Do I need a bigger one?

[lune ranger]

My rear brake is a little under powered.
160mm on a loaded Krampus.
Surly state 160mm as max rotor size but there is room to add a bigger rotor. Leaving aside the extra torque and frame breakage/warranty issues, should I get a bigger rotor?
Will I even notice the difference between a 160mm and 185mm rotor? (don't think 200/203mm will fit)

[In Reverse]

Puerile lol

[Bearbonesnorm]

Due to the weight transfer under braking back brakes really don't / can't provide much stopping power. Larger rear rotors will generally lock the wheel sooner thus providing even less stopping power.

[lune ranger]

Fair point. What is it 2/3 front 1/3 rear distribution? something like that.
At the moment the rear is not really locking st all. The Krampus has a big footprint and a lot of traction so there is room there to improve the braking power, not just locking up quicker.
I think I just answered my question!
I wonder though if 185mm rotors have a real world increase in power over 160mm when all else in the system (lever, cable, caliper etc) remains fixed.

[whitestone]

Worth noting that having too small a rotor can lead to overheating of the rotor which then glazes the pads meaning ineffective braking performance.

Just noticed you've said "rear" rotor. 160mm should be fine for most purposes, I'd be more inclined to have at least 180/185 on the front, maybe 200mm

[Rasta]

I was under the impression that rotor size was irrelevant when it came to stopping. The brake pad and the rotor surface that it touches are the same regardless of diameter.

A larger brake pad surface will stop you better.

A larger rotor dissipates heat better due to its larger surface area. So it is better on longer descents.


I could be wrong.

[Bearbonesnorm]

A larger rotor obviously has a greater diameter with the pad acting further away from the axle ... which is the reason for the increase in potential stopping power.

[psling]

A larger rotor obviously has a greater diameter with the pad acting further away from the axle ... which is the reason for the increase in potential stopping power.

Which is undoubtably true. Which is why I can never get my head around why rim brakes (with effectively a 26"/27.5"/29") rotor are usually less efficient

[lune ranger]

The outside edge of the large rotor is also moving faster than a smaller diameter one so you'd think it would be harder to stop. Patently not though.
It's only £30 for an upsized rotor and mount so I'll give it a punt.
What about pads? I've always bought metal sinitered pads. Will organic/resin/carbon/whatever pads be better stoppers? As I understand it metal pads last longer but organic brake better with a shorter life.

[Alpinum]

Worth noting that having too small a rotor can lead to overheating of the rotor which then glazes the pads meaning ineffective braking performance.

This is why I ride 200/180 rotors front and back on my trail bikes.

I think of it that more contact area passes the pads giving you benefits in a few areas.
A powerful brake will be eaier to modulate with big rotors and then there's also the heat issue. I've had issues with many brakes (especially Shimano XT and 160 mm rotors) until I went for bigger rotors. (Especially on the long descents in the Alps, but not sure how much this translates to the UK). In every case the results were better feeling brakes and noticeable more power, not just when they seriously heat up.

Fun fact: I remember weighing a size M 2018 Krampus w/o fork against a Trek Slash size L including rear shock and hardware; the Slash was lighter. It's stiffer too.
The Krampus' frame is a heavy beast.
I can't imagine it being an issue. And it also has a reinforcement tube at the brake mount (at least 2018 models do). Should be alright.

Only downsides are slightly heavier weight and more potential for bending.
TRP makes some fantastic rotors. Basically any with an aluminium spider will deal better with the heat and is less likely to go untrue.

No matter what theories and experience we come up with, it's so easy to find out yourself, I'd give it a try if I were you.

I'd be happy to read about your experience if you up the size.

[lune ranger]

The Krampus is heavy alright. When you add the humongous wheels and tractor tyres onto the frame. Plus Rohloff, Brooks saddle and all the other gubbins it weighs about the same as a 10 year old girl... fully clothed and wet through.
It does roll over pretty much anything though and feels satisfyingly attached to the ground (heavy weight + gravity = not a natural sport climber)
Probably won't notice the extra weight of an extra 20mm if rotor will I...

[fatbikephil]

Used 180 rotors on the back of my Kramp for years - didn't know 160 was the max size - its been fine and for brakes, bigger is always better

[Alpinum]

Probably won't notice the extra weight of an extra 20mm if rotor will I...

Very likely not...

I ride one my self and it truly is a tractor.

Heavy wheels also have quite an effect on braking. And much more obviously...

and for brakes, bigger is always better

Reminds me of wildlife photography talk. Focal length can only be replaced by more focal length.

[TheBrownDog]

Late in here. As above, 20mm bigger on the rear won't make much difference - Ive got 160mm on the back of all my bikes and it's plenty, as the front end does most of the braking. But 203mm on the front will make a big difference, especially if you've got 160mm there at the moment.

[lune ranger]

No, got 203mm up front.
My other MTB style bike, Thorn eXXp has 203/203mm and stops very well indeed. I have been known to ride that with a bike/rider/luggage all up weight of 140 or 150kg and never lack brake power even in the wet. My total on the Krampus is never more than 105/110kg. I'll chuck on the bigger rotor and see. It won't be worse will it.

[Rasta]

Interesting.

A larger rotor will allow for a greater torque to be applied. Force x distance.
Nearly all modern & quality disc brakes will "lock" a rotor very easily. Almost instantly. This means that the kinetic energy being transferred into heat no longer goes through the rotor but through friction between the tyre and ground. A skid.

If you want to scrub speed on a long descent then you need to dissipate heat efficiently without skidding. This means a larger disc area and avoiding locking up - which means torque is not relevant.
Torque is only relevant if you want to stop your back wheel from turning. This is not relevant in mountain biking. Unless you like skidding.


But I could be wrong. Still learning.

[Alpinum]

Nearly all modern & quality disc brakes will "lock" a rotor very easily. Almost instantly. This means that the kinetic energy being transferred into heat no longer goes through the rotor but through friction between the tyre and ground. A skid.

On long downhills I often experience riders even struggling with their hands on very powerfull brakes like Codes and Saints. We like to buy -
because it's easier and more instant - more powerful components instead of training our hand force...

Also in such cases big rotors win too.

I think the rear wheel control is larger with bigger rotors. For 3 years I rode a pure bred DH bike with initially 200/180 mm rotors and Sram Codes and struggled to keep the grip up under braking on the rear. 200 mm rotors made it easier. Perhaps it was down to struggling less with arm pump, anyways, I felt more ease to apply the right amount of braking force.

[lune ranger]

I'd be happy to read about your experience if you up the size.

Sized up from 160 to 185mm rotor.
Have ridden the Krampus with the new rotor quite a bit now including a decent 100km ride across Dartmoor with my BB200 kit strapped on.
The power and modulation on the rear is noticeably better than previously. On steep descents I'd be able to control the bike with the rear brake alone if I chose to.
All good.

[Zippy]

I haven't done the calcs (but i have sortof studied some post braking efforts using my thermal imaging camera, as you do).

Whilst you do get more potential braking force due to the extra lever arm through a larger diameter disc, I wouldn't mind betting that the advantages of a larger disc are mostly due to increased thermal efficiency.

[lune ranger]

Either way the brake appears to be better.

[wriggles]

Curious what brakes you are using Lune Ranger as have read the thread and not seen mentioned, surprisingly

[lune ranger]

BB7