Originally Posted by
hueyhoolihan
and i may be way off base here, but i suspect that many problems that can't be reproduced on the stand are due to the fact that there is less slack for the tensioner to take up on the stand, in that people rarely pedal a bike with an unloaded wheel by hand as they do when propelling it along the ground with their legs. leaving slack on top and the bottom. the slack is more equitable above and below the chainstay. just a thought. could be way wrong about this.
As you said, you could be wrong here ---- and are.
With a working freewheel, the upper and lower loops are totally isolated from each other. The upper (drive) loop would work identically if there was a mile of chain behind the bike, and the chainrings spooled
used chain onto the ground. The tension there is 100% the result of driving forces.
So the lower loop is simply a return loop to return the chain the chainrings are spooling forward back to be recycled. The tension in the lower loop is purely from the RD idler cage spring. If it weren't for the RD, the lower loop would be slack and sag under it's own weight (as it does on single speed bikes) regardless of how easy or hard one pedals.
However there is a real difference between the stand and streets. That's the bouncing around that bad pavements and potholes create.