I don't know if this makes sense or not but I've been doing a lot of spinning classes. Due to time constraints and weather, I haven't been putting as much miles on my road bike as I'd like.

How well does spinning class (on a spinning bike, of course, not my bike on a trainer) translate into being on the bike? I ride a LOT harder in spinning class but since the spinning bike isn't adjusted exactly like my bike (though I try), am I working different muscles and it isn't as beneficial as being on my own bike?

Right now I'm not factoring in spin class as far as bike training. What are the general feelings on this? I'm doing a century in May and my weekly mileage is about 60 +/- but I usually do 2-3 spin classes on top of that. (That and some running and weight training...)

Thanks