Calibration

So I have been mentally been telling myself that, "I need to check the calibration of my MTB powertap" for a couple of months now, but just haven't got around to it. I know a number of the guys with them have started having various problems so, I thought now was a good time to check the accuracy.

So today, decided was the day and I created the spreadsheet and set aside 1/2 hour to do the testing. The testing procedure is also the same as for a SRM, except with the powertap you test various gear combinations. The only equipment required is something with a know weight, either youself or a couple of lifting plates of at least 20kg and a spirit level for leveling the cranks.

The spreadsheet is just a series of basic calculations, Torque= weight * crank length * 1/25.4 * (rear sprocket/ front chainring).

The yellow powertap headunit displays the measured torque value if you hold the select button for 2 seconds.

So this is the result,


Firstly, the powertap is reading ~ 3.0% low aross the gear range or about 8.9w at threshold. Secondly you can see that the 12t & 36t produce the highest levels or error. This is a consequence of the chainline and way that torque is applied to the hub. 3.0% is double the quoted accuracy from Saris for their hubs, but at this stage I am not worried enough to send it back to be adjusted.