Originally Posted by
Voodoo76
The earlier road Quarqs, ones that required a magnet, had multiple switches (as I recall 4). They primarily used that to verify direction of crank rotation, you could zero the meter by pedaling backwards.
This is a potential advantage for wheel based systems like Power tap. Update frequency for wheel rotation speed as the "v" in the power equation is 3 to 3.5 times the crank "v" for typical gearing.
True, but the downside of wheel-based systems is that they are very far down on the kinetic chain. Power meters are supposed to measure what the body is doing, so ideally you'd want the meter close to the body to get the best readings. Without actually being on the body, the closest we can hope for is a shoe power meter, next best would be cleat, then pedal platform, then pedal spindle (LOOK, PowerTap), then crank arm, then chainring spider (SRM), chainring, chain, cog, rear hub (PowerTap). Some power is lost as friction at every step listed.
BUT, on the other hand, the rear hub is very close to where the power is applied to the ground. So, it could be argued that it's in a great position to measure what really matters: How much power was produced that made it through the bike and into the tire that propelled the bike.
