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Window shopping for road bikes, can't see many that have disc brakes, any reason for this?

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This is a great question. I keep asking myself it whenever I hear commentators on the big (hilly) road races mention that they have to be careful with the braking to prevent overheating and the tire exploding. –  Colin Newell Mar 3 '11 at 12:01
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Tandem bikes going down long hills actually can generate enough heat from braking on the rim to cause a tire blow out. The bead of the clincher softens up enough to allow the pressurized inner tube to squeeze out and thus pop. –  Angelo Aug 17 '11 at 4:48
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Also the glue that holds tyres to tubular rims will melt if the brakes are overused, causing the tyres to separate from the rim –  Mac Sep 15 '11 at 5:48
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For "normal" riding (non-mountain, non-tandem) there's simply no need for disk brakes and they add weight and cost. Simple cost/benefit equation. –  Daniel R Hicks Nov 19 '11 at 22:57
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The title of the question contradicts the actual question. The text of the question implies that there are road bikes with disc brakes, while the title implies that there aren't. –  AndreyT Oct 18 '12 at 20:31
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13 Answers 13

up vote 64 down vote accepted

I think it mostly comes down to one primary thing: disc brakes weigh more and road bikes are supposed to be light. Also, you need a heavier wheel and heavier fork to handle the forces of disc braking, which compounds the weight.

Additionally, the advantages to disc brakes (working better in mud/dirt, easier to work with a suspension, work with really wide tires) generally don't apply to road bikes or aren't what the road bike was designed for. For any bike on the road the only likely advantage of disc brakes would be that they're more likely to work after you've ridden through something nasty (mud, puddle with oil on it, etc) that's deep enough to get on the rims, and you probably want to be avoiding any puddles you can't see the bottom of anyways.

Probably the final reason is simply that disc brakes (generally) cost more than rim brakes.

I have seen bikes made for the road that had disc brakes, but they're generally not the typical "go fast" road bike. (Example: Kona Sutra)

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+1 for Weight. However I disagree with the bit where you suggest that bikes used on the road don't need to cope with the wet... Commuter/city bikes - the heavy kind - are oft equipped with drum/hub brakes in part to help deal with wet conditions. Frame issues hadn't occurred to me - that's a good point too. –  Murph Oct 10 '10 at 18:31
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Tandems generally have them, too. –  Jeremy McGee Oct 10 '10 at 21:10
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I assumed by "road bike" that the poster meant the usual US definition of "road bike" (drop handlebars, narrow high pressure tires, lightweight). Not necessarily a racing bike, per se, but designed to go fast on paved roads. I absolutely agree that weight isn't that important for a transportation/commuter bike; in fact, my own bike is a Surly LHT (beefy steel frame) that probably weighs about the same as two lightweight road bikes. –  freiheit Oct 11 '10 at 1:25
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The advantages of a cable actuated disc brake over that of a cable actuated caliper brake might not be there. Add to this that there were, prior to the past few years, very few hydraulic systems that were small enough to work seamlessly with an STI style body and there is just very little logic to the whole endeavor. Road cyclists also tend to be a surly bunch who embrace certain types of change but are repulsed by others... –  tplunket Oct 12 '10 at 1:39
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Sheldon Brown seems to focus his argument on fork construction. sheldonbrown.com/brandt/brakes.html –  memnoch_proxy Aug 16 '11 at 23:55
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Road bike don't need disk brakes.

I ride a Marin lightweight hybrid and a touring tandem. Both use V-brakes on the rims. The tandem has tungsten-carbide rims, but there's nothing special about those on my solo.

In both cases, unless I'm carrying a lot of luggage, I can brake hard enough to make the wheels skid on a dry road, so any additional power would be wasted, but then I am fairly light.

The reason rim brakes can be so powerful without hydraulics is simply because they act further from the pivot point, the hub. This means they get more leverage. It's also the reason that larger disks are more efficient than smaller ones. A larger piece of metal as a braking surface also helps with heat disapation.

In theory you could get the same advantages by having a disk nearly as large as your wheel, but this would be heavy and impractical. Instead it makes sense to use the rim you already have.

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Yep, in a sense rim brakes are disk brakes. –  Daniel R Hicks Oct 25 '12 at 11:24
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In addition to the other reasons given here, most framebuilders are also wary of using curved steel fork blades with disc brakes. Since many folks want a raked fork on a road bike for handling or aesthetic purposes, that tends to rule out disc brakes as an option.

I had a touring bike with disc brakes made by John McBride of Roseland Cycles, and ended up using straight fork blades (actually a picture of my fork is on the top of their page at the moment). After talking to several other framebuilders, John said it wasn't a good idea to do a curved/raked steel fork with disc brakes. Any curving of the fork blade allows for flexibility in the direction that the brake pushes on the fork blade. That can affect the ride (since only one fork blade is bending while braking, and also can un-spring when you let up on the brakes) and might also contribute to a chance of the fork blade breaking.

All that said, however, I know of at least one mainstream commercial example of raked steel fork with disc brakes, the Jamis Aurora Elite. Maybe they just made the forks blades heavier to compensate for the flexing.

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As a reference point, last week I was on a 6-day ride with about 120 other cyclists, from several states. There were bikes of every shape and description, a few near museum pieces (ca 1980) and maybe 30 brand-new fancy carbon bikes. Maybe 20 recumbents, and maybe 5 tandems, both recumbent and upright.

Of these, I did not observe a single case of a non-recumbent, non-tandem bike with disk brakes (and I specifically looked for this, when we were camped). This is not because of economics -- several very wealthy people were on the ride and most of the rest would be considered upper middle class, plus, as I said, there were a number of very expensive bikes. If they believed that disk brakes made sense they would have them.

The tandems and about half the recumbents DID have disk brakes, since there are valid reasons to have disks on those bikes.

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One thing that hasn't been mentioned, and probably trumps any mechanical advantages or disadvantages of disc brakes, is the fact that the UCI currently doesn't allow disc brakes in professional road races. This has a trickle down effect to you and me.

Most, if not all, of the national racing federations follow the UCI's lead on equipment rulings. This means that they don't allow disc brakes either. Since bicycle companies want to mass produce frames that are going to be useful to the widest consumer segment, they don't produce road frames with disc brakes.

Someone else mentioned that the UCI recently started allowing disc brakes on cyclocross bikes and there's a good amount of speculation that they will allow them on road bikes in the future. The pros would actually love them for mountain descents since rim brakes heat up the rim and can cause their tubular tires to come unglued. If the UCI does allow them, I suspect that we'll start to see them on road bikes more and more. As that happens, manufacturers will sink more money into R&D and the mechanical disadvantages will begin to dwindle and perhaps disappear.

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This answer does a good job of describing that the UCI doesn't allow disc brakes in professional races and why that means bike companies don't produce them. But, it doesn't address why the UCI doesn't allow disc brakes. Saying the UCI doesn't allow them yet is the same as saying sausages taste better than cabbage to a non-vegetarian. Rephrased, the question is, "Why haven't the UCI changed the ruling on allowing disc brakes, thus meaning bike companies will manufacture them?". Other answers didn't address this, they answered the question as it was phrased. +1 vote from me :) –  Scott Langham Sep 22 '12 at 14:48
    
@Blam It's not really that people pay to ride what the pros ride. It's more that the national federations follow UCI rules and bicycle companies manufacture bikes that are race legal. People wouldn't buy them if they couldn't race them, no matter what the pros are doing. –  jimirings Jun 23 at 13:20
    
I meant rules in general not just the UCI. I think some non racers want to ride a bike that looks like the bike they see racers ride. –  Blam Jun 23 at 13:57
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I wouldn't want hydraulic disc brakes on a road bike due to the chance of failure due to heat buildup on long descents caused by boiling of the hydraulic fluid. See related article at BikeRumour.

If I were to ride disc on a road bike, I'd keep it mechanical.

However, due to recent changes in UCI regulations allowing disc brakes for cyclocross racing, many cyclocross bikes have been adapted for disc brakes.

This will probably drive the evolution of lighter, more road friendly disc brakes in the next couple years. It will happen, there is a demand, but there are still some technical hurdles for parts manufactures to overcome before it will be readily adopted. Also, until it's UCI legal in road racing, a lot of big names will avoid touching disc brakes on their higher end offerings.

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Good rim brakes perform just as good as a light disc brake would. I love disc brakes on my mountain bike, And up till last year, I hated rim road brakes. But then i upgraded my road bike and the new one has Ultegra brakes. Wow they stop amazing. Flying down 2000ft descents over 5 miles with no issues at 45mph.

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I think it's because they haven't been invested in much so far by the big bike companies. But, I think they are coming, and will become more common place in the next few years. See:

http://www.bicycling.com/bikes-gear/bikes-and-gear-features/big-squeeze-road-disc-brakes

http://www.bikeradar.com/news/article/disc-brakes-the-future-for-all-bikes-32770/

After cycling my mountain bike which has discs and then moving back onto my road bike, I find I miss the discs. On any future bike I get - road or otherwise - I will definitely be looking for discs. Even if they add a little bit of weight, I just trust them to stop me more reliably than I do with any other type of brake, and I think that is worth a bit of a weight penalty.

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As a sometimes bike mechanic in a multisport (read triathlon, road, cyclocross) shop I think this is THE answer to the question.In the past, UCI has not allowed disc brakes. This year they are allowing for 'cross. I expect to see approval w/in 3 years for road. The big companies will invest and suspect final weight will be comparable with current breaking systems. Try breaking on a wet carbon rim while descending and you'll want the disc! –  Ken Hiatt May 17 '12 at 22:35
    
This answer does a good job of describing why disc brakes would be a good development for road bikes, as well as a potential future for disc brakes. But it doesn't address why they haven't been developed. Saying "I think it's because they haven't been invested in much so far by the big bike companies" is the same as saying "I think it's because they haven't been manufactured yet." Rephrased, the question is, "Why haven't disc compatible road frames been manufactured yet?" Multiple other answers have addressed this. –  jimirings Sep 22 '12 at 5:16
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Keep it Simple

Not everyone knows how to setup and maintain disc brakes. I know that it does not take a lot of work to read the manual but people sometimes prefer to stick to what they know and are hesitant to purchase a bike they do not feel they can confidently work on.

Flawed Technology

Disc brakes are far from perfect. We all should know that the front disc brake design on most mountain bikes that have discs is flawed. Apply the brake and the force wants to shoot the spindle out of the dropout or at least push the wheel across to one side. 'Lawyer lips' go some way to remedy what is a far from sound design but you do not want 'lawyer lips' on a road bike (even if they are mandatory on new bikes).

Fair weather cycling

Most bicycles are designed for fair weather cycling in California, not year round riding in Scotland. They do not have mudguards and their owners are unlikely to ride in the rain. Mudguards have been around for almost as long as the bicycle yet very few bikes are sold in wet places, e.g. the UK, with mudguards. With this, the simplest wet-weather adaptation, not considered a good idea by the cycle trade, why include disc brakes?

Strong/Light/Cheap - have any two of the three

Everything on the bike is a choice of strong/light/cheap - have any two of the three. When it comes to brakes light and cheap are the preferred options except for with a few niche bikes specifically designed for commuting.

Not much point with the rear brake

When decelerating the centre of gravity moves forward. (Some people learn this the hard way by going over the handlebars.) Even with poorly maintained rim brakes in the pouring rain it is relatively easy to lock up the back wheel, resulting in an uncontrolled skid. This may take 5-10 metres to do in very wet conditions, what possible benefit could there be in having a rear disc brake that has the potential for an even quicker 'loss' of rear braking?

Contact patch with the road

The tyres are where the real braking happens and on a road bike you have coin sized contact patches with a smooth surface. This contrasts with the mountain-bike situation where you have chunky tyres and an uneven surface.

Inherently weaker wheels

The disk brake takes up space in the hub. As a consequence the spokes are not spaced as far apart. This loss of 'dish' means that a disk brake equipped wheel is not as strong.
As mentioned by kevins there is also a problem with slowing down just the middle of the wheel with the disc brake, twisting the wheel and going against how the spoked wheel works.

In summary, a disk brake on the rear is not needed (given that maximum braking on the rear wheel can be readily achieved with a rim brake) and a disk brake on the front is flawed (due to how it pushes the spindle out of the fork dropout with current designs). Then it is going to cost more, weigh more and impose a learning hurdle come maintenance time. Plus you have lost the design elegance of the rim brake and the forces placed on the spokes are plain ugly. Admittedly there is more control when braking on discs and there isn't that scary feeling of 'my brakes are not working whatsoever' when applying the brakes in the wet but most bikes are designed for fair weather cycling in daylight hours and anyone experienced at riding in the wet soon learns to ride within the limits of their brakes.

If there really was a problem with rim brakes on bikes then the papers would have corresponding tales of woe. Actually there have been more stories of woe regarding front-wheel ejection from disk brake equipped bicycles over the last few years.

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I agree with everything you say apart from the 'flawed technology' part, surely that is a failure of fork design - not brake design. My forks have entirely enclosed dropouts (the axle slides out one side) so the problem doesn't exist. –  cmannett85 Feb 14 '12 at 7:57
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Re. the flawed technology bit. Mountain bikes have evolved into using a quick-release bolt-through axle instead of a traditional drop-outs. They're stronger, stiffer and you're not going to lose the wheel under hard braking. I don't know how feasible it is to build one light and strong enough for road bikes though. –  Olly Hodgson Jun 21 '12 at 13:32
    
+1 for fair weather cycling. –  olee22 Jul 4 at 4:53
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Check out this link for a recent post by ex frame-builder Dave Moulton about disc brakes. He discusses the reverse-directed stress to the spokes due to disc brakes as a potential problem.

He also points out that the standard caliper brake can be viewed as a disc brake with a much larger diameter disc (the rim) and without the problem of transferring the force through the spokes.

EDIT

Another good point brought up in the above linked article is the following

A disc brake means you can’t have radial spokes. Not only must there be crossed tangential spokes on the non drive side of the rear wheel, you must also have tangential spokes on the front wheel.

Even most lower level road bikes have radial spoke patterns. Changing to a tangential spoke pattern would increase the weight of the wheel.

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He obviously doesn't commute in winter in Canada - disc brakes are the greatest invention ever. –  mgb Oct 20 '10 at 21:37
    
I've never felt the need for them. –  user973810 Dec 15 '11 at 20:05
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+1 for "no radial spokes". I also hear disc-brake set ups are worse for aero. –  James Bradbury Dec 3 '12 at 11:06
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The tiny surface area of the tire doesn't give the most grip. It's pretty easy to lock up a road bike tire with STI or modern Cyclocross Cantilever brakes. The power of a disc brake would often leave one skidding out of control.

I also agree that the extra forces applied to the hub, spokes, fork, and head tube could cause structural defects or failure.

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When you say 'road bike' I think you just meant "not an off-road bike" ... you didn't necessarily mean "racing bike" ... you might have meant to include, I don't the term, a 'town bike' ... for commuting.

On that kind of bike, if you're not seeing disc brakes, I think that's because of the price range that you're looking at. When I looked into a bike shop full of bikes which cost up to about $650 then I don't see disc brakes. If I look at bikes whose prices start at about $750 then maybe they do have disc brakes.

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My commuting bike has discs - I think they're great (for these reasons)

Weight is an issue, but I think the downsides outweigh (sorry!) that - but it's fairly flat around here and I'm doing it for the exercise anyway!

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