Discussion
Been given a Trek as a commuter bike. Needed some tlc but pretty much got everything sorted now, with the exception of the rear cable disc brake.
I've put in new pads and cables (inners and outers), front brake is now reasonably good but the back is shocking. I've adjusted it to the extent that the pads are dragging slightly but still I can pull the lever all the way to the bars.
I've used the same decent quality cables and pads on both the front and the rear so why is the back so poor?
Does anyone have any advice? I'm not up to speed with cable discs, it were all v-brakes or hydraulics back in my day....
I've put in new pads and cables (inners and outers), front brake is now reasonably good but the back is shocking. I've adjusted it to the extent that the pads are dragging slightly but still I can pull the lever all the way to the bars.
I've used the same decent quality cables and pads on both the front and the rear so why is the back so poor?
Does anyone have any advice? I'm not up to speed with cable discs, it were all v-brakes or hydraulics back in my day....
Accelebrate said:
Are both pads touching the disc, or just the one that's attached to the equivalent of the piston/the moving side? The other pad is often only adjusted by winding it in or out using an allen key on the back of the caliper.
This is key.For the brake to work, BOTH pads have to end up squeezing the disc (obviously).
In your case you could have the moving pad touching the disc all the time but the other pad miles away, hence no braking.
Or you could have the stationary pad touching but the moving one miles away, hence no braking.
If Accelerbrate's adjustment doesn't work (a few clockwise notches with the allen key) then you might need to move the moving pad closer to the disc which means tightening the brake cable at the caliper.
How are you on physics?
Levers, forces etc?
As well as the getting the cable tension correct you also need to consider that the power of the brake will be affected by the orientation of the 'lever' (the little arm that spins round with the brake cable clamped to the end of it) at the point when the pads make contact.
For optimum power you need to make sure that the bite point occurs when the lever arm is normal to the axis along which the cable is pulling (at right angles to it). An acute or obtuse angle between cable pull and lever (at bite point) reduces power.
This is similar theory to how the pwoer adjusters used to work on the old Avid brake levers - loads of power but a bit wooden feeling, or a bit more progressive but ultimately less power.
Cable disc brakes can be good but are a royal pain in the ass to setup correctly.
Levers, forces etc?
As well as the getting the cable tension correct you also need to consider that the power of the brake will be affected by the orientation of the 'lever' (the little arm that spins round with the brake cable clamped to the end of it) at the point when the pads make contact.
For optimum power you need to make sure that the bite point occurs when the lever arm is normal to the axis along which the cable is pulling (at right angles to it). An acute or obtuse angle between cable pull and lever (at bite point) reduces power.
This is similar theory to how the pwoer adjusters used to work on the old Avid brake levers - loads of power but a bit wooden feeling, or a bit more progressive but ultimately less power.
Cable disc brakes can be good but are a royal pain in the ass to setup correctly.
Edited by snotrag on Wednesday 10th February 10:37
I have some old Hope M4's but the seals have gone and I'm loathed to spend too much cash on a commuting machine at the moment - I have other bikes I want to spend on instead! 
I'll check the positions of the pads; I think they're both equidistant from the disc, definitely worth checking again tho.
Levers match and the front is ok so sure it's not them. Ideally I'd get rid and swap with V-brakes but that would mean replacing wheels which I can't do.
I'll check the positions of the pads; I think they're both equidistant from the disc, definitely worth checking again tho.
Levers match and the front is ok so sure it's not them. Ideally I'd get rid and swap with V-brakes but that would mean replacing wheels which I can't do.
Whole bike cost me about £25, including new pads and cables.
What am I talking about, the new pads and cables cost £25, bike was free! Did my fist commute on the bike this morning and actually I can see why people like cabled discs. Ok, they don't have the same power as hydraulics but they felt remarkably progressive in the light dusting of snow we had. I think a bit more time and effory setting them up (I think it is the second pad that needs adjusted) and they'll be ok.
I'd not have them on my 'proper' bike but for this purpose they're fine!
What am I talking about, the new pads and cables cost £25, bike was free!
Did my fist commute on the bike this morning and actually I can see why people like cabled discs. Ok, they don't have the same power as hydraulics but they felt remarkably progressive in the light dusting of snow we had. I think a bit more time and effory setting them up (I think it is the second pad that needs adjusted) and they'll be ok.I'd not have them on my 'proper' bike but for this purpose they're fine!
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