Drilling a fork crown "brake" hole...
I have a Co-Motion Americano steel fork on my Airborne Carpe Diem. When I bought the bike (in 2001), that fork had bosses for V-brakes and consequently no hole in the fork crown.
Recently I've gone to dynohub-powered lights (B&M IQ-X). I currently have the headlight mounted on the left-side brake boss, but I would like to mount it on the fork crown. These are my questions: 1) Is there a structural reason *not* to do this? 2) Is 8mm the usual diameter for such a hole? 3) Is there a "standard" position for the hole? I'm thinking of making a calculation from the axle center to a place on the fork crown dictated by the wheel radius (at the brake track) and the brake reach. Alternatively, is there a standard relationship between the V-brake bosses and the fork crown hole? 4) I'm assuming that the axis of the hole is at 90 degrees to the axis of the steerer. 5) Would I be better off taking the fork to a frame-builder for this? I also have access to a machine shop that I trust. Thanks for any advice! Steve https://cimg6.ibsrv.net/gimg/bikefor...36c253ea3a.jpg Co-Motion Uni-crown steel fork needing hole. |
Mark B |
Thanks, Mark!
Looking at my own image (D'oh!), I see the bolt holding the fender to the rear of the fork. That hole is probably in just the right spot and could be extended. :thumb: |
2- 6mm is very close to 1/4" and that's a lot more common in drill indexes here in the US. Andy
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Originally Posted by Andrew R Stewart
(Post 22353368)
2- 6mm is very close to 1/4" and that's a lot more common in drill indexes here in the US. Andy
* Andrew knows this but not everyone here will: I have to say "often" because it varies, depending on the tolerance on the bolt, whether the bolt is all threads or has an unthreaded shank, and even depends on how sharp your drill is and your drilling technique. Drills can make holes bigger (or smaller!) than the size they claim to be. I'll admit the difference between 15/64" and 1/4" is hardly worth mentioning for this job. It's more of an aesthetic preference of mine, than a real engineering issue. Especially for a "mere" headlight/fender mounting hole, as opposed to a brake mount. I just like removing less metal when I can, as a general rule. You can always drill it bigger later. Mark B |
Instead of driving a hole that requires a nut and bolt, one could just tap a hole M5 or M6 on one side only. That way only one bolt without a nut and washer on the other side can hold whatever. I have also sometimes countersunk the hole (even though it is not for a brake bolt) because the same aesthetics apply that define why a recessed bolt head looks better.
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I’d put center of hole 1.5 times diameter of hole away from the edge of the steel.
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I don't think you'll find too many fork crown brake holes with proper edge distance, but that's been going on for over a century now with no ill effects.
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