trainer road question on accuracy.

Sounds like you know going in that it's not all that accurate when compared to an actual power meter but you're looking for consistent training measures.  Same tire inflation and same wheel tension/resistance should yield consistent measurements.

That stated, it's still an algorithm based on what's basically a made up number (speed on the trainer) so the "garbage in, garbage out" philosophy still holds true as far as the numbers are concerned.  If you go in operating on the theory that the numbers are more than likely crap but it's consistent crap than you can train effectively with TrainerRoad's Virtual Power.

I still think HR and RPE trump virtual power numbers but you can do all three.  Add a HR monitor with the TR VP and you have a pretty good training system for just a few bucks. 

The trainers have a resistance curve that relates to speed. The faster you go, the higher the resistance is. That's how they use speed. They don't try to relate it to outdoors. The accuracy from the curve can be questionable, but you can help the consistency to an extent by keeping the tire pressure the same and the tension of the resistance unit the same every time. Having the accuracy too would be better still, but if can at least keep the consistency you can keep track of progress and work where you intend to in regards to intensity.

just air your tires up to the same PSI and turn the knob for the trainer wheel the same number of times every time you use it and you will be fine.  i used virtual power for a while before getting a powertap and it worked great for me.  don't worry about speed either, as long as you set up the trainer pretty close to the same way each time it doesn't make a difference; you already know that the power numbers are "virtual" so don't take them positively or negatively, just a baseline for you to work from that doesn't correlate to anyone else's numbers.  when i got my powertap my real FTP was about 30 watts higher than my virtual power FTP because of how much tension i was setting on trainer, which caused my "speed" to be slower and give a lower power value, but again that doesn't matter because you can't take virtual power out on the road or use it for pacing during a race; it's just for the trainer. 

Having trained by HR for a few years before starting TR I disagree with that.  It wasn't until I tried the VP that I realized inefficient my efforts were on shorter intervals.  

Having said that, I did a few rides with a borrowed powertap wheel before I decided to trust the VP reading.  

I've been using TR for about three years now.  I'm "lucky" in that my indoor speed numbers on my Kurt Kinetic Road Machine are pretty close to what I can match outdoors and "feel" right (especially through the important training speeds of 18-22 mph).  It doesn't work that way for most people (I'm borderline Clydesdale).

As others have said, Trainer Road's key is the consistency of your trainer (the power curve) and the variables you can control (psi and turns of the wheel, more or less consistent temp, etc.).  Do the 8 or 20 minute test, accept the virtual FTP, and have at it.

Nothing to worry about as long as you are consistent with the setup. I've been using VP successfully for 5 years.

If using a Kurt Kinetic fluid trainer its been stated that the "calibration" is to have about a 13.5-14 sec Spindown from 20mph to 0mph (wheel stops spinning) after ~10' of warm-up.

I guess I should have expounded on my comment a little more.  The reason I stated that I think HR and RPE trump virtual power numbers is because the VP numbers cannot be used outside.  So you'd still want some kind of baseline to gauge your outdoor efforts.  If you can correlate your indoor VP number with a HR or RPE then you can replicate (for the most part) that effort outside with HR/RPE.

I agree with that completely.