Wiggly disc brakes?

I thought that the headset on my MTB was dead when I did the good old brake test and tried to wiggle the wheels back and forth. However, my dear cyclist hubby tested and he just found out that the headset's fine, it's the wheel that moves. Quite. So the front disc brake obviously doesn't lock the wheel properly? No. The brake locks the disc, yet when wiggling it the wheel moves a tenth of an inch or so back and forth by the rim. However, the movement is not even visible in by the hub. Weird? The brakes are firmly fixed to the fork, the wheel is properly mounted and the bearings in the hub are tight. :confused:

There seems to be some sort of adjustment that isn't right, but what adjustment could it be? We're noobs in the disc brake department, and we believe it has something to do with the brakes, mainly because they're the only component that we aren't fully familiar with...

Any ideas?

The hub cones are loose. Remove the wheel from the bike, grab the axle and move it side to side, back and forth. You'll feel the axle moving relative to the wheel. The cones need to be adjusted. Should only take 10 minutes to make the adjustment if you have the tools and know how to do it. But overhauling the hub with new bearings and grease might be recommended as well. Drop the wheel at your LBS to get this done right away. You damage the hub by riding it like this.

OK, that's worth checking more closely!

However, wouldn't loose hub cones make the wheel wiggly sideways as well? DH reckons the wheels are fine, and he's normally quite careful with his bike maintenance, but he never took the wheel off, and so you might be right. If so, even he gets to learn something new about bike maintenance!

Can you wiggle the rim side-to-side with the brake not engaged? Or do you mean that the rim moves forward and back when the brake is fully engaged but otherwise has no play? If the latter, it's probably the disk attachment to the hub. Either the lock ring that holds the disk has become loose or the hub flange or disk itself are damaged.

The latter. We're on it now!

I'll keep you posted - thanks a lot!

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Originally Posted by Trigress

[...] my dear cyclist hubby tested [...]

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That'll be me.

The disk is firmly attached to the hub, and there is no visible damage to the flange or hub. Also, the brake body is firmly attached to the fork. Trust me on this. When I say there isn't a wiggle, there isn't one. When I say there is, there may be - or it may all be in my head. :o (Trigress sometimes employs pop-psychological jargon about this...)

Something seems to move ever-so-minutely within the brake body itself. And it's directional, meaning that repeatedly locking and unlocking the brake whilst moving the wheel in one direction only produces a wiggle on the first locking. Then there'll be a new wiggle when I change direction. It's as if the slack needs to be taken in. In contrast, the rear brake is quite tight.

So what's in there? And should I try to take it apart?

There are 2 removable brake pads. You could see if one or both of the pads has wiggle room in it's seat. Each pad had a metal handle that you can grasp with a pair of pliers to remove the pad. There is a spring that holds the pads in place. If that's not it, then I'd guess you might have to replace the whole brake. Is it hydraulic or cable-activated? I've never known anyone to disassemble and repair a disk brake caliper, and parts may be hard to find, but maybe it's possible.

Ah! I'll have a look at the Shimano tech-site for graphics. The brake is hydraulic - which is a bit more intimidating than wires, I admit. Hopefully, it's just the pads...

since Deb didn't get back to you today - am I reading you right that when you "pump up" the brake it works fine? Did you bleed the line?

Don't think that's it. If I press the brake, it locks the disk immediately. No slow response or weak breaking power. No pumping necessary. However, there's still some play back and forth.

What I mean is that if the brakes are applied and released repeatedly, the wiggle only occurs on the first locking in either direction. Change direction, and it wiggles the first time the brake locks - and only the first. Keep the brake pressed and move the wheel back and forth, and it wiggles every time.

It seems whatever-is-loose stays in its forward or backward position until it's pushed to the other.

Brake pads are still suspected. We'll look into it next week.

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Originally Posted by Triger

The disk is firmly attached to the hub, and there is no visible damage to the flange or hub.[...] Trust me on this. When I say there isn't a wiggle, there isn't one. When I say there is, there may be - or it may all be in my head. :o (Trigress sometimes employs pop-psychological jargon about this...)

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Well, it turns out that this might be applied the other way too - when there isn't a wiggle, there might not be a wiggle, or the lack of wiggle may be only in your head. :) There is an infinitesimal chance of your hearing this again, dear.

After a fair amount of dismantling, wiggling of various bits and bobs and tightening of various bits and bobs, it finally turned out that the disc was in fact a little wiggly. It should be mentioned that this wiggle was undetectable by wiggling the disc, wheel unmounted and using only bare hands, we could only feel it and see it when applying the brakes and then wiggle the wheel. Anyway, we tried to tighten them to the wheel, but the wiggling persisted. There seems to be a slight wiggling room between the locking teeth on the wheel and the locking teeth on the disc.

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Originally Posted by Trigress

It should be mentioned that this wiggle was undetectable by wiggling the disc, wheel unmounted and using only bare hands [...]

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...It really should! I feel half excused :o

The larger question is, is it the disk or the hub? Hopefully it's the disk. I'd definitely try to fix it soon, because even if it's the disk, it could be wearing the hub and eventually require you to replace both hub and disk.

By your description, I wonder if they are Shimano brakes? If so, you should be sure to check that the connection where the disc mates with the hub doesn't some sort of wear problem. If it's a Shimano system, then the disc locks onto the hub similar to a cassette locking onto the freehub. If any wear is present there, then you'll need to address that. I have seen a number of these systems, and it definitely was a design flaw from Shimano. You should have your LBS take a look at it before it becomes a bigger issue.

It is indeed a Shimano system, and your description is quite accurate, it's just that I didn't know the words (it is most definitely a handicap not knowing these parts names in English...)!

Will it be enough just to buy a new disc or is there something else I need to have sorted out?

No, buying a new disc will not help. The problem seems to be that the connection with how the disc and the hub mate is loose... it's very similar to having a loose hub. I would definitely have your LBS take a look at it, you could develop wear in that area and cause all sorts of other problems. If you are trying to take care of it yourself... then you'll need to pull it all apart and tighten everything. Let me know how it goes!

I have already taken it to bits and tightened everything, with no effect whatsoever... There is simply too much air in between the disc and the hub, and I can't really see how that can be mended - unless I get a hub and a disc that fits snugly. I'll see my local bike repairman about it and let you know what happens!

It will be interesting to see what your LBS finds. It almost sounds as though you are missing a spacer, if you have that much room. Time will tell:):)