Is there a disadvantage to testing your FTP on a trainer? I know that cooling is a lot worse when on a trainer, but other than that is there any other reasons against testing my FTP on one?

As long as you do subsequent tests on the trainer also, it should be fine. If you're going to use these numbers to set power ranges for on-the-road training, however, you should probably do the test outdoors, as the indoor numbers will likely be at least 20-30 watts lower.

Prepare to be miserable. Road vibration and changing conditions allow one to shift the body unconsciously on the road. On the trainer we ten to be immobile. This will lead to all new respect for a good saddle. Mentally, FTP testing on the trainer is among the toughest things I have done in bike racing.

my FTP on the trainer is 30-40 watts lower than on the road, although dialing in my power is easier on the trainer because of the lack of rolling hills.
I also find that my PT does not record data as consistently on the trainer. I'm guessing this is because there is not as much weight on the rear wheel and it confused the hub's sensors a bit. I get patchy readings because of this.

That said, the PT on the trainer is great for dialing in wattage goals and nailing them down for X amount of time. I did a 45 minute SST interval and it was great motivation to keep those watts stable through out the workout. I use PE and HR to get a better sense of where my wattage should be for the given interval, as it will be lower than on the road.

I personally dont mind trainer time, but usually keep it under 2 hours and 1-2 times per week. A good movie or Ipod playlist helps. Oh, and my terry fly saddle is nice, too.

Is the 20-30 Watts loss primarily due to the heat and motion of the trainer?

I will be doing this to set my training levels, so it sounds like doing it outside is the best bet. I was having a hard timing finding an 8 mile stretch on which to do my 20 minute FTP test, but I'll manage something.

Almost all due to the rider overheating.

Even with my garage door most of the way open when the temperature was about 35F, I was only able to manage 310w for 12minutes a while back.

Riding outside, I can do that for most of an hour, and 325-330 for 20min.

That, and the complete lack of anything to distract you sucks too. I watch DVDs on my laptop and I still can't take my mind off of the pain.

Learn to love the pain!

The reason for my initial question was actually due to seeing you post about overheating in the "Training Status" thread. I'm surprised that there is an effect even with only a 20 minute test (plus 20 minute warmup I guess).

I think I will do both within a week or two of each other just to have an idea of how much power I lose on the trainer. In terms of boredom, good music goes a long way in keeping me motivated. We'll see what I think after a month or two, though.

i am typically overheating after 7 or 8 minutes at lt indoors. i put a massive fan in front of me to try to help, but it's usually not enough.

When I ride in my garage, the temperature goes from about 35-40 to room temp ~70F in under two hours. This is a large two car garage with a storage space in the back, same volume as a 3 car garage.

The amount that I sweat is pretty gross as well. When I open up the garage door, and roll down the driveway to cool down, shirtless, my mom says I look like a steam engine going into a headwind; my entire body is steaming and there are pretty big clouds of water vapor rolling off behind me.

I think I've seen that Madonna video.

I test indoors and dont really have any problems. I used to have problems with it mentally but never from the heat. A few months of trainer only riding during the summer may have helped me adapt.

Now I actually prefer to do testing or specific power related training on the trainer.

It takes a bit of practice to get used to using the trainer, especially for intervals. But having spent a year living in Chicago, it was an absolute necessity.

Although I've heard of people having more difficulty maintaining a given power indoors, I've found that I'm able to attain the same wattages indoors. Mostly, I think, it's a matter of getting used to the feeling of working hard on the trainer. Without the sensation of a moving road, and without good ventilation, you're a lot more aware of your response to the intervals. Given enough practice, you ought to be able to bring the two together.

That said, I like to do threshold tests indoors, as it's both reproducible, and something I'm pretty used to at this point. Just this Saturday, I did 1X60 at a new all-time average wattage. It's really just something you have to get used to.