Or whatever they are called... The things you put your back tire into to make your bike into a stationary bike. Whats the general consenus on these things, are they worth the money? Will they work with big mt bike tire(2.5)? And do the ride well? Ive never actually used one and am just looking for something for active rest periods during workouts indoors. Just wondering if I should drop the money on one of these or get one of those really sweet airdyne stationaries for my home gym. Just looking at them its seems like they might be alittle flimsy or jump around too much if doing anything more than light pedaling. Any opinions?