Many new frames come with cable guides, which are basically hollow tubes you put the end of a cable into to snake it through the frame, already installed. If your frame did not, go to your LBS and ask if they have some they will give you or you can purchase. My LBS builds a lot of bikes from bare framesets, so they have plenty.
Snaking those guide tubes up through the frame is a PITA for sure. You start at the ferrules on the DT where the front and rear shifter cables pass through to get to the Bottotm Bracket area and fish them out through whatever opening is in the BB to get access to the cable guides. Then, after all your cable housings are measured and cut, you carefully insert the cable into the guide tube and fish it through the frame. After you get both shifter cables to the BB area, it is relatively easy to put the guide tubes through the drive side chain stay for the rear der. cable and through the front cable area where it exits the top of the BB to get the front der. cable out of the frame.
Same process for the rear brake. Just be sure to use the correct size of cable guide tube because shifter cables are smaller than brake cables.
After you do it the first time, you just use the existing cables to fish the guide tubes through the frame to use for installing a replacement cable. Unless you are unfortunate enough to have a cable break inside your frame which rarely happens.
It can be daunting running cables the first time on a new bike. Especially if you didn't have the benefit of removing the old ones.
Hopefully your frameset came with the necessary hardware? Cable guides, covers, etc?
I'd suggest two things.
1. See if you can find a manual from Giant. For the BMC I recently built, they had a fairly simple infographic showing the proper routing for the cables. It really helped.
2. Go to a shop that sells the same bike built and look at how the cables are run. Take some pictures if needed. Alternately, you might see if you can find some images or videos online (via a search) that might show some detailed pictures.
What I used to do on a vintage Klein with internal shorting cables, was use long sections of brake housing to snake into the frame. When I could see the housing end near to where the cables would enter the frame, I would use a finishing nail to fish the housing end out of the frame.
I only had to do this once. For all the other times I needed to snake new cables, I would use the existing cable as a guide for the brake "fish" housing.
Newer frames, like the Chinese carbon I purchase last July, came with noodle housing installed and made installing cables very easy.
I just built up a new bike with internal cabling. Bike-building newb, but decently mechanically inclined. The frame had no internal guides. It took maybe a minute to fish each cable through. Maybe some frames are worse than others, but it really wasn't that big of a deal.
- use a nice, unfrayed brake or derailleur cable without a head on it.
- bend the end of the cable about 45° an inch or so from its end.
- route the bare cable through the frame.
- tape the cable in place if necessary and use it to route housing through the frame.
- remove the cable from the housing and route new cable through.
I did not know about this Park Kit when I had to route cables on my Fondriest TF2 1.0, but I did something similar using a couple of rare earth magnets that were strong enough to attract the stainless shifter cable inside the DT and guide it to the BB opening. They use a reverse polarity magnet that goes inside the tube to fish the attached cable. If I did much work with internal routed frames, the Park tool would definitely be on my workbench.
That kit seems nice. I just use a big rare earth magnet to guide the bare cable where I want it in the frame - very fast and effective. You can also use a shop vac to suck thread through a frame then super glue the cable end to it and pull it through.
My older Orbea frame has permanent cable housing stops, with just a tiny hole for the cable wire itself. There's no way to grab onto the end of the cable to pull it through. And I foolishly pulled out the brake cable to shorten the front housing, assuming there was a guide inside the frame.
Thin tubing makes it easy
But it was easy for the bike shop to push some thin tubing from the rear exit up to the front hole in the frame. With the fork removed, they could use fingers to guide the end of the tubing out of the frame hole.
Then, with tubing sticking out from the front and back of the frame, I can easily push the new wire cable through the tubing, and then pull out the tubing. (For replacing cables, push the same tubing up the old cable wire from the back until it exits out the front frame hole, then pull the old wire out of the tubing--easy!
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