abdon 

Bad terminology on my part, sorry about that. Simply using a hand reamer to clean up holes made by an undersized drill bit. On something like this creating a perfectly calibrated hole is secondary to removing any jagged edges and stress.

For the record I'm going to end up fabricating clamps but honestly most of the people against the idea don't really provide much to support their position. I'm not saying they are wrong but they are not offering much to prove that they are right either. There were some good comments on the importance to avoid crushing forces and vibration that would create unduly stress in the hole.

bulgie 

Yeah I remember when the Oregon framebuilder's org, (OBCA I think) made a fork-testing jig to the European standards for Oregon FBs to use. They tested a perfectly good steel fork made of, I dunno, Columbus SL or some such. The kind of fork that typically goes 70 years or more of continuous riding and which has won all those Tours de France and every other race, has been used for loaded around-the-world touring etc etc. Needless to say the fork bent on the first "wham". Carbon forks pass that test, but we've all seen the pictures of those forks with both blades snapped off while JRA, so the test is not useful IMHO.

Here's one of my favorites,


Adam Yates in a solo breakaway in a Classic, ~1 km from the finish and almost guaranteed to win, crashed because his fork snapped from hitting a pot-hole. Yes, the fork breaking preceded and caused the crash, not the other way. Being a true pro, he couldn't talk about it, but I can imagine what his opinion was of that maker!

Mark B

guy153

If you remove material you make things weaker. The fork is going to be weaker. Fact. This position is easily supported. Is it going to be too weak? You will probably get away with it. But it's up to you. I would not want to take the risk myself.

abdon 

Again; respectfully, that doesn't really says much. Take your entire statement and replace "hole" for "light fork". The statement is equally correct but doesn't address whether it is significant enough.

And once again; I'm not argue to defend the point. I'll be the first one to tell you "I don't know". Heck; will the fork be prone at breaking here during a frontal crash? Even at the lower leverage point it builds a point where this outcome is most likely but on the other hand the crash itself takes the cake, not whether the fork broke post facto.

Doug Fattic 

Some of us like myself are professional builders. We make a modest amount of money and never want to jeopardize our business with any kind of unnecessary risk. Neither do we have the funds or time (which is the same as money) to go about testing something to find out where is the limit before something breaks. We are smart enough to realize that drilling holes in a fork weakens it and increases the possibility for it to fail. And if it does fail the consequences can be severe. In this era of everything can be online, all a potential customer has to hear is that a fork broke and they are lost as a customer. And if the builder is sued it is unlikely they will ever recover. It only makes sense to build in enough safety to insure nothing bad happens. So the sensible approach is to never do something like drill holes in forks.

in Albon's case he is taking the risk himself. He doesn't need to worry about marketing and the likelihood the fork will break isn't high so it might be worth the risk to him while it wouldn't be for us. One of the risk factors in the will-it-break formula is the wall thickness of the fork blades. They can vary from .9mm to 1.2mm. The thicker they are the less the chance of a catastrophe.

guy153

My point is really just that the "burden of proof" is on the guy drilling the holes. When I make a fork I buy the blades from Reynolds on the assumption that they've done the math and they're strong enough for the intended purpose, even if they are light. If I plan to make modifications to the design like removing material it's up to me to work out whether that is going to be OK. The same if I was making something less well-known than a bicycle-- I would have to work out what tubes to use and estimate the safety margin.

These links might be some places to start, but it seems like quite a complex problem. Especially if you have a bolt on both sides squeezing the tube. That will add some residual stress, which will be in tension in some places (the front and back of the tube I guess) and this might lead to fatigue.

Intuitively it just feels like something I would want to avoid because a fork failure is so unpleasant. If it was somewhere else on the frame experimenting a bit more might be justified.

https://www.fracturemechanics.org/hole.html
https://www.physicsforums.com/thread...weaker.952298/

unterhausen

Did OP say if this fork was from a crummy bike with a heavy fork? That would change my answer a little.

Drilling a hole in a fork is nothing like using lightweight blades as far as the likelihood that dangerous fatigue failures will result. Lightweight fork blades are much safer than a fork with holes drilled in the blades. You don't have to know much about fatigue failure to know that. Nobody (except the people that designed the CE bike tests) care about what happens to a fork in a crash. It's not an airplane.

abdon 

Yeah, those are 100% fair points, and that first link is actually pretty darn good.

Forget about the hole for a second as it is proving to be a distraction; have you seen any resource measuring the amount of load a fork experiences along it's length? Interestingly enough steel forks tend to break/bend much higher because while that area has more metal it is also the one experiencing the most stress due to leverage. Well that and at the drop casting but those are due to material defects, too hard/brittle of a casting.

In any event; it may feel to go against logic but drilling a hole on the lower part of the fork, where there is less metal, should also be less of a toll on the overall strength of the fork as this area experience smaller loads. Drilling holes on the upper part of the fork ought to be of more concern.

Chances are I'm not going to do it on this project but I may start chucking dead steel forks in a pile and eventually test these assumptions. Basically rig the forks so I can crush them on both a hydraulic press and under sudden shock, see where they like to break even with a hole on them.

guy153

A lot of forks have a small vent hole on the inside near the dropout anyway. But it's very small-- like a couple of mm. Would be interested to know the results of your experiments! I guess you could also just try with some regular cheap steel tubing to get an idea.

unterhausen

I know the people that have done fatigue tests on unicrown fork found that they failed at the weld. And it's true that most "death forks" have had a problem at the crown. OTOH, it's possibly not directly analogous to a rack mount, but there are plenty of pictures floating around on the internets of failed fork blades due to disc brake mounts. These are approximately where a low rider rack mount would go. Those are buckling failures though, not fatigue failures. At one time I was collecting bike failure pictures, but I gave up on it.

I don't know what use a hydraulic press would be to tell you if a hole in a blade would be safe, you need cyclic stress. The failure you would experience in practice is only loosely correlated to the buckling stress.

abdon 

that's because a disk brake stresses a fork in ways a regular fork will never see. Short of a crash a regular fork sees a lot of vibration and the braking forces pushes on the blades from the brake pads to the drops, absorbing the stress. On the other hand disk brakes attempt to twist the tip of the blades off.

As I mentioned were I to kill some forks I would do both, a hydraulic press and a shock test. It may be pointless but half of hobbies are like that.

JohnDThompson 

Another option that doesn't require drilling is a U-bolt to secure the rack, although the rack in your picture doesn't look ideal for this.

Andrew R Stewart 

I don't have any links to, but the Euro fork testing standards are available (or were a few years ago) for the public to view. For a person skilled in math and mechanics (engineer...) to calculate the stresses the fork goes under during the tests shouldn't be too hard (and I am not that person). IIRC some of this data has been posted here and/or in other framebuilder's forums (and there have been quite a few over the last 25 years).

I tend to be far more seat of my pants and taking spoken word, written word and/or what is actually done by the masses as a giideline. I learned long ago that modern bikes are very refined in their designs and there's very little new to add to the mix. Andy

unterhausen

Best advice I ever got about framebuilding was from my friend who told me never to mention I have a Ph.D. in a field related to framebuilding. I do have a Ph.D. and I've done a lot of work that can be directly applied to framebuilding. Would I ever do such a thing? No. There are so many factors that a builder has no control over. Could I inspect every frame part I buy for potential flaws that might lead to frame failure? Yes I can. I even own some equipment to do that. Will I use it for that? Let's be realistic here, no I wouldn't, it's too much work. I just look at the failures other people have had and how frames have been made for the last 60 years and base what I do on that accumulated experience.