UKC

I am not a cyclist but have just noticed
Why do mountain bikes have disc brakes in the centre of the wheel, and road bikes have calipers mounted on the frame?
Is this to do with performance?
Cheers

In reply to astley007:
There's a variety of reasons why rim brakes are more common on road bikes and now that some companies have released disc brakes for road bikes the debate has heated up.

Disk brakes are more powerful and work well in dirty muddy conditions where the crud would otherwise make rim brakes almost useless. The also prolong the lifespan of the wheels by reducing wear on the rims and allowing you to keep riding on somewhat bent and bashed rims.

IMO disk brakes just aren't necessary for road bikes. I find the grip between the tyre and road to be the limiting factor with road brakes although there is the issue with big long descents where you can overheat and pop a tyre due to rim temperature.

In reply to astley007:

In addition to what Gethin has written: tandems tend to have disc brakes these days as do many but not all commuter and tourer bikes so it isn't quite so clear cut. Mountain bikes began using disc brakes as there's better and more consistent modulation, that is you can control the braking better than with rim brakes. Rim brakes can suddenly grab or fade depending on what has built up on the wheel rim or overheating, while disc brakes can do this, it's a lot less prevalent.

Road bikes, well racing cyclists, tend not to need such stopping power and the biggest benefits for those riders/bikes are no build up of heat in the rim and to a lesser degree being able to continue if a spoke breaks and the wheel goes out of true and the team car can't get to the rider.

In reply to astley007:
There are benefits from having disk brakes on a road bike - the rims can be made even lighter and a stronger profile, and will also last much longer as the use of rim brakes is one of the main limiting factors in the lifespan of a wheel. Its true that the smaller surface area of a road tyre means that higher braking forces would just result in skidding, however if a small disc with a single pot brake were used then that would be similar to the force of a rim brake.

In reply to ow arm:
So going back to road bikes, with rim brakes...are there different rubber compounds you can fit on them, harder wearing? softer for descending..if you doing more hill work than flat cycling?..bit like sticky rubber on rock boots?
and which rims wear out faster..front or rear?..or is that down to which brake lever one tends to grab?
Cheers

In reply to astley007:
As other have said, its to do with a combination of things.

Historically (before the mid/late 90's) both road and mtb used rim brakes.

Mtbs were the first to widely use disk brakes which typically offered better stopping power and modulation, plus performance of discs was less effected by muddy conditions. Mtbs tend to be heavier anyway, so the additional weight of disc brakes was seen as less of an issue vs the improved performance.

Road bikes are more 'weight sensitive', and for a long time disc brakes have been too heavy to be widely seen as worth adopting, and rim brakes have generally given suitable performance (where flatter rolling surfaces, thinner tyres, and less mud meant disc brake level of performance/power has not been seen as critical).

The 'integrated' nature of road bike brakes and shifter have meant the engineering aspects of a working hydraulic brake are more tricky. It doesn't help that rim brakes have never been adopted/allowed on the professional road race scene, so the 'trickle down' of technology that has occurred in the mtb market hasn't happened (which has lead to hydraulic discs becoming cheap and widespread on mtbs).

Things are starting to change as road bike disc brakes (both hydraulic and cable actuated) are become more widespread. Another 5 years I think it will be rare to see new road bikes without discs.

In reply to astley007:


Yes, you can get different compounds (typically dry weather or wet/winter compounds). You also get different compounds for brakes to be used on carbon rims, as brake pads that work well on traditional aluminium rims don't work very well on carbon.

You get different compounds with disc brake pads as well.


Front brake/rim tends to do most of the braking work due to the forces involved when you apply the brakes. Most riders learn to rely on the front brake far more than the rear, both with rim brakes or disc brakes. I've only ever worn out one rim and that was the front, but other riders might have different experiences.

In reply to Phil79:

>I've only ever worn out one rim and that was the front, but other riders might have different experiences.

I've never worn a rim right through but I normally destroy my back wheels falling down pot holes and they split along the wear line.

In reply to astley007:

Nothibg to do with the topic - but a little surprised to see someone else with the same unusual name!