Apologies if this was already asked. I currently ride 170mm cranks and do all my TR on my Wahoo Kickr in ERG mode.
I will switch to 165mm cranks next month and I will continue to train in ERG mode. I think the short length will mean reduced torque… but will this have any affect on my FTP, in ERG mode? Thanks for any feedback.
I’m not the physiology expert on this, but I can definitively say your trainer/power meter will not notice the difference. All the trainer cares about is how fast the cassette is turning, and how much force the chain is pulling on it with.
Now, can your BODY put out more power with shorter crank length? Who knows…but from what I’ve read it may help depending on your physiology.
Some good info in this podcast…
Torque will drop a few percent for the force you regularly put down on the pedals. Almost negligible and in ERG, the trainer doesn’t care. You might see yourself spinning at a higher cadence to make up for the lack of torque. I made the same move and while I saw an initial drop in power, I gradually made up for it with the higher cadence. It opened up my hips, too…which made for a better fit all around.
Thanks for the feedback. I am still finding my balance between power and cadence and as my FTP increases, I hope my cadence at power output points will decrease a bit. For example at 150 ~ 170 w, my current comfortable cadence with the 170 crank is about 93. I hope with the 165 crank I won’t have to spin too much faster. I tried out a 165 at my fitting today and it just felt better.
TT bike has 155mm cranks. Road Bike has 175mm cranks. If I do a ramp test on either bike (TT Bike not in aero position) I will get the same result and honestly don’t think I could tell if somebody extended my cranks 20mm. Then again I seem to be very compliant with positions in general vs. somebody like @Jonathan who would notice if you changed his cranks 1mm 
The trainer won’t care, but your body might is the simplest answer (I can think of). Any FTP change should be to how your body responds, the trainer is just ready the power you are producing.
I went from 175s to 170s. The thing I needed was an extra easier gear so I could climb while spinning rather than using the longer crank to put more leverage on the pedals at a lower rpm.