Originally Posted by
cachehiker
I read the same thing in Bicycling Science, 3rd Edition.
If I recall correctly, efficiency started to go down as cogs got smaller than about 15t. By the time you got to 13t, the loss was about 1.5% or something like that. There was no data for an 11t but I assume it would be 3-4%. The efficiency drop also decreased as torque increased and you went from being a spinner to a masher. I'm a spinner though so I'm likely to remember the spinner numbers.
At 5 mph on a mountain bike, that's 0.15 mph. At 20+ mph on a road bike, that's probably 0.25-0.35 mph depending on your aerodynamics. That's a good enough reason for me to stay on the big ring and avoid spinning out the small one.
At the time, I suspected the chain wrap around the rear derailleur was responsible for a large portion of this loss but I'm not sure that's true. I need to go back and read that section again but my friend Kara has borrowed my copy.
I haven't got a link to reference this to but I believe that the theory goes something like this.
A chain drive with rollers has a very high efficiency because there is so little friction involved in getting the chain around the gear. The bulk of the innefficiency is in the form of friction from the rollers sliding up and down the gear teeth when entering or leaving the gear. The "sliding time" ratio is higher relative to gear speed on smaller cogs. Thus a slightly (very slightly) lower effeciency. At least I think this is right........
