Disk Brakes
#1
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Disk Brakes
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.
In addition, are the mechanical disk brakes as effective as the hydraulics?
Thanks a lot.
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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.
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#3
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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.
Last edited by AndreyT; 05-11-20 at 11:55 AM.
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The diameter of the disc rotors is not a limiting factor in the design.
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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". 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.
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". 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.
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.
#6
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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?
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Is there a point?
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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.
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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.
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.
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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.
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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.
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.
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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.
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#14
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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.
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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.
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.
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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.
Another parameter that might differ is heat resistance. Mechanical brakes usually resist overheating better than hydraulic ones.
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.
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.
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..
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.
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.
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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.
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.
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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.
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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.
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.
Last edited by Kapusta; 05-11-20 at 09:04 PM.
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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.
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.
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.
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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.
I was curious so though I should look it up.
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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.
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.
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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...?
#25
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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.