I don't get rotor size ...

[Valbrona]

***Scenario: Unladen riding. High Alps. CABLE operated disc brakes.***

Is heat dissipation the main reason for large rotors? If so, does heat dissipation matter far less in the case of cable discs because there is no fluid to get over-heated and spoiled. In fact, does heat dissipation matter at all with cable discs?

Thanks.

[tim-b]

Hi

Excess heat will make a brake rotor expand and it could be permanently warped

Hope make vented and floating rotors to alleviate these problems (info available here). Hope's vented rotor in "Dyno tests show up to 15% less heat build up compared to our floating rotors". Their "Floating design deals with changes in temperature" (references here-click on "more info")
In motor vehicle braking the discs can crack due to heat changing the metal's properties

Brake pads can glaze over due to incorrect bedding in / heat, which doesn't help you to stop

You've got to work hard to cause problems with modern brake fluids in modern brakes that are used and replaced as recommended

Regards
tim-b

[pwa]

Hi

Excess heat will make a brake rotor expand and it could be permanently warped

Hope make vented and floating rotors to alleviate these problems (info available here). Hope's vented rotor in "Dyno tests show up to 15% less heat build up compared to our floating rotors". Their "Floating design deals with changes in temperature" (references here-click on "more info")
In motor vehicle braking the discs can crack due to heat changing the metal's properties

Brake pads can glaze over due to incorrect bedding in / heat, which doesn't help you to stop

You've got to work hard to cause problems with modern brake fluids in modern brakes that are used and replaced as recommended

Regards
tim-b

Brake fluids boil pretty easily with tandems ridden in steeply hilly terrain. It is advisable to back them up with a third, non-disc brake.

[reohn2]

***Scenario: Unladen riding. High Alps. CABLE operated disc brakes.***

Is heat dissipation the main reason for large rotors? If so, does heat dissipation matter far less in the case of cable discs because there is no fluid to get over-heated and spoiled. In fact, does heat dissipation matter at all with cable discs?

Thanks.

Larger diameter rotors mean more efficient braking,we have 203mm rotors on our tandem,and I have 160mm on the solos*,I/we haven't ridden discs in the very big hills but I reckon 160mm on a solo would be sufficient.
Some manufacturers specify maximum rotor size,especially front,so check yours out.
Heat kills brakes and causes fade,there are reports of plastic bits such as adjuster knobs on BB7's melting due to heat.
On long descents don't drag the brakes,I prefer to pulse brake and alternate between front and rear,giving each one the chance to cool a little.Discs,especially rotors cool quite quickly.


*I reckon a 140mm would be enough on the rear.

[andrewjoseph]

the rotor material/design makes a difference as well. the avid 160mm rotors that came with our bb7's 5 years ago warped very easily and i was always truing them.

i put shimano 160mm rotors on 3 years ago and they warp very little.

i still fancy getting 160 floating rotors though. i have them on our mtb's (203 front 180rear) and they hardly need to be touched after years of use.

[stewartpratt]

You can look at rotor size in one of two ways:
1. For a given force at the lever, you get greater retardation.
2. For a given desired retardation, you need less force at the lever.

Basically a larger rotor gives a brake that most people would describe as "more powerful". The greater your speed and/or all-up weight, the larger a rotor you'll want.

Also note that for a given retardation, a larger rotor gives less force at the axle. So if you happen to be worried about dislodging the front wheel, a larger rotor may help.

As for heat in cable brakes, yes it can make a difference. The first time I used discs was in the Alps and halfway down the more technical descents the pads were doing nothing (and stinking like hell). Fitting different pads pretty much fixes this, though.

[Brucey]

bigger discs = lower loadings everywhere in the wheel, the brake, and the fork, for a given level of retardation. Bigger discs mean less heat build-up too.

Heat build up can cause;

- pads to go away (can cure with different pads if this happens, assuming you don't crash in the meantime)
- discs to blue up, wear faster than normal, warp, crack or even melt/fail catastrophically (last one fortunately very rare)
- plastic knobs to melt
- lube to melt in caliper
- pads to become disbonded from backings
- fluid in hydro/part hydro brakes to boil
- seals to melt
- damage to non-metal parts attached/adjacent to brake parts

Heat build up matters quite a lot. I think that many riders assume that their disc brakes 'must work perfectly under all conditions' because they seem like a new and spangly idea. But neither thing is true; I don't think that there are many bicycle brakes (of any kind) that you can use willy-nilly on a long descent with no fear of any problems. R2's alternation policy is a good idea with many brake types, including discs.

140mm discs might well be OK as a rear brake but the only reason they are looking to fit them on 'road bikes' is that otherwise, the weight is going to put the (nutty, weight-obsessed) roadies off the idea.

BTW when most of the available disc brakes failed a representative test used by one of the bike manufacturers, a few years ago, they 'changed the test' so that the brakes (the ones they wanted to use anyway) 'passed' instead.....

I anticipate that once disc brakes are fitted on more everyday bikes, you may well get some nasty accidents and then some major recalls/class action lawsuits in the USA. Relative to how 'normal people' are likely to use them, many bicycle disc brakes have a smaller margin for error than (say) car brakes.

cheers

[mark a.]

So if I'm right, smaller discs have better modulation? i.e. for a given range of lever travel you get a more gradual change in retardation. So smaller discs would be better in low grip conditions. They'd also be lighter.

Otherwise large discs are the way to go.

[beardy]

Do you actually get a choice?

Isnt the size preset by the cutouts in the frame and the position of the mounts?

[Brucey]

Do you actually get a choice?

Isnt the size preset by the cutouts in the frame and the position of the mounts?

not entirely; calipers mount to an IS using an adapter, and you just get a different adapter to suit a different disc size. You can't go too nuts with this but you can certainly go up a couple of sizes.

So if I'm right, smaller discs have better modulation? i.e. for a given range of lever travel you get a more gradual change in retardation. So smaller discs would be better in low grip conditions. They'd also be lighter. Otherwise large discs are the way to go.

I think that if you have high MA overall and some sticking friction in the mechanism then this could make for a brake with poor modulation.

Up to a point more MA in a hydraulic system doesn't seem to get to the point where the modulation is bad per se, but it might take a bit of getting used to. I think that if you ride lots of different bikes but one of them has exceptionally powerful brakes, you can chuck yourself over the handlebars with that one perhaps if you have to brake suddenly or something and you do so in a reflexive fashion.

cheers

[beardy]

But many frames come with only enough clearance for a certain size of disc, especially at the rear.

[andrewjoseph]

So if I'm right, smaller discs have better modulation? i.e. for a given range of lever travel you get a more gradual change in retardation. So smaller discs would be better in low grip conditions. They'd also be lighter.

Otherwise large discs are the way to go.

In my experience larger disks give better modulation/control as well as outright power. i suspect a lot has to do with the system used, i didn't like magura hydraulic mtb brakes, shimano deore were better, hope mono mini better still, all on the same size 160mm rotor.

203mm front allows be to drag a bit when I was teaching skills to novices.

[andrewjoseph]

But many frames come with only enough clearance for a certain size of disc, especially at the rear.

you don't generally need as big a rotor on the rear.

[stewartpratt]

So if I'm right, smaller discs have better modulation? i.e. for a given range of lever travel you get a more gradual change in retardation. So smaller discs would be better in low grip conditions. They'd also be lighter.

IME larger rotors give better control: having to use less grip force means you can more quickly modulate your braking if required, because the difference between a locked wheel and a rolling wheel is just a slight reduction in grip rather than a noticeable movement of your fingers.

As per Brucey's remarks above, if you came from a bike with poorly-configured cantis to a bike with hydraulic calipers and 203mm rotors, you'd initially find them rather grabby. But once you adjust, you realise you have more control with more power (or at least, I've certainly never had brakes so powerful that that ceases to be the case).