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Bicycle Coaster Brakes

("back-pedalling brakes", "foot brakes")


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Sheldon Brown photo
Revised by John Allen
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Coaster Brakes

A coaster brake is a special rear hub for a bicycle, which performs two functions:
  • It allows the bicycle to roll without forcing the pedals to turn. This is the "coaster" part. It is similar in function to a freewheel , but uses a different sort of mechanism to accomplish it.
  • It is also a brake, operated by turning the pedals backwards.
Coaster brakes were invented in the 1890s, and have continued to be popular in some areas to this day.

Pros and Cons

Pro: Con:
Coaster brakes work just as well in the rain as they do in dry conditions.

Coaster brakes generally require less maintenance than any other type of brake.

There are no cables running from the handlebars, giving a tidy and simple appearance.

The lack of cables is particularly advantageous for folding or take-apart bikes.

Coaster brakes can be a good choice for handicapped riders who lack sufficient hand strength, or for arm amputees.

Coaster brakes are usually quite narrow, fit in 110-114 mm spacing , though they can be fitted to frames with wider spacing by adding washers along the axle.

A bicycle with at coaster brake can coast backward with stationary cranks (useful in acrobatic/extreme sports).

Coaster brakes make it awkward to get started up, and prevent the use of clip-in pedals or toe clips and straps, since there's no easy way to rotate the pedals to a good starting position.

Coaster brakes often cause skidding, resulting in excessive tire wear.

When coaster brakes fail (usually the result of chain breakage or derailment) they fail suddenly and completely.

Coaster brakes are prone to overheating and fading when used in mountainous areas.

Too many bikes are equipped with only a coaster brake, so there's no back-up system available in the event of brake failure.

To be safe, any bicycle needs a front handbrake and some sort of rear-wheel braking system. This could be a handbrake, coaster brake or fixed gear.

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The Reaction Arm

A wheel consists of rotating parts (tire, rim, spokes, hub shell) and stationary parts (axle, cones.) The braking action of a coaster brake basically consists of creating friction between the rotating and stationary parts.

As the brake is applied, this friction will cause a twisting force to be applied to the axle. The normal axle nuts by themselves can't resist this force, so all coaster brakes have a "reaction arm" which runs below the left chainstay.

The back end of the reaction arm is attached to the left cone of the hub, which is also connected to the brake shoes.

The front end of the reaction arm is clamped to the chainstay, either with a metal brand that wraps around the chainstay, or by a brazed-on fitting underneath the chainstay.

[Note: the metal band in the photo below is only looped loosely around the chainstay, a common error. If the brake is used to hold the bicycle from rolling backward (for example, when stopped facing uphill), the brake arm will rotate upward. Then when the brake is used to slow forward motion, the brake arm will rotate downward again. Repeated motion will loosen the bolt holding the metal band, or the axle nuts, leading to failure. The metal band has multiple holes to fit different-size chainstays. Cinch it tightly around the chainstay -- John Allen]

DESCRIPTION

The attachment of the reaction arm to the chainstay is vitally important. One of the most common causes of coaster brake malfunction is failure to properly secure the reaction arm after removing and re-installing the wheel.

If the bike is ridden without the reaction arm properly secured, the arm will rotate downward, effectively loosening the left cone. The result will be a shaky wheel, and an increase in the amount of reverse pedal travel required to activate the brake.

The reaction arm is usually the place where the make and model of the hub is embossed.

Tip:

If you come upon a bike that has been ridden this way, you will see the reaction arm out of position, and may be inclined to remove the wheel to adjust it. This is often not only unnecessary, but sometimes nearly impossible!

As the disengaged reaction arm and left cone turn, they screw the left cone outward. This squeezes the forkend hard against the left axle nut. As a result, if you try to unscrew the left axle nut, you may find it impossible to turn because of the pressure of the cone. Bike shops frequently see bikes come in where the left axle nuts have been butchered by unsuccessful attempts to loosen them.

The trick is to treat the cause, not the symptom. Instead of immediately going for the axle nut, use a hammer to rotate the reaction arm back to its proper position. This will often bring everything back to proper adjustment with a minimum of trouble. At the very least, it will ease the binding of the axle nut, and allow you to adjust the bearing cones in the normal manner.

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Disassembly

Single-speed coaster brakes are pretty simple. To disassemble a coaster-brake hub, all you need to do is to unscrew everything from one end of the axle or the other. All of the parts will then come out of the left side of the hub shell. See the links at the end of this article. Links to parts lists and rebuilding information are at the bottom of this page.

Two-speed coaster-brake hubs may shift by means of a "kickback" mechanism which shifts when backpedaling, or automatically by means of centrifugal weights. Links to parts lists and rebuilding information are at the bottom of this page.

The brake mechanism of multi-speed coaster-brake hubs is similar to that of single-speed coaster brakes, but they can be quite complicated. They are covered on this site on the Sturmey-Archer, Sachs/SRAM and Shimano internal-gear hub pages.

Operation-How A Coaster Brake Works

Though there are differences in detail, all coaster brakes are fundamentally similar. Different manufacturers use different names for some of the parts, I'll try to use terminology as generic as possible:
  • The chain sprocket is attached to the "driver" usually by a snap ring (though some older models used threaded sprockets with lockrings similar to those of a fixed-gear hub.)
  • The driver has a coarse thread which is screwed into the brake cone.
  • The brake cone fits loosely over the axle. It typically has conical projections at each end.

    When the sprocket turns forward, normal pedaling, the driver's threads pull it to the right, where the conical projection on the right end of the brake cone jams into the narrow part of the hub shell, transferring the pedaling force to the hub shell, and thence to the rest of the wheel.

    When the sprocket/driver turns backwards, the driver threads push the brake cone to the left, where the conical projection on its left side spreads the brake shoes, causing them to press against the inside of the hub shell.

  • The brake shoes are curved to match the curvature of the inside of the hub shell, and they have tapered inside edges. These tapered ends fit against the brake cone on the right, and the left side bearing cone on the left. The left bearing cone will have some sort of projection that will make it impossible for the brake shoes to rotate around it.
  • The retarder spring comes in many different forms for different brands of brakes, but it always serves the same purpose: it keeps the brake cone from spinning too freely with the driver. If there isn't something resisting motion of the brake cone, the driver threads wouldn't be able to move it back and forth...it would just rotate with the driver.

    Failure or wear to the retarder spring is the major cause of erratic braking/driving of a coaster brake. Sometimes they can be tightened by bending, sometimes they just need to be replaced, if you can find one.

Lubrication

Coaster brakes are intended to be pretty much packed with grease. There is no part of a coaster brake that can be harmed by grease, so be generous in applying it. You should use a grease with tolerance for high temperatures, such as automotive brake grease, but even so, coaster brakes used in mountainous terrain can "cook" any common grease.

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Adjustment

There's basically only one adjustment to be made on a coaster brake, the adjustment of the bearing cones. This adjustment can usually be made without even removing the wheel from the bike.

The left cone is attached to the reaction arm (the arm that runs below the left chainstay, and is secured to the chainstay by a clamp of some sort.)

If you loosen both axle nuts, and stick a cone wrench onto the right side cone, or its locknut, you can turn the whole axle, Since the left cone is immobilized by the reaction arm, turning the axle will adjust the cone.

If you get it too tight, the wheel will tend to bind up. Too loose, and the wheel will shake from side to side, and the brake will require a lot of pedal movement to engage. .

English Coaster Brake Adjustment:

Sturmey-Archer SC and most other English coaster brakes adjust differently from other coaster brake hubs. There's a square end on one end of the axle. The right cone is fixed to the axle; the left cone, as usual, is attached to the reaction arm.

To adjust the cones on an English coaster hub, you leave it in the bike, but loosen both axle nuts. Use an adjustable wrench to turn the whole axle, screwing it in or out of the left cone, then re-tighten the axle nuts once the cone adjustment is correct.

Any coaster brake that has a square end on the axle is intended to be adjusted this way.

Sprockets ("Cogs")

Three-splined sprocket Most newer coaster brakes use the same 3-splined sprockets as are used on internal-gear hubs. Such sprockets are available in sizes from 14-24 teeth. Changing the sprocket to customize the gearing is often very worthwhile.

These sprockets are held on by a snap ring, which you can pry off with a small flat-blade screwdriver. When you replace the sprocket, it is a good idea to use a hammer and punch to seat the snap ring into its groove so that the sprocket can't accidentally pop off, leaving the brake inoperative! Sprockets available here.

Older coaster brakes used threaded sprockets and lock rings, as with track hubs. Although they are the same in theory, they commonly used different threading, so they are not usually interchangeable with track sprockets. Threaded sprockets made for coaster brake use commonly had a radial slot at one point, to make it possible to replace a broken spoke without needing to remove the sprocket.

Note that with a coaster brake, the gear ratio affects both the forward pedaling and the brake operation!

  • Using a larger hub sprocket (lower gear) will permit faster acceleration, easier hill climbing and stronger braking.

    However, a larger sprocket will limit your top speed, and will also require moving the pedals farther backward to cause the brake to engage.

  • Using a smaller hub sprocket (higher gear) will permit you to go faster without having to pedal faster (just harder!). It will also provide quicker brake engagement (you won't have to turn the cranks as far before the brake begins to grab.)

    However, a smaller sprocket will make hill climbing more difficult, and will also require pushing harder on the pedals to get powerful braking.

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Information and Diagrams for specific brands/models:

Sutherland's Handbook of Coaster Brake and Internal-Gear Hubs, now hosted on this site, covers many common models.

Note that sometimes these would be "private labeled" or there might be "knockoffs" so you might find an unlisted model that is identical to the ones shown.

Axles and axle nuts are generally fairly generic.

Other, internal parts are generally not available for purchase, please don't waste our time by asking!

Single-speed Coaster Brakes:

Two-speed Coaster Brakes:

If you have information/diagrams of other makes/models that you would like to share with others, please send them to us. We prefer scans of 200 DPI or better, or if you mail me the material we can scan it and return it to you.
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Links:

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Thanks to Steve Birmingham and Henry Brunelle for their assistance.

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