I have my current cross bike set up on the trainer but I may have to stop using it due to ongoing health problems with my back. Would my power output on the trainer go down with my mountain bike? A very slack Rocky Mountain Instinct, or maybe I could use my XC mtb(commuter) instead since it at least has a good gear range.
Also, can I assume the Eagle 12 Speed drivetrain would be insufficient for Zwift?
I found my road bike uncomfortable and changed to my mountain bike. I put a wood block in the shock to lock it out and a slick tire on the rear. In the end watts are watts so the power is the same. You may want to adjust the settings on the trainer to account for the smaller gears. It is a little louder, but more comfortable.
Don’t believe this! Watts at 55rpm are not the same than watts at 100rpm for example. 
Changing from one bike to another can change the way you produce watts. But it depends more on the way you’re seated on your bike. On a mtb or a tt bike for example, also can make changes in your watts. But the changes are due to your position and the way you push on your pedals. Just try to imitate the best position on the bike that you mostly ride. For the rest, keep cool on rollers with a fan, drink enough 'cos the sweat and climb to get strong. 
The watts themselves are the same, 100W is always 100W no matter how it’s produced. The difference is in how you are producing the watts.
Yes, 250W done at 90rpm doesn’t feel as taxing as 250W mashing the gears at 60rpm but in the end you are still producing the same power.
Ok. Whatever you want. You’d better prepared to use watts succesfully. You’ll get more power using them in a right manner.
So the way a person sits on a road bike allows your muscles to put more force into the pedals right, and therefore getting more power than on trail bike. So if I switch bikes I should redo the FTP test(as if it was super accurate anyway lol)
Hi Douglas,
I suggest you’d better train in the position you want to be at your best. Whether is that on a roadbike, mtb or any other bike. Train specifically to get the most out of it. It has to do with the angle your muscles are working while pedalling.
Let me give you and example. If you would train a whole winter indoor on your mtb, wide handlebar, “little” gears… And you do that in function for better riding outdoors on a roadbike with a Road handlebar, 53t chainring…You see…That can never be functional, even if output (watts) is the same on both.
My Pivot Mtb didn’t have the right gearing for Zwift. I was fine on the climbs but really frustrated on the flats. It’s worth buying a road bike