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We've purchased a new e-bike for commuting, its very heavy and carries a heavy load, and the commute is through a very hilly neighborhood. The mechanical disc brakes have become a constant issue, loosing nearly all braking power in usually about a week's time. I've cleaned the pads to remove the glazing (light sanding) and 91% isopropyl alcohol on the rotors, and even replaced the pads, all to the same result - good for a few days, followed by diminished to total loss of braking power over the course of the week. What is wrong with my disk brakes?

https://blixbike.com/products/aveny-electric-city-bike Tektro Aquila Mechanical Disc Brake, 160mm Rotor

scai
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jfulton
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  • Are you getting a lot of wear on the pads? I'm wondering if you just need to adjust pad position regularly. – Argenti Apparatus Oct 14 '19 at 01:46
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    Tough to say without numbers for 'very heavy' on 'very hilly' and how far: needing to turn the pad adjuster (big flat metal disk with a very shallow hex fitting in it on the inside of the caliper) in weekly sounds completely normal to me, regular maintenance for that bike. – Affe Oct 14 '19 at 01:50
  • Website doesn't say what the bike's weight is, but it does say 500W motor which is double the legal maximum permitted for my country. How heavy is your bike, and how heavy is your load on top? – Criggie Oct 14 '19 at 03:14
  • Braking technique may be a factor - do you 'drag' the brakes on down hills?. Bigger rotors would help.Switching to sintered pads (if not already) would be beneficial as they handle heat better and wear slower. – mattnz Oct 14 '19 at 03:21
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    Glazed pads, what kind of bedding-in procedure do you follow, if at all? – Swifty Oct 14 '19 at 08:08
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    Is there a difference between front and rear? – Chris H Oct 14 '19 at 11:41
  • It's a farce that manufacturers don't implement properly working regenerative braking on such a bike. – leftaroundabout Oct 14 '19 at 15:44
  • If I were in your shoes, I would upgrade to some quad-piston hydraulic brakes. – Paul H Oct 14 '19 at 18:16
  • @leftaroundabout how would such a thing be implemented? – Paul H Oct 14 '19 at 18:17
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    @PaulH For what it means, they do have such systems available on some high end e-bikes such as this design by Microchip for brushless DC hub motors http://ww1.microchip.com/downloads/en/devicedoc/regenerative%20braking%20of%20bldc%20motors.pdf However they are expensive, complex and usually massively increase resistance while peddling with the motor off making the e-bike infeasible for use as a normal pedal bike. – Vality Oct 14 '19 at 18:26
  • @Vality there's nothing very expensive or complex about regen braking per se. It essentially just needs the hardware you need anyways for an e-bike: motor, driver and battery (or even just a power resistor for dissipating the energy). Every electric car or motorcycle has regen, it's a no-brainer. The actual reason why most E-bikes don't have it is that the common middle-mounted motors sit before the drivetrain, and derailleur drives can only transmit power in one direction. (And the mid-mount is desirable particularly for MTBs (low unsprung mass), who would benefit most from regen braking.) – leftaroundabout Oct 14 '19 at 18:54
  • @leftaroundabout Fair enough. I only have experience with hub motors in commuting, cargo and touring style e-bikes. Middle mounted motors tend to live on higher end bikes in the first place, and I shall admit I haven't used one due to the increased maintenance requirements. (My e-bike is my only commuting vehicle so I need it to work, hub motors are reliable and mean I have two separate power sources that would have to both fail to leave the bike unusable) – Vality Oct 14 '19 at 19:47
  • What's trickier on (hub motor) e-bikes is that you have to be able to coast and to pedal without power. So the regenerative braking has to be controlled by a brake lever. But most of the stopping power comes from the front brake, while the motor normally drives the rear, so you need a different braking technique as well. On the other hand plenty of e-bike riders barely use the front brake as the extra weight at the back makes it easier to get away with back braking – Chris H Oct 14 '19 at 20:11
  • Comments should not be used for extended discussion. – mattnz Oct 14 '19 at 20:15
  • UPDATE: I don't feel like I am getting lots of wear on the pads, I'll have to double check, but stats: the bike weighs about 50 lbs, loaded with about 250 lbs., the brake failure is definitely grip at the rotor, adjusting the cable pull has no effect. It definitely feels like contaminated pads, but seems unlikely since I've cleaned or replaced the pads almost weekly. Could overheating be an issue to cause pad failure? – jfulton Oct 15 '19 at 02:46
  • Overheating could easily be an issue on organic pads (or as I found out to my painful cost, resin-bound ceramic). With a total of 300lbs on hill the heat load is rather high – Chris H Oct 15 '19 at 15:01
  • Thanks for all the comments! I think the consensus is could likely be the brakes are being overloaded. I just did one last overhaul last night. If this doesn't last, I'm going to try an upgrade to Avid BB7s with a 180mm rotor - as was suggested by another fellow bike geek (former mechanic) in my office, not ready to go full hydraulic yet. – jfulton Oct 15 '19 at 21:18

5 Answers5

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Adjusting the pads is a normal part of maintenance for mechanical disc bikes.

I have a tektro mechanical e-Bike that is ridden with rider+cargo > 200lbs 8 mi/day with 4 steep (> 50 meter elevation change exceeding 20% grade) hills.

I move the adjuster for the moving pad in one "click" weekly when filling tires and readjust the static pad monthly. I invested money in buying a set of long ball-end hex wrenches, and stopped trying to work a mini bike multi-tool through the spokes to get at the adjuster, it's not really a big deal.

Moving pads in weekly or even twice weekly depending on just how heavy and hilly your idea of "very" is, does not seem extreme.

Criggie
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Affe
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In general I've found tektro brakes to need a lot of tweaking (V as well as mechanical disc). With discs this can be minimised by getting the fixed pad as close as you can without it dragging. If your rotor is bent this won't be as close as it should be

Sintered pads are definitely better for loads+hills; it's important to bed them in but they don't glaze. Keep an eye on rotor wear, as this is greater with sintered pads, especially with softer rotors.

If you're finding that cleaning alone makes a big difference, look for how the rotors could be getting contaminated, e.g. spray chain oil, greasy puddles. Even with disc brakes, mudguards help if you're flicking up greasy water.

Chris H
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  • The calipers are not self-centering as the car brakes are? Interesting... – Crowley Oct 14 '19 at 19:36
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    @Crowley hydraulic ones are, but (nearly all) mechanical systems have a fixed pad and a moving pad. The rotor is only 2mm thick, and flexes under pressure from the moving pad until it presses against the fixed pad, which was probably less than 0.25mm away at rest – Chris H Oct 14 '19 at 20:08
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How much do you consume the pads? If they wear too much you will simillar braking power but you will need to pull the lever closer and closer to the handlebars. Measure the thickness of the brake pads after you cleaned them and then after their power vanishes.

  • Exceeding wear: Your brakes are too weak for your style and riding profile.
  • No wear: Your disc and pads are greased.

If the first is your case, try to change braking strategy. Do not brake for long time to maintain speed. Brake hard for short bursts then let the brakes cool down. If not possible during very long and steep descents, make stops and let the brakes cool down. If it does not help, look for upgrading the brakes, try better pads first, then bigger disc.

If the later is your case, clean thoroughly all discs and pads and keep them clean. Do not hesitate using strong solvents like acetone or methyl-ethyl-ketone or brake-cleaning fluids. For the first run I would remove the disc from the bike and clean it in ultrasonic bath.

Crowley
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  • Unless OP is downhilling on their e-bike the brakes are unlikely to get hot. It's more likely their braking style is causing the pads to become glazed – James Oct 15 '19 at 10:35
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If you've been replacing pads, but not rotors (only cleaning them) then you could just not be cleaning the contamination of the rotor, therefor continuously contaminating the pads. That being said a 160mm rotor with a mechanical disc brake with a heavy bike is probably a lot more demanding on those brakes than they were intended for. Having a larger rotor and/or hydraulic brakes will increase your stopping ability.

jacob
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I would try replacing the rotors. They could be contaminated with something that the alcohol won't fix. That would then in turn contaminate the pads, which would explain why sanding and replacing pads has only a temporary reprieve from your issues.

Steven Eccles
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  • There are more solvents to try - alcohols, acetone, toluene and if elese fails - methyl-ethyl ketone (MEK). But be very careful, MEK is hallucinogenic and cancer-inducing when inhaled too often. Other option is brake cleaner from a garage store. – Crowley Oct 14 '19 at 19:39