HOW BRAKES WORK

A brake system consists of 3 elements; 1) brake levers, 2) cable/housing system, and 3) brake calipers. When we squeeze the brake lever a cable is pulled through the housing sheath, which moves a set of brake calipers, thereby squeezing a pair of brake pads against the rim. The pads grab the rim, as it spins through the brake calipers, generating friction. In order to slow your bike down the frictional force created by the brakepads at the rim must be great enough to slow down you, your bike, and all the gold bullion that you usually carry with you, as you ride along Shattuck Ave. at 20mph. You need to be able to stop quickly in a lot of situations, but you also need to be able to slow down gradually, by what is called modulating the brakes. Modulating the braking force at the rim is a key requirement for maintaining control of a bike. If your brakes are designed to be too powerful you lose a degree of modulation, as well as the extra risk they put you at flipping yourself over the handlebars. A designer building a set of brakes for the market has to stay within a narrow range in terms of the power or mechanical advantage that the brake exerts on the rim. Friction between the tire and the pavement or ground plays a big part in slowing us down as well. Skidding actually increases the stopping distance, as well as causing a loss of control of a bike. The most effective technique for stopping a bicycle in the shortest distance is applying a force at the brakes at just below the level where the tire begins skidding. Once the skid begins you need more distance in which to stop the bike. In either case we need our brakes to be adjusted properly and all the brake system components to be in good condition to be effective. Other parts of the bike play a part in how well our brakes are able to stop our bike effectively. A dent in the wheel (caused by hitting potholes or curbs) can flare out the sides of the rim, causing the wheel to skid more easily than usual. An out of true wheel will require a looser brake adjustment to prevent the rim from rubbing on the pads. By keeping our wheels straight we can keep the brakes adjusted so that a small amount of lever travel will stop the bike. A loose headset will adversely affect stopping by causing the fork to vibrate back and forth as the front brake is applied, making the bike difficult to handle. Riding conditions also affect how well our brakes function. In wet weather, water collects between the rim and brake shoes, as well as between the tire and pavement causing 'hydroplaning'. Hydroplaning reduces the amount of friction at both locations and therefore diminishes stopping power. In order for the brakes to work properly the water must first be 'squeegeed' out from between the pad and rim before enough friction can be generated to slow the bicycle. This creates a delay from the moment when you squeeze the lever to the moment when the bike begins slowing down, and when it does begin slowing there’s often enough power to lock up a tire on wet pavement causing a skid to occur. When riding in the rain it is a safe practice to lower your speed, as well as to anticipate situations where you will have to stop and ‘pre-braking’ in order to squeegee the water out from under the pad well before stops. When riding in dirt or when there is gravel on the pavement, the frictional coefficient of our tires is reduced requiring better braking technique to prevent the tires from sliding out from under us.

TYPES OF BRAKE SYSTEMS AND MECHANICAL ADVANTAGE

‘V-type’ or linear-pull brakes are found on most mountain and hybrid bikes. These brakes have a high mechanical advantage. Mechanical advantage of a braking system can be thought of as the amount of force applied to the rim relative to the amount of force applied to the lever. Since the braking system consists of levers and calipers, the mechanical advantage of each of these components must be considered. The next figure illustrates the concept of mechanical advantage. Factors affecting mechanical advantage are the position of the pivot point, and the length of the lever arm. Imagine trying to lift the weight pictured. If you extend the bar, the weight becomes easier to lift but requires a longer movement (arc A) to lift the weight the same distance. V-brakes, by virtue of their long caliper arms possess a relatively high mechanical advantage compared to other types of brakes, but require more cable pull to move the pads to the rim. Brake systems must fall within a certain range of mechanical advantage. If the mechanical advantage is too high it will be too easy to lock-up the brakes, too low and it will be hard to stop. So if you have calipers with a high mechanical advantage you must reduce the mechanical advantage of the levers to get the total within an acceptable overall range. V-brake levers are designed to be used only with V-brake calipers unless they have a cable pull feature which toggles on either a v-brake and cantilever setting. Then they may be used with either cantilever or V-brake systems.

As with any brake system once you have applied the pad to the rim by pulling the brake lever, the pad needs something to pull it back, away from the rim once the lever is released. Otherwise the pad will continue dragging on the rim, slowing you down when you don't want it to. This is what return springs do. Their job is simply to bring the calipers back to their starting place away from the rim. The return springs need to exert an equal amount of force to bring both calipers back away from the rim by the same amount. Small screws on the side of one or both calipers adjust the amount of tension in the return springs. If one caliper hangs up on the rim then you must either increase it's spring tension, reduce the other calipers spring tension, or perform a combination of the two.

Pad adjustment is crucial to properly functioning brakes. A properly adjusted pad will contact the rim squarely in the middle of the rims braking surface when looked at from the front.(see fig.1). If the pad contacts the rim too high it can rub against the tire when you apply the brakes, causing a dangerous problem if the pad wears through the sidewall of your tire (see fig2). If the pad contacts the rim too low it will wear the pad unevenly causing a small lip to form over time, which can hook against the bottom of the rim after releasing the brake lever, causing that caliper to get hung up against the side of the rim (see fig3). When looked at from above, the front of the brake pad should hit the rim first by a small amount, this is called toe-in (see fig4). Toe-in is required to prevent brake pads from squealing. If the pads hit the rim without toe-in, or flat against the rim, there is a tendency for the pad to vibrate causing it to squeal. When you look at the brakes from the front, as the brake caliper is activated it swings through an arc. The pad follows the same arc. Depending on several factors the pad may be on either a downward or upward arc when it contacts the rim. You must adjust the pad tilt to accommodate for this so that it contacts the rim squarely (see fig5 ). The position of the brake caliper is important as well.

The series of convex and concave washers (fig. 6) that allow you to adjust the tilt and toe-in of the pads come in two different thickness with each caliper having one thick set and one thin set. You can switch one set for the other between the caliper and the pad in order to adjust the vertical orientation of the brake arm. The arm should be very close to vertical when the pads are in contact with the rim. When the lever is released and the pads return away from the rim, the arms form a ‘V’ shape.

Regular maintenance will keep your brakes functioning properly. Brake pads require more maintenance than any other component of the braking system. The friction between the pad and rim creates heat, causing brake pads to develop a glazed surface. The pad material becomes hardened where it contacts the rim. A glazed pad will have a smooth, shiny surface as opposed to the original dull surface, and reduces its frictional coefficient. To remedy this situation it is good practice to sand the pad surface with sand paper or emery cloth. Emery cloth is similar to sand paper except that it has a cloth backing. A fairly rough grit paper or cloth is effective at removing the glazed surface. Pads can also get small pieces of debris as well as small chips of aluminum sloughed off the rim embedded into the pad material. Every time you use your brakes, small pieces of debris stuck in the pad can gouge out a channel from the side of the rim. So one should occasionally inspect pads for flakes of aluminum or debris embedded in the material, and remove any found with a sharp pointed knife blade. Tightening the cable to accommodate for cable stretch is periodically required, particularly when a new cable has just been installed since it stretches the most at the beginning of its service life. The vinyl covering on cable housing can develop cracks, which will allow dirt and water to enter into the cable housing system. Replacement is recommended when this occurs. Occasionally oiling the cable where it passes through the housing will keep it sliding smoothly and prevent corrosion of the cable.

There are two different types of cable housing, one for brakes, and one for gears, and it is important to understand the distinction between the two. Cable housing serves as a brace between the brake lever and caliper in the case of brakes; and between the gear shifter and the derailleur in the case of gears, and allows the cable to slide through its center, transferring motion at the lever to motion at the caliper or derailleur. In the case of gears, this motion needs to be very precise. Single-wire, helical brake housing compresses a small amount under pressure, so it is not well suited for indexed gears. For indexed derailleur systems, a bundle of lengthwise arranged wires are encased in a hard plastic cover. This arrangement eliminates compression, and results in accurate shifts. This type of housing is essentially held together by the plastic casing, making it fine for low force gear shifts, but too weak to brace a brake cable under high braking forces. A brake cable threaded through gear housing will rupture the plastic casing, resulting in brake failure.

Single-wire helical housing does not depend on the casing for any of it's resistance to rupturing, and is therefore well suited for brakes. Today, nearly all cable housing has a narrow teflon liner which the cable threads through reducing friction between the cable and housing. So when you are installing a cable, greasing the cable will actually make the cable action stickier. It is better to use oil to lubricate and to protect the cable from corrosion. There are different cutters for the two types of housing. Diagonal cutters should be used for brake housing. As the two flat jaws of the diagonal cutter come together they wedge through two helical turns of the wire and cut the wire without flattening the spiral. Cable cutters are needed to cut gear housing. Each jaw has a circular shape and as the two jaws come together they wrap around the housing, cutting evenly around the entire circumference of the housing. Using a diagonal cutter on gear housing usually flattens rather than cuts the housing. Once you have cut brake cable housing with a diagonal cutter, it is good practice to finish the end with a grinding wheel, to remove the jagged burr and create a flat end that will butt up neatly to a cable ferrule. Once you have finished the end, you still need to reopen the inner teflon liner, which gets smashed flat by the cable cutters. This is done with a sewing awl or sharpened spoke end. Now the cable will be easy to thread into the housing. The same process is used for gear housing, except that you do not need to finish the end of gear housing with a grinding wheel, since the cutters for gear housing make a square cut that is ready to be capped with a ferrule. Gear housing only requires reopening the inner teflon liner with the end of the sharpened spoke.

SIDE-PULL BRAKES

Most road bikes use side-pull brakes. As their name implies, the cable attaches to the two arms on the side of the brake. The two caliper arms are bolted together to form one unit, and attach to the bicycle at one point. The dual-pivot brake, found on most road bikes today, is a variation of a side-pull brake. Side-pull brakes are much more responsive than V-brakes or cantilever brakes, but do not offer the tire clearance of the latter two types., so they are used for bikes that are not intended for off-road riding. Because side-pulls have shorter arms than V-brakes, the calipers are stiffer, vibrate less, and require less toe-in than V, or cantilever brakes. Centering of the brakes can be accomplished either by loosening the bolt that attaches the brake to the frame and centering the brake using a cone wrench on the flats of the pivot bolt, or by a centering adjustment screw on the side of the caliper. If you happen to have an older bike without either of these features, then a punch and hammer may be required to center the brakes. Single pivot brakes are relatively easy to overhaul, while dual-pivot brake overhaul should be considered much more advanced. The adjustment of the calipers in both single and dual-pivot brakes should be as frictionless as possible without any noticeable side-to-side play. If your brakes have a sluggish return after release of the lever, either the pivots are too tight, or the cable is sticking somewhere in the housing. As with all brakes the cable should be oiled and sliding smoothly through the housing. Another possible cause of sticky or sluggish brakes is poor cable routing. Most road bikes feature cable routing underneath the handlebar tape. The housing should be cut just long enough for the bars to be turned all the way through their side to side arc without any pull on the cable housing. Housing ends should be ground flat so they fit snug into the ferrule. The piece of housing running from the frame cable stop to the brake caliper should have a smooth arc to it.

CANTILEVER BRAKES

Cantilever brakes are found on hybrid, touring, mountain, and cyclocross bikes. They are powerful, and offer more tire clearance than either V-brakes or sidepull brakes. Clearance becomes an important feature when one is riding on muddy trails. Sidepull brakes get clogged with mud when riding on muddy trails. Cantilever brakes also have an advantage over V-brakes in that they work relatively well with road bike brake levers. V-brakes require a special adapter for use with road levers, or a special V-type lever. As with all brakes, the way cantilevers are set up will determine how well they function. A cantilever brake is activated by the cable pulling on another cable, called the yoke, or transverse cable. The yoke cable attaches directly to each cantilever brake arm. The crucial part of cantilever set up is the angle that the transverse cable forms with the brake arm. This angle can be adjusted by changing the position of the pads, or by raising or lowering the position of the intersection of the yoke and main cables. As with V-brakes the pad swings an arc to the rim, so the pad must be adjusted so that when it contacts the rim, the surface of the pad is square with the rim when looked at from the front. When viewed from above the pad will have a bit of 'toe-in' to prevent squealing. With cantilevers there is more adjustability in the distance that the pad extends out from the caliper. This distance will affect the angle that the caliper makes with the rim face, as well as the amount of cable pull that will effectively pull the brake towards the rim. Low profile cantilevers generally require more extension of the pad from the caliper to the rim.