I race a 650 bike but my trng bike is 700. Should I use the same crank length for both bikes or is a slighty larger crank length (172.5 vs 170) for the smaller wheeled bike ok?
No. Use the appropriate size for your height/inseam.
Chris
There’s a thread going on here right now where there’s considerable discussion about crank length. The thread title is about Fabian Cancellara’s bike being at a bike shop in Canada or something like that. The discussion just evolves into a crank length discussion/argument.
My road bike is a 700 with 172.5 cranks. I race a TT bike that has 650 wheels. I keep the crank length the same, but I use a 56/42 front chainring setup and a 23x11 cassette to compensate for the smaller wheels. Works for me.
Like Bob said, if you want the gearing to stay the same you change the chainrings to (I think) 55/42 (look at a gear-inch calculator online), but don’t worry about crank length… it’s mostly irrelevant.