One issue that I never heard them overcome was resistance to turning along one axis. Industrial flywheels are fine, because they're usually fixed in place. Ca

One issue that I never heard them overcome was resistance to turning along one axis. Industrial flywheels are fine, because they're usually fixed in place. Cars tend to want freedom along every axis. My distant memory recalls some discussion of buses fitted with flywheels for regenerative braking, but they had issues with hills.