Strange Disc Brake Problem

[MrJamie]

how can that happen? fork drop outs are recessed so that even when the skewer is undone, the wheel cannot drop out.

It's not so much that the wheel falls out, as much as the force from the disc brake being applied pushes the wheel down and the caliper hence left fork upwards almost as if the wheel were rotating around the caliper, as far as I understand. This is one of the best all round explanations and linked to by Sheldon Brown on his Skewers page when he talked about the problem http://www.ne.jp/asahi/julesandjames/home/disk_and_quick_release/#slipping Its basically a massive force levering the wheel out of the dropouts, more than the skewers need to be rated for to meet standards. So everytime you brake on a front disc brake (with caliper behind the fork) its trying hard to eject the wheel from the forks. Even with the lips on the dropouts, supposedly the dropout can sheer or the skewer can fail enough that the force will push it out. Im fairly sure that what mine was doing was pushing the wheel out until it hit that lip, but the lip was doing enough to keep it in. Presumably a suspension fork would be easier to jump out of too.

There's loads of articles about it, but i guess given the proliferation of disc brakes it cant be that common to actually result in serious injury. If you google "disc brakes quick release" youll find countless articles/posts about it and loads of information.

 

[MacB]

I wouldn't sweat it, as long as you give your skewers regular checks then you'l be fine. They can be sneaky buggers loosening even when they look not to have moved at all.

Oh and a +1 to Shimano skewers I was warned off my original 'blingy' choice by the guy that was building a wheelset for me. He recommended basic Shimano and as he was selling me all the kit he was losing a fair amount of markup by nudging me to the much cheaper option.

 

[andrew_s]

how can that happen? fork drop outs are recessed so that even when the skewer is undone, the wheel cannot drop out.

When you brake, the QR gets pushed down until it's stopped by the lip, then when you stop braking it gets pushed back up to the top by your weight. Each time it moves back and forth it tends to turn a little, in the unscrewing direction. Lateral movement is a well known reason for threaded fasteners coming loose. Eventually it will undo enough to allow the brakes to force the QR over the lip unless you've retightened the QR first.

The time I saw a wheel come out there was no restraining lip.

I wouldn't expect that it makes any difference which Shimano QR is used. It's having the enclosed cam design that is the important thing. There are others (eg some Campag), but all Shimano are good, easy to find, and there are cheap options.