Sorry no details, typed & polished a bunch o fluff, then lost it when tried to send over a dead connection. (EDIT - I came back & added some detail )
Physics definitions. Work statement:
Work = force * distance
or, for angular motion, Work = torque * angular displacement.
(gosh I hope that's right
Equivalent power statement (power output equals time rate of doing work):
Power = force * velocity
or, for angular motion, Power = torque * angular velocity.
If going the same speed, then to a good approximation, the bike is using the same power, same work per time interval. From the above, not too hard to show that changes in average force are proportional to changes in the gear ratio. (Sorry, that's the part that got lost, and I aina doin it again
For shifting to the 13t cog from the 11t, force is 11/13. Suppose shifting the front from 40 to 52, force is 52/40.
This ignores mechanical friction changes. These are probably small so can be ignored for this discussion.
Also neglects biomechanical factors - accelerating & decelerating your thigh, calf & foot - consensus seems to be that 80 rpm is the sweet spot for most people - anyways this is beyond the scope of the question.
Can't know 'exact number of pounds' without a whole lot more info, some of it difficult to measure. Unless you have one of those BBs that measures it directly. I spose could make a stab at it -- need to know the crank arm length, the average number of watts it takes to propel an average (mtn/road/take yer pick) bike at (pick your speed), could take the diagram I saw years ago of average force profile for the power stroke and do some integral calculus on it... to estimate force at a specific angular position ... but life is too short...
Last edited by duffer1960; 04-09-09 at 06:47 PM.
Reason: clarification - why the heck am I still writin on this?