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Please could you explain why this might be an issue for use in a velodrome or in a fixed tt situation with P2M? Shouldn't the p2m auto zero only when there is 2secs of no power which is unlikely to happen on a fixed? thanks
Goyt-
The trouble is that power meters tend not to recognise negative power figures and report these internally and externally as zero. So when you're slowing down, the power meter thinks that there's no power, and starts an autozero even though in fact there's load being applied, albeit reverse load. This skews the zero, and gives you inflated power figures.
Brommers
The USE chainring means you can use road cranks on a track bike, but it doesn't solve the problem of the auto-zeroing giving bad calibration results on a fixed gear bike. You need some way to turn off the auto-zeroing, which AFAIK was the only difference between the normal P2M spider and the track one - the track one had auto-zeroing disabled, which you can't do with the normal one.
These days I believe it's SRM or nothing for track crank-based PMs.