The problem is fluid viscosity. When you start cold, the fluid in the resistance unit is thick and high resistance. As you ride, the fluid heats up until it stabilizes. As it heats up, it becomes less viscous and therefore less resistive.

The nut is that there is no fix for virtual power for any fluid trainer. But, if you have an external PM or your trainer has real power, this solves for the variable resistance of fluid or whatever the braking mechanism.

My fluid trainer (older el-cheapo fluid trainer from Nashbar, I think) actually does the opposite: After 7-8 minutes, there is a fairly quick jump in resistance... as if the fluid got thicker. My guess is that it expands as it heats up, and with higher heat/pressure, it is more resistive.

It doesn't really matter in the end. I used to train by HR (now I have a powermeter), so finding the ride load for a given cadence is just a matter of shifting. I know I'll have to shift to an "easier" gear sometime in the first 10 minutes, so it is no big deal.

---

-Mark Rebuck, http://www.markrebuck.com/