Looking for some scientific reckons. If I take 170mm cranks from my road bike and put on my TT bike which currently has 165s, how much should I raise my seat height?

My question is as per the subject line, going out to the TR forum for some accurate guesses before I decide whether I need to go back to the bike shop for fit advice.

I’ve currently only got power on my road bike with 170mm cranks. For some upcoming TTs and then next year 70.3 season I’d like to have power on my P-series. A cheaper alternative to buying a 165mm crank power meter could just be to switch the cranks over for race days etc. I have thought about a Favero Assioma etc but I’ve got Shimano pedals on my road bike, P-series and my Shiv which lives on the trainer.

How much do people think I’d need to adjust my saddle height and fore aft to accommodate?

You need to LOWER your saddle 5mm to start. This will close your hip angle, yes, but you need to think about the bottom of your pedal stroke, which will now be 5mm lower (used to be 165mm below BB, now 170mm).

Start there, then experiment.


That’s logical. I was definitely thinking about it the wrong way around. Thanks!