IGH

Two of my bikes have cabled disc brakes, but that's where the similarity ends. My MTB has SRAM, and I love them. I can easily lock the wheels with gentle pressure applied by only a couple fingers, yet still modulate the braking force to much lower levels under typical use. The other bike has Shimano disc brakes (not sure exactly which model), and to say I'm less than impressed with them would be a gross understatement. Even with all four fingers pulling them as hard as I can, I'm unable to lock a tire; they feel more like rim brakes on wet steel rims.


I'm willing to replace them entirely if need be, but I struggle to accept that Shimano would release such a terrible product. It made me wonder if there's something else going on, e.g. an oily disc, but even after cleaning the disc braking didn't improve. My next thought is maybe the pads themselves are faulty or deliberately made to be low friction to prevent people who aren't used to disc brakes from going over the handlebars. I feel like I'm playing a guessing game though, and at this point I should seek advice from people here before I spend money on something that won't fix the problem.


I'm happy to provide pictures of anything you need to help diagnose the problem, just let me know what you'd like to see.

Koyote

If you have cleaned the rotors properly – which means using brake cleaner or something similar - then the next step is to look at the pads. I have found that, even if they still have surface left, replacement can help. And with new pads and clean rotors, you need to “bed in“ the pads properly. If that doesn’t solve your problem, I don’t know what will.

if you choose to go this route, I suggest starting with just the front brake pad, since that does most of the work. Try a semi-metallic pad from Shimano, as that should give you the most bite.

andrewclaus

If the pads are badly contaminated (or burned), you won't get them clean. Replace them.

Also be sure you have the right levers for the calipers--road for road, mtn for mtn. A mtn lever on a road caliper will have much lower force.

And take a look at cable routing--excessive cable length with extra bends will make a difference.

dsbrantjr

As will too short with overly tight bends. The housing ends must enter/exit the stops and ferrules straight without excess friction. What is the overall cable condition? Re-cabling with quality cables and housings can often make a dramatic difference.

Iride01 

I'd still be a little concerned about the brakes that lock up with little force. You have to stop quick from doing 20 mph or faster on a slight turn, you'll be swapping ends on the bike if your back locks up on you and your front is giving you it's maximum stopping force without locking up.

I've done it a few times. Luckily I was able to recover from what seemed like my rear going almost 90 degrees to my direction of travel. Maybe I should look and see what my HR went to on those! <grin>

IGH

I used isopropyl to clean the rotors, but that was after I already used the brakes. I'll buy some semi-metallics and clean the rotors again before I swap them in. Thanks! Also, do you have any advice on how to bed them in, or will that happen with normal use?
Yeah, from reading a few articles online it sounds like if the brakes were used while the rotors were contaminated, that's game over for the pads as well. I suspect they might have gotten some oil on them, as I purchased the bike directly from the manufacturer and never cleaned them before I rode it. As for the lever/caliper matching, I should probably double check that; although I didn't change anything from what the bike manufacturer put on it, there's always the possibly they got it wrong on their end. I'll have to figure out where the model numbers are and match them up with what Shimano says.

Finally, the cables are a little long, but not alarmingly so. I'm not too worried about this, as I don't notice much difference in the feel of the front vs rear disc, even though the rear brake has a much longer cable.
Even though the bike is not new, it's condition is nearly new as I haven't ridden it much, namely because of this braking issue. The brake levers are a bit harder to pull compared to my SRAM discs, but I believe that to be due to a stronger release spring and not friction. I'll double check the ends of each section of cable housing to make sure I don't have any sharp bends.

Many thanks.

cyccommute 

Frankly, I wasn’t aware that Shimano even made mechanical disc brakes. Can’t say I recall every seeing them. That said, rotors are seldom the problem in braking issues. Pads usually aren’t the problem. Set up is usually the problem. Disc brakes work best if the pads bite early in the lever throw. For mechanical, that means the pads need to be as close as possible to the rotor without rubbing. The lever throw should be half way to the bars like rim brakes are usually set up to work. (Dirty secret: rim brakes work better with less lever throw too.) Move the pads closer to the rotor and see if the braking improves. This article is for Avid brakes but most all mechanical brakes work the same and can be set up in the same way. Follow the “Setting the Pad Distance” part of the article.

dsbrantjr

Isopropyl alcohol is not a great solvent for petroleum-based greases or oils. I recommend Brakleen® or similar from an auto parts store. While checking the cables, ensure that the housing ends are properly prepared: https://www.sheldonbrown.com/cables.html#cutting

Koyote

IGH : You can use google to find out how to "bed in" the brake pads plenty of sites explain it better than I can. Even if you keep the old pads, and simply adjust them as per cyccommute 's recommendation, I think you will want to bed them again -- the process is to put some pad material onto the rotors.

Iride01 

Do you have a good bit of mileage on both bikes and brake pads? It took somewhere between 100 and 300 miles before my discs even began to stop better than my rim brakes. Till then, I was wondering whey people thought they worked better than rim brakes.

cxwrench

I'm guessing the problem is set up. There has been some pretty bad advice in this thread concerning this and that. My bet is that one of two things has happened. First would be whoever initially set up the brakes used the barrel adjusters to 'tighten' them up. Or they did the same thing as the pads wore. You absolutely can not shorten the cable at all w/ mechanical discs. Any time you do this you're reducing the leverage in the system and very quickly will end up w/ very little if any leverage. The arm that is pulled by the cable needs to all the way back in it's most relaxed position. Then attach the cable. If the brakes are too 'loose' make the adjustment w/ pad position, not cable length. You'll also have to adjust caliper position. You can go from fantastic, powerful brakes to useless brakes in less than 15mm of cable movement. I set them up like this:
1) With the brake arm in the fully relaxed postion attach the cable.
2) Turn one of the barrel adjusters out a couple of turns, doesn't matter if it's the one at the lever or the one at the brake.
3) Turn in the fixed side piston til the pad touches the rotor.
4) Snug down the caliper mounting screws.
5) Turn the barrel adjuster back in.
6) Back off the adjuster on the fixed side a little bit.
7) Final adjust as necessary.

You can't have the rotor perfectly centered on normal, one arm mechanical disc brakes, the fixed side will always be closer to the rotor and the movable side will have more clearance. You can have the rotor centered on brakes w/ 2 moving pistons.

mack_turtle

your problem has zero to do with the brakes themselves and everything to do with how they are adjusted and maintained. there are a dozen things that could be wrong with them at this point, but they can be fixed for a lot less than the cost of replacing them.

IGH

I mainly went with isopropyl because it was the only thing I was sure wouldn't leave residue. The Brackleen claims to not leave any, so I suppose I can give that a try, but if the pads are already contaminated I'm not sure it'll matter.
I haven't ridden it much because the brakes are so bad. I live/ride in an urban environment most of the time, so quick stopping is a must. Maybe my expectations are higher, but on my MTB (which was also purchased new) I was able to stop very quickly or lock the wheel with the SRAM brakes from the first moment I used them.
I did check the the cable length because I had a similar problem with my MTB disc brakes at one point. It's fully in the "open" position when the levers are released, and I only ever use the barrel adjustment for very fine tuning after pinching the cable with the brake screw itself. I will check the piston position, though it's not obvious (or easy, as far as I can tell) how to adjust these, unlike SRAM's that are trivial to adjust by hand.

cxwrench

Wait...you're saying it's very easy to adjust SRAM brakes but you can't figure out how adjust Shimano brakes? You need to look at things a little more closely or learn how to utilize si.shimano You can find pretty much anything you need to know about Shimano stuff here. Part numbers, instructions/manuals, and compatibility charts.

IGH

Well, on the SRAM I can easily see where to adjust the resting position of the pads, and adjust it by hand in mere seconds. On the Shimano, I have to figure out which bolt adjusts what, check if there's a set screw, get the right allen wrench, etc., and look up the documentation on them. I prefer to do the latter first, but other than the Shimano logo it's unlabeled, and it's not obvious which of their disc systems it is. I think I've figured it out, but I'm waiting for the rain to end before trying it again.

cxwrench

You have rain?!? What I'd give for a nice storm right now...orange skies and near darkness at noon is not good.

IGH

I wanted to provide an update on this now that I've gotten a chance to ride it, as I think I've finally discovered a big part of the problem. Let me start at the beginning.

I screwed in the barrel adjusters, and loosened the cable where it's pinched by the bolt at the brake to ensure the calipers were fully opened. They were both close, but not completely open, so I figured this might have been contributing a bit to a loss of braking force. I also tightened up the adjustable pad on both brakes, re-centered them, and deliberately left them rubbing a little bit. The reason for this is I wanted to ensure they were "bedded in" and mated well with the disk, and if I rode a little slower for a bit to make this happen, then so be it. The rear brake ended up being a bit too close, and as soon as I lightly tapped the brake, it would actually self-energize (apply a large braking force even when the lever was no longer depressed). I suspect the reason for this is thermal expansion of the pads and/or the disc as it heated up from the initial braking, which further pinched the disc between the pads and further heated up the disc, causing more expansion, etc. As I didn't have my tools with me, I simply had to stop, let the rotor/pads cool off, and only use the front disc for the remainder of the ride.

This ended up being somewhat fortunate, as when I got back, I knew I had the rear disc pad just a bit beyond the absolute limit of where it could be and still be rideable. I backed off one click, as the pad adjustment is indexed, and tested the brake lever before heading out again. It was quite loose now, and the only way to take up slack at this point was to shorten the cable. So I unscrewed the barrel adjuster, and was shocked by how far I had to do so to get it so the brake would engage shortly after the lever was pulled--about 7mm! As the calipers have better mechanical advantage when they're wide open, this is quite concerning as I'm losing much of that just to get the pads in the correct spot.

In short, the indexing on the pad adjustment is quite coarse, which is what I believe is robbing me of a lot of the mechanical advantage I could otherwise be getting. It surprises me that this would be a problem, as I've come to expect quality products from Shimano based on other parts of theirs I've used. I'll look and see if I can swap out my pads for others with more bite (i.e. semi-metalic, assuming I don't already have those), but if that fails I'm just going to replace the entire set, from lever to disc.