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Why you need a custom aero solution to get faster on your bike.

Updated: May 2

If you find yourself thinking that cycling aerodynamics is much more complicated than it ought to be, I can explain.

Cycling looks like it ought to be very simple – after all, it’s just a rider on a bike. It doesn’t go that fast, it’s not expected to fly (at least outside BMX, which is not really my area anyway) and unlike motorsport, it’s not looking for difficult compromises about drag and downforce. By all common logic it should be a matter of “make it pointy, make it smooth” and we’re done.

It’s not. And the reason it’s not is that cycling happens at just about the most awkward possible speed.

I shall attempt to explain. The key to most cycling aero problems happens in what’s called the boundary layer. This is the thin layer of air passing over the surface of you, your bike, your helmet and so on. It’s the air that’s being affected by friction against you. Outside the boundary layer, the air is going at the same speed relative to you that it would have been going at anyway.

The boundary layer can behave in two different ways. It can flow smoothly, its speed increasing progressively from almost nothing against the surface to the full speed of the passing air. If you drew a graph of the speed within the boundary layer moving away from the surface it would be a smooth curve. This is laminar flow.

The other possibility is turbulent flow. This is what it sounds like – the boundary layer is turbulent, with tumbling swirling flow.

A few things to know about the different flows. Laminar flow produces less drag. But only up to a point, because a laminar-flow boundary layer gets thicker the further over the surface it flows. More and more air is being pulled along by the rider. And, at a critical point, this big thick layer of air will transition to turbulent flow, and detach from the surface forming the wake.

Turbulent flow is intrinsically draggier – it produces more friction against the surface. But a thin, turbulent boundary layer has less air in it that a laminar boundary layer, and will stay attached to the surface for longer, flow further around it, and produce a smaller wake. And you save a lot more in drag from that smaller wake than you lost in the turbulent boundary layer.

So the game, at least as far as clothing and rider position is concerned, is to trip the laminar flow into turbulent flow sooner than it would transition naturally, so that the boundary layer stays thinner and stays attached for longer. Hence textures and carefully placed seams on skinsuits and such like – it’s about creating just enough turbulence in just the right place to change the boundary layer.

Unfortunately, when it comes to the questions of “just enough” and “in the right place” there is something that makes it even more complicated. Whether flow is laminar or turbulent, and when it might naturally transition from one to the other, is not simple. It depends on something called a Reynolds number.

The Reynolds number (abbreviated to Re) is what predicts whether flow is laminar or turbulent. At low Re, the flow is laminar. At high Re the flow is turbulent. You calculate Re by (and I’m simplifying pretty grossly here) multiplying the size of the object by its speed and dividing by the viscosity of the fluid. Since as far as fluid is concerned, we’re only interested in air, we can ignore the last one, and worry about the first two.

So the bigger the object (or the further over it the flow travels) the more likely it is to be turbulent. Similarly, the faster it’s going, the more likely the flow is to be turbulent.

It’s just our luck that cyclists are just the right size and speed to fall into the area of Re where the flow is transitional. Slower and it would be much more laminar; faster and it would be largely turbulent.

Either of those would make life simpler. But the fact that we’re just where we are means that the optimums of how and where you trip the boundary layer become very complicated. The answer for a sprinter will be different from the answer for a time triallist. Differences in body dimensions mean the perfect answer will differ from a small rider to a big rider even if they’re going at the same speed.

The fact we’re all different shapes and ride in different positions feeds into this as well – in fact even two similarly sized riders going at the same speed will have different flow structures. It means that interactions are important too – helmets, shoes, handlebars, their influence extends to more than just themselves, they affect the air flow over the rest of the rider.

The practical upshot of this is that there are very few universals in aerodynamics about rider position and clothing. At the Olympics, several of the top team pursuit squads fielded teams in mismatched helmets – each rider was using the one that suited them best, despite them all riding the same event at the same speed. The equivalent skinsuits looked similar, but were bespoke fitted in a wind tunnel.

In many ways most of the current advances in cycling aerodynamics are coming from a more detailed, individual approach to making each rider’s set-up work. It’s not so much about skinsuits or helmets as it is about the ability to measure aero drag reliably.

To put that another way, if you want to go faster you don’t need a new bike or a new helmet, you need better instrumentation.



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