Difference in feel on a spin vs road bike

I have a few bikes (road, hybrid) as well as a Keiser Mi3 spin bike. For some reason the Keiser always seems so much more difficult in “feel”. It feels heavier and my legs are left tired from a session on it.

I decided to do an experiment. To eliminate the power meter I used the exact same calibrated power pedals. I also triple checked the fit was the same measurements. Heck I even have the same seat. Both rides were continuous pedaling steady state rides (I biked up a steep hill to completely eliminate gliding). Same wattage. Harder on the spin.

Obviously they are different bikes (between a light road bike and a weighted flywheel), but it’s the same power output at a continuous pedal exact same power meter and crank length.

So I became curious why at the exact same power can a ride feel so much more difficult on the spin bike. Then it occurred to me that there is no side to side motion on a stationary fix spin bike therefore no upper body transferring of energy.

So that raises a different question…am I better off with the static (Spin bike) for more of the training or the dynamic (Road bike). If you only could pick one which would you chose and why? Should I be adjusting up or down for each bike?

To me a watt isn’t just a watt in that you’re using more of your body on a road bike to generate it where as spin isolates the legs more. To illustrate this to the extreme imagine 200 watts on a road bike vs 200 watts on a recombinant bike…different muscles different exercise really. Not as extreme in my case but still some what a different work out.

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Keiser uses a lightweight (8lb) flywheel. Heavier flywheels provide more road-like feel and realism. Simply put the Keiser’s lightweight flywheel will force you to apply power throughout the pedal stroke. At first that will likely increase HR and decrease power output, relative to outside or with another trainer or spin bike with heavier flywheel (e.g. Stages SC3). Over a period of time you may adjust. Read this article:

Regarding preference - mine is a heavier flywheel like on the Stages SC3 spin bikes at our gym, or at home on my Wahoo Kickr (direct drive). I’d rather have my indoor training match outdoor as closely as possible.

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Yeah I always wondered what a heavier flywheel means. It carries more inertia I guess.

The main difference is the Q-factor most of the time, which changes the feel quite a lot