Why do disk brakes generate "more stopping power" than rim brakes?

Question

Bicycle's brakes generally fall into two broad categories: rim brakes where the braking pads pinch against the wheel's rim, and disk brakes where the pads press against a disk (rotor) that is rigidly attached to the wheel's hub.

General wisdom is that disk brakes are higher quality and offer better stopping power. But disk brake pads are much closer to the center of the wheel, which means that they have to exert more force to produce the same amount of torque. Wouldn't this be less stopping power?

Answer 1

For both rim brakes and disk brakes the limiting factor is the friction between the tyres and the road i.e. both types of brake can apply enough force to make the wheels skid. So it is untrue that disk brakes offer more braking force.

Where hydraulic disk brakes win is that rim brakes require a cable¹ to run from the lever to the brakes and this introduces compliance since the cable and the housing both deform under loads. A hydraulic disk brake replaces the cable by an incompressible fluid, which reduces the compliance and improves the feel, and this gives the rider better control of the braking. So braking is improved, but because of greater control not greater braking force.

You are quite correct that to get the same torque from a disk brake a greater frictional force between the pads and disk is required. This is easily done by giving the disk brakes a greater mechanical advantage i.e. the same lever motion produces a smaller displacement of the brake pads.

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¹ I'm assuming the comparison is between cable rim brakes and hydraulic disk brakes. A comment points out that hydraulic rim brakes exist but these seem to be rarely used. Cable disk brakes exist but these do not offer the advantages of hydraulic disk brakes.

Answer 2

First off, disk brakes do not offer "better stopping power", that claim of the article is incorrect. However, it doesn't matter. Both disk and rim brakes are able to provide enough stopping power to lock the wheel - the limiting factor is the traction force of the tyre, not the brake pad.

There are several advantages of disk brakes over rim brakes that do matter.

• Rim is closer to the road and is more likely to get the surface dirty, thus reducing the braking power, sometimes unpredictably.
• When using rim brakes, one must manufacture the wheels with some very flat braking surface that can't be painted and must provide enough friction. So no full-carbon wheels. A disk can be attached to any wheel.
• Disk brake can be swapped to one of different diameters to suit your preference - applying more or less pressure to produce the desired braking force, something that can't be done with rim brakes. Further, disk brake pads can use hydraulic cylinders, applying more than a 1:1 ratio.
• Wheels with disk brakes can be made lighter because the structural surface area is smaller.

Answer 3

Most answers here focus on braking force.

On the other hand, the Question is about the power and we are at Physics.SE so this is what happens with the braking power:

Braking generates heat. A steep downhill may require continuous braking that generates heat in the 1-2 kilowatt range.

Almost all of this heat goes to the rims (when using rim brakes) or in the disk (in disk brakes).

Rims are limited in their heat dissipation abilities first by the presence of the tire (limiting the safe temperature) and second by the pads material (rubber in the general case, also suffering from the heat).

Disks, on the other hand, can be made of steel or even cast iron, so they can go almost red-hot without getting in deep trouble. In fact, cast iron can go more than red-hot without deteriorating, it is the pads that limit the safe temperature.

Because the disks serve a single purpose, they can be made with much tighter geometric tolerances than the rims, allowing the use of heat-resistant materials in the pads as well.

As we know, hotter surfaces are better at dissipating heat than the less hot surfaces. All types of heat exchange (heat conduction, convection, radiation) intensify at higher temperatures.

Answer 4

A large part of why disk brakes are seen as superior has significantly more to do with engineering than physics.

In particular, one of the braking surfaces for rim brakes is the rim of the wheel. The rim is a key structural component of the wheel, is not cheap to replace (new rims are not expensive, but they require building new wheels, and building wheels is a reasonably high skill endeavor that takes a nontrivial amount of time to do right, so it’s not cheap), and also has a limited safe amount of heat it can disperse (because heating of the rims heats the tires, tubes, and possibly spokes, and those getting too hot is a very good way to cause things to fail, possibly catastrophically). This puts a practical upper bound on the amount of force (and therefore torque) that can be applied with rim brakes, because you need to not generate too much heat and not wear away too much of the rims (this second point is why pads for rim brakes are almost always hard rubber, they need to be the only sacrificial component here). This also, rather importantly for competitive cyclists, means that it’s absurdly expensive to use rim brakes with a carbon-fiber rim, because you need to specially reinforce the braking area and design it to dissipate heat away from the rest of the rim.

Disk brakes, OTOH, have both their braking surfaces completely decoupled from any key structural components, are generally less expensive to replace (good rotors are more expensive than good rims, but the labor costs are at least an order of magnitude lower because anybody with the right tools and some basic mechanical knowledge can replace brake rotors on a bike), and can safely dissipate much more heat (in fact, most are specially designed to dissipate heat). This means that even though they need more force to produce the same torque, they can apply significantly more force safely than can be done with rim brakes.

There are a handful of other aspects that make disk brakes popular though that have nothing to do with braking power:

• Hydraulic disk brakes are much more readily available than hydraulic rim brakes. These eliminate a huge amount of regular maintenance if installed correctly (they’re self adjusting and the lines don’t lengthen or slip like Bowden cables eventually do), and also provide much more precise control over the exact amount of force applied.
• If using tires wider than your rims (relatively common on modern bikes designed for off-road usage), you need to adjust or disengage rim brakes to take off the wheel, but don’t need to do anything special for disk brakes.
• Because disk brake rotors are further from the edge of the wheel, they tend to not get dirty as much as the rims do, meaning they often behave better than rim brakes in wet, icy, or muddy conditions. Additionally, even if they do get dirty, the higher force involved in braking means that the grime often does not matter as much.
• Because the rotor is also a sacrificial component, the pads for disk brakes can be made much more durable than the pads for rim brakes, which helps offset the cost of the more expensive brake pads.

Answer 5

Another important consideration in favor of disc brakes in cars, motorcycles and airplane landing gear is that they get rid of frictional heat faster than a drum brake can, which reduces brake fade (where the stopping power goes down as the brake heats up with hard use).

Answer 6

Tolerances and lever ratios

A basic limitation of brake force is that you can only move the brake lever at the handlebar by 4cm or so. Using lever ratios (or piston diameter ratios in the case of hydraulic brakes) this is translated to a few millimeters of travel at the brake but with correspondingly greater force.

Rim brake systems have much looser tolerances. The trueness of the wheel is limited, the stiffness of the wheel and frame is limited, the alignment of the brake pads is limited. This means that you need more distance between pads and rims to avoid brake rub. Unfortunately this increased distance also means that your brake pads have to travel a greater distance at a lower force.

Disc brakes usually have much better tolerances which means the pads can be closer to the disc and don’t have to move as much, so you can design your lever ratios for smaller movement and greater force at the brake pads.

Answer 7

In addition to all the other answers, also consider where in the system there is potential for flex when the bakes engage.

Rim brakes:

• Long brake arms because they have to clear the rim and tire
• Brake track on the rims is at least partially located on the open profile
• Brake arms brace against fork or the - often relatively thin - seat stays that are then pushed outwards

Disk brakes on the other hand:

• Solid metal disk
• Caliper can be a narrow chunk of massive metal

On my old hardtail I have installed a "brake booster", a U-shaped metal profile that is supposed to limit frame flexing when the rear brake is engaged. Even with that, frame flex is clearly visible and limits the force that can be applied through the brakes.

_{Brake Booster from https://bicycles.stackexchange.com/questions/72312/}

Answer 8

Consider how the two are designed.

Rim brakes use the existing rim of the wheel. This area is not specially reinforced, thus cannot be clamped too tight.
Secondly, it is far from center of the wheel, and will, in all cases "wobble" (who never had a slightly wobbling rim - please come over fix my bike?). This wobble can also be caused by wheel deformation - its just a rim strung on wires, things can happen.
So you get a surface that goes up and down* by as much as millimeter at time, and are trying to apply a constant force on it.

Now look at the disk brake. It has a dedicated heavy-duty disk that you can clamp with a ton of force (you would need a hydraulic press to squish it). There shouldn't be any wobble, since it is close to center of the wheel, and is a solid piece, not hanging on strings of wire. There is a ton of potential for braking power here, enough to rip the wheel of the hub.

Thus, one-by-one disk brakes win.
Of course there is always the matter of quality (good rim brakes can outperform shitty disk brake), and the total usable stopping power before tyre starts slipping.

(*) it actually goes left and right, and depending on type of rim brakes, will cause a problem. V-brakes that just clamp over tyre - will just shake them left and right with no harm done, but if you got those awful ones that have cord being pulled up, it will cause the sides to pull and loosen at the same time, and since the midpoint is fixed and triangles are not compressible, youre only braking at full power at the center position.

Answer 9

But, as more than one person has stated, ultimate clamping force is not relevant, because any kind of brake can lock the wheel--any increase in force after that point is useless.

Interesting how many of the comments mentioned carbon wheels: as a non-racer riding aluminum wheels on the road, I am perfectly happy with rim brakes and tend to think that messing around with thru-axles and hydraulics would not be worth the trouble.