Mechanical disc brake becomes weak after few stops

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Dec 27, 2021
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4
I got this bike:

https://www.amazon.com/gp/product/B09GFDQN8D/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

The brake is a mechanical brake. It says "Peak" on it. I have read that they are similar to the Avid bb5's.

I have watched probably 2hrs worth of tutorials altogether. I have spent days on this and I cannot figure it out. I have set them up and tuned them using multiple different methods. Each one ends up with the same result.

When I adjust the brake lever and tighten the retention bolt, the first time I use the brake, it's good. I squeeze the brake a quarter of the way to the handlebars and I stop. After 5 squeezes, suddenly I am getting no braking power. The brake arm moves the same amount as always, the pads seem to contact, from what I can see anyways, but no power. By the 5th squeeze, the brake lever is also touching the handlebars.


I tried replacing the cable, replacing the brake line, taking the cable out of the housing, all results in the same thing. I cannot figure it out.
 
Moto-roto-electrico said:
When I adjust the brake lever and tighten the retention bolt, the first time I use the brake, it's good. I squeeze the brake a quarter of the way to the handlebars and I stop. After 5 squeezes, suddenly I am getting no braking power. The brake arm moves the same amount as always, the pads seem to contact, from what I can see anyways, but no power. By the 5th squeeze, the brake lever is also touching the handlebars.


I tried replacing the cable, replacing the brake line, taking the cable out of the housing, all results in the same thing. I cannot figure it out.
Unless you're having to brake so hard against so much speed or torque that the pads are simply wearing out that fast, then:

Is this happening on both front and rear identically?

If the housing is the right length and the right kind, and the cable is good, then once tensioned that part should stay the same for a long time.

I'm not sure what you mean by "taking the cable out of the housing"; unless the cable is directly routed to teh caliper in a straight line then there isnt' anything to create the tension against between the lever and the caliper.

Pads just contacting isnt' sufficient, they have to be pushed against the rotor with enough force to cause sufficient friction to cause the braking you need. So even though they "look" like they contact the same, they're not, becuase the tension of the systme drops (which it shouldn't, not this quickly).

If your levers have adjustment barrels on them, are they steel, or aluminum? Some of the cheap aluminum barrels I have had just crush inward or spread open with the force of brake pulls. When this happens, even a little bit, it takes significant tension out of the system.

Same thing for the adjustment barrels on the calipers, if they have them.

Those, and the cables and housing, are typically the only places to adjust or lose tension in a simple system like these.

If the pads themselves are crushing under the force, that would also cause it, but they should be hard enough for that to not happen.

The only other thing I can think of ATM is that the place in the lever that you put the cable "stop" end in could be distorting or stretching, if it is soft enough metal.
 
When I adjust the barrel on the lever outward, the lever gets harder to press but the wheel still moves with little resistance no matter how hard I squeeze.

Same thing for the barrel adjuster on the other end at the caliper.


I just replaced the lever and the cable with new ones but are aluminum, squeezed the lever, wheel doesn't budge (good), squeezed again, doesn't budge, squeezed again, wheel can now move a little, squeezed again, little resistance, no braking power, lever is now able to touch handlebars when it couldn't previously. This is indoors btw.

After 5 squeezes, if I tighten the arm all the way up as far as it can go, the brake still does not work. Even though the lever is tighter/harder to squeeze. If I unscrew the brake cable at the brake arm, pull the arm down, then re-adjust it and screw it back in, I get 5 more squeezes until it loses it's brake power again.
 
I'd send it back to the shop, you can get lots of cable disk brakes used on YT for 10-20 from avid and generic ones. When the cable is tight it's tight. Brake lines are one of the most reliable elements of a bike, steel on steel, they are 1mm accurate once tensioned. I'd search google for similar issues using precise word combo's... someone in the world posted the same issue on google once? did you see? Perhaps take pliers and find out where the movement is happening on the brake lever/caliper, you should be able to bring out 2-3 mm of cable and find the fault. good luck.
 
Cheap generic mechanical disc brakes are a crapshoot. They might work okay, or not. It seems to me that your pad adjusters might be backing off a little with each application.

Avid BB5s are not good enough to warrant being the almost-as-good-as version of. Honestly, they suck.

If you want to stop fooling around and just have good brakes, Avid BB7 and TRP Spyke are the ones to have. We can quibble over their relative merits, but they're both fine, and easy to adjust.
 
First, does this happen with both front and rear brakes the same way?

If they don't both have the problem, comparing the two setups should help find the problem.

Next, when you used just the cable, how was this setup? (knowing this may help eliminate some parts from the problem)

Next, see inline responses below:

Moto-roto-electrico said:
When I adjust the barrel on the lever outward, the lever gets harder to press but the wheel still moves with little resistance no matter how hard I squeeze.

Same thing for the barrel adjuster on the other end at the caliper.
That sounds like something is wrong with the caliper itself, or it's pads. With cheap ones that's what I'd guess, given the lack of results I had before getting the Avid BB7s. But...


I just replaced the lever and the cable with new ones but are aluminum, squeezed the lever, wheel doesn't budge (good), squeezed again, doesn't budge, squeezed again, wheel can now move a little, squeezed again, little resistance, no braking power, lever is now able to touch handlebars when it couldn't previously. This is indoors btw.

After 5 squeezes, if I tighten the arm all the way up as far as it can go, the brake still does not work. Even though the lever is tighter/harder to squeeze. If I unscrew the brake cable at the brake arm, pull the arm down, then re-adjust it and screw it back in, I get 5 more squeezes until it loses it's brake power again.
...that sounds like the cable itself is stretching, or the housing is compressing into the adjusters, or the cable stop inside the lever is deforming, or the cable's endpiece is distorting, etc.

So nothing has really been eliminated. :(

By "brake arm" do you mean the caliper at the wheel, or the lever at the bars?

If you mean the caliper, then if the cable adjustment you're doing is moving the clamp part further up the cable each time, it sounds like the cable itself is stretching. (but any of the other deformations or distortions could cause the same thing).

What cable and housing are you using?

There are at least two different types of housing. One uses a spiral wrap of metal under an outer jacket. The other uses full-length "wires" stacked as a cylindrical tower from end to end, under an outer jacket. The former has some compression when used, though not a lot on short lengths. The latter should have no compression, but has the disadvantage that the outer jacket must resist the outer bowing of the "wires", and if it fails then the wires burst the jacket and you lose all the tension.

I've had some pretty crappy cables over the years, but none have stretched so badly like yours (snapped while braking hard, though). Right now I am using these:
https://www.amazon.com/gp/product/B0031GWK2G
SUNLITE Lined Brake Cable Housing, 5mm x 50ft
https://www.amazon.com/gp/product/B001GSSCRI
Jagwire Mountain Brake Inner Wire1.5X3500mm Slick
(I'd recommend just getting the Jagwire premade user-installable cable/housing kits instead, though, if you choose these' it's a lot easier!)
on Avid BB7 MTN calipers
https://www.amazon.com/gp/product/B00553YPEY
but with 203mm rotor on the front of SB Cruiser, and can easily brake hard enough to skid the wheel even in great traction conditions. Super easy and fast to setup even without the instructions, and really easy to adjust the pads with the big red knobs, too, as they wear out.

They worked fine with the OEM pads, but when I wore them out I bought these
https://www.amazon.com/gp/product/B07GCW273S
and they work but wear out really fast. Next time I'll get Avid pads. ;)
 
New cable housings? I have seen some that compress too much, then spring back.

I would still look at the calipers adjuster, perhaps they are slipping. I mean the one on the pad, not the cable. I have had loose ones back off on a ride, but not inside the garage in a test. It could be so bad it jumps a thread when you apply the brakes.

But I'm betting this problem is not the caliper itself, or the pads, but the cable and housing, or the handles and their adjusters.
 
I had a similar issue. It'd not happen often at first, but in time it happened more frequently. One time it happened when going downhill, thankfully a bike has two brakes. Anyway I could hardly stop because the remaining brake wasn't properly adjusted.

Mine was written "Radius" on it. Your link doesn't work but I think they look similar.

Here is how mine looked like: https://www.aliexpress.com/item/33036534286.html

In my case, I believe those mechanical brakes are faulty, and I also read other people having the same issue with these "Radius" calipers. In my case, sometimes when I squeeze the lever, it would go down but calipers would not grab the disk strongly (then the lever would be operated easier). The caliper would randomly "restore" itself, but if doesn't, the braking power is probably less than 40%.

It really looked like there was a problem with cable but when you inspect the system during the problem, it's with that caliper. The caliper doesn't want to go back from that "half-stuck" state.

I replaced them with Shimano mechanical brakes, and problem disappeared. Nowadays I have Shimano hydraulic brakes.
 
I would say if this is the same problem (and the calipers are just rebranded version of this "Radius" ones), they can fail in the worst time. Just replace them.
 
G'day Moto
How's the disk to pad clearance? I had a problem where the braking was weak even when the cable pulled the caliper arm as far as it could go. Take a look at the back of the caliper. Most have an adjustment via Allen key to bring the pads closer to the disk so the full travel of the caliper arm isn't used up. If that doesn't work, you'll need to shell out for quality brake as the experts have already said.

AussieRider
 
New to the forum, so hello to everyone.

I've recently solved a problem very similar to the OP's.

amberwold touches on the cause near the end of the 2nd post. On my bike the small plate that holds the cable in place against the brake actuating lever, on the rotor end, has grooves that run its length, and it was installed so that the grooves ran parallel to the cable. These grooves are smaller than the diameter of the cable so some clamping pressure can be applied to the cable between the lever, which is flat, and the grooves of the cable clamping plate.

What I found was that, once too much force was applied at the hand lever, the cable slipped and caused the paint (which was quite thick initially) to wear away. After readjusting it a number of times eventually the metal on the lever also wore to the point that a shallow groove formed, reducing cable holding power even further. I tried a few things like adding a shim to the plate to effectively make the grooves smaller and soldering the end of the cable to make it less compressible. Those things helped but not permanently.

I got disgusted and spent the next few days researching and then ordering hydraulic brakes. Problem resolved and this is what I'll use once they're delivered, but if you read a little further, I think I've actually found a permanent solution using the original components.

I didn't want to go without a bike while I was waiting for the hydraulic brakes, so I took the bottom lever off the bike to see if there was anything else I could try.

I wanted to rotate the clamping plate 90 degrees to see it would hold any better with the cable crossing the groves instead of running parallel to them, but I discovered that, while the clamping plate was more or less square, the hole was off center, and the plate would only fit one way. I ended up filing some metal off two sides of the plate to make it fit in the cross-cable orientation. After reassembly the brakes work almost as well as when new and better yet, I haven't had to readjust then once in about two weeks. Problem solved; I hope.

A couple lingering concerns: 1) It's possible that the grooves were intended to provide some relief from too much pressure at the hand lever (to prevent damage downstream) but if that's the case they shouldn't have been so deep. 2) The small groove worn into the lower lever. If I was going to continue using these original brakes, I'd probably try to get a new lever.
 
My experience with Avid BB5 is that they were hard to adjust with only one side moving, I replaced with avid BB7's. which adjust both sides.

Lately, I installed a set of HB100 cable operated hydraulic calipers on one bike. Very easy to adjust and plenty of clamping force. LEss expensive than AVid BB7;s, I believe their tradeoff is that they blow the seals and fail. We will see.
 
docw009 said:
I believe their tradeoff is that they blow the seals and fail. We will see.

That is exactly what you don't want happening at whatever speed you plan to ride at. This is potentially deadly even if the failure occurs at 10 mph. So how fast do you plan to ride?

I plan to try hydraulic brakes soon, but none of the readily available systems made for ebike applications meet my needs and I've so far stuck with the Avid BB7s, as greatly as I dislike their worst faults. I can trust these to work every time at 35 mph. I can't say the same for any type of hydraulic braking system that is readily available and works for a bicycle or ebike application.

I still plan to keep a cable-pulled e-brake with disc rotor for the rear with the hub motor also configured for light regen, once I do try hydraulics for the two front wheels. But the hydraulics I'm going to try aren't a widely or readily available solution, and they're specially built for my application, and on an experimental basis. Hopefully they'll be "good enough" regarding stopping performance at car-appropriate speeds and still match or exceed my BB7s regarding overall reliability. If not, I can always go back to the BB7s and settle for 30-35 mph cruising as my default way of riding. But I really want increased possible cruising speed while retaining reliability, and the braking systems readily available that are in the weight range I need are one of the current bottlenecks. I'm trying to keep the entire vehicle a pedalable weight if one shuts the motor off, so a heavy motorcycle braking system is not compatible with the goal I have for the vehicle, and is really way too much braking system for what I need. I don't need massive calipers or rotors, but they do need to be more substantial and engineered for more stressful applications than any of the bicycle or ebike stuff readily available.

Have some sort of backup brake, whenever/wherever you decide to test your new braking system's limits. A suboptimal brake is better than no brake, and you need to test its limits as well in the event you ever need to rely on it.
 
The Toecutter said:
docw009 said:
I believe their tradeoff is that they blow the seals and fail. We will see.

That is exactly what you don't want happening at whatever speed you plan to ride at. This is potentially deadly even if the failure occurs at 10 mph. So how fast do you plan to ride?

I plan to try hydraulic brakes soon, but none of the readily available systems made for ebike applications meet my needs and I've so far stuck with the Avid BB7s, as greatly as I dislike their worst faults. I can trust these to work every time at 35 mph. I can't say the same for any type of hydraulic braking system that is readily available and works for a bicycle or ebike application.

I still plan to keep a cable-pulled e-brake with disc rotor for the rear with the hub motor also configured for light regen, once I do try hydraulics for the two front wheels. But the hydraulics I'm going to try aren't a widely or readily available solution, and they're specially built for my application, and on an experimental basis. Hopefully they'll be "good enough" regarding stopping performance at car-appropriate speeds and still match or exceed my BB7s regarding overall reliability. If not, I can always go back to the BB7s and settle for 30-35 mph cruising as my default way of riding. But I really want increased possible cruising speed while retaining reliability, and the braking systems readily available that are in the weight range I need are one of the current bottlenecks. I'm trying to keep the entire vehicle a pedalable weight if one shuts the motor off, so a heavy motorcycle braking system is not compatible with the goal I have for the vehicle, and is really way too much braking system for what I need. I don't need massive calipers or rotors, but they do need to be more substantial and engineered for more stressful applications than any of the bicycle or ebike stuff readily available.

Have some sort of backup brake, whenever/wherever you decide to test your new braking system's limits. A suboptimal brake is better than no brake, and you need to test its limits as well in the event you ever need to rely on it.

I think 'hybrid hydraulic' concept as it is done now (mechanic handles, hydraulic pistons) is highly bizzarre concept, EXPECIALLY for something high-speed.
Basically you have WORST of both worlds - 'spongy' brake lever feeling of bowden mechanical system and possible calbe lubrication/water ingress woes, and possibility of boiling the brake fluid of hydraulic!

You want something different... I've yet to test it personally, but there are motorcycle/moped brake/clutch converters that use hydraulic handle and line, but attached to a hydraulic piston that pulls on a cable.
Now THAT is best of both worlds... likely considerably more powerful so far as pulling force is concerned, too.
 
Note however that if you overheat the mechanical brake, you are likely to boil out the lubricating grease of the ball bearings... and melt the plastic parts.
 
BalorNG said:
Note however that if you overheat the mechanical brake, you are likely to boil out the lubricating grease of the ball bearings... and melt the plastic parts.

Mechanical parts don't have to be made out of plastic, but hydraulic brake fluids and seals can't be made out of metal.

polish-klamper-is.jpg
 
Yup. Brake fade due to overheated pads/rotors is still a thing however... this is why 'finned pads' make sense, though it relates more to 'drag braking' than emergency braking of a heavy bike from very high speed, I don't think that heat transfer will be nearly fast enough in case of latter.
 
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