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Rally XC100 reading low

I installed by XC100 on my gravel bike a couple of months ago. I noticed immediately the power reading seemed to be 20-30% low. I have a Stages PM on both my road bike and mountain bike and I have a Hammer trainer. The Stages and Hammer are very close in readings. I have a 1 hour ride I do often on the identical route. I have probably done this route 200 times. My TSS readings are consistently around 50-55. With the XC100 they are always around 35. Having had PMs for several years now I have a pretty good feel for what a certain wattage feels like and I can tell the XC100 is reading significantly low. I had heard that not having them tightened correctly would cause this so I bought the Park 15mm clawfoot to make sure they are tightened to 34nm but that made no change.

My whole point of having the XC100 on my gravel bike was to have consistent training across all bikes and be able to use power to pace long gravel events. But I need reasonable consistent power readings to be able to do that. I hate to change the factor in the settings to adjust the power - for this much $ I expect accurate numbers and given I have three other PMs that are consistent my assumption is the XC100 is off.

Any possible solutions I am overlooking?

0 Luko79 over 3 years ago

Is the crank length setting right in the head unit too? (The value in the head unit overwrites the preset value what You set via Garmin Connect Mobile) rarely it sets to 140mm (instead of 172.5 or 175) and that is already 25% undermeausuring. And of course dont forget to manual zeroing your Rally before every ride.

0 johnmtx over 3 years ago in reply to Luko79

Crank length is correct.

0 Luko79 over 3 years ago in reply to johnmtx

then try the static weight test validation: get a known weight with a hook, put your bike on a wheel-on trainer (if You have Hammer then set an ERG mode at max power to brake the drivetrain), go to the head unit (i hope You have an Edge model!) calibration menu, push the manual zero offset calibration, the head unit will show "0.00", stay in this calibration menu, immediately hang the known weight with hook on the left pedal, and align it to horizontal position (pulling the rear wheel to lift the weight or pull the right crank arm to get this horizontal position),You should get a different number on head unit than 0.00, if the pedalmeter is accurate, then You should get : weight in kg * crank length in meter * 9.80665, an example, 20kg weight with 9gr hook: 20.009 * 0,1725 * 9,80665 = 33,85 NM, so the head unit should show this value, if it is much less, than something wrong with the pedal's slope calibration. (You need to play a bit with rear wheel or with right crank to get the absolute horizontal crank position which leads to the maximum value on head unit as maximum force).