[nkaminar]

Years ago, before lithium-ion batteries, some experimental busses were built using large flywheels that were used for acceleration. There was a complicated gearing system involving the engine. The flywheel was spun up during braking and then used to get the buss going again after the stop. The engineers discovered that a flywheel that was tapered from the hub to the tip could store more energy because the stress in the flywheel was more uniform. The flywheel shape was very much unlike the flywheels used on old hit and miss engines or old steam engines. The flywheel ran in a vacuum.