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Drilling a fork crown "brake" hole...

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Drilling a fork crown "brake" hole...

Old 12-27-21, 03:45 PM
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sweeks
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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


Co-Motion Uni-crown steel fork needing hole.
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Old 12-27-21, 04:09 PM
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  1. I haven't examined your fork but I can't think of any reason it wouldn't be OK structurally. Millions of forks very similar to yours have such a hole without problems.
  2. 6 mm, not 8.
  3. I'd place the hole basically as low as you can, just up from the bottom edge enough that the washer or light mount will be on the steel, not "underhanging" off the bottom, which would impinge on your fender. The higher the hole, the more likely it is to become a structural problem, such as a stress-riser that initiates a fatigue crack.
  4. Yes
  5. A machinist or framebuilder will do it better than a typical bikeshop bozo, but it's not that difficult, nor too critical, if it's just for a light. You're not going to mount a brake there, right? A decent home-handyman with a drill press is usually good enough. Some have even done a respectable job with a hand drill, though that's risky for a typical amateur.
Good luck!
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Old 12-27-21, 06:10 PM
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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.

Last edited by sweeks; 12-27-21 at 06:15 PM.
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Old 12-27-21, 08:10 PM
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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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Old 12-27-21, 08:57 PM
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Originally Posted by Andrew R Stewart View Post
2- 6mm is very close to 1/4" and that's a lot more common in drill indexes here in the US. Andy
Nit-picking: in the absence of a 6.0 mm drill, I'd try drilling it to 15/64" (5.95 mm) and test-fit the bolt, before drilling to 1/4", if the extra wiggle room isn't needed. A nominal M6 bolt will often* fit fine in a hole drilled 15/64".
* 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.

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Old 12-28-21, 01:07 AM
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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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Old 12-31-21, 11:37 AM
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Id put center of hole 1.5 times diameter of hole away from the edge of the steel.
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Old 12-31-21, 02:24 PM
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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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