Big_h,
“Maintain” effort during your spin class and your HR will be way up there. In teaching my classes we regularly hit and maintain 85-95% MHR and even top out the last song of the class. The "problem" with spin classes is mostly due to music patterns and the tempo of the instructor. Bouncing up and down for 20+ jumps listening to some techno will not cut it if you are really looking for an "interval-type" effort above LT. If you do not have access to a spin class taught by a cyclist rather than an aerobics instructor, try this: push through the "transition" of the tunes. Typically, most instructors require that you let up, grab a drink of water and towel down when the song winds down. This keeps you away from higher levels as is allows you to cool. The continuous effort (2LAP) that racing offers and delivers in the form of high heart rates can be replicated by “combining” song sets. When the rest of the class reaches for the towel, you should reach for the tension knob, gear up and crank it !
To really crush yourself, keep your eye on your monitor and the time. When you hear in you are in the last song set…jam , jam, jam. Try some level 10 out of the saddle sprinting until the last song ends. Not only will you get into 90-95% rates, you may even peak. Best thing is when you peak on a spin bike you don’t have to worry about crashing. You can “pop” and just collapse on the spin bike...like I did this morning.
You can, and should, be able to replicate your HR training levels during training on a spin bike. It is, however, 100% up to you.
CatSpin