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Originally Posted by beerco Every 1.26 seconds (or something like that for the Powertap) the hub sends out the average power for all of the hub revs in the prior 1.26 seconds. I don't know what it does with the half finished ones, it probably sends them with the next packet. One interesting thing about this is that the time base doesn't always line up with cadence so you can see your power oscillate between a number below and above where you're actually riding. |
Otherwise known as the "precession" problem, i.e., the data recording rate and the pedaling rate are different, such that the temporal relationship between the two is constantly changing...unless, of course, you pedal at exactly 47.6, 95.2, or 142.8 rpm (i.e., take exactly one, two, or three full pedal strokes in 1.26 s), in which case the power displayed by the PowerTap "magically" smoothes out. (Because of issues like this, I can usually immediately identify the type of powermeter used to generate a particular file just by looking at the graph in CyclingPeaks...SRM is almost always the smoothest, PowerTap almost always appears noisier, and Ergomo and especially Polar noiser still.)
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Originally Posted by beerco The funny thing is that SRM lets you set recording rate which is quite a bit faster than cadence (e.g. if you're pedaling at 60rpm, the SRM should only be able to calculate power every second. It however lets you go to I think .5sec or perhaps even faster). So, SRM's either got more than one crank position sensor or they are simply synthesizing a crank position based on the last crank velocity to come up with those power numbers. |
The SRM calculates power using the average torque data over the last full revolution with the time required to complete that revolution (i.e., there is only one reed switch in the crank). If it is time to record data again and a full revolution hasn't been completed, it will record the last available values...consequently, if you set the recording rate to be very fast relative to your pedaling rate, what you actually get is a rev by rev report, not an X sec by X sec report. As a result, a plot of power-vs-time for, e.g., a standing start performed with the SRM set to record data at 0.1 s intervals will resemble a staircase.
12-16.-2005
Re: A watt is a watt is a watt? Trainer vs. Road Power. 
(Zipp 808 tubular). Disk rear will have to wait till next year.
Linear Mode
