>>1612303the steepness and length of the hill helps push up the speed along with the weight of the bike
but also the wind conditions make a big difference, on a windy blustery day sitting upright will slow one down to about 40 kph
the same hill when the air is still nets 50 kph from sitting up and 60 kph if crouched closer to the top tube
but this is with ordinary gearing .eg 48x16 @ 168 crank rpm
using a chris froome style of descent posture I was able to get 70 kph out of 48x14 with 26x1.75 (47x559) tires, but only with perfect weather
I stopped after the lack of tire grip and having to pass cars on the outside of corners became an issue
