To the best of your knowledge, has anyone lab tested braking performance (modulation, fading, power) comparing a heavier 160mm rotor to a lighter 180mm rotor (or heavier 180 to lighter 200mm).
My limited riding feeling experience is that a heavier smaller rotor offers better performance than a lighter, bigger rotor, probably because it's a better heat sink, dissipates more and has better modulation.
But I cannot find any properly executed and repeatable testing on this.
Thank you.
Care to show some examples here?
A larger diameter rotor of the same design as a smaller rotor will ALWAYS be stronger.
The surface area of the braking track is what matters next. There is a very limited area to increase braking track, as calipers and pads are a specific size.
Right. Making some assumptions on OP's vague theoretical question, I'm assuming OP is comparing, say, a larger Ashima rotor to a smaller rotor that would be fairly standard with a major manufacturer's brake. Say, for example, a SRAM Centerline.
Part of why Ashima rotors are so light is because the vent holes in them are so big. This reduces the surface area of the brake track, reducing friction between the pads. The material of the rotor itself (since there's less of it) will heat up faster. But it'll also vent somewhat faster since the rotor has a higher surface area to volume ratio. This isn't a rotor you'll want to use for sustained, high speed braking efforts.
Brake manufacturers know there's going to be heat generated during braking efforts. They need to balance how that heat is managed. How much capacity the rotors, pads, calipers, and fluid have to absorb heat before adverse effects (like fade) begin to occur, and also the capacity of those parts to dissipate that heat (transfer it to other parts, or to the ambient air). Vented rotors, aluminum heat sinks in rotors for faster thermal conductance of heat from the braking track to ambient air, pad size, pad material, pad backing plate material, heat sinks on the pad backing plates, ability of the caliper body to dissipate heat, material selection, rotor thickness, etc are all part of it.
It's a fairly complicated answer. For me, there are too many drawbacks of a lightweight rotor for my riding like an Ashima to even consider it. And honestly, I don't even consider rotor weight. My main concerns are that the brake has power appropriate for the riding that I'm going to be doing, and that the brake will be reliable and fairly consistent. I'm not going to be using a smaller rotor, either.
Now, for someone who's riding the kinds of trails that don't require extended braking efforts (they use short pulses of the brakes, which gives the heat plenty of time to dissipate before using the brakes again, minimizing the chances for fade), and don't require utilizing large amounts of power, sure, a lightweight rotor like an Ashima might make sense.
Comparing a larger, lightweight rotor like an Ashima with a smaller, heavier, "something else" in a theoretical sense is kinda pointless, because the details matter. It's quite possible that the final conclusion is going to vary from one rotor to another.
