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I bought a used bike a few months ago, and I've noticed a drastic reduction in brake power.

At first, I was easily able to lock-up the rear wheel while squeezing the brake lever only half-way to the handle bar.

Now, I can squeeze the brake lever all the way to the handle bar and I'm experiencing negligible stopping power. I've shortened the brake cable at the caliper several times, and it's now as tight as I can make it without causing it to close slightly and rub the rim. Squeezing the lever slightly causes the brake to close and contact the rim, but I can continue squeezing all the way to the handle bar and still not apply any real stopping power. It feels as though the cable has become more elastic with time, like the whole rear brake system is softer to the point of being nearly useless.

Is this "normal"? Is it an indication that it's time to replace the cable? Is replacing the cable likely to make my brakes more responsive?

Update

Upon closer inspection, my break pads are worn down to plastic-on-metal; there is no rubber left at all. This is very likely the cause for my perceived soft braking, and definitely the cause of the lack of brake power.

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have you considered replacing the brake pads? –  fbo Nov 14 '11 at 18:54
    
Is that something that would help with the overall spongy feeling of the brake lever? –  meagar Nov 14 '11 at 19:21
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Yes, that would do it. Plastic doesn't brake nearly as well as rubber. :) –  Stephen Touset Nov 15 '11 at 20:49
    
The cables might be dirty inside their housings. Specially if you ride in rain or mud, you should replace cables AND housings from time to time. It makes a HUGE difference. –  heltonbiker Nov 16 '11 at 15:14
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5 Answers

up vote 14 down vote accepted

Cables will stretch over time, but they won't become elastic. They're made of twisted strands of metal, and metal isn't generally known for its elasticity.

This sounds to me like your brake pads are shot. When they become spongy and glazed, you can squeeze your brake lever quite far and feel like little pressure is being applied. Get thee to a bikeshoppery.

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Replaced my brake pads, rear-wheel stopping power feels amazing again. Thanks for pointing me in the right direction. –  meagar Nov 16 '11 at 15:19
    
Metal is actually very well known for its elasticity. When is the last time you saw a spring made of anything besides steel? –  whatsisname Nov 22 '11 at 16:43
    
The definition of "elasticity" is different depending on context. Its physics definition (ability to resist deformation) is almost the exact opposite of its common usage, which is synonymous with "stretchiness". This was not a physics question. –  Stephen Touset Nov 22 '11 at 18:17
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Given what you are describing, it is likely that you are looking in the wrong place for your solution.

Cables will stretch a small, but finite amount. The do not flex to the level you are describing. That would break them first.

Brake levers, calipers, and pads can allow some spring effect into the system, but that typically doesn't creep in over time. It usually is a function of poor product design, or poor setup.

I would bet that 2 things are happening to cause the situation you describe:

  1. Your brake pads are wearing down causing more movement in your brake lever.
  2. Your rims (the section of your wheels that the brakes touch) are working out of true, which prevents you from readjusting the brake to its proper distance from said rim, without it rubbing on the rim.

If I am correct, it will not rub on the rim evenly at every point on the rim, but will touch it one or two (or more) places, and the first step in getting your brakes readjusted is to have your LBS true and tension your wheels.

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Cables will actually "work harden" slightly and become very slightly less elastic as they age. If they really do get "stretchy" it's because several strands have broken and the cable is near failure.

Of course, many other things bend/stretch in the cable path, one being the brake calipers. As the bushings get worn and the brakes abused over time they sometimes begin operating at a sort of an angle that amplifies their inherent springiness. Worn/misaligned brake pads will contribute to this effect as well.

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Brake pads, when first set up, should hit the wheel rim square-on in the vertical plane and almost square on in the horizontal. I say 'almost' because you ideally need a millimeter of gap at the back to make sure the brakes don't squeal.

The brake calipers move the brake block in an arc and, as the brakes wear down through use, they no longer meet the wheel square on. Hence they get to feel spongy, even if they were perfectly setup when new. Contaminants in the brake blocks also build up over time and these reduce the effectiveness of the brake, meaning you compensate by pulling the brake harder. In so doing you notice the 'sponginess' in the system more than you did when the brakes were new.

To mitigate against this try to clean your wheel rims every time you have dirt showing on them, particularly after a rainy spell. This will reduce the contaminants on the rim that get into the brake block. As well as having more use out of your brake blocks your rims will last longer and your bike will be a lot cleaner.

There are plenty of posh bike cleaning things, if you want to quickly and effectively clean your wheels without investing in these bike specific cleaners, get a bucket of warm water and add a small amount of washing powder (for clothes) to it. Then use a brush - one from a dustpan and brush will do - and give the wheels a scrub. Wash off with a bucket of cold water and you should be done. A pair of rubber gloves is worth it if your skin is sensitive to washing powder, but, if you use a brush then you should not need to get your hands wet or dirty.

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Just finishing work, will be closely inspecting the break pads when I arrive at home and update my question if that is indeed the answer. –  meagar Nov 14 '11 at 21:28
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As the brake pads are used can become glazed reduceing brake performance.They can also become hard when exposed to sunlight.You can try sanding the pads,if you see an improvement the pads should be replaced.Also inspect the brake surface of the wheel for oil or residue.

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