Bicycle Technology: Rim Brakes

Although the coaster brake and other forms of hub brake still find a use on simple utility bikes, the over whelming majority of all bicycles sold for serious use these days are equipped with some kind of rim brake. Even on bikes with a coaster brake on the rear wheel, the rim brake frequently finds application on the front wheel in those countries where two independent brakes are prescribed by law (as it should be everywhere).

All rim brakes are based on one of two principles, which are depicted in Fig. 12.2. By means of a lever and usually a cable (sometimes a pull rod or even a hydraulic system), two brake blocks of a wear-resistant high-friction material are pushed against the wheel rim. In the US, only the type that applies force from the side is used. It is also possible to apply the force radially outward towards the rim, something that is still found on the roller-lever (or stirrup) brakes used on utility bikes in many parts of the world. Neither system is inherently superior to the other: the roller-lever system is just heavier and harder to adjust.

12.1. Brake test in rainy weather. With the right choice of rim and brake block, you should not overshoot the mark.

12.2. The two rim brake principles: Caliper brake; Stirrup brake

Braking Theory

In the moving bicycle, a certain amount of so-called kinetic energy (the product of mass and the square of the speed) is stored, which is absorbed when the bike is brought to a standstill. This can be done by letting it roll until friction losses have depleted this energy, which may take a long time. Or it can be done by crashing into a fixed object, which will do the job much faster, though with detrimental results to bike and rider.

To stop before an accident happens, the brakes are used. The brakes apply friction between a moving and a fixed part (in the case of rim brakes, between the rim and the brake blocks) to dissipate the energy at a rate controlled by the rider via the force he applies to the brake lever. The energy absorbed by the brakes is converted to heat (rim and brake blocks become hot). The amount of energy to be absorbed is calculated as follows:

W = 0.5 x m x v²

where:

W = energy in J (equivalent to Nm)

m = mass of bike, rider and luggage in kg

v = initial riding speed in m/sec.

The relationship expressed by this formula explains why it is harder to stop a heavily loaded bike, and why it is so much harder to stop it from a higher speed, as evidenced by longer braking distances.

The speed is reduced more or less gradually. Just how gradually is expressed by the deceleration in m/sec². A deceleration of 1 m/sec² means that the speed is reduced by one m/sec after each subsequent second of braking. When rolling to a gradual stop, the deceleration is very low (in the order of cm/sec while it is very high (several thousands m/sec when crashing into a fixed object, the damage being roughly proportional to the deceleration.

During controlled braking, decelerations of several m/sec² are achieved. If the brakes apply a deceleration of 1 m/sec² (either because they are not more effective or because the rider does not apply more force), it will take 8.3sec to stop from an initial speed of 30km/h (20mph, or 8.3m/sec). The braking distance S is then calculated as S = (Vo)² / 2a = 8.3² / 2= 34.5m.

If the deceleration is increased by more powerful braking, the braking time and distance become proportionally less. In practice, decelerations of 1—6m/sec² are typical for the range from gentle speed reduction to hard braking, resulting in braking distances that range from 35 to 6m from an initial speed of 30km/h.

12.3. Effect of braking on location of mass center.

12.4. Auxiliary brake levers flex too much to be used when high braking forces are required.

12.5. Move back and keep the weight low to avoid toppling over.

Another situation occurs when riding downhill. In this case, the bike also has a certain amount of potential energy, expressed as the product of weight and height (or mass, height and mass constant, or gravitational acceleration, G). Part of this energy is absorbed by the resistance of the air, mechanical friction and rolling of the tires. Any difference that remains between the resulting ‘free fall’ speed and the desired safe handling speed has to be taken up by the brakes. The steeper the slope, and the heavier the bike, the more energy has to be absorbed (or the more power has to be applied) to maintain the same speed, in addition to any speed reduction that may be required.

While braking, the effective mass center of bike and rider is transferred forward as shown in Fig. 12.3 above: The front wheel is loaded more, the rear wheel less. Due to the relatively high mass center and short wheel base, this imposes a severe limitation on the deceleration possible as distributed over the wheels, at least on the conventional bicycle. The maximum deceleration that can be reached with the front brake is approx. 6.5m/sec² while it is approx. 3.5m/sec² with the rear wheel. Anything in excess of those figures would lead to tipping forward and skidding of the rear wheel, respectively, both causing loss of control. To limit this effect as much as possible, the cyclist should place his body weight as low and as far back as possible during sudden braking, as shown in Fig. 12.5.

12.6. Gran-Compe lightweight racing side-pull brake. The levers are intended for cable routing along the handlebars.

12.7. Wilderness Trails roller-cam brake. Note the springs connecting the pivots with the rollers.

The braking force required to achieve a certain deceleration can be calculated. The interesting thing is the insight gained that braking, like so many other bicycle phenomena, is not some kind of black magic, but can be rationally determined. Although the average cyclist will rarely feel the desire to figure such things out for himself, the calculation is shown here for reference. Here is the formula to determine the effective braking force that must be applied to the wheel to achieve a given deceleration:

Fb = m x a / G

where:

Fb = the effective braking force in N that must be applied to the wheel

m = the mass of bike and rider in kg

a = the required acceleration in m/sec²

G = the gravitational acceleration (9.8m/sec²)

The force that can be achieved is calculated as follows:

Fb = Cf x Fl x Sl/Sb

where:

Fb= the effective braking force in N that can be achieved under the given circumstances

Cf = the coefficient of friction between brake block and rim

Fl = the force applied to the brake lever

Sl = the amount of travel of the brake lever

Sb = the amount of travel of the brake block

The ratio Si/Sb is referred to as mechanical advantage. This is one of the most unscientific terms used in engineering -- leverage would be more correct.

With these two formulas, the unknown factor can be computed. Any manufacturer worth his salt should take the trouble to verify whether the normal human can apply enough braking force, or conversely to make sure parts are dimensioned correctly.

12.8. The four most common rim brake types. All of the brakes shown here are known as caliper brakes, as opposed to the roller- lever or stirrup brake. In addition to the types shown here, various other models will be briefly introduced in the sections that follow. Center-pull brake; Side-pull brake; Roller cam brake; Cantilever brake

Brake Design and Construction

As should be clear from the preceding section, in order to brake more forcefully, it does not suffice to pull the brake lever more firmly. In addition, the components must be able to convert the force into friction between rim and brake block. The following factors come into play:

+ Maximum coefficient of friction between the materials of brake block and rim

+ Rigid (as opposed to deformable, or ‘spongy’) construction of brake, lever and braking surface

+ Favorable relation between brake lever travel and brake caliper travel (mechanical advantage).

The coefficient of friction between natural rubber (as well as most synthetics and composites used for brake blocks) and steel or aluminum used for the rims is quite adequate when dry. However, the harder and smoother the surface of the rim, the more severely the friction is reduced when wet. Smooth hard anodized rims brake poorly when wet, and chrome-plated steel rims are a catastrophe when used with natural rubber or most synthetics. Even non-anodized aluminum suffers greatly when wet: with the same applied force, the effective braking force on the wheel is reduced to less than 50%, resulting in half the deceleration or twice the braking distance.

The rigidity of the brake, the lever, the cable and any anchor points is important because all these factors could otherwise limit how much force can be applied. If the components bend, rather than transmitting the force directly to the contact point between rim and brake block, the amount of lever travel may not suffice to apply the force necessary for effective braking (note that neither force nor power gets lost, as is often suggested: it just limits how much force can be applied and consequently how powerful the resulting braking is). The first step to take when brakes feel ‘spongy’ is to replace the cable with a version that has a thick core and a stiff mantle.

The effective leverage, or mechanical advantage, between lever and brake should be selected with the dimensions of both parts in mind, as well as the size and the power of the rider’s hand. Not enough leverage is just as bad as too much: in the first case not enough force is applied; in the latter, the distance of caliper travel is inadequate to apply the available force.

12.9. Universal center-pull brake. Unfortunately this one is no longer made, but excellent center-pulls are still available from Dia-Compe and some other manufacturers.

Rim Brake Types

Rim brakes can be divided into two distinct categories: squeezing and pulling. The pulling type, represented by the roller-lever, or stirrup, brake with pull rod operation, is becoming increasingly rare, even in Britain, where it used to be standard equipment on most utility bicycles. All other brakes, referred to as caliper brakes, operate by squeezing against both sides of the rim simultaneously. The four most common types are shown in Fig. 12.8 above.

12.10. U-Brake with s4ffenthg bracket to prevent twisting of the seat stays.

On all caliper brakes, the brake arms must return to the inactivated position when the handle is released. This is done by means of one or more springs. As will be explained more fully under Brake Controls, these springs are not always powerful enough to overcome the friction in brake, cable and lever. This has some thing to do with the fact that some manufacturers use the method first introduced by Dia-Compe, integrating a spring in the lever as well. In this case, the spring in the brake need not do all of the work — but this brake must be used with the matching lever.

The Side-pull Brake

This is the most common brake on lightweight bicycles. On this model, the two brake arms pivot simultaneously around a common central mounting bolt. This type of brake is illustrated in Fig. 12.12, while Fig. 12.11 defines the critical dimensions that determine whether a particular brake will fit a certain bicycle. The inner cable is attached to one brake arm, while the outer cable is connected to the other. When applied, the cable pulls the lower brake arm up towards the other, causing the lower ends of the brake arms with the brake blocks to push from both sides against the sides of the rim. Generally, a quick-release is installed on one of the brake arms to untension or tension the cable for adjustment or wheel removal.

12.11, 12.12 and 12.13. Critical dimensions of side-pull brake. Off-set pivot type side-pull brake. Construction of side-pull brake.

The inherent disadvantages of the side-pull brake are twofold: the distance between pivot and brake block is relatively great (with given overall dimensions), and the brake is hard to center. The first is overcome by building the bike on which this brake is used to such close clearances, using narrow rims and tires, that the dimensions remain small enough to assure adequate rigidity: fine for racing and fitness bikes. But this problem becomes apparent when a big version is used on a bike with fat tires and big clearances, as it is done on cruisers.

Some recently introduced versions of the side-pull brake differ somewhat from traditional models. On these, the pivot point no longer coincides with the mounting bolt, being off-set to one side, while a support pad, off-set to the other side, supports the upper brake arm against the lower one (Fig. 12.14). This reduces the effective brake arm length that is free to flex.

12.14. Construction of a typical center-pull brake.

This and some other models are sometimes equipped with ball bearings in the pivot. Although at first this may seem an unlikely location for a ball bearing, it turns out to be quite effective, being an axial bearing. It does not support the brake arm around the pivot bolt but the two brake arms relative to each other, where otherwise high contact forces cause considerable friction when the brakes are applied.

The Center-pull Brake

The center-pull brake, illustrated in Fig. 12.14, is a symmetrical model. Here the brake arms each pivot on their own bushings mounted at opposite ends of a common yoke, which in turn contains the central mounting bolt. The upper ends of the two brake arms are joined by means of a transverse (or straddle) cable to which the inner brake cable attaches, while the outer cable is anchored against an adjustable stop mounted on the frame. When the lever is applied, the upper ends of the brake arms are pulled together, causing the lower ends with the brake blocks to squeeze from both sides against the rim. The adjustment mechanism is contained in the anchor that holds the outer cable, which is generally installed at the headset or the saddle binder bolt, for front and rear brake, respectively. This brake works best if the angle of the straddle cable alpha is about 120 deg.

As in the case of the side-pull brake, the critical dimensions of the center-pull brake are those illustrated in Fig. 12.11. Generally, the center-pull brake is designed for larger clearances and wider rims and tires. This is due to the more favorable (i.e. shorter) distances between the brake block and the pivot for each brake arm. It is important to keep the angle between the two legs of the transverse cable at least 120-deg to achieve adequate leverage. It is unfortunate that in recent years the center-pull brake has been largely ignored: the fashion of using side-pull brakes overlooks the fact that even a cheap center-pull brake offers superior braking and ease of maintenance to what can be achieved with the side-pull brake at the same price.

The Cantilever Brake

This model, formerly only used on cyclocross machines and tandems, has really taken over with the introduction of the mountain bike. Illustrated in Fig. 12.17, this symmetrical brake consists of two separate brake arms that are each mounted on a pivot bushing brazed or welded to the frame. The ends of the brake arms to which the transverse cable is attached reach outward, which makes these brakes protrude laterally beyond the rest of the bike. The critical dimensions are illustrated in Fig. 12.16.

12.15. Shimano cantilever brake

12.16 and 12.17. Critical dimensions of a cantilever brake. Construction of typical cantilever brake.

12.18 and 12.19. Construction details of cam-operated brake and U-brake. These two brake types were popular on mountain bikes for some years.

Just about everything said about the center-pull brake applies to this model as well. Once again, the angle between the two ends of the straddle cable must be at least 120 degrees to allow enough leverage. This is particularly critical on some of the newer models that do not protrude outward as much. If a brake like this does not seem to work, just check and correct this feature and you will not believe how much difference it makes.

The adjusting mechanism for the cantilever brake is usually integrated in the brake lever. It is also possible to use an adjuster on the anchor that holds the end of the outer cable, as is the case on the center-pull brake. Most cantilever brakes have no quick-release as such: instead they are released by lifting one of the cable nipples out of the open-ended one of the brake, arms, which can be done once the brake blocks are simultaneously pushed against the sides of the rim.

The Cam-Operated Brake

The cam-operated brake is based on an idea that surfaces every so many years: the brake arms are pushed apart by a roughly triangular-shaped cam plate attached to the end of the inner cable. In the late seven ties, it was available from Shimano in the form of a sell-contained caliper unit and was not exactly a big success in that form. In the mid eighties, another version was introduced by that group of creative California mountain bike engineers that call themselves Wilderness Trails, that has since been licensed to SunTour.

On this version, the brake arms are mounted on individual bosses, just like the U-brake and the cantilever brake. Illustrated in Fig. 12.18, the critical dimension are the same as those shown in Fig. 12.16 for the cantilever brake. Another version of this brake is the Odyssey, on which the brake arms are installed on a common mounting plate again. It is very suitable for use on BMX-bikes and wherever space is limited, such as on other small-wheeled bicycles (it is the only brake suitable for the Moulton mountain bike with small wheels and suspension).

The U-Brake

The U-brake, illustrated in Fig. 12.19, is essentially a center-pull brake on which the pivots are not installed on a yoke but are brazed or welded to the fork blades or rear stays for front and rear brake, respectively. In many ways, what was said about the cantilever brake applies here too, except that this model does not protrude as far on both sides of the bike. The critical dimensions are as shown in Fig. 12.16. The mounting boss location is different from that for the cantilever, so these brakes are not mutually interchangeable.

12.20. Shimano U-brake

The Pivot-Link Brake

Never heard of this? It’s the generic term I use to describe the rather rare models operating by means of a complex linkage mechanism as illustrated in Fig. 12.22. Campagnolo’s Delta brake, which forms part of that company’s top-of-the line C-Record group, is one — very sleek and expensive. Weinmann makes a cheap (and generally unsatisfactory) version based on the same concept. As Fig. 12.21 shows, it’s an old idea.

12.21 and 12.22. Left: Early pivot-link brake, introduced in France around 1910. Right Weinmann’s mass-produced version of today.

The Roller-Lever Brake

Instead of cable-operation and two-sided force application against the sides of the rim, this brake, also known as stirrup brake, is characterized by pull rod operation and the application of force radially towards the inside of the rim, as illustrated in Fig. 12.23. The rigid rods allow very effective force application, and little maintenance is required. On the other hand, the weight is nearly twice that of typical side-pull brakes, each including their respective controls. You have to loosen and readjust the brake controls to adjust the handlebars — not to mention removing the wheel.

12.23 and 12.24. Left: Roller-lever; or stirrup, brake assembly. Right: Typical adjusting details for this kind of brake.

The Hydraulic Brake

In recent years, both Magura and Mathauser have introduced very satisfactory hydraulic brakes. The former, made by a renowned German motorcycle brake specialist and illustrated in Fig. 12.25, is actually surprisingly affordable. Their recently improved mountain bike version now incorporates a quick-release, making this a very interesting alternative.

12.25. Hydraulic brake

12.26 and 12.27. Two modem hydraulic brakes. Left: Mathauser’s lightweight brake has an internal bellows, eliminating leak.

Right: Magura’s affordable mountain bike model.

On the hydraulic brake, the force is transmitted by compressing a liquid. The Mathauser brake uses a flexible bellows unit contained in an unsealed cylinder, whereas conventional hydraulic brakes use a hydraulic cylinder, often leading to leakage. Compared to cable controls, hydraulics have the advantage of very direct, rigid and light operation, with negligible friction. The argument that damage to the liquid-filled tube connecting brake lever and brake unit seems far fetched: these tubes are not particularly sensitive and should last a long time with normal use. After all, a regular brake does not work with a broken cable either. It should not be your choice for a world tour (because it may be difficult to repair or replace), but it is fine for normal use.

The Spindle Brake

This is another rare item, illustrated in Fig 12.28 and developed by the Swiss-German bicycle brake specialist Weinmann. Here the cable turns a spindle with a helical groove that then pushes the brake blocks against the rim. Since there is not less but more friction in this set-up than in conventional caliper brakes, there is not the slightest technical justification for their use.

12.28. Construction of spindle brake

12.29 Dia-Compe racing brake lever with return spring for light action.

Brake Controls

With the exception of the hydraulic brake and the roller-lever type, all rim brakes are operated by means of a bowden cable that connects the lever with the brake mechanism. A typical brake lever is illustrated in Fig. 12.31. The inner cable’s end nipple is hooked in a recess in the lever, while the other end is clamped in at a movable part of the brake. The outer cable is anchored against the fixed part of the lever on one end and either a stop on the frame or one on the fixed part of the brake mechanism on the other end. Sometimes the outer cable is not continuous but consists of interrupted sections installed between stops on the frame.

12.30 and 12.31. Left: Dia-Compe mountain bike brake lever. Right: Assembly of typical brake lever for drop handlebar bike.

There are several different brake levers, each designed for a particular model of handlebars. The most significant difference is between those made for drop handlebars and those for mountain bikes, the latter being shown in Fig. 12.32. These would be suitable on any kind of straight handlebars, although cruisers, three-speeds and other utility bikes usually come with more primitive and less sturdy versions.

There are several variants. In the first place, there are models with a built-in retraction spring as mentioned before, intended for use with matching brakes with weaker return springs. Secondly, it has become fashionab1e to use so-called aero levers, on which the cables don’t project from the front but are run along 1I handlebars under the handlebar tape.

Then there are those with extension levers, shown in Fig. 12.33 and often incorrectly referred to as safety levers. Although they are not inherently dangerous, they are not particularly safe either. The auxiliary lever can be reached from the top of drop handlebars, a position frequently used by inexperienced cyclists. Unfortunately, these auxiliary levers are not rigid enough to allow the application of full force: they simply bend. Their other disadvantage is that they interfere with another, more suitable, hand position, namely ‘on the hoods’ — i.e. on top of the brake lever mounts. A better solution for those who want to reach their brakes from the tops of the bars are the French guidonnet levers, shown in Fig. 12.35.

Finally on this subject, a few words about the roller- lever brake. Operated by pull rods, it relies on a number of pivots mounted on the frame and several adjusting connectors between rods. The attachment of the pivot mechanisms to the handlebars, the frame and the front fork must be checked occasionally. These brakes are adjusted by means of the rod connectors.

12.32. Mountain bike lever with cable nipple and adjuster detail

12.34. Magura lever for hydraulically operated mountain bike brake.

12.33. Extension lever

12.35. Guidonnet lever

12.36. BMX freestyle brake lever. The adjusting screw allows the rider to limit the brake force by reducing the lever travel

12.37. Aztec composite brake blocks

12.38. SunTour’s Petersen self-energizing brake. Wheel rotation energy squeezes the brake against the rim.

Brake Shoes

A distinction should be made between brake blocks and brake shoes: the former is essentially only the piece of friction material, the latter includes mounting hardware. Nowadays, one rarely finds replaceable brake blocks, so that the entire shoe usually has to be replaced when the brake block is worn. Not all brake shoes fit all brakes. Some manufacturers cleverly design them to match, often precluding replacement by other makes and models. In addition, there are essentially two types, respectively adjustable in one and multiple planes, referred to as directly arid indirectly mounted, respectively, arid illustrated in Fig. 12.41.

On the directly mounted brake shoe, the mounting stud is screw threaded and is held in the brake arm by a nut (or, alternately, the brake shoe has a threaded hole and is mounted by means of a bolt). On the in directly mounted version, the stud is plain and is held in a kind of eye bolt that is in turn mounted in the brake arm. Although the latter design allows more flexibility of adjustment, the use of concave and convex washers in combination with direct mounting can achieve similar flexibility and is used on many mountain bike brakes.

The material used for the brake blocks is either natural rubber, a synthetic material, or a composite of several materials. Natural rubber is quite poor, especially in wet weather, as pointed out before. Some of the synthetics give higher friction coefficients, but those that work better in wet weather operate poorly when it is dry. The only universally satisfactory material for use on aluminum rims seems to be the sintered material first introduced by Modolo, which is being picked up by some other manufacturers as well now.

Some of the other supposed improvements are mere farces. Brake shoes with cooling fins are a typical ex ample. While braking, the heat is mainly absorbed by the rim, which gives it off to the air. Although the brake blocks get hot too, the measured effect of external fins is just about zero: the heat transfer from the brake block surface to the metal of the shoe is so poor that this does not provide effective cooling.

Another fallacy is the assumption that longer brake blocks, or any other design that offers more contact area, would be more effective. The friction is a function of the force and the coefficient of friction, and the area has no effect. Although the larger brake block may wear better, the braking may even be worse, since the contact pressure is inversely proportional to the area, which may lead to vibrations arid to the build-up of water between rim and brake block in wet weather.

Braking a bike with steel rims is a different kettle of fish altogether. When dry, this material works as well as aluminum, except that some of the homogeneous synthetics tend to leave deposits on it and are therefore not recommended. But when it gets wet, trying to brake such a bike becomes a true adventure: the coefficient of friction is reduced to about 25% of its dry weather value, resulting in dramatically increased braking distances. Although Fibrax and Altenburger offer special materials for use on steel rims (which may never be used on aluminum because they are extremely abrasive), the use of hub brakes, described in Section 13, is a better solution for bikes with steel rims.

Brake Maintenance

From a maintenance standpoint, the brakes should be considered as complete systems, each incorporating the levers, the control cables and the various pieces of mounting hardware, as well as the brake itself. In fact, brake problems are most often due to inadequacies of some component in the control system. Consequently, it will be necessary to approach the problem systematically, trying to isolate the fault by checking off one component alter the other.

When the brakes work inconsistently, often associated with vibrations or squealing, the cause is frequently found either in dirt and grease on the rims, or in loose mounting hardware. First check the condition of the rims, then the attachments of brake blocks, brake arms, brake units, cables, anchors and levers. If the rim is dented, there is usually no other solution than to replace it, while all other causes can usually be eliminated quite easily.

Adjusting Brake Shoes

This simple job is often not only the solution to squealing, rumbling or vibrating noises, but may also solve inadequate braking performance and prevent serious mishaps. As it wears, the brake block’s position relative to the rim changes. On a cantilever brake the brake block moves radially inward, further away from the tire towards the spokes, while it moves up towards the tires on all other brakes. If left unchecked, chances are the brake blocks will hit the spokes or the tire.

To prevent this, it is not enough to follow the systematic brake test described below regularly: you also have to check the position of the brake blocks as they contact the rim, and readjust them when they don’t align as shown in Fig. 12.40. In addition, it is preferable if the front end of the brake block is about 1 - 2mm closer to the rim than the rear. This is to compensate for the deformation of the brake arm as brake force is applied, which tends to twist the back of the brake shoe in. Only when you adjust them this way, called toed in, will the brake force be equally distributed over the entire length of the brake block.

Brake Test

In order to verify their condition and effectiveness, test the brakes according to the following systematic procedure at regular intervals — about once a month under normal conditions. The idea is to establish whether the deceleration achieved with each brake is as high as the physical constraints of the bicycle’s geometry will allow. Tools are not needed for this test.

Procedure:

1. Ride the bike at a brisk walking speed (about 8km/h, or 5MPH) on a straight, level surface with out traffic.

2. Apply the rear brake hard. If the rear wheel skids, you have all the braking you can use in the rear, reaching a deceleration of 3.5m/sec².

3 Repeat the procedure with the front brake. If the rear wheel starts to lift off, a deceleration of 6.5m/sec² has been reached. Let go of the front brake again.

Brake Check

If one brake or the other flunks the test described above, check the entire brake system and adjust or correct as necessary. Usually, no tools are needed for this inspection, but you may have to use a variety of items to solve individual problems uncovered this way.

Procedure:

1. Check to make sure the rim and the brake blocks are clean. The presence of wet or greasy dirt plays havoc. Wipe clean or degrease the rim or scrape the brake block with steel wire wool.

2. Check whether the cables move freely and are not pinched or damaged. In the case of special controls, such as hydraulics or pull rods, check them for correct operation and installation. Clean, free and lubricate or replace cables that don’t move freely. Repair or replace anything else found wanting.

3. Inspect the levers — they must be firmly installed and there must be at least 2cm (3/4”) clearance between lever and handlebars when the brake is applied fully. If necessary, tighten, lubricate, adjust.

4. Make sure the brake arms themselves are free to move without resistance, and that they are re turned to clear the wheel fully by the spring tension when the lever is released. If necessary, loosen, adjust, lubricate, overhaul or replace.

12.39. Holding center-pull brake cable

12.40. Brake block alignment detail

12.41. Brake block attachment details

12.42 and 12.43. Left: Details (left) and method (right) used to center simple side-pull brake.

12.44. Use of Weinmann centering tool

Adjusting Brake

Roughly the same operation is followed for all caliper brakes, although the adjustment mechanisms may be installed in different positions. By way of tools, it is handy to have a pair of needle nose pliers to adjust a center-pull brake, while you may need a wrench to fit the cable clamping nut on all models.

Procedure:

1. If the brake does not perform adequately, the cable tension has to be increased. Do that initially by tightening the cable adjuster by two turns (it may be easier to first release tension with the quick- release or by removing the nipple of the transverse cable, not forgetting to tension the cable again afterwards).

2. Verify whether the brake now engages fully when 2cm (¾”) clearance remains between lever and handlebars. Repeat point 1 if necessary.

3. If the correct adjustment cannot be achieved within the adjusting range of the cable adjuster, first screw it in all the way, then pull the cable further in the clamping bolt and tighten it again. On the center-pull brake this can be done by wrapping the cable around the needle nose pliers and twisting it further as illustrated in the photograph. Now fine- tune with the adjuster.

4. If after all this adjusting the brake finally applies enough tension but does not clear the rim adequately when disengaged, check all parts of the system and replace or overhaul as necessary.

Centering Rim Brake

One of the most frustrating problems cart be the off- centered position of a brake, always dragging the rim on one side while clearing it on the other. Depending on the kind of brake, this problem is solved differently.

Center-pull Brake:

1. First make sure the brake is firmly attached, tightening the mounting bolt if necessary, while holding the brake centered.

2. If the brake is properly fastened and still off-set, take a big screwdriver and a hammer. Place the screwdriver on the pivot point that is too high and hit it with the hammer. Repeat or correct until the brake is centered.

Side-pull Brake:

Here the problem is due to the stubbornness of the mounting bolt, always twisting back into a rotational orientation by which the brake is not centered. Straightening is easier said than done: it will find its way back to this wrong position the next time the brake is applied. Some brake models come with a special adjusting tool with which the mounting bolt is repositioned, each of them with its own instructions.

On simpler models, no tool is provided, and the procedure illustrated in Figures 12.42 and 12.43 may do the trick, always turning two nuts simultaneously. To twist the brake clockwise (seen from the top of the brake), turn the top nut and the one in the back. To twist it counterclockwise, use the second nut in the front and the one in the back. There is also a universal tool called Take-a-Brake that may work on brakes without their own specific centering tool: place each of the pins inside a loop of the spring and twist in the appropriate direction.

If after all this the problem remains or returns, install a flat, thin steel washer between the brake body and the fork or rear stay bridge (or the shaped spacer in- stalled there). This will provide a smooth ‘unbiased’ surface that can be twisted into the desired position, rather than getting stuck in existing incorrect indentations.

Cantilever Brake:

On this kind of brake, the solution is usually rather primitive: simply bend one of the springs that spread the brake levers in or out a little using needle nose pliers. In other cases, there is a hexagonal recess in one of the pivots that is turned one way or the other, tensioning a spring that is hidden inside the bushing.

U-Brake:

This type usually has a small adjusting screw in one of the brake arms that can be turned in a little to bring that arm in, or out to bring the opposite brake arm in towards the rim.

Roller-Cam Brake:

This is the most popular form of what I generically refer to as a cam-operated brake. These usually have a similar adjusting screw on one of the brake arms that is turned in or out to center the brake arms.