Ah, I hadn't thought about the poor derailleur. You're saying that of course the bottom chainline will tighten when the wheel drives the cranks.
Yeah, I learned that the hard way, fooling around as a kid. We all put fixed gears on for the winter, leaving the derailleur in place but bypassing it with the chain. Being smarter than everyone else, I saw no reason to go to all that work so just threaded on a fixed cog and reinstalled the wheel. One less Super Record rear derailleur in the world.
Well, you make a good argument, also. But what about my HR dropping while out of the saddle on the spin bike fixie?
You mean in spin class? Dunno, for sure. My HR goes up when I'm out of the saddle. I think it's a very individual thing. Some folks love to climb out of the saddle, others hate it.
You probably can remember that it was a trick learning how to pedal out of the saddle on the flat, at low cadence against small resistance.
Well, sort of. I have to hold my hips a bit low and hold the bike upright. I also don't have any reason for every doing that, though.
There's a tendency to have a slack chain at about 11:00, so one has to consciously push that top foot forward at the top of the stroke, which is quite hard to do out of the saddle as there's nothing to push against. The fixie removes that bit of difficulty and your foot is automatically presented with a pedal at the appropriate position for a power stroke.
I can see that. I know that trying to spin out of the saddle under low resistance is fatiguing to muscles of my legs that normally aren't heavily stressed. But my critique still stands: if the fixed gear is helping the rider through some component of the pedal stroke, then the fixed gear must be robbing power from the drive train to do it!
I noticed that when some folks tried to revive the "retro" technique of fixed gear training, they neglected the point that, back in the day, the fixed gear would be very small. Like 42x23. So these guys spent a winter on the 48x16, pedaled around at 80 rpm -- or slower, up the hills -- and then wondered why they were pedaling squares when they got back on the freewheel. What happened, of course, was that they got lazy and began allowing the fixed gear to carry their legs through the "dead spot" at the bottom of the pedal stroke. Once back on the freewheel, they had to retrain their legs to eliminate the dead spot through muscle power.
Good stuff, this thread.