I made this move last year, BB90 with C-bear ceramic BB (24mm internal diameter) and ordered a 24mm alu Rotor axle with 155mm Rotor Aldhu crank arms and the MAS Aero spider. Best investment I made.

I bought the bike with 170mm cranks but found a lady willing to swap out the 165mm Ultegra crank on her small sized road bike for the 170's that were on my Speed Concept. Felt good but now with the 155's it's a lot better. I'm 5'7" with only 30" inner leg length (seat height now is 704mm) so quite short legged and the short cranks really allow me to spin more smoothly. No more/a lot less of a dead spot in my pedalling.

The required spacers to fit it to your BB are in the same box the 24mm Rotor axle is shipped in.

Our JCOB cranks are a 24 /24 axle and will work, they are available down to 145mm in Std. Mid compact and compact gearing.

I am 6'3" with a 34" inseam and I am currently on the JCob 155mm crankset. I really have enjoyed the switch. My cadence has slightly increased but the biggest difference for me has come from the top of the pedal stroke. Previously, I could feel a slight him impingement. After switching, it became more fluid pedal stroke. I feel like I could have went down to a 145mm crankset and would have had no negative consequences.