there isa comercial flywheel system being pushed to road manufacturers now, apart from weight a big advatage is there is no danger of a short circuit / fire / battery explosion but it has some disadvantages namely that if it breaks up it will do a very passable imitation of a handgrenade and it also has to run in a near perfect vacuum. williams apparently just repump the air out at the last mintue as the seal are more than adequate for a 2 hour race but road cars will need an air pump which will have to have completed its job before the flywheel spins, this pump also adds weight. further disadvatage's for racing are that you can't charge the system before you leave the pits so it may be impossible to have a fully charged system for a qualifying lap or race start and it also concentrates all its weight in 1 unit, an electrical kers could be built with it's components in differing parts of the car eg bmw fit their batteries in 2 packs, 1 each side of the fuel tank rather than a single unit under the tank