Crank Length?

170 on road bike and fixed gear (high cadence), 175 on brompton, 180 on ss mtb (low cadence high leverage)

i can't run 165s without it feeling like a toddlers bike -

Pretty much ditto but 5mm shorter all rond.

175 ssmtb
170 cx commuter
165 fixed and roadie

Spinning the 175s on the way to a trail is far from ideal.