Why is my bike uncomfortable on the trainer?
 

I've got a trek 2.3 that I can do 40+ miles on comfortably. I've also got a cheap steel road bike that I commute on that is ridiculously comfortable for me.

When I put either on the trainer, my backside is sore, parts go numb (yes, those as well as hands, etc), and I'm just about miserably uncomfortable after 15 minutes.

I'm using a riser block in the front to level it off and my trainer is a Bell motivator mag trainer.

What gives?

Same here. I find that it is the lack of variation in my position on the bike. Been toying with the idea of betting rollers to see if it helps. Pedaling a bike and going nowhere is just wrong......

I think bidaci is right. It might also be the lack of sensory stimulation.

Maybe I've got it clamped down too tight, a little looser mounting, perhaps?

Not if it is a standard trainer that you clamp your rear wheel into. Mounting it loose will just give the possibility of the bike popping out of the trainer. I seem to remember seeing somewhere a regular looking trainer mounted on a curved platform to allow some movement.

As far as sensory input, it probably does factor quite a bit into it . I do find when I have a good movie in the DVD player I ride longer on the trainer.

For me, I stand way less on the trainer. When I'm making my way through town I'm stopping lights, etc, and on longer rides I get out of the saddle every so often. It's hard enough sometimes to force yourself to slog away on the trainer, let alone remind yourself to switch positions when you don't really have to.

Basically, you're spending more time on your butt.

That might help - I find the trainer pretty uncomfortable as well, and seem to have less pain/numbness on rollers, though my friend has been 'borrowing' my trainer for quite a while so who knows how accurate my memory is. But at least rollers are more fun!

It seems like any fit issues you have just get magnified on a trainer. I used to ride a too-big bike and the lower back pain it caused was 10x worse on the trainer.