The TFA bars are designed to be cut to avoid sticking out to much at the back. The official instruction manual from Vision says:

'It may be necessary to trim aero extensions if the rear of the extension protrudes behind the mount. Aerobars can only be cut from the rear. Do NOT cut the aerobars extensions from the front.'


In practice this means you need to be super careful if you cut the fronts the extensions to bring the shifter back your hands. Ideally you should bring the shifters into a comfortable position by sliding the whole extension and removing the bit at the back. Cutting the backs is often a very good idea but where you cut them depends on your cable run and if you are running a electronic group set or not.

If I am envisioning this correctly you are dealing with something like an older Cervelo P2 in terms of side run cables. There is neat integrated solution for electronic groupsets where you are going to want to cut the beack of the extensions as short as possible and then re-route the cable back through the bars and into the junction box in the stem. Things then pop out and the stem and run down the sides of the bike. With mechanical cabling its a bit more messy and it better to keep the extension a bit longer and then control the cables with zipp ties or heat shrink.

you mean something like this? trim to the redline area and route it through?
then i won't be able to use the cover that goes on top?

I am not sure what you mean by cover? The extensions have rear end plugs which you pull out before cutting them and then put back in the ends. The amount of extension sticking out doesn’t look problematic to me but I can’t tell if you will hit them with your knees or not.

Your cabling issue is two fold: you are using a lot of spacers and you are using a road bike frame with cable ports designed for road bars. I think it’s going to be a trial and error process of optimizing the cables but you will always end up with a decent amount of exposed cable.

scott, i think you're right.. either way i will have cable exposed and probably more than the original BMC TM frame. i checked the TM frame originally has the hole on the top tube, whereas mine is on the side. guess i'll play around with it and see how to make it work the smoothest without exposing too much. thanks for the advise..