Bicycles/Maintenance and Repair/Brakes/Adjusting Rim Brakes

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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, to press the brake blocks (also known as pads or shoes) 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.



Overview

Details of typical rim brakes can be seen in Figures 1, 2, and 4. A summary of the expected braking distances can be found in Figure 3. The adjustment sequence below seems formal, but is only a guide. It is likely that some steps, like spring balancing, will need to be done after each adjustment. The general procedure for a full brake adjustment is as follows:

  • Alignment of the brake blocks onto the metal wheel rims is carried out first.
  • Major adjustments to block clearance are made by setting the brake cable clamp.
  • Balancing the brake arms is done with balance screws.
  • Fine-adjusting the brakes is done with the handlebar barrel-adjusters.
  • Testing is done before going onto the road.

[edit] Preliminaries

  • Inflate the tires before adjusting the brakes, since flat tires will affect the results. Believe it or not, the metal rims move in and out a bit.
  • Before doing any elaborate brake adjustments it is a good idea to decide whether a simple barrel adjustment will fix it. If you think that this is so, refer to Fine Adjustments below.
  • If further brake adjustments are required then first set the barrel adjuster, (Fig 1), so that it is out by three full turns, instead of being all the way in. This allows some fine adjustment in both directions.
  • Check that the rims are free from corrosion, oil, and dirt. If necessary, clean the rims with detergent and wire wool, avoiding the tires.
  • Make sure that the brake block contact areas have not been badly reduced. If the surface grooves on the blocks have been removed then the blocks should be replaced. If replacing them then be sure to replace both at the same time. Leather-faced blocks are best for steel rims, though rubber on aluminium rims is more common. Clean the blocks of any lubricant. since this obviously reduces braking efficiency.

[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

Assuming that the block alignment is done, spin the wheel and decide whether the block clearance is correct by trying the feel of the brake lever. 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 handlebar barrel adjusters, as described in the Fine Adjustments below.

[edit] Major 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. 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 information about the bike, 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 manufacturers brakes manual can be found in pdf format here, and its parts list here.

[edit] Fine Adjustments
Fine adjustments may be made with the 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 you need.
After undoing the barrel locknut, unscrewing the barrel will move the brake block inward, and screwing the barrel inward, (assuming it is not all the way in already), will move the brake block outward. This gives a much finer adjustment of cable length than using the cable clamp. Tighten the locknut again after any adjustment.
A comment on the initial setting of barrel adjusters seems in order. Screwing the barrel adjusters all the way in before making any brake adjustments will allow the widest range of inward adjustments of the blocks.
By first leaving a few threads showing on the adjuster before any adjustments, you will also permit a slight outward adjustment of the blocks, in case it is needed. Decide which is best for your adjustment routine.
Also, after the brake is adjusted, it should still be possible to adjust the barrels in both directions, at least by a little. In this way the cable can be slackened to allow the release of the cable bridge, or tightened to firm-up the brakes. Aim to still have the barrels out by about two full turns when you are done.

[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 breaking. Adjusting spring tension does this.

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, some might bind the brake arms. 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 the same 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 breaking 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, or a wheel that is not centred in the frame. 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, undoing the cables at the brake levers, and at the cable clamp, 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 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. Rub some oil also onto any other visible steel cables. If necessary, consider removing the brake arms to lubricate the moving parts in the pivot heads.
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 first case it is simple to release the screw a bit and reattempt the balance. A better fix for a binding screw is to add a few 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 spring has slipped, and if there is not too much play in movement, it might be possible to position it back onto the screw. If this does not work then perhaps bending the end of the spring into better alignment might do it. The spring is difficult to bend, and to do so, should be completely removed.
This adjustment may be too difficult for some so decide whether or not it is for you. To fix the spring, proceed as follows. Undo the cable clamp, and remove the brake arm by unscrewing its central mounting bolt. This is usually a 5mm hex socket head bolt. Gently ease the underside of the pivot head apart with a thin flat edge and separate the spring from the other parts. Make a slight bend in the end of the spring until it fits directly behind the adjustment screw. Springs are quite tough. It has proved possible with two pairs of pliers and much effort, though the use of a bench vice might make things easier.
There are no doubt other fixes out there. The ideal solution, if a convenient one exists, would better constrain the spring end so that the chance of the spring slipping becomes negligible.
During reassembly, avoid damage to the location pins and take care not to over tighten the brake arm mounting bolts. 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.
Brake arms have location pins as well as pivots. The pin often has three holes to choose from but some frame mountings have only one. The hole chosen decides the spring strength. If the pair of brake arms use different positions then the resulting tensions cannot be balanced. If the arms were recently reassembled then consider this possibility.
Wheel not Centred
If the wheel position is not centred in the arch of the forks then the brakes will seem faulty. This is easily checked by viewing the bike end-on and if necessary, by measurement. If a quick release skewer is loose or unlocked then the brakes could seem misaligned. This could also happen if the wheel is not settled-in properly with the bike in a vertical position before the skewer is locked. If in doubt, release and then lock the wheel correctly.
Balance Screws at Extremes
If the brakes balance, but at the balance screw extremes, first consider the above solutions. Failing those, 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.

[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. 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 wheel locking. 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, the issue of wheel balance is more critical. Widening the block clearances to accommodate a slight wobble makes softer brakes, but these might still be safe. However, if the problem worsens, continuing to widen the clearances will lead to unsafe brakes long before the wheel itself becomes unusable. Brakes designed for wider clearances will accommodate more wobble before the brakes suffer.


[edit] Changing a Brake Cable

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. This is almost impossible using ordinary electrical tools. A special cutter exists for the task. Be sure to leave at least 10 cm of spare cable when cutting. Crimp a cable termination onto the cut end to prevent fraying and to act as a stop during adjustments.

Bear in mind that a new brake cable will stretch in use. You will need to take up slack after the first hundred miles or so, by adjusting the cable length in the usual way.

[edit] See Also

[edit] External Links

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