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?

  • 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

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.

  • 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
  • @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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