Run external.

 

I just want some opinions on how much big of a risk I'm taking on doing this.

A decently big risk. Holes in tubes (production frames) are reinforced in some manner. You're tempting fate, some spots more than others.

Also, carbon fiber, like foam/carboard, is murder on cutting tools. You'll likely destroy whatever (cheap) twist bit you use. This also assumes that you don't chip off pieces of the layup, either inside/outside the frame...which can be avoided (on the outside) with more expensive brad-point bits. Ideally, for a bicycle, you would use an abrasive stone, since that would have the least chance of tearing at the layup anywhere. Regardless of what you chose, though, bit or stone, it would have a tendency to walk, which is bad for obvious reasons. You only get one chance to do a good job, and if you've never done it before, well, you might be buying a new frame earlier than you wanted to.

Short answer: zip tie the housing to the frame, and be done with it.

 

I researched this fairly thoroughly when I drilled a frame several years back with no stealth dropper routing & concluded drilling the down tube to be less than ideal. Seemed many eventually developed stress cracks at the drill hole. I drilled a hole a bit above the BB on the seat tube & is still going strong. This obviously won't help your situation, but figured I'd throw it out there.

I'd try to either stabilize the cable to minimize the friction or go external. They make all kinds of cable holders if you go external. Check universal cycle, they have a good selection.

 

Unless you're looking to buy a new frame, either live with it or buy a new frame but don't drill it. Well, unless you like wondering just when it's gonna break, because it will.

 

I wish I had a good answer for you. I have a newer Air 9 Carbon with the rubber inserts. I can't get the rubber insert to stay in place. We here at work often reinforce our cables with sheathing of some sort so that the bend isn't concentrated in one small area. Good luck.

I have drilled "access" points in carbon (and other materials). The biggest issues with making a hole in the carbon is having the "raw" ends where the hole is. So maybe after drilling the hole, might try coating the opening with some epoxy to seal in the area so to speak. Hope that helps a little. As long as the hole is not too close to the joints, you should be OK.

 

Get stick on cable guides and run external, unless of course you don't like that frame and want it to crack. Any holes in the DT on bikes are designed to be there and reinforced so they are strong.

 

I have done some carbon drilling, but not on bike frames. I would be cautiously comfortable doing this, but you're the one exercising that option.

My experience with drilling carbon is moderate on both tube and sheet applications. It does not seem to any effect on the bits anymore than anything else that I've experienced. Preferably use a new sharp bits and incorporate a slow, gentle feed pressure. Lighten up on the feed before breaking through the other side. The trick is a very slow gentle feed with a sharp bit. You don't want 'peel-up' or 'push-out' delamination. Slow feed with very light pressure. Be careful and prepared to avoid letting the bit take too much of a bite and 'tearing through' the other side. Once through, let the bit flutes 'polish' the fresh cut hole.

Make sure to start with a pilot hole and work your way up to the desired dimension hole. Do not attempt to begin drilling with your desired hole sized bit.

Followup with securing or dressing the hole with epoxy or like resin to seal the laminations. This might be several coats or applications.

 

......and do it outside wearing a mask. Not long back from a H + S course, the instructor thought carbon fibre will be the new asbestos. 'hope he's wrong.

 

How about enlarging the existing holes in the head tube so you can get a sleeve/grommet/bushing of some sort around the cable housing? That would be safer than drilling new as that area is probably already reinforced.


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@wschruba @WHALENARD @noapathy @gmats @LyNx @Cleared2land @BLUFF @Stray Mutt,

Thank you all for the advice and recommendations! Definitely more to it than what I expected. I will route the cables externally. Some Gorilla tape should help.

 

I've been putting internal front derailer cable routing on old or nos 26er frames that I've upgraded to the Shimano side swing. I've put about 500 miles on a Fuji SLM with no problems. I use a dremel to cut out the cable stop with a vacuum hose mounted above it to suck up the carbon particles. The exhaust air is routed through a foam filter then a HEPA filter so I hope that's enough.

I don't think a small hole is going to be a problem. A 32 hole carbon rim has 66 holes drilled in it. A carbon frame has bottle cage holes, drain holes, cable holes.

Worst case I'll have to buy another $150 carbon 26er frame.

 

I'll take my chances...

Never ceases to amaze me ... always an opinion that something terrible will happen with no experience to back it up (referring to the "your frame will crack" response). I have an Epic 29er I've been using an external dropper on and will be upgrading this Fall to a stealth - and moving to stealth for multiple reasons. I've thoroughly researched drilling CF on the web and every mfg that's chimed in on a similar discussion with a negative response all said the same thing "the frame has not been reinforced in this area for drilling like other areas that have been drilled". So, why not just reinforce it? If you can reach the area internally, and there is room, should be easy enough with some glass / CF cloth and CF assembly epoxy (I'll be using 3M DP 460). I'll probably be drilling somewhere in the BB area, but depends on what I see when I pull the cranks / spindle. I can't imagine a force that would crack a frame through a hole in the middle of the BB shell ... but I'm not an engineer so maybe it's possible! If it still cracks through the hole after doing the reinforcement I'll be shocked but take it like a man. I'm betting though it won't, and since I bought the bike used I have no warranty anyway with Specialized. Even if I had a warranty I would still do it. I'd be surprised if they denied a warranty for a crack remote from the modification.
I admit I have no experience with CF frame repair / building ... but I have a lot of experience with whitewater kayak FRP boat building (what we did back in the day before plastic boats) and CF kayak paddle repair so I know how to work with the stuff and know a bit about dissipating stress risers. Also my plan has been backed by at least one custom frame builder friend. He works with steel and AL but knows a great deal about frame stresses - made me a bit more confident with the plan.

 

^^^^ Go for it.

I'm in support of your successfully doing this. It's not rocket science. I believe your probability of doing this with no resulting problems or issues is very good. Given your intent to gain access to the inside and reinforcing the hole properly, I would have no issues with your plan.