You hear all sorts of factors for what you should multiply your trainer time by to make it equivalent to outside riding. I’ve spent many hours over the past few years thinking about this since I spend over 50 percent of my bike time on the trainer. It also pertains to some of the calculator inputs so has a double meaning to me. What I believe is that in terms of speed potential improvement (ability to increase 20 min power/weight ratio), the factor should be about 1.3. The reason for the factor is that if one sets someone sets out to ride 1 hour at Zone 1 on the trainer, they can set their HR at the middle of the zone for the duration and just crank along…no interruptions. When riding outside, due to stops, down hills, etc, your zone 1 ride will result in an HR at the bottom of the zone (if ridden properly without going into zone 2). These 4 or so BPM extra on the trainer represent addition oxygen passed through the system and further aerobic development. Based on what I have seen, its about 30 percent more development. Now, its not that simple otherwise I wouldn’t be discussing it. Anyone who understands my training philosophies knows that I believe in a two part equation for race success. One, is increasing speed potential (through fitness, body comp, efficiency, etc), the other is durability through volume for your race distance (meeting critical volume). The durability allows you to meet your speed potential curve at longer distances. What I have found is that in terms of durability, you just need to sit your butt in the saddle for at least the time suggested by critical volume. This is independent of if you are inside on the trainer or outside. Therefore a 1.0 factor should be used when working on the trainer in terms of durability. So, what does this mean? What I have done is use a 1.0 factor during the final 6-8 weeks leading into a major race when durability is important for race day. Prior to that, I use a 1.2 factor so that when I switch to the 1.0 during the final 7 it doesn’t become too much of an overload. I hope this helps and doesn’t further confuse things….