Originally Posted by
DannoXYZ
There are some equations in this paper that helps explain it:
Cycling Up and Down hill.
It has to do with how resistance increases with speed. On flat ground, power-required increases to 3rd-power due to overcome air-resistance. On hills, power-required increases to 2nd-power due to lifting weight vertically. In a way, you can consider the flat ground power equation to be an integral of the hill-power equation (which is a derivative of the other). It's similar to the difference between power and torque for an auto-engine.
That's a very nice paper. I particularly like it because it validates many of the methods I've hit upon for lowering ETs.
We know that on-course time is reduced by going harder on the hills, as described in the linked paper. I have observed that on climbs lasting about an hour, my best speed of ascent is at exactly a 78 cadence when not limited by gearing, while my most comfortable speed on the flat is at about 88 cadence when cruising and in the mid 90s when TTing.
My guess is that there are curves of VO2 vs. cadence and of glycogen consumption vs. cadence and that these curves cross at a sweet spot for each particular length of climb, and that each cyclist will observe a different intersection depending on their talents. For a 3 hour climb, I'm sure I would find a cadence closer to my on-the-flat cadence to give the best rate of ascent, but I've never been on such a climb to look into it.
I am unconvinced by the theory that cadence slows because we have a dead spot in the pedal stroke. I suggest the opposite: that pedal stroke is not as important on the flat where momentum is high and a dead spot would not be noticeable. However when climbing, preservation of momentum is all important. Accelerations sap our strength. So another theory would be that we slow our cadence in order to fire our muscles precisely enough to maintain a constant torque on the bottom bracket. This would fit in with pros being able to pedal faster on climbs. Naturally they are more talented and more experienced in the details of pedaling.