exxxviii wrote:
The downside to single-sided power is consistent inaccuracy. It is like a broken analog watch-- it is correct twice per day, but just don't know when those moments occur. If he has a power imbalance, then the power will be incorrect by about 2x the % off. For example, if his L/R imbalance is 48%/52%, then the PM would read about 4% low. For example, if he was riding at 250W with a 48/52 imbalance, the PM would read 240W. At least it would be consistent, as long as his imbalance was consistent.
That's not strictly true. The 2% refers to the range of accuracy, not how inaccurate the power meter is. So the reading could be anywhere between 2% low or 2% high. It could be absolutely spot on. So using the example above, the left handed power meter is reading the left leg at 120watts and doubling it to 240. 2% either way means that the actual power being put out by the left leg is anywhere between 117.5 watts and 122.5 watts. This then means that the overall reading could be anywhere between 235 watts and 245 watts. If the 48/50 is consistent at all powers then that reading is as accurate as a double sided power meter as the left leg will always be putting out 48% of the overall power and the power meter will give this reading, +/- 2% (which is the same level of accuracy as almost all power meters out there).
That's the issue with single sided, it's whether any imbalance is consistent across all power ranges. The only way to tell is with a TRUE double sided power meter (such as power tap P1 or Garmin Vectors). SRM is not a true double sided power meter, for example. So, you could spend a chunk of money on a true double sided power meter to find that your leg balance is consistent across all power levels. End result, your readings from a single sided power meter will be just as accurate (save for silly situations where you deliberately pedal harder with one leg, or do single leg drills) as a double. Single sided are a compromise and, for some people, not particularly accurate. For many, however, they work almost as well, if not as well as, a true dual sided power meter.
I have no vested interest. I started on a stages and using a stages I improved my FTP by around 25% from when I first bought it. I now own power tap P1s and they give me the same FTP readings as my stages and I've found the only time I am biased is high power, high torque (i.e. climbing a 20%+ gradient). Any other time, I am 50/50. The end result, I could have kept my stages and it would have served me perfectly.