag created by turbulence, as each protruding item breaks up the boundary layer airflow over the rider's body. Today, smoothness is all, and pros wear full-face visored helmets for solo rides.
We've done what we can to lower 80% of the drag, and now we must look at the remaining 20%, namely frame and wheels. Here, we're saving seconds rather than minutes, according to the dictum of a famous British coach, Dave Brailsford: the “aggregation of marginal gains”. If you make many tiny losses in drag, they combine to build a useful advantage.
A key moment in the aero revolution came in 1984 when “Cecco” Moser broke the 51-kilometer barrier for the hour, in the thin air of Mexico. Every aspect of his drag had been researched in a wind tunnel, and marginal gains came from bull horn bars, disc wheels, a skin suit, and skin hat, plus overshoes. Some of Moser's kit has dated since then. Today his plunging “funny bike” top tube has too much frontal area to please designers, and a smaller disc at the front brings handling problems – especially in a crosswind. Another key moment was in 1989 when Greg Lemond beat Laurent Fignon in the final short time trial of the Tour de France. After thousands of kilometers, the winner's margin was eight seconds. And some commentators put Fignon's loss down to the drag of his ponytail hairstyle!
As the aero revolution got started among rank and file cyclists, the first turbulent items to be tucked out of the airflow were brake cables. Taped to the bars and fed through the frame, they gave a marginal loss in drag and made the rider-bike combination a few seconds faster over 40 kms. Then came the sculpted carbon frame. The drag of any design could be tested in a wind tunnel, and shape followed shape in progress that continues to this day. In 2019, a carbon-framed bike for a clubman's budget has drag-fighting features that would have been impossible ten years ago.
Top pro time trial bikes make no compromises for comfort. If you can afford such a bike, it might be possible to trim your personal best by up to a couple of minutes. Every part of the frame is designed for a smooth aerofoil section and low frontal area – which is why the “top tube” of the frame is horizontal. To cut turbulence, components such as brakes are sheltered from the wind behind parts of the frame. A steep seat post angle allows the rider to “get on top of his work” in a streamlined position. The “seat tube” diverts airflow smoothly on either side of the rear wheel, as do the short, stiff seat stays. And let's not forget a smooth aero bottle, which might be worth 30 seconds over 40 kms. A TT bike, with carbon wheels, aero bars and a rider in a skinsuit and visored helmet, yielded a drag coefficient that almost matches the Tesla: 0.244. That was in wind tunnel tests. The UCI has banned drag-beating measures such as the arms-flat-forward “Superman” position, which gave a drag figure of 0.18. But it's hard to see where you're going!
Now we come to the rotating components of the bike-rider combination: cranks, pedals, and wheels. The standard bike wheel with a box rim and 36 spokes is a nightmare for the aerodynamicist. It is a miracle of strength, capable of supporting 300 times its own weight – but, boy – does it generate drag! Get a rear disc wheel, and you'll cross the line up to half a minute earlier.
A smaller number of flat spokes will help, but the biggest non-disc marginal gain comes from a deep aero-section carbon rim – the deeper the better. It's designed to function as an aero unit with the tire in place – a teardrop section. A good ICAN wheelset can cut off almost as much drag as a disc, with the added security that the machine won't get twitchy in a crosswind. Research goes on into carbon wheels, with promising deep-rim three-and two-spoke designs. Trust your cranks. The top brands are already optimized to keep drag as low as possible. Use overshoes to cover all the things that create drag when your feet are spinning at 85 rpm. And use the smallest pedals you can find.
When you plan your dream bike, think small. Many little changes add up to a big result. Low frontal area, aerofoil shape and non-turbulent smooth surfaces cut drag. And less drag means you have surplus watts of power which result in higher speed, and better performance. And that's not such a drag, is it?