For years, running rotors that measured 200 or 203mm in diameter was the norm for downhill bikes, while 160-180mm was the typical trail bike rotor size. Those dimension are still common, but we're starting to see a shift towards even larger rotors, especially on the World Cup DH circuit. It's partially due to the arrival of 29ers – those big wheels take more power to slow down, especially when the course is super steep.
Just how big can rotors get? Well, SRAM's athletes have had access to 220mm rotors for the last few seasons, and companies like Galfer, Trickstuff, and TRP, among others, have rotors in that size range as well. It's not just the diameter that's increasing – in many cases the actual rotor thickness has been increased in order to improve heat dissipation, and to reduce the likelihood of the rotor getting warped.
Bumping up a rotor size creates more power, which means that riders don't need to expend as much energy pulling on their brake levers. That's an important factor, especially on those near-vertical tracks – arm pump and overall fatigue are directly related to how hard, and how often, a rider needs to grab those levers. Of course, there are limits, and it'd be silly to put a gigantic rotor on an ultralight XC bike, or on a bike that's not ever going to see steep terrain – it's all a matter of picking the right tool for the job. Plus, fork manufacturers typically have a maximum recommended rotor size that's worth keeping in mind.
All that being said, if 220mm rotors work well, why not go even bigger? That seems to be the route Galfer are taking - Baptiste Pierron was rocking a prototype super-sized 246mm front rotor last week in Vallnord. He's since downsized for Les Gets, where the real challenge is going to be resisting the urge to grab a little brake on the ridiculously high speed, open sections of the course. Val di Sole is the next race on the calendar, a rugged track full of boulders, roots, and plenty of sections where as much stopping power as possible will come in handy – it'll be interesting to see what solutions are employed to find the ideal balance between speed and control.