zeroing torque on a trainer?

[kopride]

If I remove the contact between my wheel and trainer drum, and let it spin loose, can I zero the torque on my PT inside by "coasting" on the trainer. I usually zero outside by coasting, so is a zero torque on a trainer accurate? I just had a battery change on my CPU and I think it is reading a bit high. . . either that or I picked up around 18 watts on my FTP after having my bike in the LBS for a few days while I rode my fixed gear and did intervals on a PE scale. I will probably not be able to get outside between trainer sessions this week. I'm just wondering if the torque reading would be accurate since there would literally be no weight on the wheel if the bike is clamped on the trainer. Probably a stupid question, but I'm wondering if anybody has a good reliable way to zero out their torque inside, or it is the same process as outdoors?

 

[swampy1970]

It's the same procedure as outdoors.
 
My trainer has variable settings for the resistance. I just put it on the lowest setting, wind it up and then freewheel and zero the torque.
 
In addition, I check the tire pressures before each ride and check the time it takes to "roll down" from 25mph to 10mph and adjust the pressure of the rear roller accordingly - just to keep as much as I can the same.

 

[kopride]

Thanks. I have a fluid trainer so the resistance is not variable, but I will try and wind it up so that it does "coast" long enough to zero out the torque. I wonder whether I can just take the drum pressure off entirely to "coast."
 
As for tire pressure, I do bring it back up to 120 psi before each session, but the contact between the drum and the tire is a bit ad hoc. I basically adjust the roller pressure by making it tight enough to keep contact without slipping but not so tight that it makes any real imprint in the tire. Given that I am reading watts based upon a strain gauge on the hub (PT), my sense is that drum pressure doesn't affect the actual watt reading. I do understand that it can affect a CP or other device that reads at the drum, but I would think that it is not a real factor with a hub device like the PT, or a crank based gauge. If I am wrong, let me know, and I will spend a few more minutes trying to get that drum pressure consistent.
 
Again, I am hoping that I just made a big jump in power, but it is very suspicious after a battery change. I rode 2 x 20s @ 258 last night, and they just seemed too easy. I will zero tomorrow and see.