Julius Naim wrote:
> I'm pretty rubbish at picturing this, has anyone got a picture to
> point me at to help me get it right?
How about a mental picture? This one works for me: Think about the load when you stand on the pedals with the cranks horizontal and stationary, as when coasting over some rough pavement or down steep hills. If you coast with the left foot forward, then this is the same loading on the cotters as with pedaling. (The left crank is trying to make the axle go 'round in the normal, forward direction and the right crank is resisting that force, just as in pedaling, except the forces exactly balance so there's no acceleration)
Both cranks are pushing their cotters against the bottom edge of the BB axle, and of course one cotter is facing down and one up, so only one can be bearing on the thick part of the taper.
This is where Sheldon is a little bit wrong when he says "Note that all of the loading on both cotters occurs when you push on the left pedal." That's true for the case of pedaling, which is most of the ride, true, but doesn't account for the time you spend coasting over bumps or downhilling with the right foot forward. There the forces are fully reversed. And if you hit some big bumps in that position, the magnitude of the force can be higher than even strong pedaling. Thus you can get TWO dents in each cotter (if they are a little loose, allowing them to become dented), not just one dent each which would be the case if only the left crank forces mattered.
Maybe that's why I got in the habit of coasting with the left foot forward? ;-)
Mark Bulgier
Seattle WA USA