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I am replacing the cable for the rear mechanical disk brake on my brothers MTB, because it is all chewed up. Now when loosening the cable on the rear brake (barrel adjuster on lever and brake all the way in), I noticed that there was a preload on the brake arm that pulls the pads together, meaning that the cable was clamped while the lever was already moved up a bit.

This was as it originally came from the factory. The brake is a Shimano (Alivio controls) and looks something like this with only 1 adjustable pad.

Rear Brake

Every tutorial I found for brake installation doesn't put a preload on the brake arm, even the official Shimano service document doesn't mention to move the lever up before tightening the pinch bolt.

On the other hand I can see why it came from the factory like this, because when clamped in the arms neutral position, even with both barrel adjusters turned out, the brake would only start to grip with the lever already fully pulled towards the handlebar.

So should I preload the brake a bit when installing the new cable or what am I missing?

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  • Did you try replacing the pads? There is a lot of lever uptake/preload needed on worn pads. – Chris Watson Oct 31 '20 at 16:29
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Without having serviced that exact design, I'd expect that at the very least you'd need to take up the slack in the mechanism. Some slack is to be expected as in similar designs the pads sit back into the housing but pass in through the rotor slot. That means more travel is needed fitting new pads than in operation, easily dealt with by a little preload on the cable. It doesn't surprise me that it's not mentioned, though maybe it would be helpful. Setting the cable position in the pinch bolt with the barrel adjuster wound in is normal for many brake types.

Preloading slightly against a spring also helps keep cable ends nicely seated.

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  • There already was no slack in the system, I pulled the cable with pliers before tightening just to be sure. – Erik Aigner Oct 24 '20 at 13:51
  • I was thinking of slack in the brake mechanism itself, rather than the cable. Slack is perhaps not quite the right word, but the soft movement in the spring up to the point where the pads (are about to) bite – Chris H Oct 24 '20 at 15:47
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    @Chris H Probably free stroke you’re referring to. – MaplePanda Oct 24 '20 at 18:31
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    @MaplePanda I think so. If I'd been thinking about pistons I might have recalled the term but not with mechanical brakes. I'll stick with "slack" though, I think - I try to use layman's terms where possible, as I know some engineering terminology so can't get a feel for what's jargon and what's useful – Chris H Oct 24 '20 at 19:09
  • FYI I regularly watch Seths (Berm Peak) channel and I actually found a vid where he did preload it on a touring bike youtu.be/60sCS6eFRZo around 4:35. Mystery solved! – Erik Aigner Oct 25 '20 at 16:13
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I've had the same exact issue with similar Shimano brakes and Avid BB7.

I normally put "preload" in the actuator arm, as I learned before on linear pull rim brakes (a.k.a. V-brakes).

What I do is to fully screw in any barrel adjusters and back up a turn or two, then with one hand I move the actuator arm to the point where it engages the disc, then tighten the pinch bolt just enough to barely hold the cable relative to the arm. There should be no slack in the cable. At this point I release the arm and pinch the cable with my fingers just before the actuator arm, as to prevent it from sliding too much in the next step. Now I press the brake lever roughly half of its travel, trying to reach just the desired engagement point. (since the pinch bolt is barely tight, it will let the cable slide). Then, without releasing the cable, the other hand releases the brake lever and fully tightens the pinch bolt.

The fine adjustment of the engagement point is done with the barrel adjusters and the pad position adjuster bolt(s) if present.

I use a similar method for v-brakes since those do not have a hard stop for how much the return spring moves, i.e. the brake arms opens as much as the cable allows. When I installed disc brakes I just assumed the same thing as the actuator arm usually moves way more (require more cable travel) than the cable travel provided by the lever.

Which brings me to a very important note: MTB Mechanical disc brakes require the same cable travel as linear pull or v-brake. Levers designed for road brakes or a very old MTB style of brake known as "cantilever" brakes are not suitable as they offer much less cable travel and cannot be adjusted properly in most cases.

Road disc brakes only became popular in recent years (answer written in 2021), so now there are many options of disc brake calipers that are suitable for road bike brake levers, as they are designed to work together. There also exists road brake levers that are designed to work with MTB types of brake.

Even though it is possible to mix Road and MTB brake components, it is really not advisable, because they are designed with different cable pull and leverage ratios (different mechanical advantage).

Just to clarify: I do not think this would be the case for the O.P. since he mentions the only changed component is cable, everything else is "as from factory". But someone mixing a normal Road lever with MTB brakes will experience a similar issue.

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  • Are you saying that something like Tiagra 4700 shifters shouldn't be used with mechanical disc brakes? – Wilskt Apr 11 at 7:09
  • @Wilskt Some particular combinations of lever and disc brake caliper may work due to tolerances and design, but as far as I know it's trial and error, and the adjustability is not "standard". There are, however, road brake levers that are designed o produce the cable travel required for linear pull, and disc brake calipers designed to work with road brake cable travel (Avid BB7 for example does have a road specific variant). – Jahaziel Apr 12 at 13:40
  • A friend of mine has installed road handlebar and brake levers on his mountain bike. His setup works well enough for him. I tested that bike once but I would not ride it as it was. The levers almost bottomed out before engaging. He built that bike way before road disk brakes became as popular as are today, so it is possible to mix road levers with mtb brakes, although not advisable. – Jahaziel Apr 12 at 14:52
  • Ah okay, so "Mechanical disc brakes require the same cable travel as linear pull or v-brake" is incorrect, because there are road-specific mechanical disc brake calipers available. Could you edit the answer to reflect that? – Wilskt Apr 12 at 15:52
  • @Wilskt, I've updated the answer and tried to be more specific about the pull ratio mismatch. My background is MTB and I tend to forget that disc brakes are becoming standard equipment for road. – Jahaziel Apr 12 at 17:34

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