"They just collapsed because of the pressure": Giro d'Italia hopeful blames disc brakes overheating for ending GC challenge

I don't have a physics answer, but I'm just wondering how close his spokes are to the disc? bearing in mind that they're rotating so will be getting a pretty strong cooling effect, those discs must be getting super hot, or he's been tialling specialized's new cheese based spokes....

The rotor doesn't touch the spokes, and so can only conduct heat by radiation and convection. There is likely to be minimal transfer by radiation unless/until the rotor is literally red hot, and given the cooling effect of air passing at speed, the transmission of heat to the spokes via convection would be absolutely minimal.

So no, basically.

If you want to test it, feel your spokes at the bottom of a long descent. They will be air temperature or even cooler. Don't touch the rotor, because you'll burn yourself, but if you really want proof... 

Discs in general do cause a bit more stress to spokes, but that's mainly due to dishing, and has nothing to do with heat.

It's also worth noting that not 1 single other rider - and AFAIK everyone on the road that day was riding discs - had the same problem. If it was systemic/a general issue as Kelderman suggests, we'd likely see/have seen a lot more of it.

Physics is wrong, conclusion is right. Main heat transfer to spokes will be conduction via hub body. Dodgy spoke will fail from stress, not heat.

So the rotor will get hot enough to transfer heat to the hub body then to the spoke? 

That's theoretically possible but for any transfer to be meaningful you would have to get the rotor seriously, seriously hot first. 

Next, all that heat in the hub (conducted via the rotor) would cause seals and grease to catch fire way, way below the temperature needed to weaken spokes. 

You would have to heat steel spokes to hundreds of degrees to appreciably weaken them. Meanwhile your tyres have caught fire and your hub is emitting smoke.

It just won't/doesn't happen in the real world.

Mechanical stress is a totally different argument.

I'd say very unlikely. But you do get stresses in the wheel with a disc brake that you don't with a rim brake, so wouldn't rule it out as a factor

Is it said which model wheel? It would be good to know the lacing pattern.

When discs first came in for MTB a few wheel builders tried radial lacing, but there is too much twisting and that caused spokes to break.

You'd think that we would have heard of this happening more often in MTB given disc brakes have been used for over 20 years

That was my only thought, and when looking up what they were likely on, Alpinist or Rapide. They are both radial for non-brake side and one cross for the braked side. They also run DT Swiss Aerolite T-head spokes. Straight pull. 

There is no way in my opinion would the heat coming off a rotor ever get hot enough to radiate over to a spoke for long enough to cause failure. At the end of a race, unhappy with his performance, his back pain....etc. Was just looking for answers that he didn't have.

Agreed. MTB hs been smashing bikes down alps for decades with disc brakes. Dissipating the heat so the brake fluid doesn't get too hot is important (hence finned brake pads) but no one has ever cooked a spoke. 

Agreed regarding the excuses. If spoke failure 'due to heat from disc brakes' was the cause then surely plenty of other riders would have had similar issues, particularly his team-mates riding identical team bikes.

dave atkinson wrote:

I'd say very unlikely. But you do get stresses in the wheel with a disc brake that you don't with a rim brake, so wouldn't rule it out as a factor

Indeed, a disc brake requires torque to be transferred between hub and rim, via the spokes (hence tangential spokes). Rim braking does not.

Given that braking forces are unlikely to exceed 1G, the spokes in a rim braked wheel will not be under any greater tension due to braking than they already experience due to the weight of the bike and rider.

I'm none too sure how much tension the torque transfer imposes on the front wheel's spokes during disc braking, but I suppose it exceeds the case of the driven wheel's spokes by the same factor that a rider's braking exceeds their acceleration, which is to say not by much.

Sriracha wrote:

braking forces are unlikely to exceed 1G

If memory serves, it's more like 6ms^-2 or 0.6G. Try and brake any harder than that and you go over the front.