Tony's Cycling Blog: BMC TM01 Cable Routing

One of the challenges in building the TM01 (or most of the latest generation of TT bikes) is routing the cables. Internal cable routing has been commonplace on TT and Tri rigs for some time now, but the latest generation of bike design goes to great lengths to minimize the aerodynamic drag caused by exposed cables. While this is good from a go-fast point of view, it makes getting the cables just right difficult. With Di2 cables the job is a little easier because observing the proper bend radius for the cable is not as hard. However, with mechanical shifting too tight of a bend can mean cable drag and poor shifting. Even with Di2, there is still the issue of cable drag with the brakes so even those with electronic shifting are not home free.

The BMC TM01 routing brings 3 of the 4 cables behind the stem and down into the down tube from a hole in the top tube. This is not groundbreaking by any means as many bikes use this routing. What does make it somewhat unique is that BMC designed a plastic stem cover (it actually covers the steerer, but they call it a stem cover) to hide the cables before entering the frame. To utilize the cover you need a nearly horizontal approach angle. If you use the negative wedge and a couple of spacers (basically like using a long negative stem on a regular bike) you are going to be in pretty good shape as the approach will be relatively flat. But with a shorter stem or a positive stem angle you might be challenged to make this work with traditional cables. Of course, I had this problem so I had to spend some time coming up with a solution.

With my bar position, I would have to bring the cables straight down out of the extensions until I hit the stem, then back toward the top tube and make another 90 degree turn straight down the head tube. From these pictures you can see that there is excess cable and I certainly could shorten it, but I felt that even with the shorter cable housing I would have had bend-radius issues. My big fear would be that the brakes would not release properly and I have enough challenges when racing, rubbing brakes would not be one I would want to overcome.

The obvious solution was to go with Nokon cables. If you are not familiar with these cables, they use a pretty standard steel inner cable, a fairly thin Teflon coated liner that is intended to be be run the entire length of the cable and finally, there are small alloy links that make up the housing. Nokon touts the reduced friction and better braking performance by the compressionless housing as the big benefits of the cables. The reason they make sense here is that the alloy links allow for very tight bends.

Nokon cables are slightly more challenging to work with than regular housing, but after installing several sets I think they are only marginally more difficult. The big downside is cost. Retail for brake and derailleur cables is around $200. If you bought the two kits you would still be at least two kits short of having enough housing to cable the entire bike. You can buy extension kits, but I'm not sure you could cable the entire bike for less than $400 retail. I had no desire to drop that kind of money on cables so I decided to try to use the existing housing for the long runs through the frame and only use the Nokon sections where I needed to have the really tight bends in the cables.

What to buy?

So what is needed? Well, that would depend on your frame size I suppose and where exactly you cut the cables. It would also depend on what you want to do with the brake cables. The front cable runs through the head tube and it is easy to make that work with standard cables. The rear break is a challenge, but the way most base bars route the cables you might be able to get away with standard housing there. That would leave one Nokon kit for the shifters. You will also need something to join the standard housing and Nokon housing together. I used Jagwire housing connectors which you can find here.
I owe a big thanks to Brian @pfbikes for suggesting the connectors.

The portion of the project that concerned me the most is the point where the two cables join. If they are not aligned or do not stay aligned under tension, this is the most likely place to have cable drag. I thought a lot about where to find a straight run that would be optimal for the joint section. I considered the down tube, but then I thought about the challenge of joining the sections and the potential noise from the connecters hitting the frame. I decided the best spot would be just below the top tube. The cable is vertical at this point and also pretty easy to access.

Installing the cables

Cable routing through the frame is not too difficult. The one really challenging section is the rear derailleur because you must go through the chainstay. For this reason, it is critical that you always leave yourself either the metal cable or the housing in the frame. If you do this you can always get the other section attached.

Step one is to pull out the existing metal cable. For bar end shifters this is really easy, just loosen the 5mm bolt on the derailleur and cut off the cable tip. Push the shifter down and you should see the end of the cable come free. Just pull it all the way out. Then pull the cable housing from the rear of the extension until it is totally out.

The next step is to mark the housing for your cut. I chose to make the cut 2cm below the entry point on the top tube.

With all of the cables, you can push them in a few centimeters from the back which will allow you to work with them above the top tube.

The next step is to determine how much Nokon housing you will need. The best way to do this is simply run the liner down the length of the extensions and then down to the point where the cables will be joined. Make sure to have enough cable to make the two bends under the stem cap. A little extra cable is much better than not enough.

Once you get the length right, put on the alloy sections and go ahead and run the steal cable through the shifter and through the liner. When you are finished you should have something like this.

Place the Jagwire housing connector on the housing in the frame and run the metal cable through the housing. I should note, there are two end pieces that come with the Nokon cables, one is 5mm and one is 6mm, depending on the housing connector that you bought, use the appropriate bit so that you do not have any movement. Worst case, take a small bit of electrical tape and rap it around the cable end so that it fits snug in the connector. Push the cable through and then pull it through the derailleur or brake depending on which cable you are working on. Make sure that as you pull this tight and that the two sections of cable fit correctly into the housing connector which should now be a little below the top tube.

After you tighten everything down make sure that you turn the bars from side to side and check the shifting and braking. You do not want to get everything back together and realize that your brakes grab when you turn the bars.

Once you are sure everything is working well, route the cables under the stem cap. Note in the picture below that I had to pull the rubber ends off of the extensions to give myself some extra room for the cables.