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Discussion Starter · #1 ·
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.
 

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The smaller rotor will heat up faster due to the lower surface area and more friction needed for an equivalent braking force at the wheel. This is why in auto racing they use bigger rotors diameter rotors for increased braking performance and obviously there's some consideration to heat capacity. It's also why no DH racers are using small heavy rotors (they all use 200mm+ rotors).
 

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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.

If you take a 180mm vented rotor, and compare it to a 180mm non vented rotor, the non vented rotor will likely have a greater braking force just due to the fact there is more materia in contact with the brake pads creating friction. The drawback is greatly increased heat retention.

Long and short of it - if you want braking power, large rotor.
If you want weight, smaller rotor.
 

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yes it does, just in a different plane not relevant to this discussion...
well again, dependent on compressibility of the material and how it is compressed,
but yes not relevant here to nit pick about material science and physics of making atoms closer to each other, and especially in regards to 40 dollar bike rotors
 

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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.
 

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Care to show some examples here?

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.
About those ashimas..... If there is more air than braking surface, that rotor is not going to work very well for any real serious braking needs.

people need to remember a brake is nothing more than a mechanism to convert kinetic energy into heat. There is a balance to it for sure.
 

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About those ashimas..... If there is more air than braking surface, that rotor is not going to work very well for any real serious braking needs.

people need to remember a brake is nothing more than a mechanism to convert kinetic energy into heat. There is a balance to it for sure.
That's my point. OP didn't say which "larger, lighter rotors" he was talking about. So I made an assumption with a somewhat obvious example.
 

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Discussion Starter · #16 ·
I do, but I was looking for a lab testing report for some hard data.
As said there are endless designs and one cannot test them all.
Regarding mass, given the same thickness of 1.8mm, a heavier rotor will have a bigger surface.
Heat is dissipated through radiation as well, not just convection, plus big part goes through the arms to the hubs, to the point that bearings are often damaged by that heat, that "cooks" the grease in the bearing.
But then again, my question was if anyone knew about some lab testing result available online on this matter.
 

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I do, but I was looking for a lab testing report for some hard data.
As said there are endless designs and one cannot test them all.
Regarding mass, given the same thickness of 1.8mm, a heavier rotor will have a bigger surface.
Heat is dissipated through radiation as well, not just convection, plus big part goes through the arms to the hubs, to the point that bearings are often damaged by that heat, that "cooks" the grease in the bearing.
But then again, my question was if anyone knew about some lab testing result available online on this matter.
Sure. Every company does their own testing to ensure their product meets their own expectations.

Let me guess, you like reading the German bike websites where they test every damn thing? Me, I hate that ****. Life's too short.
 

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Trying to make some sense out of the question, you seem to be comparing a small rotor like this:
Rim Auto part Motorcycle accessories Automotive wheel system Circle

with a lot more of cooling area,

To a rotor with larger diameter, that looks like this:
Rim Spoke Line Circle Line art


I suspect you'll not find any studies given the number of variables involved: riding speed, material of the pads, etc. Samewise, the solution that applies best for you may not be the best solution for me.

In general, people worried about modulation will prefer resin pads, and people worried about temperature will prefer metal pads. Heavy people will likely worry about rotor size. And you'll be somewhere between these three.
 

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Here's my issue with the small rotor idea...The only place I've had actual significant brake (pad) fade is riding actual DH trails. This was fixed with a pad upgrade. I'm 215 lbs btw. The real issue I run into at the bike park is finger fade (hands and arms fatiguing). That's the real reason I'm running Code RSC and 200mm Centerline rotors on my enduro bike now. I have the big rotors for braking power, not heat dissipation. IOW, heat isn't the problem. Smaller rotors don't need better heat dissipation.

I'm actually thinking about upgrading the 180mm rotors on my hardtail to 200mm for more power. I haven't had any issues with heat on that bike. Better heat dissipation on that bike would help zero.
 
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