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It’s taken a lot longer than expected but, at last, Bikemagic editor Mike Davis has published the piece he’s has been promising to publish for some time. Davis has been a regular contributor to the various QR/disc brake topics on the BikeBiz.co.uk bulletin board but has waited before ‘going public’ with an article of his own.
Part of the reason is no doubt the web-savvy nature of (probably) the majority of BikeMagic readers. They will be well aware of Annan’s theories: Usenet newsgroups have been awash with claim and counterclaim for many weeks.
Many consumer sites have also been lifting huge chunks from the various QR/disc brake articles published on BikeBiz.co.uk.
And the bulletin board of Singletrackworld.com has also been abuzz with QR/disc brake discussions, especially as one of Singletrack’s best known contributors was disabled in a bike crash, cause as yet unknown but which has been linked to Annan’s theories.
The Bikemagic report is flagged with a danger symbol: an exclamation mark within a red triangle. It’s titled ‘Are quick releases safe?’
Here are some choice quotes from the article, with the full link below:
Conventional wisdom says that a quick release skewer doesn’t have to work all that hard on a bicycle. The weight of the frame and rider sits on the axles, so all the QR has to do is to support the weight of any wheels that may find themselves off the ground. But, says Annan, conventional wisdom doesn’t know about disc brakes…
Back in the comfy world of conventional wisdom, front wheels can only come out if the front quick release isn’t properly secured. It’s not an uncommon problem with novice riders. But many of the people reporting certainly weren’t novices, having years of riding experience and certainly knowing how to operate a quick release skewer. Engineering theory shows that repeated transverse loadings on threaded fasteners – like the nut end of a quick release skewer – can cause them to become loose. Additionally, heating and cooling cycles can have a similar effect due to the threaded parts expanding and contracting. So, Annan says, what happens is this. Due to the repetitive loads placed on the axle by braking, the QR skewer gradually unwinds itself. Eventually it unwinds sufficiently that the downwards force from heavy braking is sufficient to pop one end over the retention lips – and remember that the brake calliper is only on one leg and thus applies more force to that side of the axle. With one end out, it takes very little for the other end to come adrift too, and down you go. Well, we’re pretty convinced that an approximately vertical dropout and rear-mounted brake calliper is a poor design which leaves the quick release and retention lips having to do things they were never intended to do. The whole dropout/skewer system has gradually evolved, and its presence on modern bikes is really just a convenient hangover from bikes passim. A subtle redesign of the fork would remove the possibility entirely… These aren’t things that we can do ourselves, though. It’s something that the manufacturers would need to address. Trek have said they’re investigating, although it’s the legal department that’s involved. Which is not Trek saying, "Here’s a problem that we need to fix" but "Is there a problem here that’s likely to lead to legal action?" Of course, it’s all a rather interesting legal paradox. Say that a manufacturer decides that this is a big problem and redesigns something to make the problem go away. The new design is safer, but by redesigning it they’re effectively saying that the old design wasn’t as safe as it could be and leaving themselves open to legal action. What’s pretty clear is that having the open end of the dropouts roughly in line with the forces on the axle under braking isn’t terribly clever. If the skewer breaks or comes undone for whatever reason, having things arranged so the axle is driven downwards under braking isn’t what we’d call fail-safe.
If any engineer sat down with a clean sheet of paper, they wouldn’t come up with a design like the one we have for holding wheels into disc-equipped bikes. The forces of tradition and the demands of backwards compatibility are strong. But generally, it works as long as the various components are satisfactory. There’s really no rational objection to coming up with something failsafe, though, and we wouldn’t be at all surprised to see some fiddling with dropout angles and calliper position in the near future. Clamp-up through-axles obviously make the whole thing a non-issue, but they’re currently aimed at the DH market and are fairly beefy as a result. Perhaps someone’ll come up with a lighter, smaller, XC/enduro version?
