On the wheel, it's unsprung mass. That extra mass hurts the ability to the wheel & suspension to quickly respond to bumps, and causes energy to be lost into the

On the wheel, it's unsprung mass. That extra mass hurts the ability to the wheel & suspension to quickly respond to bumps, and causes energy to be lost into the suspension and tire. It's even a problem on trains - early electric trains had connecting rods between the motors and wheels just like steam trains. Later ones have the motors in the bogeys, but the motors are sprung and can swivel on the same axis as the drive gear.