UKH

cable disc brake trouble

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 G. Tiger, Esq. 14 Aug 2020

Hi all,

I'm trying to fix up my neighbour's bike for him, and have got to the brakes. Some sort of Tektro cable actuated disc brakes. The front works fine, but the back one is awful, so I'm trying to work out what the issue is. t doesn't seem to grip - even spinning the wheel by hand and putting the brakes on doesn't stop it immediately, let alone rolling on the road with a fatty like me on it.

I've burned off all the crap from the pads and given both them and the rotor a good clean. no help.
I've swapped the front and back pads to see if they are the problem - they are not. Front still works well with rear pads and rear still shonky with the front ones.
I've tightened the cables to try and get more purchase, but no luck. So I am after suggestions.

Replace the cable? and the housing/ferrules?

Get into the guts of the calliper and do something? do what? hose it down with WD40?

Anything else? New brakes is an option, but if I can fix it I'd prefer that.

Ta

GTE

In reply to G. Tiger, Esq.:

What lever and caliper? Specifically is it a road lever and a MTB caliper?

 LastBoyScout 14 Aug 2020
In reply to G. Tiger, Esq.:

Assuming they are of the more common type where the lever only activates the outer pad, has the inner pad been wound forward to compensate for pad wear/is the rotor hitting the calliper and not the pad?

There will be a thumb wheel/hex key somewhere to do it - you might need to do it through the spokes/take the wheel out to get at it.

Random thoughts:

- are the brake disks the same type? I've seen an issue with cable callipers and disks with particularly wavy edges where the pad can "wobble" on the edge of the rotor. Feels like the brake "pulsing" at low speed/pressure, but should bite enough to stop if you pull harder.

- As it's rear, I assume you haven't got an incorrect setup with a 180mm disk mount adapter and a 160mm disk?

- if not used for a while, would be worth checking the cable and outer for rust and lube them, but probably not the issue if it's actuating the pad.

Post edited at 16:41
 LastBoyScout 14 Aug 2020
In reply to beardy mike:

> What lever and caliper? Specifically is it a road lever and a MTB caliper?

I didn't think that made any difference in "most" circumstances, but happy to be corrected.

 G. Tiger, Esq. 14 Aug 2020
In reply to beardy mike:

It's a full suspension mountain bike, but looks to be made out of scaffold poles, so definitely not top end. I don't think it has ever been upgraded in any way.

Caliper is Tektro HO, lever has no identifying marks, but I would guess that it came with the caliper.

If I pull the arm off the caliper and use a spanner to turn it, the brakes work well. I'm going to replace the cable and see what happens.

GTE

 G. Tiger, Esq. 14 Aug 2020
In reply to LastBoyScout:

Good thought - let me get my allen keys

In reply to LastBoyScout:

Absolutely does because a road lever has around half the pull due to the position of the cable in the lever. It means that if it's an MTB brake caliper there is also around half the leverage compared to a v-brake/mtb lever. But sounds like it's not a mismatch anyway. If it's an MTB style lever it's highly unlikely to be a non v-brake lever. Could just be that it's a junk caliper. Most cable pull calipers are...

Post edited at 17:08
In reply to G. Tiger, Esq.:

Next think I was going to ask was about the static side pad. If it's wound too far out due to wear, when you pull the lever it won't pinch the disc... 

 dovebiker 14 Aug 2020

Could also be gummed up due to brake pad dust and any other dirt. Remove the caliper and pads and see if you can give it a good clean and ensure that the actuation lever / piston works across the full-range. The internal mechanism is very simple if you need to take it apart and clean the insides.

 nniff 14 Aug 2020
In reply to G. Tiger, Esq.:

The fixed pad can be stiff as hell - it may have a thumb wheel to adjust it, with an allen key hex in it.  THe thumb wheel is usually too stiff for words,.  A bit of brake cleaner and winding it in and out a few times with the allen key will free it off.  Wind it in until it just touches the disc, then back it off a bit.  Then adjust the cable to bring the moving side in

gezebo 14 Aug 2020
In reply to G. Tiger, Esq.:

If it’s a crap bike it’ll have crap cables. I’d put new cables on, particularly outer and you’ll probably find that with reduced compression from the outer when loaded it’ll be much better. A £5 fix. 

 G. Tiger, Esq. 14 Aug 2020
In reply to LastBoyScout:

I adjusted the inner pad and it's working like a dream. Thanks everyone.

New cables/housing on order anyway so might do that as well when the time comes.

Front derailleur indexing next, but looking at the state of the chain rings, I think it might need a whole new drive train. The chain sits on to of the teeth rather than dropping into the gaps.

Gte

In reply to G. Tiger, Esq.:

How many speed is it? If you want a decent range and are buying a new drivetrain but not new shifters, I've just bought a 1x9 drivetrain for my boys bike with a Ztto 11-40 cassette aft 15 quid, a second hand 9 speed derailleur (any 9 speed one will do if it has a long cage) and a single ring up front. THere are IXF chainsets including a ring for about 30quid on fleabay (square taper BB so cheap bb if needed) then you wont have to worry about the front derailleur and you can bin the shifter. The only hing you need to watch when you install is that you wind the B-tension screw all the way in to allow the jockey wheels to clear the cassette as usually the derailleur will say 36 tooth max, but it'll do 40 if you push it...


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