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My brakes were working normally until I adjust them today. I thought the brake pads were too close to the rims so I increase the distance a bit. Now every time I brake, they make horrible squeaking sound. However, when I test them in slow motion, they won't make that noise. My guess is only when the bike is moving fast, the brake pad cannot hold the wheel strong enough; consequently, the wheel slides on the pads causing the squeaking noise. Please correct me on that if I'm wrong.

Now, I wonder if there's any golden rule on the optimal distance between brake pads and rims?

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Can you specify the type of brakes you are needing assistance with? (caliper, v-brake, canti) –  Tha Riddla Jun 14 '12 at 20:08
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I don't think the problem you're experiencing is really about the distance between the pads and the rims. @Kibbee's answer most likely describes the culprit. It might be worth changing the title to 'What causes brake squeal' or something similar. Although that would mess with Stephen Touset's accurate answer to the question heading as it currently exists –  Mac Jun 14 '12 at 23:25
    
Please explain what type of brakes you have and exactly what you "moved" to help you better... Post a picture if possible. –  Jahaziel Jun 15 '12 at 1:17
    
Question -- When you brake do the brake levers squeeze all the way down to the handlebar, or is there still some space under them. It sounds like perhaps you're simply not able to squeeze tight enough, due to cable slack. (Note that if the wheels do not maintain a constant distance from the pads you need to get the wheels trued.) –  Daniel R Hicks Jun 16 '12 at 12:34
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7 Answers

up vote 7 down vote accepted

From what I've read, you should have a slight angle (called "toe-in") between the rims and the pads to prevent them from squeaking. Angle them so that the front of the pad angles towards the rim. When applying full force, the entire brake pad should come in contact with the rim. See this question for more information about relieving squeaking from your brakes.

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Thank you very much for the links. They are very helpful. –  chepukha Jun 18 '12 at 17:54
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I'm not aware of any established best practices other than "as close as possible, without any rubbing through a full revolution of the wheel".

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I'll share my personal formula, which I use myself and in a couple bikes I often perform maintenance to, and that are ridden by other people. These tips are for V-brake or "linear pull" type, but the same can be applied to other types too, at least caliper and cantilever, as long as they are cable actuated (i.e. no hydraulic).

First of all, I check brake pad alignment. This step is critical for good braking performance, and this bit is the one that can affect your noise problem. It also affects the perceived "correct" distance between pads and rim, but this topic slightly falls outside your question, so, I'll just assume you have your pads aligned.

In most a linear pull (v-brake), cantilever or caliper type brakes, the distance between pads and rim is determined by the "length" of brake cable relative to the length of the casing, adjuster barrels, cable stops and other items in cable routing. Cable slack increases brake lever "travel" which means how far you have to move the lever until it effectively starts braking. This distance is the critical parameter for me, this is personal because it depends on your hand size and the combination of lever/grips you use. Also, your hands have a natural point in which they "squeeze harder". This is the point where it takes less effort for you to actuate the levers. Your levers should move so they begin the real braking action just a little before you reach this "hard squeeze" position. This way you have your hand in optimal position when you need to brake hard.

I start by screwing down any adjusting barrels or bolts to a minimum (or near minimum). This will loosen the cable a bit, increasing pad to rim distance and also the "travel" of the brake lever, this means you'll have to press deeper the lever for the brakes to actually touch the rim. These barrels are usually located between the lever and the cable, or between the cable and the caliper (if any).

Now I loosen the bolt that holds the cable to the caliper or the V-brake arms, and while pressing the pads against the rim, pull the cable so there is no slack, and tighten the bolt.. but just a little! only enough to avoid the cable from slipping out from just the spring that separates the pads from the rim. Here is the trick: with one hand, hold the cable between your index and thumb, in such manner as if you loosen the bolt again, your fingers will prevent it from slipping out. Now, slowly depress the corresponding brake lever until you almost reach your "hard squeeze" distance, meanwhile, the cable will slip between your fingers, but the pads should remain pressed against the rim. Without releasing the cable, tighten the retaining bolt again, this time tighten it fairly tight. Check the lever travel and give the final tightening to the retaining bolt. Repeat for other other lever if needed.

Now spin your tires in the air to check whether it rubs the rim. Re-check any bolt you have loosened for proper tightening.

Don't panic if you don't achieve the perfect adjustment in the first try, you have a second resource: the adjusting barrels.

The adjusting barrels have two purposes: One is to fine tune this distance. They work by taking up or giving out cable slack. Usually they consist of a "hollow bolt" the cable goes trough, and is bolted to the lever assembly or the caliper. The other end of the bolt receives the casing. By "unscrewing" the barrel, they add distance between the casing and the lever or caliper, thus reducing cable slack. Almost all these adjusters have also a locking nut that you screw all the way down to the lever or caliper, so it prevents the adjuster from moving "on its own".

The other use of this is to re adjust the brakes as pad wear increases the distance between rim and pad surface, so you keep the brake lever travel in its optimum.

In an ideal setup, this is it. But there are other considerations:

Rim truing and dishing: The rim should be straight and properly centered. Untrued rims will rub the pads in certain part of a wheel revolution, but not all of them. A rim that's not correctly "dished" (i.e. centered) will rub against one pad all the time. V brakes can be easily adjusted toward rim centering because they have two independent return springs, each with an adjuster bolt that regulates it's tension. By balancing both sides' tension you get the pads the same distance from rim. Also a deformed rim may have bulges or cavities that complicate things further.

Cable condition: cable and its casing should be in good condition too, not rusted, not overly worn, not sharply bent.

Bike fit: Levers should be positioned properly, according to bike style, riding purpose/technique, sizing and personal preference.

Component combination: In some cases, certain combination of specific components, brands, designs, can make it very difficult or even impossible to achieve correct adjustment. This is very unlikely to be your case, as you stated your brakes worked fine before.

As you see this is a practical approach that doesn't deal with millimeters or such. When you finish go for a safe test ride and repeat the procedure if necessary. The first times you'll net a few tries before you conquer it, when you get used to it, you'll be able to do it in one attempt and without needing to move the adjusters.

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tl; dr: one mm. –  user973810 Jun 15 '12 at 3:01
    
Wouldn't misadjusted centering of the brakes be a more likely cause for one to rub than dish error? I'd think the latter would be fairly unusual... –  Reid Jun 16 '12 at 18:59
    
@Reid: Your point is right, the problem is the mismatch between rim dishing and brake centering. For example, problem will arise if someone swapped wheels with different dishing without readjusting brake centering. –  Jahaziel Jun 17 '12 at 23:26
    
+1. Thank you very much for your detail answer, Jahaziel. I'll definitely try out your tips. –  chepukha Jun 18 '12 at 17:56
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So the squeaking, as revealed in other answers to similar questions (e.g.) is down to the block not getting a reliable 'grip' on the rim and bouncing slightly and re-gripping.

Your first issue is probably that the pads were previously more correct than now, but distance doesn't have to be a factor in squeaking. I imagine that the angle of attack was wrong, but the whole pad was close enough for it all to gain contact with the pressure available and reduce the chance of it losing grip. Now you've moved it further away, there's more scope for the block to bounce, so the similar angle combined with greater distance has made the mistake into a genuine problem.

So you've two options:

  1. move the pads closer - as close as possible without rubbing. If you've a wheel that's true, this can be just enough to be able to see daylight.
  2. fix the problem, improve the angle on the pads. But then do option (1.) anyway because you're brakes will feel more positive and will reflect more of your effort.

That said, some people do like a little more room: if you tend to 'sit' on your brakes, that is your fingers are always there or thereabouts, you might find really close blocks forever engaging the brakes slightly, which can make the ride a little jumpy. YMMV.

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+1. You made it very clear, Unsliced. Thank you. –  chepukha Jun 18 '12 at 17:57
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Since the question has been edited, and now it focuses on the squeaking problem, I post my grain of salt on Pad Alignment:

Sometimes braking noise is caused by a vibration that occurs this way: When the pad initially touches the rim, it moves a little, due to rim's drag. But this movement causes a sudden loos in friction, releasing a little pressure thus allowing the pad to go back to its original position. This cycle repeats really fast, causing noise and poor braking performance. Here comes to play the angle mentioned as "toe in", as it can overcome the noise problem and even use component flex as an advantage.

Pad Alignment is critical for good braking performance. Start by checking general alignment: rim pads usually have a curved shape when seen from the sides of the bike. This curve must correspond with rim's curvature as closely as possible. The pad must contact the rim's braking surface in the center (between inner and outer diameter) and should "land" into the rim at an almost straight angle. This means pad wear should be even.

How to achieve the correct angle?

Loosen the pad retaining nut so it can be angled by hand. Press the arm that holds the pad, press it against the rim (like applying the brake by hand). while pressing, tighten the nut. Repeat on the other pad. This can be tricky, but once you get the hang of it, it becomes a breeze...

When you press the arm against the rim, the pad self aligns with the rim. If you apply enough pressure, that keeps the pad from rotating out of position while you tighten the bolt. This works good enough for me, but sometimes you need a little angle that makes the rearward end of the pad to touch the rim first, so the drag on the pad, and flex on the components make the rest of the pad to land down on the rim (Imagine a modern plane landing, where it touches the ground rear wheels first and then the nose touches down). To do this, adhere a little bit of cardboard (using adhesive tape) to the rim's braking surface. When you do the "press the arm" step, do it so the pad touches the cardboard on one of its ends, which end? the one that points against tire rotation.

With this alignment the "tail" of the pad touches the rim first, and the drag on the rim causes natural flex on pad-brake arm assembly causes it to "lower the nose of the pad down to the rim". This rather causes an increase in braking power, some style of self assisted brake (This concept is also used in automotive drum brake systems).

With the correct toe in angle, your brakes become very efficient and less demanding on your hands, but depending on your specific component combination can be too much, causing easy wheel lock up, which is inefficient and dangerous, so be sure to perform several tests increasing speed a little each time. If you find that your wheels lock up to easily, reduce toe in angle on the pads (by using thinner cardboard on the alignment step).

Correct alignment can reduce or eliminate the noise problem, but other factors may be involved. In my experience the rims can get dirty from a gunk that sticks to the rim when pads heat up due to friction, causing both noise and braking degradation. That's why my brake tune up procedures include rim cleanup. I use the green kitchen cleaning pads traded by the brand name "Scotch Brite" (Sorry for the ad, I don't know the generic name of this product). Dish washing paste can be used to better results but just water while gently scrubbing rim's braking surface is enough.

Note: This alignment procedure focuses on V-Brake systems that use semi-spheric spacers that allow multi directional angle changes on the pad, and the stem of the pad is threaded, other systems may use this kind of pad as well, but there are other designs where alignment has to be done completely by hand and eye and other systems do not allow pad angle adjustment at all.

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+1. Thanks again for your effort. –  chepukha Jun 18 '12 at 17:58
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The squeaking from brake pads is from dirt and rust on side of rim. Pads should lie flat against the rim.(No incline),for maximum surface contact.

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As mentioned by other respondents, brakes should be far enough from your wheel rim so that they do not rub on the rim as the wheel spins, but close enough so that you have sufficient stopping power when braking. If you're having trouble with finding this happy medium, you wheel may be out of true and need an adjustment. Brake pads should be evenly spaced on either side of the wheel (with a slight toe-in) and have equal tension on either side to ensure both sides have simultaneous contact with the wheel.

For squeaky brakes, assuming your brake pads are relatively new (not old and hardened), the most likely culprit would be an accumulation of brake pad dust on you wheel rim. As you brake, the friction between the rim and brake pad wears away the brake pad and some of this is left on the wheel rim. Excessive amounts contribute to squeaky brakes.

Cleaning your wheel rims is easy. Any (disposable) cloth or cotton pads with a bit of rubbing alcohol will work wonders, just be sure to go over each side of the wheel rim a few times to clean it, rather than re-distribute the dust. Also avoid getting the rubbing alcohol on your tire, or on the brake pads.

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