Disc brakes on road bikes?

There's absolutely no basis for the belief that disc brakes have better stopping power at high speeds. Because of the larger radius you can get more torque with less applied force from a rim brake. The larger area of the rim also dissipates heat faster than a disc rotor would on a long descent.

The real reason that disc brakes are great for mountain bikes is that they get frequently ridden in wet, gritty conditions. Disc rotors are a lot cheaper and easier to replace when they get worn down than rims.

There are road bikes with discs, Giant, Avanti/Specialised, Cannondale, to name a few.

artmichalek said:

There's absolutely no basis for the belief that disc brakes have better stopping power at high speeds. Because of the larger radius you can get more torque with less applied force from a rim brake. The larger area of the rim also dissipates heat faster than a disc rotor would on a long descent.

The real reason that disc brakes are great for mountain bikes is that they get frequently ridden in wet, gritty conditions. Disc rotors are a lot cheaper and easier to replace when they get worn down than rims.

+1, sort of. The reasons disc brakes get used on MTBs and not on road bikes is two parts. First, disc brake systems are heavy and complicated, so they're not often seen on road bikes, and the "road bikes" that gclark8 is probably referring to are probably cyclocross bikes.

The second part of why you find them on MTBs: durability. By moving the braking surface away from the wheel's rim, you're less likely to lose your brakes if you knock your rim out of true, since the disc rotors attach directly to the hub and not the spokes or rim.

So the big answer is somewhere in the middle here. Road bikes are less likely to bend their rims since they usually don't see quite as much "action" as mountain bikes, so it makes sense go with the lighter option that brakes just as well.

Oh, and I'd think that rim brakes are probably easier to modulate, though I don't have any personal experience with discs.

Well as I see it there are several reasons "against" disk brakes on road bikes.

1) Road bike forks are not as stiff as MTB forks, so they can take some of the road shocks by flexing. As the brake calipers sit at the far end of the fork, the lever is more important then with rim brakes, so the fork will bend backwards on hard braking and force you to adjust the trajectory permanently.

2) There is apossible heat transfer issue between the brake calipers and the fork. Both carbon and aluminum tend to loose most of their stiffness at temps as low as 150°C, which can easily be reached on long descents.

3) The spokes on just one side of the wheel have to transmit the whole braking force onto the rim. This puts a lot of stress on the spokes and nipples.

4) And I guess there's is definitly a weight issue involved.

Most of the problems enounced could be solved if some clever mind put itself to it, but as to now I only know of the german company "Canyon" which presented a prototype last year, with double 120mm disk brake at the front for a total bike weight of 6.8 kg. Production has not yet started afaik.

Dan

Dan512 said:

Well as I see it there are several reasons "against" disk brakes on road bikes.

1) Road bike forks are not as stiff as MTB forks, so they can take some of the road shocks by flexing. As the brake calipers sit at the far end of the fork, the lever is more important then with rim brakes, so the fork will bend backwards on hard braking and force you to adjust the trajectory permanently.

2) There is apossible heat transfer issue between the brake calipers and the fork. Both carbon and aluminum tend to loose most of their stiffness at temps as low as 150°C, which can easily be reached on long descents.

3) The spokes on just one side of the wheel have to transmit the whole braking force onto the rim. This puts a lot of stress on the spokes and nipples.

4) And I guess there's is definitly a weight issue involved.

Most of the problems enounced could be solved if some clever mind put itself to it, but as to now I only know of the german company "Canyon" which presented a prototype last year, with double 120mm disk brake at the front for a total bike weight of 6.8 kg. Production has not yet started afaik.

Dan

1) It doesn't matter where you put the brake. Unless the wheels are extremely heavy, the bending load on the fork won't change for a given weight and deceleration.

2) You're thinking about strength, not stiffness. And you would have to move a lot of heat through the caliper assembly to get the fork legs that hot.

3) Unless your front hub is made out of rubber, both sides can transmit torque. Radial spokes are out of the question though. (We wouldn't want to contradict the last ten years of marketing, would we?)

4) You could probably come up even on the weight by cutting down the rim (tubular anyway), but there's still no good reason to do it.

Giant CRX 0, http://www.giantbicycles.net/au/030.000.000/030.010.000.asp?year=2007&range=169
Avanti Blade Elite, http://www.avantibikes.com/index.php?option=com_content&task=view&id=7&Itemid=18
Yes the Cannondale is a cyclocross.

gclark8 said:

There are road bikes with discs, Giant, Avanti/Specialised, Cannondale, to name a few.

A search of the websites for these manufactures indicate that they do not make road bikes with disc brakes.

gclark8 said:

Giant CRX 0, giantbicycles.net030.010.000.aspOpen ↗
Avanti Blade Elite, avantibikes.comindex.phpOpen ↗
Yes the Cannondale is a cyclocross.

Not true road bikes.

Road bike technology is financed and driven by pro racing for which discs are too heavy. However for touring and "utility" (yuk word) bikes, discs are highly practical. One day, a genius wheel/bike builder will turn their head to the every day stuff of daily riding and a bike will appear that is comfy with efficient disc brakes (isn't Hope stuff so nice?) and then bombproof and affordable. Buggrit I woke up. I dream of selling a bike that people will take to the supermarket rather than their car. Oh well dream on

rosborn said:

Howdy,

I brought my new bike to work because I was going to ride after. One of my co-workers, who rides a MTB, asked why road bikes don't have disc brakes. His theory is that disc brakes work better in braking at higher speeds. His MTB has disc brakes and he thought that, since road bikes are generally ridden at much higher speeds, it would be better if they were equipped with disc brakes. I guess his theory is valid but I couldn't offer him an answer as to why road bikes do not come with disc brakes. Is it weight? Tradition?

Just curious.

Rob