Bike Check: Nino Schurter's Custom 10-Time World Champion Scott Spark RC

 Note that he's running a 38T chainring, compared to 32 or 34T for us mortals.

 @zedpm: I run a 32 with 51 in the rear and for sure Nino goes faster up the climbs than me + 17%

 @zedpm: Speak for yourselves. In the Old Mortal class I'm 28t everyday of the week, twice on Sundays.

 @hardtailherold: that's not how it works. With more power at the caliper you need more grip, or you'll just lock up the wheel.

 @ak-77: although SRAM brakes have modulation so they don't just lock up. Also contact patch plays a massive part it. So wide low pressure XC tyres wil have a large contact patch.

Also, if that's how it worked why would anyone bother with bigger discs and better pads if the limitation is the tyres. And why would brake uprades slow you down even with cheaper tyres and grip compounds.

 @HardtailHerold: your braking power can never exceed the traction of your tires, that’s basic physics for any vehicle. When it does your wheels lock up and you slide. Modulation helps prevent lock up, but it doesn’t change the maximum braking power that your given traction can provide (traction determined by your tires, your weight distribution, and the trail surface). You’ll generally only see the larger rotors and 4 piston brakes on bikes that have tires that support the added braking power (ie, enduro, downhill, aggressive trail, etc).

As to theories of why he runs 4 piston on an XC bike, I suspect the ability to run smaller rotors someone already mentioned is likely. Less rotating mass with the lighter rotors probably better for him than slightly lighter calipers.

 @Betacygni: Rotating mass making a sizeable difference is a myth that is hard to kill. The rotors sit pretty close to the axle, so they hardly count. Furthermore, I have done the calculations once for the difference between 1kg of mass added to the outside of the wheel VS to the frame. The difference was a few seconds to an hour on an XC course with frequent braking and acceleration. Saving weight on the rotors and adding to the caliper is not going to make any difference.
Rear brakes on XC bikes are usually not short on power. My guess is that this combo gave him the best modulation.

 @ak-77: when podiums are decided by a second....was that one 1600g tire or two?

 @ceecee: It was either one 1000g heavier tire or a 1000g heavier frame. Of course, there are very few situations where you actually trade rotating mass for non-rotating mass, but the take home message from my simulations was to save on total mass and mostly stop caring about whether or not it rotates.

Or actually, the take home message was to save on rolling resistance first and total mass second. But rolling resistance in a realistic situation is quite hard to measure.

 @ak-77: but saving rotating saves total. Did the simulations account for rider fatigue accelerating a kg heavier rear tire w/ same tread? Were they published anywhere respectable? I'm sceptical

 @ceecee: Yes, saving rotating saves total most of the time, but in this case it was suggested Nino runs heavier 4pot calipers so he can save on rotating mass with smaller rotors.

I did those simulations just for fun and to educate people about what does and what doesn't make a difference, and by how much. I can't be bothered to write it all up and go through peer review etc., especially since I usually publish in very different fields of physics. And anyway there's no new science in there, simply Newton's law.
The point here is that the amount of energy you lose from the extra power needed to accelerate rotating mass wrt static is tiny, and whenever you have to make a choice between weight and other performance factors, the fact that the mass rotates shouldn't really influence the decision.
Rider fatigue doesn't come into play, I simulated the same effort and compared times.

If you are sceptical PM me, I am happy to send you the code, it's a few simple Matlab scripts, if you find an error let me know.

 @ak-77: I'm just a guy w/ a tape measure and an elementary physics education, but thanks for the offer. The smaller rotor case was ignorable but the claim on +1kg of circumferential seemed sensational. It must come down to the force required to move the total mass vs the force required to rotate/accelerate the wheel/s. I'm keen on weight since I move it around a lot and clearing the same gap at the end of a ride can be more difficult than towards the beginning--still don't see how fatigue doesn't come into play when one can't produce the same effort, but must accept that lifting +1kg is more laborious than rotating it. Reduce weight, but not because it's rotating weight. Use lighter tires because they're lighter, not because they're easier to accelerate--and worry over rolling resistance first, esp when racing. It has the greatest effect on fatigue, followed by total mass, followed by accelerability. Never mind wind speed and direction! Tally ho