i was looking at some tips for a neat externally routed di2 cable set up and a few sites suggested 150mm cable for the shifters to the under the stem junction box (bars i think were either 42 or 44 width). I have looked at my bike and i cant see how anyone could run a cable that short, either my measuring tape is very odd or i am missing something here. What length cables do you people use just to give me a better idea of likely lengths that i will require?

i will measure against my bike once i have all the component parts on it but i want to order the cables soon as they are the last piece of the jigsaw and i dont want to strip down my current set up as its still in use.

Can you just make a mock-up using bits of string and a pretend box, battery etc? Then just measure the string. Once you kow how long a piece of string is it’s easy.

My wires are very flexible so string should be a good proxy.

You generally need 350mm from shifters to stem junction, maybe 300mm if bars very narrow.  you need to account for 20mm for the cable loop near the shifter too, to stop the cable pulling out of the shifter in a crash.

Thanks blazing saddles, that seems to tally up with my very crude measuring.

i have done a bit of a mock up guess but without knowing exactly where they connect on the shifters and how much depth they go to plug in it does leave it slightly inaccurate. I deffo dont want cables that are too long so i would prefer to be a bit more precise.

I’d not use anything less than 350mm in that case, it’s the right length really, 300 would do at a real push but it’s not ideal.

I’d agree with that. You can easily take up any slack, having them too taught will cause problems.