Making a kilometer blurry
Join Date: May 2006
Location: Austin (near TX)
Bikes: rkwaki's porn collection
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Assuming you have appropriate gearing, it's the inertia problem. On a climb, your speed loss during the weak part of the stroke is much more linear. On a descent, it's quadratic, since your wind drag (force) is proportional to the square of your velocity (e.g. you're not fighting much wind on a climb).
I think the resulting power surges take the edge off your FTP.
So, on a descent, your mid-stroke acceleration and deceleration is MUCH greater, and you're wasting more energy getting your leg speed back up (accerating the bike into quadratic resistance) every revolution.
This is the same reason trainers sap more power than when you're on the road, and why flywheel ergometers don't (think of the coast-down time on the road vs. on a mag trainer).
It's tough to fight the power problem, but it can help the speed to sprint over the crest a bit and get back up to a high speed as soon as possible (spending as little time going slow as possible).