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Mechanical disk brakes wont bite!!

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Mechanical disk brakes wont bite!!

Old 09-20-22, 12:54 PM
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Originally Posted by shawnin van
ok.... why is it that it reduces stopping power if the pads are too close to the rotor? i like a firm lever. i have a downhill bike with hydraulic discs and a firm lever and good bite... in this case you are saying to not have the pads close to the rotor.. ********** also it is super hard to adjust the stationary pad as the allen key screw is blocked by the motor.. only way is to take the caliper off.. adjust and put back,, a pain...access to adjust the brakes in general are hard,, i cant easily adjust the caliper as the mounting screws are blocked by my rack,,,,, a pain... i am really wondering if it will just take a few hours of riding for these to bed>>>.>???? although i have never needed to do that.. as of now the stationary pad is very close to the rotor... 0ither barely needs to move.. is that good or bad
???
OK, all this tells me that you should be using a hydraulic disc brake which self adjust. Mechanical disc brakes have to be kept properly adjusted. There is no way around this. If the constraints of your rear wheel make adjusting the brake calliper difficult, maybe you should think about installing a rear hydraulic disc brake system. That way, you wouldn't have to worry about fiddly rear brake adjustments
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Old 09-20-22, 03:21 PM
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Originally Posted by shawnin van
ok.... why is it that it reduces stopping power if the pads are too close to the rotor? i like a firm lever. i have a downhill bike with hydraulic discs and a firm lever and good bite... in this case you are saying to not have the pads close to the rotor.. ********** also it is super hard to adjust the stationary pad as the allen key screw is blocked by the motor.. only way is to take the caliper off.. adjust and put back,, a pain...access to adjust the brakes in general are hard,, i cant easily adjust the caliper as the mounting screws are blocked by my rack,,,,, a pain... i am really wondering if it will just take a few hours of riding for these to bed>>>.>???? although i have never needed to do that.. as of now the stationary pad is very close to the rotor... 0ither barely needs to move.. is that good or bad
???
Jesus H Christ...you asked for help diagnosing and fixing your problem. I told you exactly what the problem is and how to fix it. If you don't like the answer because it's 'a pain' to do that's your problem. Moving the actuation arm forward reduces power because it reduces leverage...like a cam being over center. This problem you're having is endemic to cheap electric bikes. The engineering on these bikes sucks. It's hard or impossible to adjust brakes, but the bikes are spec'd w/ brakes that need constant adjusting. You can bed new pads in 10mins if you do it correctly. It's good that you have the stationary pad close to the rotor but you need to set up the rest of the brake properly for it to work. I've explained this already so I'll not go through it again.
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Old 09-20-22, 06:19 PM
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Originally Posted by cxwrench
... Moving the actuation arm forward reduces power because it reduces leverage...like a cam being over center.
This is counterintuitive. Can you please explain why moving the arm forward reduces the leverage?
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Old 09-20-22, 07:21 PM
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Originally Posted by alexk_il
This is counterintuitive. Can you please explain why moving the arm forward reduces the leverage?
Imagine for a moment that you're hanging from Big Ben by a rope tied to the hour hand. At six the hand is pointing at you and you can't move it no matter how hard you pull. OTOH - at three the hand is horizontal and it's easy to pull it down to 4. Likewise, at five o'clock it's more difficult to pull the hand down to six.

Same with the brake arm. You get the maximum leverage when the cable is meeting the arm at a right angle to it's range of motion. Howver some brakes are designed with variable leverage, offering maximum travel in actuation, and greater force when the shoe nears the end of it's travel range. So my initial statement may not hold true, depending on the internal linkage in the brake, but in any case you get the maximum leverage when moving the arm produces the least pad movement.

Applying this, you might want to shim the entire caliper so the stationary pad is either backed out almost all the way, or cranked in much more. The right answer depends on the specific brake, though the former is more common. You might look at the position of the front stationary pad, since that works well, and duplicate that in the rear by shimming the caliper as needed.

FWIW- and not trying to be snide, there's a difference between hanging a component, and being a mechanic.
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Last edited by FBinNY; 09-20-22 at 07:37 PM.
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Old 09-20-22, 08:07 PM
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Originally Posted by alexk_il
This is counterintuitive. Can you please explain why moving the arm forward reduces the leverage?
I can't imagine not being able to work this out in my head, but read post 29 from FB a few times.
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Old 09-21-22, 04:31 AM
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Originally Posted by FBinNY
Imagine for a moment that you're hanging from Big Ben by a rope tied to the hour hand. At six the hand is pointing at you and you can't move it no matter how hard you pull. OTOH - at three the hand is horizontal and it's easy to pull it down to 4. Likewise, at five o'clock it's more difficult to pull the hand down to six.

Same with the brake arm. You get the maximum leverage when the cable is meeting the arm at a right angle to it's range of motion. Howver some brakes are designed with variable leverage, offering maximum travel in actuation, and greater force when the shoe nears the end of it's travel range. So my initial statement may not hold true, depending on the internal linkage in the brake, but in any case you get the maximum leverage when moving the arm produces the least pad movement.

Applying this, you might want to shim the entire caliper so the stationary pad is either backed out almost all the way, or cranked in much more. The right answer depends on the specific brake, though the former is more common. You might look at the position of the front stationary pad, since that works well, and duplicate that in the rear by shimming the caliper as needed.

FWIW- and not trying to be snide, there's a difference between hanging a component, and being a mechanic.
Kind of makes sense, but not sure if this is how my brakes work.

They come with the built in barrel adjuster that positions the brake arm at 5-5:30 o'clock, looking from where the cable goes.

I bring the arm with the barrel adjuster closer to 4, pads are touching the rotor at 3, directly at the right angle. I would assume this to be the optimal position for the arm, is it not?
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Old 09-21-22, 06:35 AM
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After all actions by the OP and still no hard bite, and if its not a broken/compromised cable housing…is there a chance the brake caliper structure itself has a stress crack that is allowing the caliper to flex rather than bite down hard? I’ve seen it happen once in a rim caliper but I’m fairly new to disc calipers.
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Old 09-21-22, 06:39 AM
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Originally Posted by cxwrench
Jesus H Christ...you asked for help diagnosing and fixing your problem. I told you exactly what the problem is and how to fix it. If you don't like the answer because it's 'a pain' to do that's your problem.
Now, you’re already one of the more knowledgeable and helpful contributors on this ‘mechanics’ forum…and if you could be more patient with your ‘patient’ you’d be veritable Sheldon Brown reincarnated…just a thought.
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Old 09-21-22, 07:20 AM
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Originally Posted by alexk_il
Kind of makes sense, but not sure if this is how my brakes work.

They come with the built in barrel adjuster that positions the brake arm at 5-5:30 o'clock, looking from where the cable goes.

I bring the arm with the barrel adjuster closer to 4, pads are touching the rotor at 3, directly at the right angle. I would assume this to be the optimal position for the arm, is it not?
I agree that any angle greater than that 3:00 position, when fully applied, is fine. The problem is that many do not understand the leverage issue and continue to adjust past that position as pads wear.

The clock analogy would be a little better, though less dramatic, if the hands were pulleys. The cable runs over a radial foot before the pinch bolt. As long as the cable is contacting that foot, the leverage is unaffected by arm position. If you start at the fully extended position, and then fine tune with the a turn or two of the barrel adjuster, that's okay. And the next time you descend a couple of mountain passes and would like better brake feel before riding home, a turn of the adjuster in the field is okay, too. It's there for a reason. But don't make a habit of it.
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Old 09-24-22, 06:03 AM
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Originally Posted by shawnin van
Mechanical disk brakes wont bite!!
I had to have a set of LX cantilevers put down because of food aggression...
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Old 09-24-22, 07:20 AM
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All of the other points aside about setting brakes up maximally, I had a wreck a week ago involving some wet concrete so I wholesale cleaned and lubed my bike. Got something on the rear disc and it took me most of the week to wash it off the disc and pads and rebed the shoes. Now they're working fine, but it was no-stop for a couple of days. Point being that I was doing some diligent cleaning and it took me a few tries to cut through whatever was causing the problem. My suspicion is that there was still something stuck in the webbing of the disc that ran out when warm and the best solution would have been to take the disc right off and soak it in a tray of solvent and then replace the shoes.

I can't imagine that an actuator arm in a slightly wrong place can have that much effect compared with a persistent spot of oil in the wrong place, and if I were the OP that's where I'd be spending most of my time on this problem.
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Old 09-24-22, 07:49 AM
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Originally Posted by shawnin van
... my rear pads have stopped biting and will come to a long stop when pulling the lever.
​​​​​​...
I might have a similar problem with my front brake, long stop and no hard bite.

The long stop seems to be due to the pads not being parallel to the rotor. The pads initially touch the rotor with only a portion of the pad and the contact area increases with more if the cable pull.

For some reason, I am unable to keep the pads aligned to the rotor in the front, it moves out of alignment when I tighten the bolts. The usual trick to apply breaks to tighten helps a bit, but is still not enough. Perhaps the brake mount points are not 100% aligned with the wheel/rotor.

Last edited by alexk_il; 09-24-22 at 07:52 AM.
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Old 09-24-22, 08:52 AM
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God I love my rim brakes…
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Old 09-24-22, 09:13 AM
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Originally Posted by mdarnton
All of the other points aside about setting brakes up maximally, I had a wreck a week ago involving some wet concrete so I wholesale cleaned and lubed my bike. Got something on the rear disc and it took me most of the week to wash it off the disc and pads and rebed the shoes. Now they're working fine, but it was no-stop for a couple of days. Point being that I was doing some diligent cleaning and it took me a few tries to cut through whatever was causing the problem. My suspicion is that there was still something stuck in the webbing of the disc that ran out when warm and the best solution would have been to take the disc right off and soak it in a tray of solvent and then replace the shoes.

I can't imagine that an actuator arm in a slightly wrong place can have that much effect compared with a persistent spot of oil in the wrong place, and if I were the OP that's where I'd be spending most of my time on this problem.
You'd be wrong. A mechanical brake can go from very strong to completely useless in less than half an inch of cable pull. You can trust me on this one, that arm has to start in the fully relaxed/back position.
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Old 09-24-22, 06:04 PM
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Originally Posted by mdarnton
I can't imagine that an actuator arm in a slightly wrong place can have that much effect compared with a persistent spot of oil in the wrong place, and if I were the OP that's where I'd be spending most of my time on this problem.
I can also confirm from experience that it actually does make that much difference. I screwed this up badly when I bought my first bike with cable disc brakes and couldn't figure out how to get them to work correctly. It wasn't until figuring out that you absolutely had to start with the actuator arm in the fully relaxed position before I got the correct performance out of them.
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Old 09-26-22, 03:04 PM
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Take it to a shop OP.
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Old 09-27-22, 05:56 PM
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Originally Posted by shawnin van
I have an ebike and i like the brakes to bite hard.. they were like that on the stock brakes... my rear pads have stopped biting and will come to a long stop when pulling the lever. i have shimano brakes with m485 pads... i have tried new resin pads,, wont bite.. new metalic pads better but wont bite... i have adjusted them numerous times.. i have tried bedding the pads... no luck,,, i have cleaned and sanded the rotor.... what is the trick to get my rear brakes like my front??? to bite!! if i pulled my front hard id flip over the bars,,,, any ideas????
Perhaps they were never adjusted correctly. Here is the tailless one's beta on adjusting mechanical disk brakes.

The inner pad does not move. Rather, when actuated, the outer pad pushes the rotor into the inner pad, deflecting the rotor slightly. It would follow that if the outer pad has to push too far, braking power would be lost and the rotor miight take a permanent bend. I've also seen brakes adjusted so the inner pad is completely retracted, which means the rotor gets pushed against the caliper itself. Not much friction there.

So, loosen the caliper mounting bolts. Observe the depth of inner pad protruding clear of the caliper. Adjust pad depth so you have about 1.5 mm protruding. Now squeeze the brake lever, letting the caliper float. It should be on the tight side, that is, with less than 1/3 of the lever pull to make contact. Loosen the cable a little if necessary. Then, while fully actuating the brake lever, tighten the caliper mounting bolts. You can use a rubber band or old toestrap to hold tension on the brake lever. After tightening, release the lever, spin the wheel, and observe that the rotor probably clears the outer pad but rubs the inner pad.. Ignore the outer pad for now. Retract the inner pad so it just barely stops rubbing. You should still have more than a millimeter of inner pad protruding. With your caliper properly centered, you can now adjust the cable tension to your liking. I recommend 1/2 to 1/3 of free play to the handlebar. When you hit the brake, you should feel a solid "thunk" of the outer pad pushing the rotor quickly and firmly against the inner pad. If you don't get the "thunk," your inner pad is too far from the rotor.

Last edited by oldbobcat; 09-27-22 at 06:02 PM.
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