If 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 performance-modelling 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 aero wheels could see your speed on the flat rise from 44.6kmh to 45.4kmh, an increase of 1.63%. At a 2% gradient, aero is still the way to go – the deep section wheels would have to be at least 2.8kg heavier than their shallow counterparts for them not to be the quicker option. But this value drops dramatically as the road ramps up. At 4%, aero wheels up to 940g heavier will still be the faster option. At 6% this drops to 390g, but by the time you hit 10% there’s just 50g in it for the aero advantage to win over shallow wheels.
So let’s take that into the real world. Zipp’s 202 Firecrest clinchers are its shallow lightweight option at 1,450g, while its 808s are the super aero choice at 1,885g, a weight penalty of 435g. According to our data-modelling, that means they would still be the better choice at gradients of up to around 5%, in line with Quan’s statement earlier.
But all our calculations thus far effectively assume that any gradient is constant and that the road is straight, when in reality neither is the case. Having to negotiate bends and changes in pace introduces a new variable into the equation: inertia, which is roughly a calculation of mass multiplied by distance to the wheel’s centre.
Wheel maker Mavic has studied the impact of inertia on wheels, and found that the higher the weight at the rim (a la aero wheels), the greater the inertia. ‘That’s important because inertia influences how quickly the wheel reacts,’ says Mavic research engineer Maxime Brunard. ‘If speed is varying greatly from one moment to the next, you want low inertia. If you ride at a constant speed, high inertia is fine.’ In short, if confronted with short, punchy climbs, go shallow. For longer gradients that require one speed, go aero.
Performance modelling
OK we’re getting somewhere, but having only dabbled in data to test our theories thus far, it’s time to go all in. Best Bike Split, which was bought by Training Peaks in 2014, has developed a performance-modelling engine that’s able to absorb a ton of data – a rider’s functional threshold, weight, bike set-up and wheel choice – before predicting a bike split for a particular course.
It’s an accurate enough model for pro teams such as Trek-Segafredo to have utilised it to choose, for instance, whether their riders should use the TT Speed Concept or Madone with clip-ons when faced with an uphill time-trial.
FLO Wheels is a US wheel company that used Best Bike Split to model its wheels over numerous Ironman bike courses to see how terrain affected wheel choice. These included flat, rolling and steep courses, plus extreme courses like the Alpe d’Huez Triathlon, which features the iconic ascent used in the Tour de France.
The company compared light training wheels (1,100g) and aero wheels (1,624g)against heavier training wheels (2,259g). On the Ironman Florida course, which featured just 300m of climbing over 180km, the heavier wheels came in at 5h 21m 44s. The lighter wheels only saved 2s, while the aeros came in at 5h 14m 10s – a 7m 34s saving. Even on the 13.2km Alpe d’Huez climb, the lightweight wheels bettered the ultimate aero combo by just 23s. ‘Our modelling showed that aerodynamics are more important than weight when it comes to wheel selection,’ concluded FLO co-founder Chris Thornham.
So for most real-world scenarios it looks like aero wins, but this lightweight versus streamlining debate is missing the point, according to Lightweight Wheels’ Chris Hewings.
‘Based on experience, anecdotal evidence and being a bit of a fatty, I’d be more concerned about wheel flex,’ he says. ‘Most of the lightest wheels will be inherently more flexible for anyone other than a really skinny racing snake. Power transfer due to wheel stiffness is equally as important as having low weight, which is where most climbing wheels lose out, especially for riders like me, who are over 80kg.’
Hewings makes a valid point, and further to the body weight issue it’s worth considering the following too: if you weigh 90kg and your bike is 7kg, with the wheelset a miserly 1.1kg, your hoops make up just 1.12% of your 98.1kg total set-up. But if you’re like Quintana and around 60kg, with the same frame and wheel weight, your wheelset makes up 1.61% of the total, which may only be 0.5% more overall, but represents a near-50% jump in its proportion compared to the larger rider’s wheels.
So shallow rims for lighter riders and aero for heavier? We’ll give the last word to Specialized aerodynamicist Chris Yu: ‘Combined with a given weight difference, the question of which wheel to choose boils down to gradient and wind – specifically yaw angle, which actually depends on rider speed, too. But depending on the specific combination of aero and weight difference, the trade-off point for gradient can range anywhere from about 4% up to nearly 10%.’
That’s hardly definitive, but it’s an answer… oh, wait, there’s more: ‘That’s before taking into account exposure to crosswinds and speed of the rider.’ Crosswinds, you say? Right. Maybe we should just forget the whole thing…