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Mechanical disk brakes wont bite!!

Old 09-18-22, 09:22 PM
  #1  
shawnin van
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Mechanical disk brakes wont bite!!

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????
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Old 09-18-22, 10:11 PM
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The "standard" for rear brakes is the ability to lock the rear wheel and cause a skid. They should do this easily and reliably.

My instinct is to suspect surface contamination of either the pucks or rotor, but you seem to have eliminated that.

Reaching blindly for straws, have you checked the cable? It may be kinked, frayed, or have a damaged housing.

Another possibility is something affecting travel of the actuating arm causing it to bottom out internally rather than against the rotor

Last edited by FBinNY; 09-18-22 at 10:15 PM.
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Old 09-19-22, 04:44 AM
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What model of brakes do you have? Also, you mentioned doing a lot with the pads, but did you ever shorten the cable?
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Old 09-19-22, 06:39 AM
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FBinNY mentioned the cable. Excess housing length and/or changes in direction will reduce power. Some say changing to compressionless helps, but I haven't tried it.

Verify the lever is correct for the caliper, road vs MTB.

Verify the lever arm at the caliper is near its fully relaxed range when brake is not applied.

If the rotor got contaminated with oil, sanding and cleaning doesn't always do it. Replace it. I've had to do this a couple of times on donated bikes at the shop.
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Old 09-19-22, 06:44 AM
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Why did you replace the brakes if the stock brakes were working as you like? When you replace the brakes the rear problem was immediate or initially they worked well?
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Old 09-19-22, 07:57 AM
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If you are constantly messing with and changing the pads then they might never bed in correctly. It took about 300 miles for my new disc brake bike to start showing that it had better braking than the rim brake bike I had previously.

While I knew of the bedding in procedures many do, I chose not to do that in a aggressive sort of way. I just braked firmly when I actually needed to stop along my normal routes. Not that this plays anything toward your issue, I'm just trying to mitigate the number of others that will "helpfully" tell me about the bedding in procedure I should have done. <grin>
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Old 09-19-22, 08:45 AM
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they bike is fairly new. Lever position is correct. the caliper works fine, the cable has no kinks and the cable length and routing never changed.. i changed pads when the brakes stopped to bite.. the front ones bite like mad. i first took original pads off and sanded them and used iso 99% on rotor.. no improvement, i tried new organic pads.. no bite. i tried new metalic pads no bite.. i tried adjusting the inside piston of the caliper all the way out.. no difference,, then moved it a bit in so the pads were closer together,, it was a bit better.. still not the bite i like,, i sanded the rotor.. not much difference. i have now put the pads very close to the rotor so the lever has a nice firm fell, but just not the bite i like.. i assume since the front bites hard the back could and should too.. how could the rear rotor be so contaminated there is no bite? i have a downhill bike with hydraulic brakes - i change those pads all the time.. never bed them and they instantly bite like mad.. whats going on here??? any other things to try? could it be the adjustment of how they calliper sits? thats like the only thing i did not do due to not being able to full losten the bolts as rack is in the way.. i take it off by taking mount off,,, should the pads be far away from the rotor?? i have the spacing tool.. they are evenly spaced. but so close i hear the rotor touching the pads when it spins. thats the only way i can get good stopping power.. and yes.. it like it to bite so it would skid... help!
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Old 09-19-22, 08:47 AM
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Eeeew! That post needs some organization of thought!

I'm not even going to try to read it.
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Old 09-19-22, 08:52 AM
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shawnin van
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answered all questions....
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Old 09-19-22, 09:16 AM
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Your posts are definitely TL;DR. That said, have you at any time used the barrel adjuster on the lever or the caliper to 'tighten' the brake? As the old pads wore did you take up the extra lever pull by doing that?
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Old 09-19-22, 10:41 AM
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yes.. that was the first thing i did.. but it did not create more bite. then i sanded the pads.. did not help.. that was when i started with new pads... but it just lead to a long stop and not the bite i like.. then tried lots of bedding... long slow pulls... did not help... then sanded the rotor lightly..then tries adjusting the pads and caliper. still did not work..
one question is: is it very important that the pads sit evenly spaced on the rotor? or is ot ok that the rotor sits to one side of the pads? say the stationary inside pad??
remember they are mechanical disk brakes - only one piston moves and one is stationary... am i missing something>**********
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Old 09-19-22, 10:48 AM
  #12  
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Originally Posted by cxwrench View Post
Your posts are definitely TL;DR.
It must be my age because the majority of stuff I read on forums (and elsewhere) is TL;DR. Either I'm lacking patience or younger people don't know how to organize their thoughts or appreciate the virtues of brevity.

In any case, the OP can do a simple travel test by removing the wheel and applying the brake against a dollar bill. (not generally recommended with hydraulic brakes). If there's adequate smooth travel, all upstream factors are eliminated because, even with a spongy housing, force in equals force out. So, it's down to the pad/disc interface.
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Old 09-19-22, 10:51 AM
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Originally Posted by shawnin van View Post
is it ok that the rotor sits to one side of the pads? say the stationary inside pad??
remember they are mechanical disk brakes - only one piston moves and one is stationary... am i missing something>**********
In disc brakes only one pad moves, deflecting the rotor and pushing it against the stationary pad. You want to minimize the required deflection by having the stationary pad as close to the disc without rubbing. Note that a warped rotor will force you to increase the stationary pad's clearance, and this can reduce brake performance.
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Old 09-19-22, 11:02 AM
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yes.. less rotor movement makes sense... so closer to stationary pad would also make sense,,, i have done that and moved the stationary pad as close to rotor as possible..i can see the piston pad moving to the rotor and working... only issue is that it does not bite on the rotor always a stupid annoying slow stop... even without the "bedding" i have always got an instant bite on my downhill bike.. never ever did the bedding thing.. is there a good way to de contaminate the rotor?? i dont want to buy a new one...
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Old 09-19-22, 11:22 AM
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Originally Posted by shawnin van View Post
..... never ever did the bedding thing.. is there a good way to de contaminate the rotor?? i dont want to buy a new one...
Read my signatures below.

First of all, do the travel test I described to determine whether it's the caliper or cable/lever.

I prefer acetone (from hardware store, not nail polish remover) for cleaning rotors. Lastly (for me) if you've done everything you could reasonably do, consider the possibility, that MAYBE in this case bedding the pads may solve the issue, EVEN THOUGH YOU'VE NEVER HAD TO IN THE PAST.


BTW- a bit of life, not brake advice. Do not solicit advice, then explain why you're going to disregard it. Even if you are going to disregard it, don't say so.
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Old 09-19-22, 12:24 PM
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Swap the front and rear brakes. If the front brake still bites hard, it’s not the brake or the pads. If the rear brake still doesn’t bite, it’s a cable/lever/rotor issue.

Swap the rotors. If the front brake still bites hard, it’s not the rotor.

It’ll take some work, but you ought to be able to isolate the problem by swapping components. FWIW, have you thoroughly cleaned the rear brake? I don’t like mechanical disc brakes very much, but they usually work OK if they’re clean. I‘ve had problems when the cable or actuating arm is sticky.
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Old 09-19-22, 03:52 PM
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Originally Posted by shawnin van View Post
yes.. that was the first thing i did.. but it did not create more bite. then i sanded the pads.. did not help.. that was when i started with new pads... but it just lead to a long stop and not the bite i like.. then tried lots of bedding... long slow pulls... did not help... then sanded the rotor lightly..then tries adjusting the pads and caliper. still did not work..
one question is: is it very important that the pads sit evenly spaced on the rotor? or is ot ok that the rotor sits to one side of the pads? say the stationary inside pad??
remember they are mechanical disk brakes - only one piston moves and one is stationary... am i missing something>**********
As someone else posted earlier, and I'll be more emphatic about it, you absolutely cannot use the barrel adjusters to tighten the brakes. It pulls the actuation arm forward and past where it can make power. You may get the pads against the rotor but you will end up with less and less power the more you turn the barrel adjuster. The ONLY way you can tighten those brakes is by adjusting the stationary piston. Usually you'll have to adjust the position of the caliper too, it needs to be very close to the stationary pad, with a bigger gap to the moveable pad. The chances of the pads being contaminated w/ oil are slim UNLESS you use spray chain lube which is a huge no-no w/ disc brakes. The arm must be in the fully relaxed position. Undo the cable and let it go all the way back. Attach the cable and adjust. I will turn the stationary piston in nearly til the pad touches the rotor, then pull the lever til the pad that moves touches...then tighten the caliper bolts. You'll then back off the stationary piston a bit and you should be done.
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Old 09-19-22, 03:54 PM
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Originally Posted by byscott View Post
Swap the front and rear brakes. If the front brake still bites hard, it’s not the brake or the pads. If the rear brake still doesn’t bite, it’s a cable/lever/rotor issue.

Swap the rotors. If the front brake still bites hard, it’s not the rotor.

It’ll take some work, but you ought to be able to isolate the problem by swapping components. FWIW, have you thoroughly cleaned the rear brake? I don’t like mechanical disc brakes very much, but they usually work OK if they’re clean. I‘ve had problems when the cable or actuating arm is sticky.
No, no, and no. You don't know enough about mechanical/cable brakes to provide good advice.
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Old 09-19-22, 06:27 PM
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You have an e-bike with mechanical brakes, that is not ideal at all. If you are wanting to be stuck with that bike and mechanical brakes I would recommend looking at TRP HY/RDs with better compressionless cables and housing and nicer rotors and that will help a lot. Personally I would look at a bike with better hydraulic braking. An e-bike that comes with mechanical brakes generally is going to have a load of other lower quality parts and construction and is not worth putting a ton of time and money into.
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Old 09-19-22, 08:52 PM
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Originally Posted by cxwrench View Post
No, no, and no. You don't know enough about mechanical/cable brakes to provide good advice.
Suit yourself, buckwheat. I’ve been doing this stuff forty years, including a couple stints in bike shops. I have a couple bikes with mechanical discs. He’s got one brake that works like he thinks it should, and one that doesn’t. The possibilities are limited.
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Old 09-19-22, 09:18 PM
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Originally Posted by byscott View Post
Suit yourself, buckwheat. I’ve been doing this stuff forty years, including a couple stints in bike shops. I have a couple bikes with mechanical discs. He’s got one brake that works like he thinks it should, and one that doesn’t. The possibilities are limited.
The OP listed all the things he'd done. I asked him if he used the barrel adjuster to 'tighten' the brake. He replied yes. He's killed the mechanical power of the brake by over tightening the cable and not adjusting the stationary piston. This is the problem. If I'm wrong I'll bow to your 'stints' in bike shops and your 40 years of experience. Even with your experience your knowledge is limited.
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Old 09-20-22, 02:09 AM
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What brand of brake pads are you buying? Are they cheap off brand? Or a reputable brand like Shimano, Kool Stop, etc...?
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Old 09-20-22, 12:01 PM
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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
???
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Old 09-20-22, 12:11 PM
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Originally Posted by shawnin van View Post
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
???
You don’t want to use the barrel adjuster to tighten the caliper because it changes the leverage you get while pulling the lever to brake. You should still set the pads close to the rotor, but using the pad adjustment, NOT the barrel adjuster.
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Old 09-20-22, 12:14 PM
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Originally Posted by shawnin van View Post
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..???
I think you must have misinterpreted something you read above. Folks are saying that you WANT the pads close to the rotor, but that you need to do it with caliper positioning, and NOT by simply turning the barrel adjusters. You want the actuator arm (the arm the cable pulls) to be as far back (relaxed, extended, however you want to think about it) as possible when the pads start to engage with the rotor. I'm sorry to hear that it's difficult to get everything configured with the hub motor and rack in place. One option is upgrading to hydraulic brakes with e-bike compatible levers (if you want to keep the motor cutout functionality).
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