Adding a set of aero wheels to your triathlon bike makes it look the part but how much extra speed do they provide for the investment?
Aero wheels are often a go-to upgrade for triathletes seeking to buy more speed in pursuit of faster bike splits because of their drag-saving potential. We take a look at how that aerodynamic efficiency works and how much time you could save to help you decide if a new set of wheels should be on your wishlist.
How Does Aerodynamic Drag Work?
Minimising aerodynamic drag is of key importance to triathletes because it means less effort is needed to sustain speed. Think about riding into a headwind – it’s much easier when you’re low to the bars rather than sat up with wind buffeting across your chest.
The shape of an object moving through the air has a huge effect on how much drag it creates. In the case of wheels, the tyre acts as a leading edge that splits the air rushing to meet it. The rim’s shape dictates what happens after that.
Deep Section Wheels Vs. Shallow Wheels
A deep-section aero wheel helps to control the airflow, allowing it to move smoothly over its surface and come together again with minimal turbulence. By contrast, a shallow box-section training wheel splits the air but can’t guide it after that, creating a lot of turbulence and drag that holds you back.
It’s for this same reason that deeper aero wheels are faster – with disc wheels being the best example of aerodynamic performance in lab conditions.
However, we ride in the real world where we have to contend with the effect of crosswinds on handling. This means you won’t even see pro cyclists opting for a front disc outside the velodrome.
This is where design based on yaw angles comes in. Yaw is the is the relationship between the direction the wind is coming from and the direction you’re travelling in.
Imagine a long ribbon tied to the back of your seat post – when stationary, a crosswind might blow it right out to one side (90° yaw), but as soon as you’re cycling this angle decreases because you’re generating a headwind against the ribbon by riding forwards.
So the faster you go, helped along by your aero wheels, the more negligible an effect crosswinds have on your aerodynamics as a whole.
for an Ironman at 25mph, you’re looking at slicing SIX to SEVEN minutes
While we rarely ride in completely calm conditions or into directly head-on wind, there’s plenty of research from wheel brands such as Flo Cycling, Swiss Side, Trek, Specialized and Mavic to suggest that the majority of riding takes place between yaw angles of 0 and 10%, so this is where wheel manufacturers tend to focus their efforts.
By using slightly wider rim profiles, designers give up a little speed in a front-on, wind-tunnel setting to improve the aerodynamics in real-world conditions where wind is constantly changing as your direction does.
This is exactly how handmade carbon wheels expert ENVE goes about its design process. By taking into account the effect of changeable conditions, a wheelset can be created to produce the fastest overall experience while maintaining handling. This is why you’ll often see a deeper rear wheel and, in ENVE’s case, a slightly wider front wheel too.
How Much Time Can An Aero Wheelset Save You?
So, how much does all this science mean you save during your triathlon bike leg? Flo Cycling looked at the effect different wheel depths and speeds would have over different distances compared to a standard Mavic Open Pro box-section training wheel.
At 20mph, a 45mm aero wheelset saves 99 seconds over 40km while the brand’s 90mm deep option ups that to 116 seconds. Or for an Ironman at 25mph, you’re looking at slicing six to seven minutes respectively.
It’s also worth remembering that the lower the speed, the bigger the time savings you’ll have. This sounds counterintuitive but because slower athletes spend longer on a course, they benefit from improved aerodynamics for longer and saving more time overall.
Whether those time savings are enough to part with the £1,500+ you’ll likely be handing over for the privilege is up to you, but there’s no doubt that there’s time to be saved with aero wheels for those seeking every advantage.