Hydraulic disk brakes usually adjust for pad wear via special seals that allow the pad to "creep" closer to the brake disk as they wear down. It seems that such seals could equally well be used with mechanical disk brakes. However, all mech disks I've seen require manual pad adjustment. Why is this so?
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Not quite sure what you're after here - the mechanical brake doesn't have any kind of seal like a hydraulic brake keeping fluid inside. Mechanical brakes are basically levers and cams/cogs inside. Are you asking about self-adjusting mechanical brakes of any sort ?– Criggie ♦Commented Jun 3, 2022 at 11:38
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1@Criggie if you look at page 4 of this document , it seems that the seal "rollback/slip" mechanism doesn't really require any fluids to operate. If I understand it correctly, it could equally well be operated by a mechanically actuated piston. Thus, I wonder if a similar mechanism could also be applicable to mechanical disc brakes.– Jussi NurminenCommented Jun 3, 2022 at 13:00
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In which case, the question you're asking really is, "Why do manufacturers choose not to add to the expense of their mechanical disk brakes by including an auto-adjust feature?", and that's a question that could only be answered by the manufacturers. Of course, the real answer is "profit margin". The other possibility is that the auto-adjust seal cannot be operated equally well by a mechanically actuated piston.– FreeManCommented Jun 3, 2022 at 15:34
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OK so it is a more-general question - the line about "such seals could equally well be used with mechanical disk brakes" was confusing. Shall we make the question more general ?– Criggie ♦Commented Jun 3, 2022 at 22:17
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2 Answers
I think the difference between hydraulic and mechanical disks is that there is no seal on mechanical calipers. You are saying that the seal "rollback/slip" mechanism doesn't require fluid to operate, but it's important to note that mechanical brakes don't have a seal. If you consider the brake pictured below you'll see that it's an open system requiring no seals and just has an actuator pressing against the pad, with a spring to retract it.
I'm also not sure what the added benefit would be, since the usable thickness of the pad material is only a few millimeters, requiring only a very small amount of adjustment over the lifetime of the pad. You could easily just adjust when needed as it's not likely that you would need to have the pad be adjusted during a ride.
Also worth noting that in order for the rollback and slip to work, it has to have a sealed chamber behind the pad, and even if it was sealed, it would probably need to be filled with hydraulic fluid, since if it was only filled with air, the compressibility of the air wouldn't have sufficient force to pull the piston back from the seal.
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Yep, you got it. I didn’t realize the need for a sealed chamber. Basically it would become too complicated and costly. Commented Jun 3, 2022 at 16:28
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1Not during a ride, provided you check the brakes before the ride, but reasonably often. Definitely quite a few times (more than 10 in my experience) over the pad lifetime. Commented Jun 3, 2022 at 19:46
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@VladimirFГероямслава The difference between worn pad and a new pad is about 2mm, so if you adjust it 10 times, each time you would be moving it about 0.2mm. You would have to be very picky about your brakes to adjust them so much. My brakes have a maximum of 3mm of adjustment, so even if you went through that full range, you would still be adjusting 0.3mm each time. Which seems a little bit excessive.– KibbeeCommented Jun 3, 2022 at 20:22
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1It is not excessive at all, the mechanical advantage is big and 1 mm is enough to have your brifters all the way to the handlebar. Commented Jun 3, 2022 at 21:45
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1Yep, my experience is also that mech brakes require pretty frequent adjustments for optimal performance. That's why I was asking. Commented Jun 4, 2022 at 13:47
In mechanical brakes, the cable is of constant length. It can't become shorter or longer.
In hydraulic brakes, you have a reservoir of hydraulic fluid. If the pad creeps on the seal (after the seal has "rolled" already as much as it can "roll"), the lever will move closer to bars the first time. Then when you release the lever, reconnecting the reservoir to the active working fluid in the brake line, the new empty space in the line will be filled with new fluid. The next time, the lever won't move closer to the bars anymore as this time the pad motion is only made by the seal "rolling" and not by the pad creeping on the seal.
A mechanical brake would need something equivalent: a device that would offer new length of brake cable, just like the hydraulic reservoir offers new working fluid.
I'm afraid such a device can't be easily made in small format, to be small enough to fit inside the brake lever. This is not a Swiss watch with minuscule forces, but rather a device that has to be capable of transmitting great forces.
Mechanical-hydraulic hybrids that use a mechanical lever, a mechanical cable, and the brake master cylinder is near the caliper, and the cable just actuates the piston in the master cylinder, could obviously have such an auto-adjust feature. But purely mechanical ones, not so easily.
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Good answer. As a point of interest regarding your last paragraph, note that adding a self-adjustment feature is not a trivial matter: the TRP HY/RD with its self-adjusting open system is significantly bulkier and heavier than say the Juin Tech R1, which is a closed, manually-adjusted hybrid caliper. Commented Jun 7, 2022 at 9:16