Spring Powered Car - Which Design Is Best? I'm in the preliminary stages of building a spring powered car to race over a distance of 10 metres. It is a yearly event held in my local area and so far we've only seen one spring powered car. I think this is because it may be difficult to design a way to harness the energy efficiently.
The previous years design was very simple: A tube with a spring inside attached to one end. The other end was attached to string which was wound around the axle thus stretching the spring ... you get the idea. 
My question is what design could harness the power of a spring most efficiently?
Here are a few ideas I've come up with:
** please excuse my lack of paint skill **
Here is where you'll find the (non)technical drawings of the two below designs.
Design 1: A lever. From the picture you can see that there are two blocks which are attached on a pivot. On the other ends the spring is attached. The two blocks are then pulled away from each other. Then when the block is released the spring pulls tight which pulls the string around the axle , which in turn moves the car forward.
Design 2: This design uses a linear gear to drive the back axle. A rack and pin system pulls the rod with the teeth (not sure what the technical name is) back which makes the spring taught. It is then released, this drives the back axle. ** the problem with this is that once the propulsion stops so will the back axle as the gear will still be engaged.
This next idea unfortunately (or fortunately considering my paint skills!) doesn't have a picture to accompany it. For this my idea is to use a piston, like in a spring air rifle. However, the problem is that I've got no idea how to harness the power of the piston to create forward motion. Using a turbine doesn't seem like it would be the most effective way. 
Another important question: What sort of materials would be best to use with high power springs? And on that note, what sort of springs are best to use? At the moment I am thinking a trampoline spring may be powerful enough.
I am also very interested in the maths behind what's going on. I'm taking A-level further maths and physics and would like to have an understanding of what forces are going on. How would I be able to calculate the the amount of force generated by a given design (provided I knew the spring constant of my spring)? And then from that how would I be able to calculate the kinetic energy transferred from the elastic potential?
Any and all feedback and support is much appreciated! 
Yours,
-Jragon
 A: The usual design of something like this would use a flat coil spring (or better, maybe a dozen of them in parallel!) https://www.google.com/search?q=flat+coil+spring&safe=off&es_sm=119&biw=1695&bih=594&source=lnms&tbm=isch&sa=X&ei=kSobVPLlNdfloASAyYLwBw&ved=0CAcQ_AUoAg  together with a suitable gear drive (the latter could be from a bicycle). For a constant driving distance the complications of gears can be avoided by matching the wheel size of the vehicle to the number of turns needed to fully wind up the coil spring. The way I see this, one can probably pack thousands of J of energy into such a design, which would move a coil spring car of e.g. 50kg (with a child as driver) at a max. speed of roughly 6-10m/s, which is a not so harmless 22-36km/h (and more, if you are willing to go to extremes, but that would require serious safety precautions against the accidental case of breaking springs... and serious breaks!!!). 
This, by the way, is what a really professionally machined spring powered toy looks like: http://9bytz.com/spring-powered-toy-car/. I hope you can enjoy looking at this just as much as I do! 
Now... getting hold of the right flat coil springs may be very hard and expensive. So what may be a good, cheap source of springs? An old coil spring mattress comes to mind. While it does not exactly have the right kind of spring, one may be able to assemble multiple mattress springs into a useful torsion spring design. I say "may", because I have never looked at the mechanical properties of those springs. If they are made from poor material, they may break easily without storing much energy. Anyway... just a thought for a source of free springs... take an old mattress apart (but be careful with the dirt - wearing gloves and a face mask may be a good idea!). 
Another source of a great torsion spring may be an old garage door spring: http://www.overheaddoorparts.com/1gadosplewot1.html?gclid=CjwKEAjwheqgBRD5vOfR45az5AESJABy4m8-wWpQernIEgumm9FLBc9YnMemrcdWllZ37s226m3Q2RoC6p7w_wcB. This looks almost ideal! 
