Bizarre Powertap numbers…comparison

[fabiosav]

Bizarre Powertap numbers…

I have been getting nutty numbers on my Powertap (wattage up about 100 watts from one day to the next, without EPO…) and so I went to a friend’s and hopped on t he computrainer. We set it for 200 watts, and I got insane numbers (we all know what 350 and 200 feel like, and it is unmistakable). Anyway, he had some other guys pick-ups there (the yellow display unit) and, at 200W on the Computrainer, steady ride, we got 50 watts, 350 watt and 110 watts. One of the three was right out of the Saris bag and it was way off… The differences were not linear, either. Some were closer at 300 watts, other at 100 watts. Brand new indicated 110W, and, again, 200 and 110 feel way different.

I can understand my hub is not calibrated, BUT it would be consistently out, not sending 50W when I connect one unit and 110 or 350 on another, at the same pace, resistance etc.

Thoughts? Do you trust their numbers?

 

[Thorman]

Bizarre Powertap numbers…

I have been getting nutty numbers on my Powertap (wattage up about 100 watts from one day to the next, without EPO…) and so I went to a friend’s and hopped on t he computrainer. We set it for 200 watts, and I got insane numbers (we all know what 350 and 200 feel like, and it is unmistakable). Anyway, he had some other guys pick-ups there (the yellow display unit) and, at 200W on the Computrainer, steady ride, we got 50 watts, 350 watt and 110 watts. One of the three was right out of the Saris bag and it was way off… The differences were not linear, either. Some were closer at 300 watts, other at 100 watts. Brand new indicated 110W, and, again, 200 and 110 feel way different.

I can understand my hub is not calibrated, BUT it would be consistently out, not sending 50W when I connect one unit and 110 or 350 on another, at the same pace, resistance etc.

Thoughts? Do you trust their numbers?

Try zeroing your torque.

 

[RapDaddyo]

I'm not sure I absolutely understand what you are describing as the watts display behavior. If I understand correctly, two candidates come to mind. Zeroing the torque as Thorman suggested (I zero my torque on every ride). Hub batteries is a second. The worst scenario is that one of the strain gauges is not working. I think this might result in the watts display pattern you are describing, where it would cycle through a repeating pattern, say, 350, 200, 100, 50, 0, 350, 200, etc. This would suggest that it is firing the correct watts then firing blanks, and trending down as it averages, then repeats the pattern. I'm just not certain how the hub behaves if one of the strain gauges goes on the blink.

 

[acoggan]

Thoughts? Do you trust their numbers?

If used according to manufacturer's instructions, yes. Although I am aware of a few rare exceptions, PowerTaps tend to be either spot-on, or clearly not working properly at all. This is different than the Computrainer or SRM, where significant but more moderate errors of 10-20% aren't all that uncommon.

 

[Woofer]

Bizarre Powertap numbers…

I have been getting nutty numbers on my Powertap (wattage up about 100 watts from one day to the next, without EPO…) and so I went to a friend’s and hopped on t he computrainer. We set it for 200 watts, and I got insane numbers (we all know what 350 and 200 feel like, and it is unmistakable). Anyway, he had some other guys pick-ups there (the yellow display unit) and, at 200W on the Computrainer, steady ride, we got 50 watts, 350 watt and 110 watts. One of the three was right out of the Saris bag and it was way off… The differences were not linear, either. Some were closer at 300 watts, other at 100 watts. Brand new indicated 110W, and, again, 200 and 110 feel way different.

I can understand my hub is not calibrated, BUT it would be consistently out, not sending 50W when I connect one unit and 110 or 350 on another, at the same pace, resistance etc.

Thoughts? Do you trust their numbers?

If you have lots of doubts - just do the stomp test described in the FAQ for the PowerTap.

 

[fabiosav]

Actually, it was zeroed. What is odd is that the same hub would result in three very different numbers from three different computer units. I do like the idea of zero-ing the torque more often. Andy C, have you really found them spot on? Do they match compu-trainer?