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Bicycle Rim Brakes

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Sheldon Brown photo  John Allen photo
by Sheldon "Sure Grip" Brown
and John "Mechanical Advantage" Allen
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Scope of this Article

This article covers topics which apply to all types of rim brakes, and points to other articles with additional information.

Why rim brakes?

Bicycle brakes fall into two general categories: hub brakes -- or rim brakes, in which the rim does double duty as an unusually large brake disc.

The rim's speed past the brake is nearly the same regardless of wheel size, and so a rim brake works equally well on any size wheel. Because a rim brake operates at nearly the full diameter of the wheel, the clamping force for a given amount of braking is much lighter than for a hub brake. Because this force is lighter, a rim brake stresses the bicycle frame and fork less than a hub brake. Because the rims offer a relatively large heat-dissipating area, rim brakes are relatively safe for speed control on long, downhill runs, on single-rider bicycles.

Rim brakes also have weaknesses. They can overheat the rims on a tandem or other heavily-loaded bicycle, causing blowouts. (See the article about tandem brakes). A misaligned brake shoe can destroy a tire or dive under the rim and lock the wheel. Some kinds of rim brakes have pivots above the tire, and work less well if they have to reach down past fat tires. Wet rims reduce brake effectiveness -- drastically with cheap steel rims; moderately with aluminum rims. Rim brakes require good wheel true. Rim brakes can wear rims down, leading to rim failure.

All in all, though, rim brakes are simple and practical, and they are preferred for economy, sporty performance and light weight.

Rim Brake Typescaliper-600.gif

Rim brakes fall into two basic categories.

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Cable Issues

Most brake problems result from excessive friction or poor installation of the cables, not poor setup, or poor quality brakes . See the article on cables for information on cable selection and adjustment of brake cables and brake levers.

 

Rim Issues

For a rim brake to work well, the rim must be clean and of of uniform width, and the wheel must be true. A lumpy or dented rim will make the brake grab. A rim that weaves from side to side will rub on one or both brake shoes and make you go slower whether you want to or not. A rim brake also can drag if misaligned, or if friction in a pivot keeps a brake arm from retracting fully..

Cross-section of a
rim with parallel
braking surfaces
Cross-section of
a rim with
angled sidewalls
rim with parallel braking surfaces rim with parallel angled surfaces

Most modern rims have parallel braking surfaces. Up through the 1970s, though, most rims had angled sidewalls. Many brakes would not allow the shoes to be adjusted to the needed angle. Also, the sidewalls would act as a wedge, making the rim effectively wider where it ran closer to the hub. A rim brake would grab if there was the slightest unevenness due to a sloppy wheel truing or a curb or pothole impact. A wedge-like rim also could promote brake chatter. Some rims still are made wedge-like. They work fine with hub brakes. Avoid using them with rim brakes.

Especially in winter or in off-road riding, grit grinds away at the rim sidewalls, producing a dark-gray powder on aluminum rims that is hard to clean off hands or clothing. If the rim sidewalls are allowed to get too thin, they split. Aluminum rims have this problem bad enough. Rims with carbon-fiber braking surfaces are light, but they wear faster, require special brake shoes and are costly to replace. Many modern rims have a small pit in each sidewall to indicate a safe wear limit. When the pits disappear, a rim should be replaced.

A rim which split due to brake wear

Rim which split due to brake wear

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 Brake Shoe Types

A common type of brake-shoe assembly has a brake block (the rubber part that contacts the rim) which is separable from the shoe (the metal cradle for the brake block).mathauser

Older brake shoes of this type were open at one end, making it easy to slide in a replacement brake block. If the brake shoe was installed backward, the brake block could slide out in use. Modern brake shoes are idiot-proofed -- closed at both ends.

It is still often possible to pry out the brake block with a small flat-blade screwdriver, or you may have to bend down the tab at one end of the brake shoe and slide the brake block out. Bend the tab back up after installing a replacement brake block. A vise is helpful to hold the brake shoe while working with the screwdriver --holding the brake shoe lengthwise in the jaws of the vise. You may also use the vise to press a brake block in or out, with one jaw of the vise pressing on the shoe at one end, and the other on the brake block at the other and.

Better modern brake shoes, like those shown below, have a metal plate molded into the rubber.

Smooth Stud

Brake shoe with smooth stud

Threaded Stud
(Plain Washers)

Brake shoe with threaded stud

Threaded Stud
(Spherical Washers)

Brake shoe with threaded stud

It is common for brake shoes, like the ones shown, to be longer to one side of the stud. These brake shoes are relatively thin, allowing them to fit into the tight space between the rim and fork blade despite their greater length.

Braking force tends to rotate the brake shoes slightly in use, so the rear presses harder than the front. To equalize pressure all along the brake shoe, the longer end should face the rear of the bicycle. The result is more uniform wear and reduced tendency toward brake squeal.

There are three different ways of attaching brake shoes to the brake arms, as shown.

Smooth-stud brake shoes are most often used on traditional cantilever and centerpull brakes. With modern brakes, they permit all needed types of adjustment. Older brakes such as the classic Mafacs didn't have any provision for yaw angle (toe-in) adjustment, but most cantilevers made since the 1970s have provided full adjustability.

Threaded-stud brake shoes with plain washers have limited adjustability. For this reason, they are not preferred.

Threaded-stud brake shoes with spherical (domed) washers do allow for all angle adjustments. There are two convex washers, which go inside and outside the arm, and two matching concave washers that mate with the convex ones.

Usually, the concave washers are two different thicknesses, so you can select two different extensions depending on whether you put the thick or thin concave washer on the inside.

Shoe Adjustment

Brake shoes can be adjusted in 5 different directions:

Sometimes it is difficult to get all 5 adjustments set at once, and to get the brake shoe tight enough without twisting it out of position. It helps if you remove the nut and lubricate the threads, being careful not to get any lubricant on the rim or the brake shoe.

Probably the best way to adjust the shoes is to use the Shimano technique:

To align brake shoes with spherical washers:

  1. Install brake shoes in rough alignment. Tighten the nut on one brake shoe post with the wrench, while holding the brake shoe by hand. Tighten the nut on the other side only finger-tight, roughly aligning the brake shoe.
  2. Place a calling card or similar thin item behind the back end of the brake shoe that is finger tight.  Clamping the brake will align the surface of the brake shoe with the surface of the rim, setting set the roll angle and toe-in.
  3. While still holding the brake lever to clamp the brake lightly, loosen the nut and slide the brake shoe post up or down in the slot of the brake to adjust theight. Twist the brake shoe on its post to adjust the pitch angle.
  4. Hold the brake lever tighter and use the wrench to tighten the nut as much as possible without twisting the shoe out of alignment.
  5. Hold the end of the brake shoe with a small adjustable wrench to prevent it from rotating while completing tightening of the nut.
  6. Repeat, starting with the second step, for the other side.

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Noises

Rim brakes that are working properly are generally quiet. Noises indicate problems and can also help you diagnose them.

Chattering

A chattering sound while braking may result from sticky brake-shoe deposits on the rim, or from a loose or flimsy brake. Chattering is common with cheap brakes that have thin, stamped-steel brake arms, and may require replacement of the brake. A rim with angled sidewalls promotes chattering, as the flexibility of the brake assembly allows it to ride upward toward wider part of the rim, closer to the tire -- and grab; then downward toward the hub, and loosen.

Rattles

You may hear a rattle and feel it through the handlebar when you apply the front brake lightly. The rattle stops when the brake is applied harder. The problem isn't actually in the brake at all: it is a loose headset. When you are not using the brake, your weight pushes the front fork forward. Braking pushes it backward, and if the forces balance, road irregularities rattle it back and forth.

A rattle that occurs when the brake is not applied and goes away when it is applied, indicates that something is loose. -- most likely, a brake pivot, or the bolt that attaches a caliper brake to the frame. Find the problem and fix it! A loose front brake can jam in the spokes, tethered by its cable, and is very dangerous. A loose brake shoe can tilt and wear through the tire, or jam the wheel.

Clunking

If you bear down with your belly on the saddle and try to roll the bicycle backwards and forwards while holding a brake lever, some of the rattles described in the previous section become clunks. A clunk indicates that something is loose. Fix it!

A clunk that occurs once per wheel revolution while braking indicates that the rim has an irregular braking surface. See the longer discussion of this topic in Sheldon's article on creaks and clunks.

Scraping

Some brake shoes intended for off-road use are made of unusually hard material. It is normal for these brake shoes to make a light scraping noise in use.

Also, wet sand on rims and brake shoes can make a scraping noise. Your rim is being worn down! Usually, releasing and re-engaging the brake a few times will clean off the sand.

Other than this, a scraping noise when the brake is applied is probably from a worn brake shoe whose metal backing plate contacts the rim. Replace the brake shoe before it wears a groove in the rim sidewall.

Musical tone, varies with speed

A musical tone may appear when you apply the brake. The musical pitch falls as the brake slows you down. Some older (pre-1980) rims had serrated sidewalls, and with these rims, this noise is normal, if unwelcome: brakes work better on smooth rims. More often, this noise results from a brake shoe that is contacting a tire. The vibration results from the pattern of the tire fabric. The brake shoe may be misadjusted, or the wheel may be misaligned -- see Sheldon's additional comments.

Some tires have smooth rubber on their sidewalls, and then this problem doesn't make a noise. Also check the brake shoes visually. Fix this problem before the brake shoe eats a hole in the tire!

Squealing/Squeaking

Squealing of brakes is a common problem, and there's no one simple solution to it.

It's caused by the friction of the brakes against the rim flexing the shoes or brake arms, which then slip back, grab, slip back, grab, etc. at high rate -- like a violin bow on a violin string. All brakes do this, but with luck the pitch (frequency) is too high for human hearing.

Occasional squeaks are annoying, but they are generally not a safety issue. On the other hand, a loud and persistent squeal makes it hard to modulate the brake. It is usually caused by a new brake shoe that has not worn in, or by grease or oil on the rim.

You might think that an oily rim would make the brake slip. Instead, it softens the brake-shoe rubber, which leaves sticky deposits on the rim. The brake shoe catches on these, and squeals.

Here are some things to try if your brakes squeal (and also see Jobst Brandt's advice):

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Centering Adjustment

When the brake is released, the brake shoes retract away from the rim. Ideally, the shoes on both sides should back off by the same amount. If they don't, the brake is not properly centered. In extreme cases, one of the shoes may rub on the rim even when the brake is not being applied.

If a brake appears off-center, check first that the wheel is installed straight in the frame/fork. If the wheel is crooked, and you maladjust the brake to compensate, you are creating two problems where there was only one before.

If the brake is, in fact, off-center, it is often the result of too much friction on one of the pivots. Unhook or loosen the cable (the transverse cable, on a cantilever or centerpull brake) and try moving each brake arm individually by hand. They should move smoothly and freely, and always come to rest near the same position. If you suspect friction, unscrew the bolt that holds arm to the boss or pivot, and remove the arm. The surface of the pivot should be smooth, free from rust, and coated with grease. If it is rusty, that is usually a sign that the bicycle was sloppily assembled by the dealer who sold it. Use emery cloth or sandpaper to remove the rust, and wipe off the dust and sand. Coat the boss or pivot with grease, and re-install the brake arm.

Spring Adjustment

If your wheels are centered, and your brakes are not, and, if the pivots are properly lubricated and free-moving, the brake shoes should be centered. If they are not, you probably need to adjust the spring tension on one or both sides Different types of brakes feature different systems for adjusting the springs.

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Mechanical Advantage and flex

Mechanical advantage is a key concept, which you must understand before you will be able to choose and service brakes. Mechanical advantage is commonly referred to as "power", when discussing brakes, and it is common for people to confuse this with quality. A brake may be designed for any amount of mechanical advantage, but there is a fairly narrow range that is usable.

A few models of brake levers have adjustable mechanical advantage. Mechanical advantage may also be adjusted by changing the brake shoe extension or the length of the transverse cable if a brake uses one, or by replacing the brake, the lever or both. Pulley devices such as the Travel Agent also are available to adjust mechanical advantage.

Too Much Mechanical Advantage

People with brake problems often think that they need more "power," when they actually need less! In particular, when modern low-profile cantilever brakes are used with drop-bar type brake levers, the combination produces excessive mechanical advantage. This problem also arises when using direct-pull cantilevers (such as Shimano's "V-Brakes") with levers made for other brakes.

If there is too much mechanical advantage, the lever will be all-too-easy to pull, but it will run out of travel and bump up against the handlebar before the brake is fully applied. Once the lever hits the handlebar, it doesn't matter how much harder you squeeze! If you try to correct this by tightening up the cable, you will wind up with the brake shoes too close to the rim when at rest, which will cause them to rub, especially if the wheel isn't perfectly true.

Too Little Mechanical Advantage

If your brakes too little mechanical advantage, when you squeeze your brake levers, you may feel a nice firm response. In fact, if you just squeeze the brakes of a bike that is not moving, your first impression may be that the brakes are in great shape, because they feel so solid and firm...the problem is, that they don't stop the bike, unless you squeeze very hard on the levers! A brake system with too little mechanical advantage will push the shoes against the rim quickly, but won't push them hard enough.

In some cases, what appears to be inadequate mechanical advantage turns out to be that the brake shoes are not "grippy" enough; they may be dirty, or dried out, or of low quality. Better-quality shoes can make a real difference. I particularly recommend Kool-Stop Salmon brake shoes, which have good grip and wear slowly.

Variable Mechanical Advantage

Mechanical advantage can vary as the brake is squeezed.

Ideally, it should start out low, so that the brake shoes can be set for generous rim clearance. The low initial mechanical advantage will allow the shoes to move in toward the rim fast, in response to a small amount of hand-lever movement.

As the shoes get close to the rim, the mechanical advantage should increase, so that a small amount of hand force will result in a strong force pressing the shoes against the rim.

Some high-end brake levers, most notably the upper-end Shimano models with the "servo-wave" feature, are designed to do exactly this.

Unfortunately, conventional center-pull brakes produce exactly the opposite variation! As the lever is pulled, the yoke angle gets sharper and sharper, while, in the case of low-profile cantilever brakes, the pivot-cable distance decreases. Both of these conditions reduce the mechanical advantage as the brake is applied.

Since the travel increases as the brake shoes wear down, braking performance degrades as the shoes wear, not because the shoes have any less grip, but because the mechanical advantage has decreased.

Flex

Preferably, every part of a rim-brake system is rigid except the brake shoes. Their compressibility allows them to conform to the shape of the rim and to take up the motion of the brake levers. If, on the other hand, the brake arms are flimsy and flexy, as on cheap long-reach sidepull caliper brakes, there is no way the brakes can develop enough "power". Most of the motion of the brake levers is wasted in flexing the arms. If the mechanical advantage is high, the brake lever will go to the handlebar before the brake shoes grip the rim very hard. If the mechanical advantage is low, the feel at the levers will be harder, but the brake will never develop enough grip.

See also Jobst Brandt's comments on brakes

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Last Updated: by John Allen