Window shopping for road bikes, I can't see many that have disc brakes. What are the reasons for this?
Disc brakes are now common on road bikes, even racing road bikes in the professional peloton. Most of the major bike manufacturers offer disc and non-disc options for most of their bike models. Some "endurance", "adventure", and "cyclocross" (drop bar bikes sold under "road" category, but intended for some off-road use) models only offer options with disc brakes now.
However, there is some opposition to their use in certain types of road racing. If a rider's leg contacts a disc rotor while the bike is going fast, that rotor will likely cause a serious injury (it's spinning fast and pretty thin, so cuts flesh easily).
The advantages of disc brakes are primarily in wet and/or muddy conditions. In dry conditions, a good rim brake can lock both wheels up just as well as a disc brake. So for road-racing under dry conditions, there's no real advantage to disc brakes, and appears to be some disadvantages.
Many of the "racer" style road bikes follow what's allowed for professional road bike racing, so it's possible that the current opposition to disc brakes in professional road racing will eventually lead to them becoming less popular on road bikes.
Personally, my next new bike will definitely have disc brakes. I don't race in a peloton, I do sometimes ride in the wet, and it's much cheaper to replace a rotor than to replace a rim or wheel. (rim brakes eventually wear down the rim and make you replace it)
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)
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.
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.
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.
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.
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.
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:
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.
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.
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.
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.
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.
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.
Here's another reason that hasn't been discussed: rim brakes are better for "feathering" - where you very lightly engage and/or disengage the brakes to make very minor speed adjustments, which is incredibly important in the middle of the peloton. You don't want to surprise anyone with sudden changes, you want to be able to respond instantly and smoothly to what's happening in front of you.
I understand (apologies, no reference) that disc brakes have better absolute stopping power, but for fine speed adjustments and feathering control rim brakes are superior, hence their dominance on road bikes.
Just to add a few points that have been missed ( or I didn't see )..
Disc brakes overheat.. This may have been an issue on really old, really crappy calipers but I've had rotors hot enough to burn my arm but I've never "boiled" the brake fluid. If you have tried to bleed your brakes an done it improperly, you will have introduced air to the brake lines. Air will rapidly expand and create the spongy feeling attributed to "boiling". If don't trust me, pour some dot5 in a sauce pan and try to boil it.
Disc brakes are for power...
Disc brakes ( aside from wet performance ) have always been about modulation and control. A well bled set of shimanos have always been able to provide linear control with one finger in any condition. Far better than any caliper, canti or mini-v setup. (Yes I ride TRP Mini-Vs on my trail bike and I love them... simplicity is king!) Keep in mind, old hayes brakes were the equivalent to a boat anchor and got love for that from the old school downhill crowd since you should either be on your brakes or pedaling your ass off.
disc brakes require a more rigid (heavier) wheel build.
Carbon wide section rims might change this for road bikes but.. a disc set up requires a lot of spoke tension and a lot of spokes since the braking force is generated at the hub and the spokes essentially act as a lever. This means it has to be strong in order to transmit the braking force to the tire since the caliper is slightly eccentric to the hub ( Class 3 lever where the fulcrum is the hub ). Compare this to a rim brake where the braking force is applied directly to the rim/tire complex itself meaning the wheel only has to content with the vertical and lateral forces of riding as opposed to the additional torsional force of disc brakes. How many radial spoke disc front wheels do you see?
Please think of your highschool physics class and forget the FUD.
Just to be clear, I ride rigid mini-v regardless of road or mountain and ditched my discs years ago... got tired of bleeding.
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.
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.