Just bought a used computrainer and noticing that while the mph reads the same on the CT as the PT, the watts are about 30-50 lower on the CT. I do a 8-10 minute warm-up calibrate it each time 2-3x to make sure it’s warmed up and then go. Any thoughts?
We’ve noticed that too. I can’t tell you much about why it is or hich set of information is more “right”. I’d be intersted to learn though.
That’s interesting, I posted about this a while back…
CT testing showed a FT value that represented a delta off 50 watts higher this winter vs. a used Powertap Pro I bought last winter.
The response here was that my Powertap must be off… Riiiight.
I would just ignore the difference and make a note of it in your training diaries; that’s what I wound up doing.
be sure your warmup is not spent in “calibrate” mode (where it shows speed and “up”), but in the erg mode showing some substantial watts. Calibrate mode has really no drag applied, and will not warm your tire nor the system well.
Do the warmup in erg mode even though it isn’t accurate yet, I would recommend above 150W -200W and work it up to levels you expect to ride at. It takes a while, like 10-15 minutes and do several spin-down cals about 2-4 minutes apart until the number stabilizes. By the end shoot for 2.0 to 3.0. Don’t worry, this number is not terribly important to accuracy, but it is to slippage. Some talk among users that 2.5 is better than 2.0. Whatever number you get, the CT uses it to compute watts due to drag, so getting a particular number like 2.5 is not critical. Don’t go below 2, you will slip.
Now compare put your CT into erg mode and ride at say 220 watts at about 18mph. This is fast enough and hard enough you won’t be experiencing the low end drag-only effect. You will certainly see your pt jumping around, but it should average around the 200, or a touch higher is normal, reputedly a slight difference between hub and tire measurement points.
After that, talk to CT to see if you can get it recal’d. There are net posting on how to do this yourself. Tedious, but it works.
Now, are you SURE your PT isn’t the part that’s inaccurate? Have you zeroed it while in the trainer??
Paul
I’d love for my Powertap to be off by that much!
I’ll take FT watts of 310 (Computrainer) vs. 266 (Powertap Pro) at 145lbs. 
How did you “tweak” your CT. Can you pls expand.