Originally Posted by
cooker
They're both perfect circles (or supposed to be).
It's going to be farther from the bottom bracket at the top, and, by the same token, closer to the bottom bracket at the bottom. So if both the top and the bottom of the circle have moved up an equal amount, it hasn't changed its diameter.
Imagine a bee settles on the top of your shoe as you pedal. Your cleat on the bottom of your shoe is moving in a perfect circle, and the bee is moving in a perfect circle an inch or two higher.
Doh! Why didn't I see that? Probably because it is not right.
Close your eyes. Imagine yourself sitting on top of a pedal. Now try to imagine how in the world the two orbits can both be perfect circles if one orbit is two inches higher than the other while they share the same axle. The imaginary bee's eyeballs are not always the same distance from the center of the bottom bracket. The bee's eyeballs are also orbiting the axle of the pedal upon which my shoe is resting. It is this extra orbit that creates the oval. Visualize .....