Disk brakes are becomming more popular on bikes now, and I was just wondering what the advantage is of this type of brake over the older rim brakes.
Working backwards from how it works:
Not using the rim means the brake is unaffected by how straight the rim is, and heating the rotor doesn't affect the tyre. The rim is not worn away by braking and the rotor material can be selected entirely for its suitability for that one purpose. This also allows smaller clearance between pads and rotor, allowing:
Increased pressure between brake pad and rotor compared to rim brakes means disks are less affected by water or other contaminants. Pads can be made of harder material giving increased pad lifetime.
Having the brake caliper further away from the tyre means it's less affected by mud, and it can be made partly or fully enclosed to further sheild it. The rotor is also less likely to hit obstacles as it's further from the ground.
Being a hub brake makes unconventional designs possible. Single sided wheel mounts are the most obvious example, but some suspension systems are incompatible with rim brakes, and building a rim braked carbon composite wheel is much harder than building one to use a hub brake.
Disk brakes can also be fitted to both sides of a wheel for increased heat dissipation, and it's easier to vary the design for different characteristics (weight, heat capacity, modulation etc). Doing that with a rim brake means the brake manufacturer has to start selling wheels as well.
These combine to make disk brakes more reliable and easier to maintain.It's also somewhat easier to remove and reinstall wheels as you don't have to release the brake first.
I have been using disc brakes on my Mountain, Cyclocross, and Road bikes for well over 10 years now. Earlier disc brake versions certainly took a while to reach excellent, smooth, and modulated braking power levels that they currently enjoy. In that context, a poorly adjusted or designed disc brake vs. a well adjusted v-brake showed less performance capabilities.
However, in recent years (5+), the technology surrounding disc brakes has increased dramatically. Particularly with hydraulic disc brake systems. Hydraulic disc systems tend to have significantly more stopping power than any rim brake system I have used. One other side effect of the additional stopping power is the ability to "modulate" that power much more smoothly. You can control how much power you use a lot more easily.
As stated above - there are different maintenance requirements with disc brake systems than with rim brake systems. However, typically they are no more onerous to learn than other maintenance protocols for any bike part.
In addition to hydraulic disc brake systems, there are mechanical versions which use your traditional cable to provide the lever to caliper clamping power on the disc rotor. These systems have the advantage of being usable with shift/brake integrated lever systems (commonly found on road bikes or bikes with "drop bars").
I have found the mechanical disc brake systems to not have as much power and modulation as hydraulic systems. In addition, they tend to require a lot more maintenance/tweaking to keep them working perfectly smoothly. The pad contact points are a lot more susceptible to making noises or dragging as they wear. Mechanical systems clamp from only one side. One pad is fixed in place, the other pad is pushed against the rotor slightly warping it into the fixed pad to provide the stopping power.
Hydraulic systems mostly have two separate calipers that push the pads from both side, (typically) providing stopping power from both sides, without warping the rotor (much).
Rim brake system tend to wear our the rim surface of the wheel if they are used in wet weather conditions. As grit gets on the rim, and you brake, the grit essentially 'sands down' the rim over time. Eventually you'll have to replace the rim as it is worn out. The thinner the rim gets, the more likely that heat transferance from the braking action can affect the rubber of the tire itself, sometimes causing catastrophic failure (exploding tire). This is fairly rare, and requires circumstances to be "just right" (i.e. well worn rims with thin side walls, possibly mis-adjusted brake pads, and long extended descents where the rider is "dragging" their brakes constantly causing heat buildup, tire casings that don't handle heat well, etc...).
Generally speaking, the maintenance requirements for rim brakes and hydraulic disc brakes tends to be about the same. Over time, you may need to have the hydraulic brake lines "bled" to keep them in good performance. However, most modern hydraulic brake systems typically don't need to be bled that often. I find my mountain bike hydraulic systems to require it only after 18 to 24 months of riding (I typically ride about 3,000 miles on my MTB in that interval).
Otherwise, it's simply a matter of replacing pads as they wear out. I do believe that disc brake pads will tend to wear out faster than rim brakes. They're simply smaller in surface area, and have a lot more heat demands placed on them. This is not true in foul weather riding conditions though - since wet/sandy conditions tend to affect rim brake pads a lot more than disc brake pads.
A lot of people will cite that disc brake systems are heavier. This is true in general. However, the majority of the 'extra weight' is non-rotational weight, which has a lot less effect on the ride quality or feeling of a bike. Since the weight is at the axle area, the weight is fairly insignificant.
Rim brake systems require thick, heavy tracks for the pads to press against. This requires that a rim brake wheel rim be significantly heavier than a disc brake rim. All of that weight is rotational mass at the outermost edges of the wheel. Any additional weight in this area is significantly increased in terms of the feeling/impact of that weight.
I often see rims made for disc brake applications that average anywhere from 100 to 250 grams lighter in weight than their rim brake counterparts. This is a significant amount of weight when it is at the worst possible location.
A nice thing about rim brakes (any type) versus disc (any type) is the fact that they provide a guide for quick truing your wheel without taking the wheel off and putting it on the stand. Hanging my bike down on each end I can quickly true both wheels to perfection. You can't do this with a disc. And rim brakes do provide a generally cleaner look without a cable running down the fork or the rear stay. And they generally are less weight and very easy to adjust. Little things mean a lot and things do add up. Oh yeah and the pads last crazy forever which is a nicety. Fifty years of riding a LARGE number of bikes with all the kinds of brakes discussed in all conditions and my winner is a rim brake of high quality suited to your tire/riding terrain.
A lot of good answers already - but one big disadvantage of disks is that has yet been mentioned is their ability to eject the front wheel when using a tradition QR axle. The torque generated on the axle (for a normal rear mounted disk caliper) is down - the same direction as the dropouts. Its been shown that even properly tightened axles can come loose - usually with large (200mm or greater) disks, but loose a QR can come undone very easily.
As a result, "Lawyer Lugs" are pretty much mandatory on Disk equipped front wheels (Unless a through axle is used). The problem with Lawyer Lugs is they somewhat defeat the advantages of a the QR, in that to remove the wheel, you have to unscrew the nuts quite along way.
Edit: (Two years later): Rather than a link provided earlier, an internet search for "James Annan disc brake quick release" will give plenty of answers and most recent discussions on the issue.
A number. Generally, they are unaffected by wet conditions, whereas rim brakes can get dicey in the wet. As well, for off-road bikes, they are less susceptible to mud, dust, and sand, and the attendant rim wear as well as poor braking. Also, on long descents or downhill sections, there is no danger of rim-heating and tire failure. Downside... You need purpose-made fork legs and wheel hubs to handle 'em , and the entire assembly is perhaps a bit heavier than traditional rim brakes.
You are seeing these now on even mid-level mountain bikes.
Oh, and you don't have to wrestle large-size tires through a tight-fit when remounting the wheel. Even with quick-release, some tires will just barely clear the brakes when putting a repaired flat back on.
One of the advantages given above is that a wheel that is out of true will not rub against the brake pads if using disk breaks. However I think that this rubbing with rim brakes is really the advantage precisely because it warns you that your wheel is out of true. It is not a good idea to ride in such conditions as the wheel may fail.
All brakes are limited by front wheel traction - you either start skidding or flip over the bars. I believe it is about 1 g of deceleration. So anyone telling you one brake is more "powerful" than another is talking nonsense.
A key difference between rim and disk brakes is that rims are essentially disks 4x as wide as your typical disk. This means a disk brake requires 4x the force to provide the same deceleration torque. While better pad/disk materials and stiffer construction can reduce the problem, the difference has to be made up by greater mechanical advantage. That is, a disk brake lever must travel through more arc with more grip pressure to get the same effect as a rim brake lever. This is touted as "modulation" but is really just the brake requiring more energy input from the rider's grip. So long as you can reach the deceleration limit, it doesn't matter, but there is less margin to work with.
It is often claimed that disks are less susceptible to the wet due to the higher forces and location, and this may be true. (Certainly the idea of a fully enclosed disk is nifty, although I've never seen it implemented.) But I'm not aware of any experimental confirmation.
I'm not sure I would get disk brakes again. I used to have v-brakes, but thought disk brakes would be better.
The advantages stated are true but I've found:
- They are harder to maintain. I was able to make simple adjustments to my v-brakes very easily, but I don't know where to start with disk brakes. (This could just be lack of knowledge on my part)
- If you get oil on the disk and it gets on to the pads they will squeak badly and the pads may need replacing (Seems obvious now, but I just needed to be more careful when cleaning my bike and use a clean cloth for cleaning the disks)
- The stopping power seems comparable to my old v brakes
My problems with disk brakes could probably be fixed by a little bit more knowledge, but if you're not familiar with disk brakes then these things are not worth thinking about.
Disk brakes are great and necessary if you use carbon fiber wheels, or are riding a mtb or cross bike in severe yucky conditions, or you have a fat tire bike, for road conditions disk brakes are not really necessary, and consider this, a rim brake is the exact same thing as a disk brake is (in fact the rim brake uses the rim which has a far larger surface area then a small rotor and thus actually runs cooler), they both have pads, and they both use a metal contact point like a rim or rotor to stop.
While an aluminum rim does wear using rim brakes those rims will last 35,000 miles on average, rotors on the other hand average 2 to 3 years, rotors cost between $10 to $90 each so lets say an average of $50, plus $20 for the average price pad which will need replacing every season; the average dedicated rider rides about 6,000 miles a year so in 5 years the disk brake user will have to spend about $130 for brake maintenance, while with rim brakes a set of pads cost $12 and will last at least 7 to 10 years, but the rim has to be replaced so lets assume the rim cost $400 for a decent aluminum rim. (these were just averages so don't shoot me if the numbers aren't exactly correct.)
So you can see how much more rim brakes will cost as was pointed out by another poster...except for one small problem: A road bike wheel makes about 670 rotations for every mile that is travelled. During that time, the rim is constantly compressing under the weight of the rider, such that every spoke is loaded and unloaded with each rotation. This ongoing cycle of radial load takes its toll on the wheel, inevitably producing fatigue. Lateral loads also contribute to wear and tear of the wheels. These forces are generated as the rider transfers their weight from one side of the bike to the other when pedalling out of the saddle. Lateral loads are at their greatest during sprinting and climbing efforts as the bike is violently tipped from one side to the other. The drive-train places extra load on the rear wheel in the form of torsion of the hub, and with disk brakes that torsion is a lot greater which is why the fork and rear stays have to be reinforced or the frame/fork too will take a hit. Once again, there is an ongoing cycle of loading and unloading for the spokes/rim, and that contributes to the development of fatigue. There are no outward signs of this fatigue until the spokes, or rim starts to break or crack as a result.
Therefore both rim brake and disk brake users will have to replace the rims due to fatigue, thus in reality in the long run the rim brake is far less expensive to use. Also if disk brakes are used in the rain a lot you could be replacing pads as often as after a couple of rides which really drives up the cost of disk brakes, whereas the pads on rim brakes don't care if it wet or not; and on long wet rides it's recommended you carry a spare set of pads for disk brakes!
Also the stopping ability for both brakes are identical, I even tested this with an experiment with a friend who has disk brakes and was confident his stopped better than my, so we ran side by side at 14 mph, 20 and 32 mph on dry pavement and we both hit are brakes at the same time, we did this 6 times at each speed; then later we tried the same thing on wet pavement at 20 mph, and the results for all the tests were a tie, sometimes he stopped a tad quicker and sometimes I did, none of us stopped shorter than a foot from the other person (at the 32 mph test I actually stopped a tad shorter during the whole test, we think the disk brake may have experienced some fade because he mentioned he had to use more hand pressure to stop the bike then before). The only thing that we could think of as to why sometimes one stopped shorter than another was reaction time, one of us would hit the brakes before the other and that person stopped a tad shorter. VeloNews reported this: “Road tire traction is high enough that in practice, the braking limit on a road bike is often not the traction limit, but the tip-over point — the point when the forward weight shift from braking causes the rear wheel to lift off the ground and the bike begins to do an endo. (You can test this yourself by riding at a slow speed on smooth pavement and clamping the front brake trying to induce a front wheel skid — if you brake hard enough, you’ll find that the rear wheel lifts off the ground before the front wheel can begin to skid). In other words, road tires often have more traction than can actually be used for braking.” Thus what they’re saying is on dry conditions it’s all about tire adhesion to the pavement, if all things are equal, body weight, bike, riders ability to stop fast, tires, road surface, etc, they both will stop in the same distance which I was able to prove in my test. all of this nonsense about how fast disk brakes can stop isn't true, it's just marketing hype.
One poster mentioned a drawback to rim brakes is oil getting on the rim, not sure how you could get oil on a rim unless you were real sloppy with lubing your chain and too lazy to clean the rim afterwards, two fails right there. But what the poster didn't mention was that oil will cause the same issue if it gets on rotors as well.
With mechanical disk brakes you have to be careful not to buy a bike with small diameter rotors which rules out buying lower end bikes with disk brakes, small diameter rotors won't stop you as well as rim brakes. With hydraulic disk brakes the lines tend to be fragile. Brake modulation with disk brakes fail in comparison to rim brakes; whats weird about this modulation thing is that the best brakes I've ever used for modulation was single side pulls! dual pivots removed some of that modulation and disk brakes removed more of it due to shorter shorter mechanical lever that requires less leverage and reacts to quickly (and this is where the fallacy that disk brakes must be better because the brakes crab fast...it's an illusion caused by the shorter mechanical lever; mechanics and physics do not magically change for bike components and braking. For road-biking applications, anyone who advocates disc brakes over rim brakes has never taken physics, and does not understand how torque works and/or what a ‘lever advantage’ is. Disc brakes are excellent for cars but bikes are not cars. People can SAY that disc brakes have more stopping power than rim brakes, but saying it (and even repeating it) doesn’t make it so. physics is not on the side of disc brakes; you generally want the braking force to be applied at the greatest (practical) radial distance from the hub, where it provides the most effective translation into torque. For bike wheels, in particular, you would not want the torque to be applied at the hub, given that the contact with the ground (which is where the drag or “slowing” actually occurs) is at the perimeter of the tire thus causing the spokes to have to translate the force from the disc out to the tire. And finally, the disc brakes are necessarily offset from the centerline of the hub which causes a lateral or shearing force that is roughly proportional to the amount of braking that is being applied (and is also proportional to the amount of the offset from the centerline of the hub), which can result in a lateral “pull” under very hard braking conditions. The fluid in a closed hydraulic system on disk brakes heats up and expands on long descents and tends to lock the brakes. Rotors bend easily if something hits them (or they get so hot they warp) making the bike unrideable, with rim if a rim gets bent you can try straightening the rim and ride on. Mechanical disk tend to need more frequent adjusting then any other type of brake system. You cannot swap wheels with another disk brake equipped bike.
Again let me stress this, if you have carbon fiber wheels you should be using nothing but disk brakes, it actually can be hazardous to be using rim brakes with CF rims even if those rims have an aluminum brake track though not as much as all CF rim. But for road bikes the added maintenance and setup hassles of disk brakes aren't worth it; I've been riding for over 40 years on rim brakes, including racing, and never had any issues with stopping, even racing in the mountains of S California. I also known people who toured all over the US and even Europe with heavy 75 pounds average loads with cantilever brakes and never had problems stopping even while descending. And speaking of descending, rim brakes suffer from less fade then disk brakes due to the much larger surface area of the rim that cools faster than the smaller surface area of the rotor.
Look, the CF wheel manufactures were having issues with their wheels experiencing brake fade, delaminating, and failing when they heated up, something of which was costing them lots of money on warranty returns in addition to R & D; plus a lot a manufacturer’s found themselves lying to customers about the cause of the delamination and failures and placing the blame on other reasons so that they wouldn’t have to pay any warranty adjustments which in turn makes the companies doing so look bad. This is why the evolution of disk brakes on road bikes came about and not for any other reason.
And even if pro cycling ever legalizes disk brakes, which they will as more and more CF wheels come onto the scene, the pros will use whatever is given to them, and they will sell the technology to us because that's what they're paid to do.
damn, I've written an rambling essay! I guess I had nothing else better to to tonight, since I didn't want to see the ball game!
They work even when it's wet (I think that's because rims get wetter because they're closer to the ground; and, because they have a bigger surface area, they take longer to dry).
Brakes that work when it's wet are especially useful when in traffic, or going downhill.
Good brakes might also be useful on a cycle path in the evening: it seems that small children may find a flashing front light attractive, and hurry into your path to get a better look at it.
I did not see any mention of wheel build requirements. I have a mountain bike with disks and a road bike with rim brakes. My front road wheel has radial spokes, a style of lacing which I do not believe could be used with a disk. My disk mountain has 3 cross lacing. The spokes creak and groan a little under heavy braking, understandable given the torque load being transfered though the spokes. I did not see any disks in the Tour de France and I presume the lighter weight of the brakeset and completely adequate power of rim brakes are significant factors, but the lighter and better aerodynamics of radial spokes is the kicker.
Some of the advantages that are claimed for disk brakes don't really make any sense to me.
One advantage people claim is it that it no longer matters how true your wheel is. I guess it doesn't matter too much on mountain bikes because that fat tire is going to take up a lot of the slack so you won't notice it as much while riding but even still I'd much rather just keep my wheels true maybe I'm just pedantic.
The whole thing about there being a tighter clearance between the pad and the surface increasing pressure doesn't make sense unless you're using a hydraulic system that actually creates a mechanical advantage which I don't think bike hydraulic systems do because pressure = force/area. I'm pretty sure bike hydraulic systems use a mechanical advantage of 1:1 correct me if I'm wrong please! If this is the case it means its simply used for actuation not increasing pressure.
Although with a tighter clearance the brakes will grab sooner during the pull. Once the pads grab, the force component of the pressure that you can apply is simply a matter of how strong your fingers are. So assuming that you use the same method of actuation, and you are only comparing disk brakes, and rim brakes you are applying the same amount of force to the brake rotor in both situations.
Having that force being applied farther away from the point of rotation should stop the bike faster because your producing more thermal energy per rotation of the wheel.
The only advantages that I can see to disk brakes are that your wheel will be more likely to get submersed in mud and water than a disk that is higher off the ground but if some debris such as mud does splash up and hit the disk it is less likely to clean itself off when you ride through a puddle.
Disks are way more reliable.
For example, I had a bike that had a disk in the rear of one of my bikes, in case my front brake broke.
One day it did; the whole right arm hit a tree and disabled it and the disk I was still using stopped me on the trail just fine. I then put a disk on the front since they hit stuff if you are into that type of thing.
My first bikes had pedal braking great for skid outs & very effective, simple & intuitive. In fact my three speed banana seat bike is still probably my top bike of all times. Later on used cheap u shaped rim brakes that squealed & sucked...many moons later sprung for an Iron horse full suspension with shimano LX hydraulic disc brakes -still not really that impressed & always worried that I would get an air bubble in the hydro lines when flipping it upside down to work on it. My light & efficient hardtail (Giant SL Aluxx) that I've been using for the last 8 years has shimano mechs reliable & good in bad weather but the braking was not nearly as good as my buddies v-brake bike in good conditions... I will soon be going back to V-brakes (Shimano BRM 422)... if that sucks I'm going back to pedal brakes & rolhoff hub or a planetary shifting crankset.
Disc will always be better. Now there are systems lighter than v's. even a cheap set of disc brakes are better. i used traditional brakes until 2 years ago when i bought a bike with dual mechanical disc brakes and immediately felt the huge difference. its not really that they stop better but how they feel when stopping. its the same feeling from braking in a car or motorcycle. with v's braking isnt as good even if it can stop the bike just as fast. disc is more superior and dominant overall. the control you have with disc is amazing compared to v. traditional brakes will never ever have the modulation and advantages that disc will always have. facts are facts. dont care if ur a v brake fan and a weight weenie. gotta move on same day ya know? dont need them? thats fine i didnt either but they are so good i want them and i got them. never going back to traditional brakes ever again :) ya wonder why u started to see more and more disc brakes on bike eh? maybe cuz they work so dam good? try them with a professional setup not one from kmart or walmart cuz those are not adjusted correctly therefore people sometimes assume they suck. have a nice day