Da Vinci cable couplers
 

Last year, I bought a pair of DaVinci cable couplers so that I could use single-bike length cables. During the season, I installed the derailleur one, but didn't need to touch the brake, so I left it for this winter's overhaul. Now I find that the brake cable "head" doesn't actually fit inside the coupler. Did I miss something here? I didn't think there were two different kinds/sizes of couplers. I got mine at Tandems East.

Another question: I learned, from experience, that putting one of these in the middle of a long span under the boom tube is a bad idea. I had to put a piece of foam tape to keep it from pinging at every bump. I am planning on putting it close to the rear BB shell to minimize that. Is there an ideal place for these?

There is two sizes of couplers one for each size cable.

The cable couplers on my tandem have 2 rubber donuts around them to keep from hitting the frame. I don't know what kind they are. You might look for couplers with donuts around them.

I have the o-rings as well, but it still makes a pretty solid ping when you have it at mid-span! Sure, it is better than metal on metal, but extremely annoying. I just put it at 4 inches from the captain BB shell. With any cable tension at all, it now cannot get to the frame.