How does padding out your bra make you faster on a bike?

The latest controversy in professional cycling is not doping, not hidden motors, or even extended sticky bottles. No, the controversy sweeping over the Tour de France Femmes is padded bras, which have led to extra clothing checks as riders sought an aerodynamic edge in the stage 4 time trial on Tuesday.

But why on earth has this become a controversy, is there any merit to the claims that padding out a bra can make cyclists faster on the bike, and how is it supposed to help?

Interestingly, the science behind why padding your bra out can make you faster on the bike is based on very sound aerodynamic science.

When cycling, especially in time trials where speeds are higher and there's nobody to shelter behind, the majority of the force you have to overcome is aerodynamic drag. The faster you go, the greater this majority becomes. You just have to look at the giant time trial helmets that have taken over the professional peloton to see that aero gains are the driving force behind a lot of developments, if sometimes maybe at the expense of aesthetics.

Those large helmets give us some clue as to why padding out bras can create a performance gain. With aerodynamic drag being a key factor to overcome, one way of doing this is reducing turbulence around the body. This can be done via textured fabrics such as you see on skinsuit sleeves and backs, or by reducing the amount of ‘space’ around the body of the rider.

This is because that space, such as between the head and shoulders, arms and head, or behind the arms and into the gap between them, the body, and the hips, creates areas of low pressure.

Air flowing over the body then enters the low pressure area and creates turbulence, which creates more aerodynamic drag. Wide time trial helmets are used to close the gap between the head and the shoulders while also creating a smoother transition, while large visors are also now commonplace to reduce this gap between the rider's arms and the helmet.

This brings us on to those padded bras. The gap between arms, legs, and body is a large area that creates a low pressure area which draws air in, rather than flowing smoothly around the body. This in turn generates more drag for the rider to overcome.

By padding out bras, you could potentially smooth the flow of air from the arms, onto the body, and around the hips, and reduce the size of the low pressure area. It’s a similar principle to that used by the men’s WorldTour riders with radios down the front of skinsuits - or indeed Victor Campenaerts' under-arm radio placment - which before the UCI intervened, could be padded out significantly. Done for ‘comfort’ on paper, the reality was to create a fairing at the front of the suit to improve aerodynamics. In CTT-sanctioned events in the UK, this was taken to the extreme a few years ago with fairing fitted behind the arms, legs, and large water bladders down the front of skinsuits all to improve aerodynamic performance. Rule28 makes a specific aero gravel base layer with a bottle holder down the front, again to reduce that low pressure area and reduce drag - verified by the Cyclingnews Labs wind tunnel team as offering a saving of 2.73 watts at 40km/h. For context, stage 4 winner Marlen Reusser averaged 43.3km/h, at a (very) estimated 400 watts.

Padded bras effectively work on the same principle, and have the potential to reduce the rider’s coefficient of drag.

The idea is that it makes one a more slippery shape, rather than all these gaps, ridges, undulations and blunt edges that bounce the air all over the place and create more drag. It’s why a car, with a much larger frontal profile, is able to have a lower coefficient of drag than a cyclist; fewer moving parts in the wind, and a sleeker profile with fewer gaps that can create turbulence.

So yes, padded sports bras can help riders get faster, especially in time trials, although the difference is small, and the exact amount of difference it makes likely differs from rider to rider.

Of course, there are various ethical issues to consider when asking commissaires to check sports bras, too. Especially in a sport still predominantly represented by male figures when it comes to staff.

The announcement of additional clothing checks drew a mixed reaction from teams at the race. Naturally, all parties wanted to prevent cheating, but that was balanced against a need to avoid invasion of privacy when performing checks.

Considering that time trial helmets have been allowed to evolve into huge aerodynamic fairings that certainly aren’t designed that way for the protection of riders' heads, it seems a slightly trivial area to focus so much attention on. Similar to the banning of ice socks down the front of skinsuits in the heatwaves for the sake of limiting fairings, there are more important considerations at play.