I assume by "spin bike" you mean one of those health club bikes used in spinning class and not one of your bike set up in a trainer. I try to train in the off-season on my rollers. But, none of those even roughly equates to moving my 17 lb bike and 187 lb body up a hill or through the air in front of me. A combination of rollers and spinning classes is about the best I can do until the weather permits road work.