Chasboy

Hi folks, I’ve been noticing the development of disk brakes for bikes and I admit I was pretty skeptical since the discs are so small compared to the actual rim. Can anyone tell me why the disks are effective? I understand that the swept area is larger but isn’t the leverage at a disadvantage with the disc being so close to the hub? I looked for answers online and got no where.

In addition, are the mechanical disk brakes as effective as the hydraulics?
Thanks a lot.

Kapusta

In short the smaller size of the disc is offset by the higher clamping force of the caliper, and the disc’s ability to withstand it.

AndreyT

You are absolutely right. Disc brakes are at mechanical disadvantage compared to rim brakes. Disc brakes have to compensate for that by generating more braking force between the disc and the pads, i.e. by exerting more clamping force. They do it. And they do it successfully.

Bicycle brake efficiency is capped by such objective limits as 1) wheel losing traction and beginning to skid, or 2) bicycle flipping over the front wheel. So, if your braking system can provide enough braking force to reach these objective limits, your braking system is obviously as efficient as any braking system can possibly be. Mechanical disc brakes easily reach these limits, which means that they are equally as effective as hydraulic ones.

The parameter that might differ here is not the efficiency, but rather the precision of the braking system, a.k.a. "modulation". I.e. how easy it is to precisely control intermediate braking forces from the braking lever. Hydraulic brakes usually offer better modulation than mechanical ones.

Another parameter that might differ is heat resistance. Mechanical brakes usually resist overheating better than hydraulic ones.

tomato coupe

The diameter of the disc rotors is not a limiting factor in the design.

Paul Barnard

I was getting ready to type out a response, but you said it better than I would have. I prefer mechanicals only because for me they are easier to work on. I have ridden rim brakes and various mechanical and hydraulic disc brakes. The only time I had an issue I couldn't resolve myself was with hydraulics.

Chasboy

Hypothetically, if the disk and caliper were located out near the rim, (the disk would be about 26” in diameter), wouldn’t less clamping pressure be required, leaving everything else the same?

Tacoenthusiast

Yeah but the disk would be big and heavy.....


Google Buell motorcycle if you want to see what that looks like

Kapusta

Yes, a 26” rotor would require much less clamping force than a 6” rotor.

Is there a point?

surak

I've been kicked off an MTB with 203mm front rotor without exerting much force. As others have said, there's no need to go larger. The advantage to smaller rotors are weight and ground clearance -- the latter results in less contamination from ground spray.

Koyote

My mechanical disc brakes stop about as effectively as a good set of rim brakes. And they are all perfectly adequate.

My hydro disc brakes generate muuuuch more stopping power than that, which allows me to more easily modulate my speed on some of the very steep descents around here -- I can do it without having to death grip the brake levers.

Troul 

the pad's surface to the rotor impacts the longevity for braking. Too small of a pad can lead to glazing while sacrificing the ability to stop as intended.

alo

Bike wheels often develop slight wobbles. If rim brakes were really close to the rim, they would rub. So rim brakes are set up with a fairly large gap between the rim and the brakes. When you apply the brakes, there is a fairly large distance for the brake to travel before it contacts the rim. Because rim brakes are designed for this large movement, less force is applied to the brakes.

With disc brakes, the pads can be less than half a millimetre from the disk. Because there is much less movement in disc brakes, the same amount of force on the brakes lever results in much more pressure on the disc pads.

Koyote

I’m not sure what this means, but I’m pretty sure it’s wrong.

Chasboy

yes, I’m just trying to get it clear in my mind that for the disk more pressure is required, contact patch is increased and the technology to exert the pressure needed is available in the caliper design, which apparently it is. Rim brakes don’t exert that much pressure and the rim contact patch as well as it’s stability is not as good as the disk.

Kapusta

Maybe not well articulated, but Alo is basically correct (if I am reading him correctly).

On rim brakes, since the brake pads need to cover more distance before hitting the rim, the mechanical advantage of the lever over the pads needs to be less less (so that more pad travel distance can be covered as you pull the lever).

Part of the reason that disc allows more mechanical advantage of the levers over the pads is that the pads can be set up very close to the rotor, so you can set up the system with the lever having more mechanical advantage over the pads without spending a whole bunch of lever throw just reaching the rotor.

Not sure If I explained that any better, but there you go.

cyccommute 

Just to be clear, the wheel that loses traction is the rear wheel. The front wheel can’t skid on a bicycle. The rider will flip over the front wheel before that happens. Rim brakes can easily reach the same limit as any rim mounted disc brake.

Your definition of modulation is correct but it’s not something that I have ever experienced with hydraulics. Hydraulics have always been powerful in my experience but it’s just raw power. Trying to control it in those places where intermediate braking is needed is not easy. Every hydraulic I’ve used has been grabby with control always on the edge of disaster. Mechanical (and rim brakes, for that matter) provide exactly the intermediate control as you describe it.

Hint, look at a rim and rim brake caliper. Look at a hub mounted disc and the caliper. Notice something? They are exactly the same. Same principle and very similar mechanism. They only differ in pad material. That is why hub mounted discs are somewhat better (but only somewhat). A rim brake could be made that is just as effective as a hub mounted disc if we were willing to use steel rim and sintered metal pads.

I haven’t ever had “a death grip” on any brake lever. I’ve mountain bike and ridden heavily loaded touring bikes...both road and off-road...with nearly every brake system around. Even with old center pull Weinmann’s, never had a death grip. The only brake that I’ve ever had to be cautious with is hydraulic. I never found them to have a middle ground...only on and off.

If the wheels wobble, fix the wobble. Hub mounted rotors can just as easily develop a wobble and rub. Removing the wobble is much harder than taking it out of a rim. Spokes are easier to minor adjustments because it’s just a simple tension adjustment. Rotors require bending that is far less precise.

You also have the distance argument wrong. Rim brakes don’t need to have a huge gap between the rim and the pad. They are often set up with a huge gap between the rim and the pad. Most bikes are adjusted so that the pad doesn’t hit the rim until about the lever has moved about half way to the bar. I don’t care what Sheldon Brown says, that makes for very mushy brakes and what many people experience when they use poorly set up rim brakes.

And here is the reason that many people dislike rim brakes. If the rim brake is set up so that the brake actuates much earlier in the pull, the brakes feel more powerful and the braking feels better. You can detune disc brakes to the same half lever pull and the best hub mounted disc will be the worst brake you have ever used.

A well set up rim brake will exert enough braking force to send a rider over the bars. That’s the limit of braking for bicycle under any brake.

cyccommute 

There is nothing that limits a rim brake from being set up close to the rim. Again, if the rim wobbles, that’s something that can, and should, be fixed. I’ve run my brakes so that the wheel is locked at about half travel of the lever which is similar to how hub mounted discs have to be set up. I currently have bikes with hub mounted discs, dual pivot rim brakes, mixed front disc/rear linear and cantilevers...Oh! The humanity! The cantilevers are actually on the bike that requires the most braking power...a loaded touring bike. I’ve even ridden that one down New Found Gap in North Carolina in a driving rain at around 50mph. I had no problem slowing nor stopping.

Chasboy

thank you all. It’s been enlightening and my questions were answered and then some.

restlessswind

Rim brakes are my preferred brake in dry road conditions. When dry, they are solid and easy to deal with. Disc brakes are at an advantage for MTB as the rim gets in the mud. Disc's stay clean. Discs also work well when wet...just like cars. Mechanical discs seem to be harder to keep adjusted, and always dealing with the rotor rub, or brushing that can be annoying. Some mechanical disc calipers are about useless too. I crashed into someone due to a weak braking system. Hydraulic brakes are much more sensitive and require less effort to squeeze. My wife has arthritis, and this is a benefit for her hands. New hydraulic brake hoses can be very costly to replace or lengthen They all have pros and cons, but some mechanical disc calipers are no good for anything.

Kapusta

You completely missed my point. Like... completely**. And I think you missed Alo's as well.

Are you claiming that you set your rim brake pads as close to your rim as your disc pads to your rotor? Please think hard before you answer this.

**EDIT: A better way to say that would have been "I don't think we are not talking about the same thing". From your response you seem to think my post is weighing in the relative merits of disc brakes vs rim. It is not.

Koyote

Here is what I meant: as long as it is impossible to pull the lever all the way to the handle bar, I think you can develop the same amount of force on the brakes whether the pads are very close to the rims (so don’t need to pull the lever very far), or the pads are just a bit further away (and hence you have to pull the lever is a bit further).

I’m willing to be corrected if I’m wrong...But even if that’s the case, I don’t think there is any advantage in having so little brake lever travel.

Steve B.

And in case anybody wondered, and as the OP asked about “Disk” brakes, with a “K”, generally the British use the spelling of “Disc”, while the U.S. it’s typically “Disk”.

I was curious so though I should look it up.

Kapusta

OK, you and Cylclo seem to be on some other subject entirely from what I (and I believe Alo) are talking about. He was not wrong, you just don't get what he is talking about.

Koyote

I understand what the two of you are talking about… I just think you’re both wrong.

Please note that you have now told two of us that we don’t understand your arguments. So, perhaps the problem is not with us...?

SClaraPokeman

For road bikes I feel that discs are a solution in search of a problem. I'm a big guy and rode in the the Santa Cruz mountains for 30 years and never felt my brakes lacked modulation or stopping power. Occasionally rims overheating could be a concern and I would pull off the road to let them cool. But that was pretty rare. The biggest advantage to discs from my experience is that they don't care what size tire you're running and they're self adjusting. Rim brakes are best up to 28m tires and require much more frequent adjustment for me. I don't like adding complexity to a bike and if I was in the market for a new road bike I would be willing continue going with rim brakes. A cross or mountain bike--yes definitely want discs. The worst brakes I had were cantilevers on a touring and mountain bike--maybe I'm not a good mechanic, but I could never get them tuned so they didn't howl and they provided lousy stopping power.