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I was wondering whether 20" wheels can get away with smaller rotors than 28" wheels, and if there is a correlation, if it is linear. If it was linear, then 140mm discs on a 20" wheel would be equivalent to 200mm discs on a 28" wheel (and it would be unnecessary to mount anything bigger for pretty much any application).

At a given forward speed, the circular speed of the rotor scales up linearly with decreasing wheel size, which on smaller wheels should help with the cooling and should provide higher friction power and thus higher braking power.

But I can't find any source confirming this, so I'm asking here.

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To achieve the same braking power, the rotor/wheel diameter ratio should be constant. (Assuming the caliper is the same, and assuming cooling is sufficient.)

If the rotor/wheel ratio is constant the leverage between the force applied by the road surface and the rotor is constant.

Also, because circumference scales linearly with diameter, the speed of the rotor surface at the caliper, and hence friction force, will be the same.

Re heat dissipation, rotation of the disc will have little effect compared to forward motion through the air, which will be much more effective at providing cooling air flow.

I would think that for most bikes that are not being used for sustained braking on descents an appropriately sized rotor and brake caliper has sufficient heat dissipation. I’m perfectly happy to be informed otherwise though.

  • An experienced user agreeing with my guess is enough for me to go with 140mm discs for my 20" build, but do you have actual evidence backing up this claim? I suspect there is something I'm/we're missing, because my fancy but down to earth 20" recumbent has 160mm discs and I'm pretty sure there is a good reason for their decision (braking is of course really, really great on that bicycle - but I don't have a 28" with 200mm discs to compare with so I don't know if it's better). – Nobody Apr 27 '18 at 13:23
  • The only thing missing from this is why we want the same braking power: It's fairly obvious, but the kinetic energy to be dissipated in the brakes is mv²/2, i.e. depends only on mass and speed. Most of the mass m is the rider, which doesn't depend on the wheel size, – Chris H Apr 27 '18 at 13:23
  • @Nobody I've got 160mm discs on both a 700C tourer and a hardtail (very much an entry-level hardtail). Both have plenty of stopping power but the discs get rather got on long decsents. 200mm discs will stay a lot cooler. – Chris H Apr 27 '18 at 13:26
  • @ChrisH Now it would be good to know how the dissipated heat relates to surface, rotational speed and forward speed of the disc. I can't calculate that because it heavily depends on data I don't have (or, well, data which would be difficult to calculate). If dissipation relates linearly to rotational speed, 140mm@20"=200mm@28", but I don't think it's necessarily linear. – Nobody Apr 27 '18 at 13:31
  • @Nobody the relation to rotational speed is almost certainly less significant than to forward speed (due to symmetry). But even that would assume a sort of equilibrium which we don't have except in sustained braking at constant input (constant speed down a constant hill perhaps). The thermal mass of the rotor is significant in real situations. – Chris H Apr 27 '18 at 13:42

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