Drilling a fork blade for a mount point
 

Like this:

https://cimg6.ibsrv.net/gimg/bikefor...026ba7c5da.jpg

I can drill a neat hole for a M5 bolt, it is not something that I would attempt to do free hand with a drill. I figure if I were to ask in the regular bike forum the standard response would be that the fork would shatter and shrapnel would hit small children 50 feet away. I'm hoping that wouldn't be so.

No reason why a hand drill can't do this or that a fork would spontaneously explode. But I don't suggest installing anything in that hole that requires any amount of securement, like a rack. Most every mid blade boss I have seen used some sort of braze on to best handle the forces.

The simplest, and likely most common, way is a hole drilled in the outer side of the blade and a bottle cage boss brazed into this. Sometimes with a star shaped reinforcement.

Still having a drilled hole in the blade is the through hole method. The hole is drilled through both the outer and inner side of the blade and a tube, usually unthreaded, is brazed into the blade. This came about with the "over the tire hoopless" low rider racks (Blackburn comes to mind) and a simple long bolt goes through the tube with a common nut securing it all.

A no blade hole method is to use a hourglass shaped seat stay rack boss placed on it's end for the low rider boss. This provides some stand off for the rack, from the blade, and the method I have used a number of times.

In each method the clamping forces are not trying to crush the blade, as would be the case with what I read the OP's idea as being.

Back when brakes were nearly all bolted on calipers some bike companies did a poor job at making a rear brake bridge mounting. Many companies would just through drill the brake bridge and using washers/spacers with a curved side clamp to that bridge tube with no hole reinforcement. Sometimes this worked well, the bridge tube was of sufficient strength to not crush down (which would then let the brake caliper rotate from the brake forces), sometimes less so (Raleigh had a reputation for this issue on their basic bikes). But the result of a rear brake not working or locking up (either end of the range of what we call function) is of less potential than that of a fork blade beer canning.

I strongly suggest not drilling a rack mounting hole that won't also be reinforced somehow and that won't create tube crushing pressures from the hardware. Using U or P clamps is a far safer method. If this isn't what you want then either shift your goals, get a fork with said braze ons already in it or learn how to braze (and paint...) Andy

Go for it...what could go wrong...

I wouldn't drill a 5mm hole for a 5mm bolt and leave it without reinforcement. It's not trivial to see, but there is reinforcement evident in OP's picture. If it's a good bike, you'll crush the fork blades with an unreinforced through bolt.

The sad thing is that I did other brazing work on this frame but forgot to do this bit. The frame is already powder coated :-\

Sounds like I can probably get away with it better if I fabricate some long concave aluminum washers in the shape of the fork blade. That way the pressure would be distributed over a longer area.

The thing is, the same answer can be given to anybody thinking about riding a bike on a road.

I'd consider stripping enough powder coat to enable drilling and brazing in an insert and then seeing what your powder coater can do blend it all in. Forks aren't something to be taken lightly - especially when drilling a hole through the tube. It needs the internal support. You could also just use external clamps perhaps?

The correct way to do this is to strip the powder from the fork blades and braze in a reinforcement or don't drill a hole and braxe on a section of tube to the front or rear of the blade. Powder coating a fork is not that expensive and having a hole through your fork blade, without a brazed in reinforcement is dangerous.

Your aluminum insert idea would protect against crushing, but it still leaves you with a hole through the fork and the insert does not provide sufficient support to prevent the tube from collapsing around that hole. I don't have any proof that this will happen, but I wouldn't be comfortable with doing it that way.

The snag is that I'm in Alaska, I ended up having to ship the fork and frame to the lower 48 to get it powder coated and that alone was not cheap. After many many many delays the bike is finally assembled, I'm not willing to take it apart and start burning even more money for this. Not on this interaction. Another snag; I'm about to fabricate custom racks for this bike, I rather build them from the get go as a 6-point bolt on (canti studs, mid fork, drops) plus over tire stiffener loop. Sounds like a fabricated long washer from billet aluminum would take care of not adding additional crushing stress on the blade, it can be made to hug a larger area.

Maybe I have lived a sheltered life but I have never seen a steel fork having a catastrophic failure without an ungodly amount of either rust of blunt force trauma. Obviously a fork without a hole is going to be relatively stronger than one without, but I use the word relative because in practicality it should be strong enough, as most are already overbuilt for their intended purpose. I do appreciate that folks want to have an overabundance of caution; Cino Cinelli himself was leery of making handlebars out of aluminum instead of steel. Everybody that has been at this long enough remembers how folks thought carbon on a bike was suicidal.

I did a whole lot of searching for pictures of steel fork failures, I could not find a single one showing a failure from this source. Interestingly enough it seems that counterintuitively, a hole lower on the blade would be less stressful than a hole in the higher portion of the blade. Most fork failures from blunt force trauma are most likely higher up the blades where there is even more steel because the higher you go, the more leverage the force have to bend it. Sort of like the same way that is easier to bend and break a stick from the middle rather than closer to the end.

I've broken a fork and know just how much damage to me that can do. (I was lucky. I only lost thousands of dollars, years of recovery and my profession.) When I ordered my Mooney a year after my accident (Peter Mooney was a clubmate and at the club annual dinner when it was announced I was in a coma) I asked for braze-ons for LowRiders. He wouldn't. Wouldn't drill a fork blade. First 4 years, the Mooney wore the U-bolts every time the LowRiders went on. Bent that fork and ordered another. (Crash, the Lowriders weren't on the bike.) Asked again about the braze-ons. Peter said it had occurred to him that the English light/generator mounts could be used; no drilling required and that he'd be happy to do that. And they've been on ever since. I have to drill out the racks for the larger bolt but that isn't exactly hard. A round file also works.

That's because the owners are too embarrassed to admit they did such a thing, and it's too much effort to post a picture using a mouth stick. As far as your stress analysis, it doesn't really matter that a fatigue crack is more likely to form elsewhere if it's likely to form where you drill your hole. "I know it broke, but it could have been worse."

Depending on the blade, steel forks are a bit marginal. That's why cheap bikes have aluminum or carbon forks. Forks undergo a lot of cyclic stress. Your hole quality will be bad, and that is more likely to promote crack initiation. Use a clamp.

If there is concern about matching the color, OP could always paint the area a contrasting color after the brazing, so it looks intentional. I'm not a framebuilder, but brazing on some rack bosses like these seems much smarter than simply drilling holes through the fork blades: https://framebuildersupply.com/colle...ses-pack-of-10

Having been on two bikes that had their forks break I guess I am a bit conservative when it comes to forks. And my brakes. Andy

If making a custom rack, maybe consider the Jim Merz style from the '70s:

https://cimg6.ibsrv.net/gimg/bikefor...a13792f3e0.jpg

Not the best picture, but if you look closely you can see the clamp on the blades started out as a close fitting hoop with two pinch binder braze-ons, which is then split. This is why Merz racks couldn't be made "on spec" and sold for people to fit to their own bikes; the fit of the clamps is unique to each bike. The hoop even has to be made to match the taper of the blade at that point, it's not a parallel-sided cylinder. It might take Merz-level fabrication skillz to make the clamps fit well enough, but this design is extremely well-proven, including around-the-world fully loaded camping with all the weight on the front rack.

Mark B

Not on this day and age. Posting stupidity is rewarded on the Internet :-/

That is incorrect all around; the industry moved to aluminum because it was cheaper to produce, not better. If anything aluminum and carbon are inherently prone to cope with stress poorly. Also there is no reason to believe my hole quality will be bad specially because I use reamers to finish taping holes on work like this.

If you don't mind sharing how did they break?

Honestly all fork failures I have seen are the consequence of hitting or getting hit by something. The fork breaking did not add much to the immediate danger of the crash. In some cases you could have argued that the fork should not have done that but the crash itself was still the number one risk to life and limb.

There haven't been enough threads like this around here lately. Another thread I enjoyed recently was on a musical instrument forum. A guy was arguing that the earlier version of a particular bass amplifier was superior to a subsequent version because the latter used updated and (in his mind) inferior technology. He persisted in arguing even after being informed by another poster that both of the amplifiers had been designed by the guy he was arguing with.

Reminds me of a time, years ago when I was much less careful in posting online, when the Jim Kelley FACS amplifier came up on a guitar forum. Perhaps you know, but for anyone unfamiliar, it ran its 6V6 output tubes at an extremely high plate voltage and spec'd a particular make and model of tube for best reliability. I blurted something about the design being "insane." Then, Jim Kelley himself popped up and replied that he thought it was a reasonable design, and had a good record to back that up. I decided to eat my words without delay. :o

The thing is I'm just asking a question, I'm not even defending what I'm asking. But if somebody wants to say that it is a bad idea I will most certainly ask why they think so and point out if their reasoning doesn't make sense. Like claiming that nowadays we use carbon and aluminum forks because they are stronger than steel forks.

#1 was on my balloon tire bomber in the mid 1960s, a Rollfast. The steerer broke at the threading, thankfully just above the stem's wedge. Can't say much more as I was only about 11 years old, the shop that replaced the fork is where I ended up managing about 10 years later.

#2 was on my first Fuji Finest, the 1973 model bought with my first checking account. The right fork dropout fractured/cracked between the slot and the blade end, much like the common Campy rear dropout failure and likely due the same reasons (too much heat during brazing, undercutting the blade end during the filing/grinding and chrome plating). This bike got a "warranty" replacement fork that in hindsight I suspect was the dealer's way of getting rid of me and likely didn't come from the Fuji distributer.

Over my years of shop work I have seen a number of fork failures. Some from poor design and/or poor manufacturing. A lot from rider "error". Andy

What's that upper bolt on the right fork blade ??

If I was forced to drill a hole for a rack, I'd braze a tube through the hole for reinforcement.
I like the braze-on bosses ThermionicScott linked to. No holes needed.

If you don't want to repaint it maybe make something like a "p-clip" that just goes around it. Just a strip of sheet metal and some kind of plastic or rubber coating.

Steel forks don't pass the tests that forks are subject to nowadays unless they are really heavy. Those tests are a bit extreme, but not after you start drilling holes. And you have a 5mm reamer?

I took notice of this "because I use reamers to finish taping holes on work like this." and read it as a tapping reamer, which I have never seen before.

My view of the industry going to Al and/or carbon fiber forks is about weight and stiffness. Both very easily measured and felt by the perspective buyer/rider, unlike strength (however the larger in cross section nature of most of these forks surely are perceived as being stronger too). Andy

It's the CE tests that production bikes have to pass. The fork tests in particular are totally unrealistic, and favor a carbon fork or an overbuilt aluminum fork. A steel fork that you would actually want to ride is not going to pass.