Why are teams using mountain bike disc rotors at the Tour de France?

As we enter the third week of the Tour de France, we've had plenty of time to look at the bikes of the riders already, and there has been plenty of cause for tech discussion at this year's race. There's been tyre talk, as the debate of tubular vs tubeless was usurped by clinchers. Wheels have had their time in the spotlight too, as Jumbo Visma and Ineos Grenadiers shunned Shimano in favour of non-sponsor selections, despite our suggestion that Shimano has something new coming.

Today it's the turn of another wheel-based component, the disc brake rotors. Riders including Julian Alaphilippe, Marc Hirschi and Adam Yates have been using mountain bike rotors.

• Interested in disc brake stoppers? Check our guide to the best road bike disc brakes

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The rotors they have been opting for are the Shimano RT-MT900 rotors from the brand's mountain-bike-focussed XTR range. This is instead of the SM-RT900 rotors that come part of the road-focussed Dura-Ace groupset that the riders have been using. 

• Want a full run down on Shimano's road offering? Here you'll find everything about Shimano road groupsets explained

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There are various differences between the two sets of rotors, the most noticeable of which is that the cooling fins are smaller in size on the XTR rotors and leave sizeable holes between the rotor's centre-lock body and braking surface. Conversely, the Dura-Ace rotors feature much larger fins which make the rotors almost solid in appearance. This more sparing use of material means the XTR rotors weigh less; nine grams less for 140mm rotors, 10 grams for 160mm. 

The holes drilled into the rotor's braking surface are larger, too, which while untested in this application, can lead to greater bite, better wet-weather performance, and increased cooling properties. Thirdly, the body of the rotor - which includes the centre-lock adaptor and the arms to connect to the rotor's braking surface - is built differently; the Dura-Ace rotors have thinner arms which curve in line with the rotation of the wheel, whereas the XTR rotors use double-strutted, almost-triangular-shaped arms that are bulkier in appearance. While this is unconfirmed, it's a fair assumption that the result is an increase in stiffness and durability. 

There has been plenty of rumour and speculation around why, which we're going to dive into, in the hope that we can find some answers.

We'll preface this with a disclaimer: We don't have the answers. Mechanics and teams keep these decisions close to their chest, but we'll dive in nonetheless in order to see what the most likely reasons are.

Weight

The first and most obvious difference is the weight. As mentioned, the difference is to the tune of 19 grams for a pair of 160mm front and 140mm rear - the most common combination. 

Despite the minor potential benefits of rotating weight, few Tour de France teams need to save 19 grams in order to hit the UCI's limit of 6.8kg. We know from our Specialized Tarmac review that a 58cm Tarmac SL7 weighs 6.89kg, drop that down to the 54cm frame that Alaphilippe is riding and you're already at (or below) the UCI limit. This suggests to us that while weight will be part of the equation, it's unlikely to be the full story as to why teams are using the rotors. 

So, what about cooling?

Both rotors utilise 'Ice Technologies' which essentially pairs an aluminium layer sandwiched between two steel layers, and 'Freeza', which extends this aluminium layer into cooling fins, these are then painted with heat-dissipating paint. With larger fins, one would assume the Dura-Ace rotors are better at cooling, albeit offset somewhat by the larger holes on the braking surface of the XTR rotors. Riders in the Tour de France are extremely unlikely to be dragging their brakes all the way down a descent, so it's unlikely that cooling has ever been an issue, so even if there is a difference, it's unlikely to have been a factor in the decision. 

Better braking?

With the larger holes drilled into the XTR rotors' braking surface, one could predict a slight increase in braking performance, especially in the wet. A theory compounded by Maciej Bodnar's choice to use the XTR rotors at the road world championships in Yorkshire last year. 

This could be a consideration for confident descenders such as Hirschi, but in all likelihood and experience, the Dura-Ace brakes offer such high performance already that we can't envisage any riders going to such lengths for more. 

Aerodynamics?

There probably is a difference, but with the turbulent air coming off the front wheel and the minimal thickness (around 1.8mm) of the rotors, any difference will be small, and it's unlikely to have been a factor in the decision. 

Crosswinds?

When Roval launched its Rapide CLX wheels, they made claims about the stability of their front wheel in crosswinds and that by reducing those moments of panic where your wheel is swept off-line, it would reduce the number of times a rider has to slow down in order to regain stability. With the increased gaps in the rotors, the XTR rotors will potentially be less affected by crosswinds, and with that, they're faster… right? 

Possibly. It would explain why Adam Yates used it on the front wheel only at the Dauphine, but also, it probably isn't the full picture, because in almost every other case they're being used both front and back. And a 160mm rotor will be much less affected by wind than a 622mm wheel.

Is it because they're stronger?

The tolerance for clearance between a rim and a rim brake pad is approximately 3mm. Whereas that of a disc brake rotor is considerably smaller at around 0.5mm. Therefore, if a rotor is ever misaligned, damaged or bent, there's much less free space before the brake pads start to rub. 

With the increased bulk in the body of the rotor and its arms, it could be conceivable that the rotor is stronger, sturdier, and better at withstanding impacts. 

With the amount of riding, crashing, travelling, transportation and manhandling, it's easy to imagine a scenario where a wheel is knocked, dropped, crashed or someone's bike leans against another, pushing the disc out of true. It makes sense that if a team is able to use a rotor that can better withstand such impacts, riders would require fewer bike changes in the long run. 

Or is it a simple case of availability?

With the COVID-19 pandemic closing factories around the world for months earlier this year, and with rumours rife about if and when Shimano plans to update Dura-Ace, could it simply be that Shimano's stocks are running low? 

We reached out to Shimano for an answer, and the response suggests otherwise, as Ben Hillsdon tells us: "Teams are also provided with Dura-Ace rotors so they are free to choose." 

He then went on to reiterate the point of weight: "Our team liaison officers tell me that some teams still struggle to get the 6.8kg weight limit so this is the reason why they request XTR rotors."

The likely answer is a combination of the above, as WorldTour teams are rarely single-minded in their approach to equipment advancements. Weight is clearly a consideration, but the increased durability could well be part of the equation, and the potentially improved wet-weather performance will be a bonus.