Big Chain Ring or Little Chain Ring that is the question?

Over the last few months I have been playing around with using the large and small chain rings on my turbo sessions. For clarity this has been on a smart trainer and used in ERG mode, so the turbo controls the power and all I need to do is pedal.

The first question that comes to mind is why does it matter what gear I use? 

Surely 200w in erg mode is still 200w no matter what gear I am in…right?  The short answer is yes and no. The power output is the same, however the difference is in the ride experience and what goes on physiologically to achieve that 200w. While using the small chainring and lower gearing to achieve the 200w you need to deliver a higher pedal force through the dead areas of the pedal stroke to keep momentum as opposed to the fly wheel inertia taking up the slack with higher gearing and a higher flywheel speed.

Without getting into too much detail, your choice of gearing ultimately impacts the trainers’ flywheel inertia and essentially this means: –

Higher gearing = Higher Faster Flywheel Inertia.

That generally in my experience means “easier” as the high flywheel inertia works to keep the drivetrain movement easier. This is akin to being “on top” of your gears when on a flat road or gentle descent happily tapping away to maintain a speed…Takes little effort right!

The other positive here is your bragging rights as it gives you higher average speed when you upload your data! 

Lower gearing = Lower Slower Flywheel Inertia 

This is more like the feel of climbing and you are constantly “in” the gear you are pushing just like climbing a hill there is no rest bite. The slower flywheel speed does however result in a quieter turbo session as the flywheel is spinning at a lower speed.

Which chain ring should I use?

In my opinion you should be trying to match the flywheel inertia to the majority of your riding or the area of riding you wish to improve, but in my experience I have felt like I have had a harder workout when I use the small chain ring as apposed to the larger chain ring on the same workout. In summary :-

Flat and Fast Road Riding = Use Higher Gearing and Faster Flywheel Speed

Hilly and Off-Road Riding = Use Lower Gearing and Slower Flywheel Speed

A final thought…I touched on early that the feel of the rides are subtly different when you use the small ring and the large chain ring, which is a result of the flywheel speed and inertia and the different use of the muscles in the legs. So whether you prefer to ride in the small or large chain ring, I think there is merit to change it up once in a while and use the muscles in a different way.