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How much time can aero kit save?

Sam Challis
22 Mar 2019

Manufacturers go to great lengths to make aero kit, but will it actually make you faster? Cyclist finds out.

If you ride a bike you know all about the effects of wind resistance. When riding at speed on the flat, aerodynamic drag contributes up to 90% of the overall resistance to forward motion. That’s because the blunt and irregular shape of a bike and rider is inherently bad at passing smoothly through air, meaning that any improvements in aerodynamics can make a big difference to saving precious energy and increasing speed.

As a result, manufacturers spend massive amounts of time and money on aerodynamic testing, claiming for instance that kit that is 10% more efficient, could save you 20 seconds over a given distance or give you an advantage equivalent to five watts. Cyclist wanted to see if these claims could be substantiated in the real world, and not just in a wind-tunnel.

To carry out our own investigation we’ve come to the Hillingdon Cycle Circuit in West London. The plan is for resident bike tester James to complete five sessions of three laps at a metered and consistent effort of 300 watts using a different piece of aerodynamic kit each time.

We take the time of each session to determine the aerodynamic effectiveness of each item, and then put them all together to ascertain the cumulative effect.

The components we’re swapping out are wheels, bars, helmet and skinsuit – in other words the ones most likely to be changed by an average road rider in search of better aerodynamics. Although there are some variables that mean our testing isn’t wholly scientific, our plan certainly appeals to Reynolds’ director of technology and innovation, Paul Lew.

Aero kit test kit

‘This kind of testing doesn’t happen enough,’ he says. ‘It’s a great way to test aerodynamics – the problem is that it is just too expensive to control the variables in the real world to the same extent that you can control the more limited number of variables in a wind-tunnel.

'It would be too challenging to achieve the level of reliability that most engineers would be comfortable putting their name too. You guys aren’t engineers though so don’t need to worry, and I see value in this type of testing.’

So with the validation of one of the most respected names in cycling aerodynamics, we get cracking.

Wheels

James’s first three laps are completed on Mavic R-Sys SLR wheels, Giant Contact SL bars, Sportful’s Pro Race jersey and Super Total Comfort bibshorts and a Giro Aeon helmet – all kit with no aerodynamic pretensions.

These three laps average 2m 33s, which we use as a control time against which to compare the laps with our aerodynamic adjustments.

Aero kit test wheel swap

The first alteration is to switch to Bontrager Aeolus 5 TLR deep-section wheels. Deep-section wheels have shorter spokes that disturb air flow less than standard wheels, plus the deeper rims also encourage a laminar (smooth) airflow over their surface.

Lew explains that at certain yaw angles, good aero rims can use wind like a sail on a boat, creating forward thrust that opposes drag. James’s average time for the three laps is 5s quicker per lap (2m 28s) than the control time – a 3.3% improvement.

This doesn’t seem as big as we were expecting, but Lew isn’t surprised.

‘The sail effect of deep rims works best with consistent side winds, for example on an out-and-back TT course. On a short race circuit the corners change the angle of the wind too much for them to have such a dramatic effect.’

Aero bars

Aero kit test ENVE aero bars

The next step is to swap out the traditional bars to flat-top Enve SES aero road bars. The Kamm-tail shape of the tops presents minimal frontal area to the wind, so has the effect of neatly parting the air to reduce the pressure differential that causes pressure drag – a key factor in total aerodynamic drag. 

The average lap time with the Enve bars is 6s down on the control at 2m 27s – a 3.9% saving. This surprises us, as we hadn’t expected the small difference in bar shape to have such an effect on aero drag, but the improvement in speed wasn’t all down to the bars’ aerodynamic cross section.

They are narrow along the tops and flare out at the drops, so while the overall width is the same, the tops present a smaller frontal area. Plus the shape of the drops encourages the rider to adopt a narrower, more aerodynamic position. 

Simon Smart, founder of aerodynamics company Drag2zero, helped develop the bars. ‘Changing from a more upright position to a lower position gives the biggest reduction in drag,’ he says.

‘The extra watts saved from good aerodynamics is easily greater than the gains you’d get from the best winter’s training you could do.’ 

Helmet 

Aero kit test skinsuit

As the rider’s head is one of the areas most exposed to airflow, manufacturers produce aero-road helmets designed to direct airflow efficiently to minimize turbulence while still having a few cooling vents.

This has lead to recent claims of impressive aero gains, with Giro claiming its Air Attack is 17 seconds faster over 40km than its Aeon helmet. With Specialized’s Evade on his head, James clocks average laps of 2m 31s – a 2s difference to our control time. When extrapolated, this could lead to a saving of roughly 26s over a 40km ride. 

Lew explains this impressive level of time saving is likely due to James’s relatively high speed. To maintain a 300 watt output he was travelling at around 38kmh, and the benefits of improved aerodynamics increase exponentially with speed.

‘If you were to plot a graph with drag on the vertical axis and velocity on the horizontal axis, you wouldn’t see a straight line – you would see an exponential curve upwards,’ says Lew. ‘Drag starts to increase significantly over 30kmh, so above that velocity aerodynamic gains really show.’

Skinsuit

Aero kit test circuit

Our last piece of aero kit is a Santini Speed Shell skinsuit. Lew and Smart agree that because the body is the biggest singular mass it contributes the largest proportion of the drag.

A skinsuit removes the surface ripples caused by seams and garment overlaps, so we are expecting quite a large effect on aerodynamics. Yet we find only a 0.8% time saving, with James setting average laps of 2m 32s.

While still a decent saving when extrapolated to a longer distance, it is less beneficial than the other items in this test. This could be attributable to the fact that James’s standard jersey and bibshorts had a fairly racy cut with minimal excess fabric. 

Other benefits of skinsuits, such as the compression they exert on muscles, have been shown to reduce fatigue but only after longer distances than we were riding here.

A final point worth mentioning is that in contrast to the other components that can be reshaped to minimise drag, a skinsuit can only work to reduce what’s known as ‘direct friction’ and does not alter the shape of a rider’s body. Direct friction is a secondary component of aerodynamic drag and is much less significant than frontal area and profile. 

The full aero 

Aero kit test bike

James completes his final three laps kitted out with all of the aero gear. Together they allow him to cleave 10s off his control lap average at the same power output, lapping at an average of 2m 23s.

This total saving of 6.5% would be worth a significant 2m 27s over 40km. Predictably this result is slightly less than the sum of the individual gains due to the exponential increase of drag with speed, plus the complex way in which multiple aerodynamic elements interact.

The whole rider and bike works as a system and it’s not simply a case of adding one element to the next and expecting them all to behave as if in isolation. Many experts in the field say component integration is where the big future gains lie.

Even though our test wasn’t as stringently controlled as a laboratory experiment, it revealed consistent trends supporting the efficacy of aerodynamic kit in a real-world setting, and showed that the obsession with aerodynamic gains is well founded. Smart agrees: ‘Once you’ve trained your body to a good level it’s hard to find that much more, but aero kit is a viable way to get faster.’

Although Smart and Lew warn that the lack of repeatability with our test should make us cautious about coming to concrete conclusions about the comparative merits of the various pieces of aero equipment, both agree that the kit would be beneficial. 

‘Whether your test controlled enough variables or had a large enough data set to make a buying decision based on the results is a decision for your readers, but I predict that the overall trends displayed would change little if the data set was increased.’ A wind-tunnel you say? Who needs one of those?

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