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?

Cyclist - - Cycle Science - Words JAMES WITTS Il­lus­tra­tion ROB MIL­TON

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

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