More Safety Fears

It seems that there may be a serious an inherent problem with using disc brakes on forks with normal dropouts (as opposed to bolt through axles) that I didn’t know about. According to James Annan using a front disk brake and a fork with quick release dropouts, leads to the disk brake exerting a huge force on the wheel, pushing it directly out of the dropouts.

A tight QR and decent retention (or lawyer) lips generally hold the wheel in place, but not all the time. When it no longer holds the consequences of failure are very nasty. James’ comments followed a spectacular wheel-fork departure, the results of which are illustrated here. More discussion can be found on the following Google group link: uk.rec.cycling from December 2002.

Now when I was planning discs, I knew that the extra braking forces put a lot of stress on the fork, so I opted for a bolt through pair of Marzocchi Z1 Freerides, but all the above discussion doesn’t make it very clear about what happens at the back end of the bike. So as James seemed to know his stuff I asked him – Are the same forces exerted on a rear wheel with a quick release? This is his response:

From: James

Subject: Disc brakes and QRs

Yes, and I’m in the same position as you. I don’t think it’s a big safety issue, but in my case it is a slightly annoying one. It depends on how the dropouts are angled relative to the disk calliper, and my bike (Ventana El Conquistador tandem) is particularly bad, with rearward facing dropouts which are at exactly the worst possible angle for the disc brake.

The maker soon realised the error and changed to more conventional vertical ones. If only major fork manufacturers were so responsive…. That is another reason why I am particularly aware of the disk brake issue – our rear wheel occasionally slips a bit although now I do the QR up bloody tight and it’s been ok for some time.

It’s similar to QRs with horizontal dropouts for singlespeeding – you can just about get away with it, but slippage is a headache (rear wheel slip while honking up a big hill is not a matter of life and death though). I wouldn’t trust my life to the rear wheel not slipping, but its not a big safety issue in my view since even if the rear wheel slips sideways a bit or pulls out completely, this is only going to cause a skid and not a headfirst plummet to the ground (especially on a tandem with the long stable wheelbase).

I don’t think different disk calliper types will make a significant difference, it’s really just a matter of geometry. The open fork ends should point directly away from the calliper itself, rather than being nearly parallel to the tangent.

So there we go. There is always the possibility of problems. And whilst talking of things disc brake related the old favourite of how to orientate the direction rotation of disc rotors came up again recently on the Singletrack forum. Craig settled the physics by saying that strictly speaking the orientation does matter. Although he doubts the stresses involved are anywhere high enough to worry either us. This is backed up by manufacturers Hope – who stated that the rotation direction was down to personal preference.

If the arms of the disc rotor are pointing backwards then they will act as a tie and be in tension. If they point forwards they will act as a strut and be in compression. Steel is more efficient in tension because you don’t have to worry about buckling of the section under compression, so it’s best to run the discs with the arms pointing backwards (or in the opposite direction to the rotation of the wheel).

War on Iraq looms closer. This is going to go down in history as the war for Oil. Why? Find out more here.

Right time for a beer.

Author: Cris Bloomfield

Usually mountain biking in the North.

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