I'm changing the brakes on my Synapse Carbon and there's not enough free cable available, so I have to replace the inner brake cables. This is easy to do except that I've never done it on a bike with internal cable routing.

The rear brake cable passes through the top tube. The housing doesn't go through, so the housing is in two sections that butt up to stops where the cable goes in and out of the frame. So what happens when I thread a new inner cable through the frame? I have no idea what's inside the tube and don't know if I'll somehow have to fish it through the opening at the rear or what.

Any tips or tricks?

If you can find a plastic tube the right diameter to slide over your brake cable and through the frame tube, that will make it easier.

Even if you can't, fishing the cable through probably isn't as hard to do as you're thinking. The trick is to position your bike so the cable exit is on the bottom of the tube. That way. gravity will position the cable where you can snag it and pull it out with something.

Sometimes the frame has an internal tube or guide to make it easy, but it's a lottery.

If all else fails, you can suck a bit of string or wool through with a vacuum (or maybe blow it through with compressed air).

That's if you're starting from scratch.

But with a cable already in place, you can just tape a bit of string to it.

Yeah, I think the rear cable stop is a plastic piece that's snapped into the tube. If I pop that out it will leave a hole large enough to find the end of the cable. And I can rotate the bike on the stand to position it as you suggest. Thanks.

The first method of routing an inner cable through the tube (besides the liner sleeve already mentioned) that I use is to put a slight kink in the inner cable about 1" from it's end (this allows you to roll/rotate the cable and it's end will flop around a bit). This and if the, usually, rear cable port plate is removable a tool made from a spoke in the shape of a "J" to act as a hook reaching in with.

These usually do the trick, although I have had to hold a flashlight in my mouth while I reach inside with one hand and spin/move the cable with the other hand.

Have I said lately how much I hate internal routing??? Andy.

This is the method i was going to suggest. So far i still enjoy internal routing.

It varies by bike. Some have continuous tubes, or (like mine) sort of funnels to catch and guide the wire to the exit hole. Problem is that you won't find out how hard it can be to thread a wire until it's too late.

Unless I'm sure that it'll be easy, I cut the existing cable to get a clean end at the front, and use a piece of heat shrink tubing to splice the new wire to the old, and gently push/pull through. Use a decently long piece of shrink tube so you have some margin in case you pull a bit too hard.

The sleeve method works great for my bike without internal tubes - snip the cable, slide the sleeve over it, old cable out, new cable in, sleeve back out and done.

The one time I messed it up I used a piece of copper wire to fish my way through the holes... the stiffness of the wire compared to a cable was a big help.

Internal routing is not for Nancys.

+1. It's not so hard to thread a fine wire through the holes and then using it to pull the cable.

Are you able to see any part of it if you remove the seatpost? I've only worked on internal cables once and I saw the opening in the top tube when I took the seatpost out but that was only after spending 20 minutes fishing around with a old brake cable.

This idea sounds like a winner. I was thinking somehow pulling the new cable through using the old one, but couldn't think of a way to connect them that wouldn't be too bulky.

The best way, of course, is to run your housing all the way through. Then it becomes a snap to replace the cables.

That decision was made by the frame builder. Whether it's designed for full length housing, or only a bare wire depends on the size of the hole and possibly what's inside the tube.

For full length housing frames have an elongated oval hole, so the housing can enter without making a sharp bend. Some have a tube running all the way through and brazed or bonded at both ends.

For bare wire, the holes will be stepped and function as cable stops, with a base hole small enough that only the wire fits through. These are the ones that are the hardest to work with, but usually not easily converted to full housing through routing.

Thanks again, FB, for passing along that clarification.