Hi, I am having difficulty finding a proper answer to a couple of questions I have regarding internal cable routing through a carbon bike frame. Every Youtube video I have looked at, with one exception, shows inner cable only routed through the frame. The exception is OZ cycle where he puts the whole lot (inner and outer)+ anti rattle foam through his frames. Now, he usually is very thorough with his videos but he makes no mention of using only an inner cable so I am now confused. 1… My frame does not have completely integrated cables and uses short bits of outer before the inner goes through the frame for the front and rear derailleur and another bit of outer at the rear of the chainstay for the rear derailleur. The hole through in the frame isn’t big enough to allow a complete cable through frame and had thin guide tubing for the inner when I built the bike which is removed once the inner is in place. Surely if there were channels built into the frame for inner cable use only you wouldn’t need the thin guide tubes? So, with this arrangement, what is stopping the cables rubbing against the frame every time they are under tension if there are no outers to keep them in place and how would you know whether you would need to use just inners or the complete cable when building a bike with a different frame.
You answered your own question; if the hole isn’t big enough for the outers then it’s inner wire only on the inside of the frame. The arrangement is very much down to the individual bike frame, I have one with inner cable internal routing, one with full cables all the way, and a TT bike where the less said about cable routing the better 
On most internally routed bike frames, there isn’t anything for the cable to rub against between the cable stop in the frame and the guide under the bottom bracket, or from the guide to the derailleur. There’s quite a lot of open space inside those tubes.
My fuji had the thin guide tubes left in place intentionally to keep the cables from rubbing and then they exited at a guide at the bottom bracket. From the bottom bracket to the rear it was all exposed on the bottom of the chain stay.
As said, there’s normally nothing in there to rub if it enters and exits in the designed places. It will be under tension and in a straight line from one to the other. It’s not like how cable bends around in the housing outside the bike. There it bends because the housing forces it to. I see no need for an inner sleeve inside the bike usually.
It’s no accident that the grommet where the cable enters the frame is small. The total cable route must have a fixed length to within probably a fraction of a mm. The housing has fixed length, but the span from the entrance to exit will have variable length if the housing is not firmly stopped. That can’t work.
But yes, you probably can actually remove or drill the grommet frame insert and run a cable housing into the frame, through the BB (but not into the external BB guide), down the chainstay, and out to the derailleur (removing the exit stop) to get a full housing. Then it’s not running straight inside the frame. The cable itself won’t rub, but the housing could rattle a bit and just be annoying.
Worked as a bike mechanic for 5 years and did many and a wide range of internal routings, initial installations as well as replacement cables. Some frames came with strings or thin plastic tubes pre-routed thru the frame to guide the first installation. Others, you were on your own. More recently, some frames, particularly mtbs, have molding in guide tubes where a continuous housing can simply be shoved through. When replacing existing cables/housings the existing parts can virtually always be used to guide in the replacements. Things that are useful to have on hand are a few lengths of liner (small diameter plastic tubing that just fits the cable), lengths of cable, a housing/brake hose coupler, and in some cases something like the Park Took internal routing kit. For example: To replace a housing or brake hose through a frame that has no internal guide, remove the cable leaving the housing in place. Stick a ~2’ length of brake cable 1/2 way into the existing housing/hose and the other end into the new housing/hose until the new and old butt up against each other. Push the new housing/hose into the frame and gently pull the old one out on the other end being careful to keep the two butted together as feed on the new housing/hose. Another example: To replace a bare cable through a frame, cut the crimped end or the head of the cable off and slide a length of liner over the cable through the frame. Pull the cable out, run the new cable through the liner, then pull the liner off the new cable. Sometimes it’s as straightforward as that, but often these methods require some careful forethought/cleverness/finagling.
As Saddlesaur said, lots of various ways have been used for routing through frames. I’ve been unimpressed with pretty much all methods other than what is on my '20 Trek Domane SLR. It uses full housing all the way, AND has an access door for storage in the downtube of the frame which also just happens to provide a way to see your housings and hoses going through and manipulate them from that point, midway down the downtube.
@ozmadman said he had the thin guide tubing through the frame when he built the bike. That’s critical to save (or get some from a shop) for some frames because you’ll use it every time you run another cable in that frame. It lets you run that guide in through the frame ON the EXISTING, OLD cable before you pull that old cable out, leaving the inner guide in place and sticking out at both ends of where it runs through the frame. You then can use that to guide your new cable in through the shifter, the first section of housing, and then the frame and then just pull the thin guide housing out once you have your cable through to whatever is the last internal section of the frame. This is what is needed on my Trek FuelEX, and makes the task at least pretty manageable.
I’m not a fan of frames that have a permanent internal tube for each cable (or hose) even though it can make cable changes a lot easier than fiddling with those removable thin guide tubes. My biggest concern with that design is what is required if the internals fail, which is probably rare.
The only downside I see with running full housings internally all the way through is the potential for noisy rattles if they are not secured well, and that can be a real problem. My Domane has channels that clamp onto the housings/hoses as they run through the downtube, holding them snugly in line. That clamping bracket is mounted to the carbon frame, and is accessible through the storage access door in the downtube. The housings run through the BB area and then the chainstays to derailleurs and rear brake caliper, and are secured where they terminate. They are very quiet and don’t rattle around. (I have not changed those housings yet, so I might learn something new in the process, probably later this year. Saddlesaur’s description of running the new one in, butted up against the old, with a cable running through them is roughly what I expect to do.)
My Diverge came with full length housings up to the rear derailleur. I prefer this over what I’ve seen with other bicycles where the cables can slap against the inside of the frame. This was the case with the brake hose on my Pinarello and it made a really nasty snapping noise.
Don’t get me started! Original STI (and similar) shifters had the shift cables exit the side of the shifter and make one easy large radius 90 degree turn to follow the frame rearward. Routing under the bar tape replaced this with three much tighter radius turns, mainly for cosmetics with a teensy improvement in aerodynamics, resulting in more friction, hysteresis, and cable/housing wear, and of course more “fun” replacing the cables that needed replacing more frequently. Shimano did resist this initially, but eventually succumbed. Internal routing just compounded the madness. Of course the solution now is throwing tech at it, tens (hundreds?) of thousands of transistors, circuit boards, lines of code, radio transceivers, motors, gears and batteries to replace what was pretty simple and elegant mechanical system as appropriate to the concept of the bicycle itself: elegant, efficient, human-powered.
Or you could switch to SRAM and get something that I didn’t have to change cables but about once a year. I really don’t like Shimano after their half-baked attempt at wireless. Either wire it, or make it completely without. They seem to be about half-there with the stuff they do. As to aero, it’s way overdone. You want to make things that way, work on your body and position. It will make way more difference than internally routed cables.
FWIW: I’ve been running SRAM Red mechanical on my road bikes since 2010. I’ve done a good number of conversions to wired Di2 with various internal, external wire routings, external battery, seat tube battery, road shifters, TT shifters, aux shifters etc… All a PITA IMO, but we charged accordingly. Another thing we did was sell Di2 diagnostic and firmware updates with tunetups. There was almost always an update to apply, and is some cases it required detaching components from the harness and updating them individually. There were quite a few Di2 bikes in the field so we also had a good number of repairs; damaged cables, failing batteries, worn out/defective derailleurs. I sold a few SRAM eTap conversions. Ludicrously simple to install, and AFAIK, there was never a firmware update needed or issued. I was def a fan of eTap, but I intend to stick with mechanical as long as practical.
Yes, I am sticking with mechanical as I cannot justify the cost of Di2 etc ridiculous cost for a micro second faster gear change. Irrelevant in my opinion unless you are a pro and get it for free and have mechanics to sort out the mess. Tubeless tyres attract the same grief as well IMO. Also as a final gripe, I think Shimano has shot themselves in the foot making 105 Di2. The groupset of the people is now out of reach for " the people"
I agree with the SRAM ‘Double Tap’ to eTap conversions. I really don’t like what Shimano did with the current flavor of Di2. I had a 2018 Roubaix with the original ‘wired’ Ultegra Di2 and I was quite pleased with how it worked. Got a Pinarello Grevil with SRAM AXS (I would have loved to convert to XPLR) that worked a ‘treat’. Snappy and never a problem. Batteries lasted as long as a small factor battery should and it was never a problem to power. I’ve heard horror stories on the new Di2.
Care to share some of these stories? As someone who has the new DI2 I’d be interested in hearing what to watch out for. Knock on wood, I haven’t had any issues with mine yet.
Electronic shifting, Shimano or SRAM, works great. As far as faster shifts, I believe mechanical is marginally faster. It feels faster to me as the shift occurs as I’m move the paddles because I’m actually moving the derailleurs as the paddles move. With electronic, moving the paddles closes a switch, and then the shift occurs. Still, any speed difference is inconsequential. With electronic, the indexing is done in the derailleur as opposed to mechanical where it is done in the shifter. With mechanical, wear/hysteresis in the cables can cause imprecise shifting. This does not occur with electronic. Personally, I want my bike to be entirely powered by me, including shifting. I don’t want to check that it’s charged up before riding, even though that’s an infrequent occurrence.
I’m avoiding going electronic for one additional reason: Theft. I just had a 9K bike stolen. I’m thinking it was on a ‘hit list’ where people get a list of bicycles to steal for customers willing to buy at a ‘good price’. Take away the glitz and they are less likely to be taken. When a customer has to get the piece serviced and they only folks who can are the manufacturers shops, they are less likely to ‘walk away’.
BTW, the 9K bicycle was recovered. It looks like the perps abandoned it in the river area and it needs to be serviced and some parts may need to be replaced. However, I do stand by the fact that the snazzier bikes are being snatched. The best thing you can do is get a cheap lock and secure to a solid object. And I stand by using mechanical as it is less likely to be grabbed.
Better to get a good lock if you have a decent bike or better still don’t leave it anywhere. If you have to leave a bike anywhere get a crap one and hand paint it black that will deter most scumbags
Maybe over there. Here, the bike thieves are dumb as can be. One tried to sell a very expensive and well kitted out bikepacling rig for less than the price of the saddlebag on it. It was understated expensive with a pricy belt drive and a very sneaky dynamo hub all.internally routed to a usb socket on the head tube. When he was asked what size it was, he said 1.5 meters. Apparently he’d had it from new, which is fascinating given it was a custom made frame. Genius. I believe the police were alerted and got it back.
Here the bog standard bikes are the ones that get nabbed. Trek, Giant, S-works. Because we are such a small country, it pays to steal stuff that there is a lot of. Anything special is easily spotted when a thief tries to sell it.
Ha, ha … here in the UK bike theft is rampant. They will steal anything that is not bolted down and even if it is then they come armed with bolt cutters in broad daylight, Druggies most of them looking for their next fix. You need a good lock which will probably weigh half the weight of the bike!!. No-one in their right mind would leave a bike even for a few minutes to go into a shop without locking it up.