southpawboston

This has been somewhat debunked, at least for people who prefer a high pedaling cadence. A flexy BB reduces the height of bi-phasic torque peaks during a revolution of the cranks and helps spread those peaks out more (imagine two peaks on a graph: one tall and skinny, the other short and wide; they both have the same peak area representing total power output), reducing strain on the rider's legs and knees. This phenomenon (which Jan Heine refers to as 'planing') seems to hold pretty true for me, as the best climbers in my fleet are the ones with skinny tubing and flexy BB, despite any differences in weight. During steep climbs with these bikes, my legs don't feel as strained and I'm more limited by my cardiovascular shape than my leg muscle tone. On rigid bikes (like a couple of touring bikes with stout tubing that I've ridden), my legs give out long before my lungs do.

The concept of 'planing' may not hold as much validity for those who prefer a very slow cadence, where the torque peaks tend to be less sharp and more distributed throughout the crank revolution.