Bicycles/Maintenance and Repair/Brakes/Adjusting Rim Brakes

From Wikibooks, open books for an open world
Jump to navigation Jump to search
Bicycles

Chapters:
01. Introduction 50% developed  as of January 04, 2011 02. What Style of Riding is for Me 25% developed  as of January 04, 2011 03. Equipment and Accessories 25% developed  as of January 04, 2011
04. Riding tips and safety tips 75% developed  as of January 04, 2011 05. Guide to safety 75% developed  as of January 04, 2011 06. Maintenance and repair 50% developed  as of January 04, 2011 07. Modifications 0% developed  as of January 04, 2011 External Links 0% developed  as of January 04, 2011

Fig 1. Typical Brake Details
A. Cable Housing.  B. Housing End Ferrule.  C. Cable Guide.  D. Housing Detent.
E. Rubber Noodle.  F. Cable Clamp.  G. Cable Tail.  H. Cable Cap.
I. Brake Arm  J. Brake Block (Pad).  K. Block Setting Screw.  L. Arm Mounting Bolt.

ADJUSTING RIM BRAKES

Rim brakes on bicycles are simple to adjust, and the term rim is used to distinguish them from hub brakes. Rim brakes include all brake designs that depend on using brake pads to close on the rims of a bicycle's wheels. The brake parts for the most common configuration are identified in Figures 1 and 2.

Although brakes are usually mandatory on bicycles, the laws and rules for brake performance vary, and some countries, while making law for new bicycles, ('at the point of sale'), have few specifications for the operation of bicycles after that point. Bike riders are advised to use the sources of law that apply in their own countries, but where the rules there are incomplete or unclear, they are advised to use any point of sale specifications that are available. Because the matter might then still be unclear in some countries, a table below repeats the braking distances of British Standard BS6102/1 for new bicycles.

Bike shops can perform any number of tasks for the bicycle owner, but the basic brake adjustments are easy to do for yourself. At the simplest level they consist of screw adjustments on the handlebars, while knowing what gaps you intend to produce. At other times, (rarely), the cable length needs to be changed, but this too can be done by anybody with a practical leaning. It is perhaps when the conventional adjustments fail to solve the problem that most people resort to such pages to learn more. This page includes a selection of the most common confusions for brake adjustment and explains how to correct them.

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, and although it lacks interaction, should at least leave the reader better informed than when he started.

Preliminaries[edit | edit source]

Fig 2. Typical Brake Lever Details
A. Brake Cable Housing.  B. Brake Barrel Adjuster.  C. Adjuster Locking Ring.
D. Brake Lever.
Consider these few things before carrying out brake adjustments. Doing so might save time in the long run:
  • 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 just need a slight gap adjustment or to balance the brake arms refer to Fine Adjustments below.
  • 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. If they are not centered, then this might be your problem as opposed to the brakes themselves; so correct the matter first.
  • Inflate the tires first. Before adjusting the brakes, make sure that the tires are properly inflated, since flat tires will affect the results. Metal rims can move as tire pressures change.
  • Preset the barrel adjusters. Before starting any cable length adjustments, first set the brake levers' barrel adjusters so that they are out by two full turns, instead of being tightened all the way in. This allows some fine adjustment in both directions at any stage, and will allow the cable to be slackened enough to release the wheel from 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.
  • 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 replacing them will improve braking greatly. When replacing them be sure to replace both at the same time.

The Full Procedure

The full adjustment procedure is summarized below but it should be emphasized that slight adjustments might solve the problem. In any case, 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.

These above points are all described in some detail in the text that follows.

Brake Block Alignment[edit | edit source]

Fig 3. Block Alignment. Note the direction of 'toeing-in'.
The brake blocks need to be aligned with the metal rims. Refer to Figs 1 and 3. 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 (Figure 1, D) or undo the cable clamp (Figure 1, F) before carrying out the work.

Block Clearance[edit | edit source]

Decide whether or not the block clearance is correct by trying the feel of the brake lever. (Figure 2, D). 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.

Rough Adjustment[edit | edit source]

  • To reset the cable length slacken the brake cable clamp (Figure 1, F), 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 (Figure 2, B), provided not too much range is expected of them.

Alternatively, to set cable tension, back the barrel adjusters off a few extra turns, squeeze both pads against the rim, tighten the cable pinch bolt, then back of the barrel adjuster until the wheel spins freely between the pads.

  • 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.

Fine Adjustment[edit | edit source]

Fine adjustments are made with the handbrakes' barrel-adjusters. (Figure 2, B). 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 (Figure 2, C) the brake blocks can be moved by turning the barrel screw (Figure 2, B). 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 a wide wheel removal, you might wish that you left more play in the cable. That is to say, provided that the barrel has always about two turns of thread showing, then tightening it all the way in will allow the cable nipple, (the end of the cable guide), to be unhooked from the housing detent without touching the cable clamp. Some find this method useful. Decide which is best for your routine.

Brake Block Balance[edit | edit source]

'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 5). It is often a posidrive screw, (M4x6mm) , with a tightening insert. Turning this screw clockwise will cause the brake block to move outward slightly, and turning it counterclockwise 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 may not be captive. At the other extreme, if a screw is too far in, the brake arm will bind; 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. Brakes usually can be balanced unless the wheel is not centered in the wheel-arch. (See Common Brake Problems on this page for more on this).

Final Check[edit | edit source]

Fig 4. 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 4 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.

Common Brake Problems[edit | edit source]

Balance Problems[edit | edit source]

Fig 5. 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 housing ferrules or nipples, or a quick-release wheel that needs re-clamped closer to the centre of the wheel arch. See these points below.
Fig 6. Typical Brake Block Assembly. Note the washer padding for a persistent balance problem. The wide and narrow concave washers are sometimes interchanged to accommodate available block designs.
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 as 'bicycle oil'. Brake lever assemblies, cables, and the brake arms should all be lubricated. A cable is oiled by dropping oil onto the cable with the brake lever operated to expose it. A single drop of oil at the point where a cable enters a housing will travel the length of the cable by capillary action. Rub some oil also onto any other visible steel cables. When the brake arms are removed, do not forget to clean and grease the mounting pins before reassembly. The rotating parts at the back of the brake arm's pivot-assembly should not be disassembled, though a drop of bike oil is useful when they seem a bit gritty in operation.
Balance Screws
The balance screws work by pushing and releasing the coiled ends of the springs, so 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 washer 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. Balance screws are usually fitted with nylon inserts to make sure that they are fairly tight, though a drop of thread tightening fluid might also help 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 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 reinstalled, then consider this possibility. Although some frames are provided with three holes for the location pins, some brakes only work on one particular position, so take note during any disassembly as to which holes were used.
Obstructed Cables and Housings
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 parts at the ends of cable housings (ferrules), become unseated during brake work; the entire brake balance suffers. Baggage on the front handlebars can kink cable housings, and a similar binding can be found in folding bicycles; in the latter case, when the bike is unfolded, make sure that any tight loops near the hinge points are straightened before making off. 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's skewer-clamp is tightened it is important to hold the wheel in a correctly centered position during the process. Bad centering can also cause the bike to run poorly, might feel a bit heavy, and can make shifts a bit less certain. The two springs, one on each end of the skewer, (both with their pointed ends inwards) are there to help the skewer start off in the center. 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 brake balancing adjustments will come into range, and the bike might even run better.
Brake Arm Spacers
Each brake arm has a brake block and on its screw thread there are various washers and spacers. One of these spacers is wider than the other, and they can be interchanged, in order to get the brake arm spacing within range for effective balancing. If the brake blocks (pads) were recently changed or removed, check that there is symmetry in their choice. If one pad has the wide washer near to the pad and the other has the narrow one then the distance of each brake block to the rim will be different and they will not balance. The Tektro brake manufacturer has a pdf file that explains how to decide which spacer to use for their products, though it has been found to have some generality.
Badly Worn Brake Blocks
If one brake block has been worn down much more than the other then that might affect whether or not the brakes balance, especially if it is combined with some other trivial factor. Inspect the blocks and if changing them be sure to change both at the same time. Generally speaking, if a brake block has been worn down so that any part of any of any of its grooves is no longer clear then they both need replaced.
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 6, 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.

Soft Brakes[edit | edit source]

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. Badly centered wheels 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.

Cables and Housings[edit | edit source]

The subject has been given a separate page at Cables and Housings.


See Also[edit | edit source]

External Links[edit | edit source]

Retrieved from "https://en.wikibooks.org/w/index.php?title=Bicycles/Maintenance_and_Repair/Brakes/Adjusting_Rim_Brakes&oldid=3929467"