Material With High Elastic Energy Density?

Hi Everyone,
I wish to calculate the feasibility of making a small, light, short-range car which runs on the potential energy of an elastic
Many hoovers (vacuum cleaners) have a button on them which you can press with your foot which will reel in the unit's power cable - with quite some force!
Using this principle, it should be possible to make a car which has, say, a light electric motor which is plugged into mains electricity, and which slowly winds up a spring, ready for you to go and drive with it. Because it is so difficult to get good information about the elastic energy density of suitable materials, I have not been able to calculate the feasibility of this system.
Lead-acid batteries, which are used in most electric vehicles, have quite a low energy density.
The energy density of hydrogen, petroleum, gasoline, diesel and LPG are all high - but it is my opinion that this high number is based on a fundamental cheat - they also use the energy of oxygen which does not have to be carried, but which is instead taken from the ambient air.
I think that latex might be a good material - simple, cheap hand-held catapults using latex can now fire ball-bearings with the same amount of energy that a Magnum 44 hand-gun can fire a bullet with (though the bullet will, of course, be far more lethal than a ball-bearing).
So - can any of you material scientists recommend a material?
Also, do any of you have any idea what energy density I might be able to obtain? This number is, IMO, the key to calculating the possible range and performance I might be able to achieve.
Thanks for any help!
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Seeker of Truth wrote:

Wisdom -- figuring out facts from available information.
Clue #1 Elastic stored energy is used in a few toy automobiles. Some toy automobiles use batteries.
The toy automobiles that use batteries probably go 10 to 100 times the distance of the wind up versions.
Clue # 2
There are a few wind up electric radios and flashlights. Battery powered electric radios and flashlights run about 10 to 1000 times longer on a set of batteries than a single mechanical winding system does.
The equation for the stored elastic strain energy per unit volume is :
Energy(per unit volume) = 1/2 # Y(Young's Modulus) * Strain *Strain (length/length)
You can get the elastic strain energy per unit weight by dividing by the density.......
However, except for the fun of digging up the numbers, you really don't need to take this much further.
You aren't the first person to ask the question of "Can I run a car on stored spring energy?"
Such articles were trendy in the 1970's push to alternative energy sources for automobiles, following the first "Oil CRisis". From that came such promises as the wonder of super flywheels "right around the corner".
HIstory...... Knowing history might lessen the need to reinvent the spring.... er... reinvent the wheel.
Good luck
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