Are my cable disc brakes knackered?

Question

Well, I bought a bike called "Iron Horse" and the bike has front and rear mechanical disc brakes. The brakes, upon inspection, have no writing at all (The manufacturer was too scared to put their brand name on them...).

Since day one, they have been absolutely terrible; Back brake only locks the wheel when I let weight off and the front brake would sooner have me hit a car in front than actually do any noticeable braking. I'm aiming to be able to lock the front brake to the point that pulling the lever makes me flip the bike like on my road bike, and the back brake cause wheel lock-up with merely about 30% action.

Cabling routing and length seems fine to me, brake lever is not bending, nor is mount for it. Caliper mount is nice and sturdy and does not move no matter how much I clamp on the brake. The pads are not half-inserted, nor are they worn out. They're brand new! Pads are not contaminated, pads are equal distances from the rotor on either side of the caliper, cable is tensioned so that the brake is pulled 15% of it's total action before pads contact the rim. The rotor is arrow-straight. The wheel hub where the disc attaches is not damaged in any way nor weakened considerably.

I've never worked on disc brake bikes, so my question is; Do all mechanical disc brakes have this much difficulty stopping? Is there something big that I'm missing? Is there a particular thing causing this? Could the pads be a bit duffed? Is there any type of pad I should try out on my bike (different compound, etc) or any specific modifications I should try?

And finally, are there any really, really risky bodges that I can try out to immediately bolster braking power? (Anything I do on the bike is not your fault, but my own.) Give me a really risky method of making the brakes powerful instantly and I'll give it a shot! :P

Answer 1

Usually what causes this (literally no braking, firm lever feel, lever nowhere close to bottoming) is an error in brake setup or adjustment wherein the brake caliper's armature is being pulled to the end of its range of motion before full braking power is reached, or in some cases before any braking power is generated.

This can happen either as a result of incorrect setup or by compensating for pad wear exclusively by way of the cable barrel adjuster without adjusting the stationary pad.

To determine armature bottoming is the problem, see if you can use your fingers to directly push the armature as far as it can go. If possible, try turning the wheel with your hand while doing so to ascertain no braking is happening. Or just undo the brake cable anchor; if the arm was obviously near the end of its range of movement, that's probably been the problem.

Most mechanical brakes perform their best when adjusted so that the armature starts out near the beginning of its range of motion. The mechanical advantage characteristics of the brake change throughout its travel. So when setting up a brake, always try to get the arm as close to that point as you can while still having all the slack pulled out of the cable.

Usually adjusting a generic mechanical disc brake with this issue is going to involve undoing the cable anchor, possibly installing new cables if the old one either won't be long enough with the arm starting down in the right spot or has more of a crimp than is safe to have in the system (usually mechanical discs are pretty good about the latter point), loosening the caliper mounting bolts, dialing in the fixed pad adjuster, re-positioning the whole caliper more inbound towards the wheel such that the moving pad starts out closer to the rotor, then re-fastening the cable anchor and doing final adjustments.

For a brake with pad adjusters for each pad, resetting the barrel adjuster and extending the moving pad in may fix the acute problem, although it probably won't be enough to get the arm back to its intended starting position.

Answer 2

You've eliminated mechanical/cable problems, and the levers sound fine.

There's some braking effect, so the pads are moving on lever pull.

There are 3 possibilities:

• the brake pads are terrible, either cos they're bad or they have been contaminated
• The rotors are contaminated (which means pads will be too)

Clean them with IPA and possibly replace the front pads.

I doubt the pad gap is too generous because your lever is not bottoming out on the bar.

BUT another thought - a cable-operated caliper is normally one-sided. So one brake pad is fixed and supposed to run very close to the rotor. The other moving pad subtly bends the rotor into the fixed pad. You may need to confirm that the non-moving pad/rotor gap is nice and small.

If its big, loosen the two caliper mounting bolts a bit, shim between the rotor an non-moving pad with a sheet of paper or similar, then hold down the lever while tightening the two bolts. Then extract shim. Your brake may have a lot of room on the other side now, so you need to take the slack out of the inner cable at that point.