Should you go deep or light?

In a sportive with equal amounts of flat and climbing terrain, what sort of wheels should you go with? Deep section wheels to eat up the flat roads like Tony Martin? Or lightweight climbing hoops to propel you up like Nairo Quintana?

f a cyclist rides 10km on the flat followed by 10km uphill, then obviously the uphill section will take longer,’ says Marco Arkesteijn, lecturer in sport and exercise biomechanics at Aberystwyth University. ‘Now let’s say [arbitrarily] that the deep section aerodynamic wheels give you a 10% increase in speed over shallower rims on the flat but, due to their extra weight, a similar decrease in speed on the uphill. It stands to reason that you should choose the shallow rims because you’ll spend more time on the ascent so that’s where you want to save the time.’

Ah, but it’s not that simple. Over to Kevin Quan, director of engineering at Knight Wheels: ‘My ex-colleagues at Cervélo did a lot of testing and calculations to answer this question. They discovered that aero gains trump weight loss for anything up to around a 5% slope for the average recreational rider and an 8% slope for a pro.’

So if you’re Chris Froome tackling the Col de la Ramaz on Stage 20 of this year’s Tour de France, a 13.9km ascent with an average 7.1% gradient, deep rims are the way to go. If you’re accountant John Smith having a go at the same climb, it’s better to go shallow.

For a slightly different take on things, let’s change our measure of comparison to the trusty watt. ‘At 40kmh, the switch from shallow rims to deep sections could be worth around 10W, which could save you 30 seconds over the hour,’ says Rob Kitching, founder of performancemodelling outfit Cycling Power Lab. ‘Let’s assume the weight penalty of using deep section aero wheels is half a kilogram. Even on a gradient of 10%, the cost of carrying that extra weight up the climb would likely be less than 5W. A course would have to have a lot of tough climbs, where there would be a big weight penalty, before it would make sense to ditch the aero upgrade.’

Life’s a drag

A key factor we need to consider is the drag area (CDA), which is the product of an object’s drag force and its frontal area. Using aero wheels has been found to reduce a cyclist’s CDA by 3-5%, so if you generate 350W of power, using