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I want to install dynamo-powered USB charging apparatus inside my fork steerer. However, it's a carbon fork that has no exit at the crown for the dynamo wiring.
Is it safe to drill a small hole (at most 5 mm I guess) from the crown area through to the interior of the steerer, so that I can pass the AC wiring through?
The fork is this: https://www.kinesisbikes.co.uk/Catalogue/Forks/Adventure/ATR-DISC-FORK
This is what I'm hoping to achieve:
When carbon manufacturers and fabricators put holes in things, they're thinking about:
The answer to whether you can feel safe choosing a random spot to put a plain hole through and call it good is no. The answer to whether a carbon fabricator could figure out a good solution here is probably. How cost-effective it would be to get them to do it is probably borderline at best. If you want to DIY it and do a reasonable job, it's going to mean putting together some passing version of the same skill set, which is attainable but not necessarily something that can be fit into an answer here (although maybe someone will show otherwise).
Here is one possible plan that risks very little: Use a flashlight or bent wire probe to test whether the fender hole could have a wire run into it up into the inside of the steerer. If it can, find an M6 bolt with an unthreaded section, cut the head end off leaving enough unthreaded length to fill up the front crown fender hole, trim the threaded end down, and epoxy it into the front side of the crown (after dialing the fit and prep of the surfaces to be glued as needed of course). That becomes a stud you can mount your fender and generator light on, and secure them down with a nut. Your charger wire can be run up the back into the open fender hole there.
I would assume it is not safe to drill into carbon, not at that location. I am not saying that it is not safe, only that I strongly suspect so and that I am not willing to assume otherwise.
To state the obvious, when you drill holes in something, you weaken the structure. A heavier fork may be overbuilt and it might be fine, but you don’t really know if the fork is overbuilt enough for this purpose. Additionally, you would have to drill the holes properly. In the case of carbon, you want to use good and sharp drill bits that won’t catch on fibers and maybe cause a delamination in the structure that could spread. In the case of metal, you’d want to file off the rough edges.
I am aware of people drilling holes for internal shift wiring in metal frames, and even in some carbon frames. I have always advised that this is at your own risk, and that I wouldn’t do it at all if the tubing or carbon is very light. I am not sure how the average stresses in the areas where you drill those ports compare to the stresses at the fork crown. I would guess, however, that the fork crown is one of the more highly stressed parts of the bike, considering that every time you hit a bump, force is getting transmitted upwards through the fork.
I assume bicycle engineers these days would use finite element analysis to simulate the loads on the components involved to design a fork with an internal routing port. That is, they break the frame and fork into many segments, and simulate loads on each one1. I would assume that most of the holes in existing forks are drilled rather than moulded - the thing is that the engineers would have designed the carbon layup2 to withstand a foreseeable range of forces after drilling. In older times, I bet these decisions were made more on an intuitive basis, but parts were heavier. This provided more of a safety margin. n principle, if you could do FEA, you might be able to get a sense of how much of a risk this is. However, your simulation would need to simulate stresses under a wide range of scenarios, and you would also need to know how the fork is constructed, e.g. how many plies of carbon, what orientation, etc. Basically, if you had the means to do this, you wouldn't be asking a question on Stack Exchange.
This is a long way of saying don’t do this, you would be much better off buying a replacement fork that already has a port for internal routing if you must use this particular charger. I know that ENVE makes one such fork, although I don’t know if its rake and axle-crown height are identical to this one. If you do this, it would be at your own risk, and I would prefer to wait for others to do it first.
Note 1: This is similar to agent-based microsimulation, except that each agent in FEA is part of a tube or other component rather than a person or geographic area.
Note 2: Carbon components are usually constructed from many bits of sheet carbon fiber that are cut up into various shapes, layered and otherwise arranged in a certain way to achieve the desired stiffness and strength characteristics, and then baked in a mould to cure the impregnated resin. This is what I mean by carbon layup. Designing the layup for a carbon frame or fork is parallel to choosing what tubes (what alloy, what wall thickness, butted or not, ovalized or otherwise shaped or not) go into a metal frame or fork.
I faced exactly this question about a year ago. I did all the internet research I could, talked to bike mechanics and emailed fork builders and repairers. The answer that emerged from all this was a clear 'no'. The fork crown is subject to large forces and drilling it will weaken it unpredictably. The one thing that's worse than not having a sweet USB charger in your steerer tube is having no face after a fork-failure induced crash.
I contacted a carbon fibre repair company in the UK and asked whether they might be able to drill the crown and also reinforce it (like they do in their various frame-crack repairs). They said yes and I considered sending them them fork but then realised that a) I didn't really trust the strength of the repair and the reward/risk ratio was pretty low; b) the cost of the job was a substantial fraction of getting a new fork built.
I've since hunted all over the internet for the right fork and, I'm afraid to tell you, a year later I still don't have the fork I want. The problem is that the current global bike shortage has got all the fork builders tied up with large contracts and they can't squeeze in custom jobs like this. The most affordable off-the-shelf option I found is this: https://bombtrack.com/parts/forks/ext-fork-2-2/
If it has all the features you're after, I'd say go for it. For me, it's missing something that I'd like to have on my fork, so I'm still hunting.
Good luck with it! If you find a solution, would love to hear what you go for.
IF you do drill a hole, I would suspect that going straight down the steerer tube (ie radially out of the tyre) would be better than drilling in through the side of the steerer tube.
Firstly, you're likely to interfere with bearing seat, and then the material covering the "floor" of the tube is probably not that thick nor structural.
You need to measure the thickness of the bottom, somehow using a depth probe and comparing the inside and outside measurements.
Either way, you will want to seal the edges of any hole, with some kind of lacquer suitable for Carbon Fibre. You also need a rubber grommit to protect the wire from abrasion, and to secure the wire away from wheel/tyre rub.
Ultimately, we can't tell you what is "safe" - that comes down to your risk tolerance and how good you are at noticing a potential problem before it develops into an actual problem.
Based on the descriptions so far, I would contemplate drilling up into the steerer tube, but I would not drill into the fork tine/blade/leg.
For a hands-on professional opinion, you should take the fork to a local carbon fibre repair specialist for an in-person inspection. There would be some costs here naturally, same as visiting a doctor.
