Missing Links

While everyone is learning how to work with the new SRAM Transmission Drivetrain, issues arise.

  1. The trim adjustment needs to include a wider range for some reason. It’s too narrow, even when using parts within spec.
  2. Don’t ignore the high torque that is put on the mounting screw for the derailleur. 35 Nm (310 in-lb, 25.8 ft-lb) is needed. Too loose and you’re going to lose. Seriously.
  3. Why does the stupid app have to connect to the internet? The app sucks balls.
  4. Someone imagined that 425mm was a short rear center on a bicycle. It’s actually quite long.

On this last point, we now have an issue, the engineering and documentation treat 425mm as a hard minimum. This is where this post comes in. I make mountain bikes for use on aggressive trail. As such, they have reasonably short rear centers, 405mm as on my wife’s new Vega and 410mm as on my new Millennium Falcon. Any optimally designed trailbike or gravel bike for average sized people will be in this range.

If we are following the very specific instructions for the Transmission installation, we are left with an issue, what do we do about the chain length for non-sanctioned frames? Below is a matrix for chain length that isn’t calculator based that is buried in the details of the documentation:

I noticed an obvious pattern in the results.

Carrying that to shorter rear centers, I can estimated a chain length for non-sanctioned frames. Remember, this is not an authorized setup and you can’t complain to SRAM if anything goes wrong for you. Still, hackers gotta hack.

So, my guess of 114 links for use on the Millennium Falcon was good. The 410mm rear center with a 34t ring falls right into where 114 links live.