Bicycles/Maintenance and Repair/Brakes/Adjusting Rim Brakes

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Bicycles

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Fig 1. Typical Bicycle Brake Details

ADJUSTING RIM BRAKES

The brakes on bicycles are simple to adjust. They are an essential piece of safety equipment, so learning to adjust them is a worthwhile skill. Remember, after adjusting brakes; try them in a safe environment before riding.

Most bicycle brakes use a lever on the handlebars to pull a cable. On ordinary rim brakes the cable pulls on brake arms, and brake blocks (also known as pads or shoes) press against the metal rim of the wheel. Friction then slows the bike.

Bear in mind that the best way to learn the adjustment of brakes is to be shown by somebody while it is being done; in this way much of the mystery vanishes.

The next best way is to follow a fairly stolid description of the sort below, which will get the job done with a little perseverance.

Preliminaries

Consider these few things before carrying out brake adjustments:

  • Inflate the tires first. Before adjusting the brakes, make sure that the tires are properly inflated, since flat tires will affect the results. Believe it or not, metal rims can move as tire pressures change.
  • Is a full adjustment really needed? Before doing any elaborate brake adjustments it is a good idea to decide whether or not a simple barrel adjustment might fix it. If you think that this is so, refer to Fine Adjustments below.
  • Preset the barrel adjusters. Before doing elaborate work on the brakes, first set the barrel adjusters, (Fig 1), so that they are out by two full turns, instead of being all the way in. This allows some fine adjustment in both directions at any stage, and allows the cable to be slackened to release the brake blocks when necessary.
  • Check that the rims are clean. Remove any dirt and oil from the rims, and if necessary clean them with detergent and wire wool, being careful to avoid the rubber of the tires.
  • Make sure that the wheels are correctly set. The wheels should be fully engaged in the dropouts and centered in the wheel arches if brake balancing is to work.
  • Check the brake blocks for wear. Make sure that the brake block contact areas have not been badly reduced. If the surface grooves on the blocks have been removed by wear, then the blocks should be replaced. If replacing them then be sure to replace both at the same time.


Method Summary

The details of typical rim brakes can be seen in Figures 1, 2, and 4, and a summary of the expected braking distances can be found in Figure 3. The general method for each set of brakes is as shown below, though it is quite usual to repeat balancing at various stages throughout the adjustment.

  • Position each brake block so that it presses cleanly onto its wheel rim.
  • Set the brake cable length to roughly establish the brake blocks' clearance.
  • Adjust the brake-arm balance screws so that both blocks close onto the rims together.
  • Fine-adjust the handlebar barrel-adjuster for the required brake performance.
  • Test the brakes both off and on the road.

[edit] Brake Block Alignment

Fig 2. Block Alignment. Note the direction of 'toeing-in'.
The brake blocks need to be aligned with the metal rims. Refer to Figs 1 and 2. The leading edge of each block should be slightly closer to the rim than its trailing edge. This prevents brake squealing and is called "toeing-in". Use a coin, a credit card, or any other thin material under the back end of the block while adjusting it. Some suggest tying an elastic band temporarily to the trailing end of the block to allow more freedom while working.

To make the adjustment, slacken the screw that holds the block. Usually a 5mm hex wrench is used. Swing the brake arm in so that the block is pressed squarely against the metal rim, and then re-tighten it while holding the block hard with your "toeing-in" device in place. Avoid the rubber of the wheel; the block should contact only the rim. The block should be parallel to the rim, noting that some blocks are curved to fit its shape. Do one block at a time, and just let each arm relax after the block is set. If there is insufficient clearance to work or if you intend adjusting the cable length later in any case, then unhook the cable bridge or undo the cable clamp before carrying out the work.

[edit] Block Clearance

Decide whether or not the block clearance is correct by trying the feel of the brake lever. The brake should feel responsive without too much handbrake slack prior to the start of braking. Some mountain bike V-brakes might need only a 1mm gap, while many other brakes need about 2mm. If in doubt, refer to your bicycle handbook. If a significant adjustment is needed, resetting the cable length should do it. If a small change will do then use the brake lever barrel-adjusters on the handlebars, as described in the Fine Adjustments below.

[edit] Rough Adjustment

  • To reset the cable length slacken the brake cable clamp, (usually with a 5mm hex wrench.), and let the brake side arms relax. Move both of the side arms toward the rims until the blocks are about the right distance away; although you can do this in any way that you like, it is easiest to hold one arm against its rim and adjust the other's gap so that it is about double the intended block clearance. Pull up any cable slack and re-tighten the cable clamp. This may take a couple of tries to get the blocks close to, but not touching, the rim. In any case, fine adjustments can be made with the barrel adjusters, provided not too much range is expected of them.
  • In the absence of documentation about the brakes, aim for the combined clearance of the two blocks on v-brakes to be about 2mm; this will give 1mm gaps after balancing. For less demanding brakes, assume about double this figure. Some manufacturers provide internet sites with technical information of this kind. A good example of a manufacturer's brake manual can be found in pdf format here, and its parts list here.

[edit] Fine Adjustment

Fine adjustments are made with the handbrakes' barrel-adjusters. (Fig 1). If not on the handlebars they may be near the brake blocks. If the brakes just need a small adjustment, this may be all that you need.
  • Barrel adjustment; after undoing the barrel-adjuster's locknut the brake blocks can be moved by turning the barrel screw. To move both brake blocks inward, simply unscrew the barrel (counter clockwise). Conversely, to move both brake blocks outward, turn the barrel inward, (clockwise). This feature gives a much finer adjustment of cable length than setting the cable clamp. Check the feeling of the brake frequently while getting the adjustment right, and tighten the locknut again when it is done. Lastly, decide whether or not to re-balance the brake arms.
  • Notes on barrel range: There are differences of opinion as to the ideal amount of thread to leave on the adjusters when the brake adjustments are complete. The main ideas are these:
    • For maximum barrel range; screw the barrel adjusters all the way in before making any brake adjustments to allow the widest range of inward adjustment of the blocks. This might be the case where compensating for block wear is the main consideration.
    • To unhook the brakes; for example, when a wheel is being removed, there is no need to slacken the cable clamp. If there is enough adjustment range on the barrel-adjuster, it can be tightened to slacken the cable as much as possible, before unhooking the cable nipple from the slot in the cable bridge (see Fig.1). Some choose to do things this way; decide which is best for your routine.

[edit] Brake Block Balance

'The brake arms should be adjusted so that both blocks apply pressure to the rim at the same time. As a result, at balance, there is no sideways displacement of the wheel during braking. Although slight imbalance is not always critical, displacement of the wheel by even a small amount can cause damage when small-clearance devices such as distance counters are installed on the spokes. Balancing the spring tensions keeps the wheel centered even during braking.

For brakes like V-brakes, there is a small screw near the bottom of each brake arm to adjust the spring tension.( Fig 4). It is often a posidrive screw, (M4x6mm) , with a thread tightening compound. Turning this screw clockwise will cause the brake block to move outward slightly, and turning it anticlockwise will cause the block to move inward. As one block moves, so does the other, to maintain the distance between them. Adjust these until the clearances are about equal. In this way, operating the brake causes the blocks to reach the rim at about the same time.

Be careful not to withdraw the screws too far since they are not captive. At the other extreme, if the screws are too far in, they commonly bind the brake arms; if a brake arm seems inactive, or unresponsive, this might be the case, or the spring may just have popped out of its slot. Try to avoid the limits and to reach a balance with the screws near their mid-range. This is easier than it sounds since making an identical adjustment on both screws will leave the balance unchanged.

For side-pull and centre-pull brakes, slacken the central retaining bolt and correct the position of the whole brake assembly.

[edit] Final Check

Fig 3. Maximum Braking Distances for Adult-Sized Bicycles. These figures can be found in CTC Standard for the Safety of Hired and Used Cycles, ( 5/3/99 ).
Maximum Braking Distance (metres) Dry Flat Surface Wet Flat Surface Remarks
Speed 24 kmph (15 mph) using both brakes 5.5 m n/a Only if safe
Speed 24 kmph (15 mph) rear brake only 14 m n/a Only if safe
Speed 16 kmph (10 mph) using both brakes 2.5 m 7.5 m Safest to Test
Speed 16 kmph (10 mph) rear brake only 7.5 m 19 m Only if safe
Rotate the wheel to check brake clearance. Make sure that there are no repetitive noises coming from the brakes.

Test the brakes on the spinning wheels before riding. If these work well enough then test the brakes again by riding the bicycle in a quiet place. The brakes should stop the bicycle decisively in a fairly short distance. Some v-brakes in particular have a short stopping distance; on these you should not need a deep pull on the brake lever for a good braking effect since this is a sign that the blocks are set too far from the rims. In any case be sure to refer to the manual if there is doubt.

Figure 3 is an extract of maximum braking distances as given by the CTC Hire Standard, that is itself related to the content of British Standard BS6102/1 for new bikes. The CTC standard is an attempt to consider used bikes, as opposed to bikes at the point of first sale. In any case, these stopping distances are useful until such time as the European standards properly address the issue.

The most common reason for long braking distances, apart from maladjusted brakes, is the degradation of the brake block surface area. Be sure if replacing these to replace both together.


[edit] Common Brake Problems

[edit] Balance Problems

Fig 4. Typical Brake Layout. Note the location of the balance screws.
Sometimes, despite best efforts, the brake arms will not balance. The spring balance screws are designed to have limited range since they are only expected to handle the difference between the arm tensions to achieve balance. So, faults that only slightly bind any part of the braking system can cause trouble with balancing. Possible faults include: The lack of general lubrication, the binding of brake arms, the slipping of faulty adjustment screws, unseated brake-arm springs, unseated cable nipples, or a quick-release wheel that needs reclamped closer to the centre of the wheel arch. See these points below.
Fig 5. Typical Brake Block Assembly. Note the washer padding for a persistent balance problem.
Lubrication
Small changes in brake arm resistance are often corrected by lubrication alone. As a last resort, to identify the source of binding, undo the cables at the brake levers, and at the cable clamp; this can isolate the three parts of the system. In this way, each section could be studied without being affected by the other parts. Any roughness in operation could then be observed. Many problems of this kind however, are solved by routine cable lubrication.
The moving parts of brake assemblies, with the exception of blocks and rims, should be oiled. Use a light machine oil of the type sold for bicycles. Brake lever assemblies, cables, and the brake arms should all be lubricated. Cables are oiled by first undoing the cable clamp, then with the cable loose, dripping a drop of oil into points where cables enter sheaths or fixings. The brake lever cable connection on the handlebars is accessed by lining up the two slots in the barrel adjuster then sliding the cable through the slots. A singe drop of oil at the point where a cable enters a sheath will travel the length of the cable by capillary action. Rub some oil also onto any other visible steel cables. If necessary, consider removing the brake arms to lubricate the mounting pins.
Balance Screws
The balance screws work by pushing and releasing the coiled ends of the springs, increasing and decreasing their strength. However, things sometimes go wrong.
In some brakes the brake arm binds up when the screw is all the way in. In others, the screw slips off the end of its spring because of bent parts or poor design. When either of these things happens it will be difficult or impossible to balance the brakes.
If the balance screws seem to lack range of adjustment. then look for these problems on the underside of the brake arms. In the case of binding screws it is simple to release the screw a bit and reattempt the balance. Bear in mind that carrying out an identical adjustment on both balance screws should leave the balance unchanged, so unless there is some other problem, it will usually be possible to correct binding in this way. A better fix for a binding screw is to add a few thin washers under the balance screw head to avoid binding in the future. This limit should be set to avoid the worst case; found usually with the block against the rim. If the screws are ever replaced, avoid using screws that are longer, unless the extra is padded out ,as mentioned above. This will avoid brake binding. Remember to use some PTFE tape or other tightening compound on the screws to avoid their getting lost on the road.
Sometimes the spring on a brake arm slips out of its groove, and this is seen as one side of the brakes doing nothing at all. This happens at times when the bridge has been undone. Fix it by moving the spring back behind the arm into its groove. At other times, the springs of cheaper brakes can slip off the balance screws, but this is largely a fabrication problem; consider a new set of brakes if this happens, since these springs are not easily reworked.
Brake arms have location pins to fix them to the flats on the bike frame. The frame often has three holes to choose from but some frame mountings have only one. The hole chosen decides the spring's strength and the work needed to pull the brake lever. If the pins of each brake arm in a set are located in different positions, then the resulting tensions cannot be balanced. If there is an obvious imbalance in the brake arms and the arms were recently reassembled, then consider this possibility.
Obstructed Cables
It has been noted that accessories such as straps, lock cables, and bungee cords are easily forgotten, and can bear upon brake cables in such a way as to make the balance seem impossible. The same condition is possible when the metal parts at the ends of cable housings become unseated during brake work; the entire brake balance suffers. Baggage on the front handlebars can kink a brake cable at the point where it enters the cable bridge, especially if the cable-guide is is of the spring type and not solid; if this is the case it can be improved with a plastic tie on the cable-bridge to give it a more direct entry, or by using a more solid guide-piece. If the change is fairly sudden, and in particular after parking the bike or unhooking the cable bridge, then look for these simple solutions.
Wheel not Centered
Many brake balance problems are associated with wheel positioning. There are at least three situations to consider:
  • When a quick-release wheel is positioned within the wheel arch it can happen that it is not fully engaged in the dropouts. This happens when a wheel is installed with the bike in a stand or otherwise above ground. A gap is often visible. This problem causes the brake blocks to close on the rubber of the wheels instead of the rims, so it is not subtle. Reset the wheel.
  • Brakes are adjusted for the quick release lever positioned on the left of the bike. When a front wheel is wrongly installed with the clamp on the right, the existing brake balance will be upset. Reinstall the wheel with the lever on the left.
  • When a wheel is installed it is important to make sure that the wheel is centered in the wheel arch. This problem also causes the bike to run poorly and it might feel a bit heavy. The two springs, one on each end of the skewer, (both with their pointed ends inwards) are there to better center it. The springs are not strictly necessary for the skewer to work, and as such they might be missing. In any case a wheel can be centered by adjusting both the lever and the locknut. One way to do this is to slacken both ends until the exposed screw threads look about equal, then tighten both the locknut and the lever a little bit at a time. Make final adjustments in the same way until resistance is first noted when the closing of the lever approaches the horizontal. Some tires have a center ridge that is useful for checking that the wheel is centered. When this problem is corrected the adjustments will come into range, and the bike might even run better.
All Else Fails
If the brakes balance, but at opposite screw extremes, then further adjustments are prevented. First consider all of the above solutions; i.e., is the tighter of the two binding, etc.? If you are stuck with unmatched springs in the brake arms, (rare, since the set that came with the bike are matched in manufacture), it may be possible to get the screws back into mid-range with a washer or two. Add thin 6mm washers, as shown in Figure 5, to the brake block assembly that habitually has the wider of the two gaps. Adjust the number of washers until the balance is achieved closer to the balance screws' midpoints.
When all else has failed, and a fix of this kind is being considered, do not forget to consider the buying of a new set of brakes, and getting the bike shop to first take a look.

[edit] Soft Brakes

If brakes are softer than intended, even after adjustment, then it might be that the toeing-in is excessive. Also, if wheel wobble causes rubbing on the brake blocks, widening the gaps will necessarily soften the brakes. See the comments below.

Excessive Toeing-in
It is impossible to set firm brakes if the blocks cannot be set close to the rims. For brakes needing narrow' clearances, e.g. 1mm to 1.5mm, the toeing-in needs also to be narrower than usual. If the toeing-in were set to the usual 1mm more than the front of the block, then for the above example, the average clearance for each block would be 1.5mm to 2mm, and the brake would feel soft. Thus, the toeing-in needs to be minimal if a firm or hard brake is required. In fact some brake manufacturers recommend no toeing-in for their products. Brakes that use wider settings by design, are affected less by toeing-in.
Wheel Wobble
Wheel wobble causes repetitive brake noise but most brake noises are unimportant. The most common cause is debris from the road. Also, new tires have rubber tails on them that rub on brake blocks, but these can be trimmed. Faulty wheel locking with a quick release can also cause some rubbing of the brakes. It is as well to check this if the problem follows recent wheel work. These noises are not serious.
If there is even a slight wheel wobble then brake block clearances will need to be widened to avoid the repetitive rubbing noise. Although some wheel wobble is inevitable, when it is excessive then repairs may be required. In that case see: Truing a bicycle wheel , in an attempt to handle it yourself, or consider getting some advice from your bike shop.
For brakes that need narrow clearances, say 1mm, the issue of wheel balance is more critical. Widening the block clearances to 2mm, to accommodate a slight wobble makes softer brakes, but in most cases these will still be safe. However, if the wobble worsens, then continuing to widen the clearances will lead to ineffective brakes long before the wheel itself becomes unusable. Brakes designed for wider clearances will accommodate more wobble before the brakes suffer.


[edit] Changing Cables

A brake cable is a braided steel wire with a small locating part at one end. This part is called a nipple. The nipple fits into a slot within the brake lever, and when the lever is pulled, the cable is pulled to operate the brake arms. The barrel adjuster pushes the part holding the nipple back or forward to fine adjust its starting position. The shape of these fittings is different for racing bikes and other bikes.

Drop-handlebar ("racing") brake levers have a pear nipple and straight handlebar levers have a barrel nipple. Universal cables are available that can be used for any bike. They have a different nipple at each end. When the cable is cut to length the unused nipple is discarded.

Brake cables run through a protective shroud called an outer or cable housing for part or all of their length. The section leading from the handlebar to the frame is always surrounded by an outer. Together, the cable and outer form a bowden cable.

On racing bicycles and some others, the cable is open where it runs along the frame. In these cases small pulleys are used where the open cable changes direction.

To remove an old cable, slacken the cable clamp bolt on the brake arm. Pull the cable through the outer until it is at the barrel adjuster. The cable end at the lever is accessed by lining up the two slots in the barrel adjuster, then sliding the cable through the slots. Steel cables can be recycled.

To install the new cable, put a little grease on the nipple and locate it in the lever. Then feed the other end of the cable through any cable outers, adding a little grease. Dress the cable through any pulleys, stops and guides until it reaches the brake arm assembly. Feed the end through the brake arm clamp bolt.

It may be necessary to cut the cable to length, and the exact length is best taken by measuring the old cable. Cutting these cables is almost impossible using ordinary electrical tools, and a special cutter exists for the task. Be sure to leave at least 10 or 12cm or so of spare cable when cutting. Crimp a cable termination onto the cut end to prevent fraying and to act as a stop during the slackening of the cable clamp.

Bear in mind that a new brake cable will stretch with use, and you will need to take up slack with the cable clamp after the first hundred miles or so.

[edit] See Also

[edit] External Links

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