Adjusting mechanical disc brakes

[stevie_b]

Hi,
A basic question no doubt, but I've googled it, you-tube'd it and searched on here for it with no luck.

My bike has Shimano M415 mechanical disc brakes. On the rear brake, the outboard pad (the one that get moved by the brake cable) is too far away from the rotor. On the You Tube videos I've seen, this is fixed by undoing the bolt on the caliper that holds the cable in place, moving the pivoting arm up a little (which is what happens when you pull the brake lever) and re-attaching the brake cable at the arm's new position. That certainly positions the pad nearer the rotor, but it weakened my braking power significantly. The effect is very similar to when you adjust cantilever brakes and reduce the mechanical advantage: the brake feels firm when the bike's stationary, but the brake lacks the power to put a meaningful squeeze on the rotor.

As far as I can tell, changing the starting position of the pivot arm shouldn't affect the brake's mechanical advantage.

I've seen a 1-page PDF guide by Shimano (attached) on how to adjust them, but it didn't help me. The adjustments it described didn't seem to move the pad closer to the rotor.

A side question (which might help with the main question): is there supposed to be much tension in the brake cable when the brake lever is at the rest position? Obviously any slack in the cable is bad so it'll need a least a tiny bit of tension, but is it OK to have a fair bit of tension?

 

[Rohloff_Brompton_Rider]

Can't you just use the adjustment screws?

 

[compo]

I have the same brakes. Have you centred the rotor correctly by adjusting the calliper. Then bring the inboard pad in so it JUST touches the rotor, then bring the outboard pad in using a 0.3mm feeler gauge to set the pad gap to the rotor. Then use the gauge to set the inboard pad. The reason I set the inboard pad to touch the rotor initially in to stop the rotor flexing when the outboard pad is bought into adjustment. You don't actually need to set it touching, I just find it easier. One other thought is how worn are your pads?

 

[Sterba]

Compo, do you really mean a .03mm feeler gauge? it would be awfully thin. 0.3mm sounds more likely.

 

[RWright]

I had brake rub on the used mountain bike I recently purchased. I adjusted the wheel position in the frame, aligned it using the quick release skewer..

 

[compo]

Compo, do you really mean a .03mm feeler gauge? it would be awfully thin. 0.3mm sounds more likely.

Oops. You are correct. I put the decimal point in the wrong place. I have corrected my post. Thanks for pointing it out.

 

[stevie_b]

How do you centre the rotor by adjusting the caliper? That sounds like there should be an adjustment that allows the caliper to move along the axis of the wheel hub (thus repositioning the gap between the pads relative to the rotor). I've had a good look at the calipers and at the Shimano instructions attached in the original post, and I can't find any such adjustment.

Assuming I can find out how to do the above, you said I then need to adjust the inboard pad (I'm OK with that) and the outboard pad: is the only way to adjust the outboard pad, to undo the cable, move the pivoting arm to move the pad closer to the rotor, then tighten the cable at the arm's new position? I can't see any other way of adjusting it. The bit about releasing the cable and moving the arm was what I initially did, but that took away most of the power in the brake.

I'll take a look at the wear on my pads, that's a potential cause of all this.

 

[compo]

With the wheel secured in the forks (or frame) the rotor should be central in the gap where it passes through the calliper. If it is the leave it alone but if not then you need to undo two allen screws that secure the calliper to the adaptor. You will find you have both vertical and horizontal movement of the calliper. Move the calliper until the rotor is central within the calliper then tighten everything back up. It can get a bit frustrating when you tighten the screws and the calliper moves, but just persevere until you get it right. You are correct about using the cable to adjust the outer pad. There is a threaded adjuster as well so it could be worth screwing this right down while you have the cable undone before refixing it. If everything is correctly adjusted and you are unable to get a decent pull on the lever I would remove the pads for closer examination.

this is the tech document for the M416 brakes which is the one I use:

http://techdocs.shimano.com/media/t...01/SI-8EA0D-003-ENG_v1_m56577569830696761.pdf