|Entry #||Driver Name||Car Name||Mass
|23||29||Kim Aaron||Lego Car||206||17||4.25||2.5||21.92||9.02|
Balloon car designer Kim Aaron writes:
The Lego car used the balloons as a twisted elastic band geared to the back axle. It seemed that it would be easier to couple the stored elastic energy to the wheels this way rather than trying to use a piston. Jet-propulsion was never a serious consideration - it's sort of an impedance mismatch.
The toughest part was fastening the ends of the balloons to the vehicle. I basically inserted a plastic nut INSIDE the balloon and screwed it onto a plastic threaded rod on the outside of the balloon. Then I had to twist the balloon the right way so the torque would tend to tighten rather than loosen the nut.
I chose to use Lego Technic since it already has many mechanical components such as gears, axles, wheels. The alignment is important so your car doesn't swerve off the track. The mechanical tolerance on Lego parts is very good. The modularity of Lego meant I could fabricate rapidly, and I could modify the design rapidly.
I found it was very important to put talcum powder both inside the balloon and on the outside. If I didn't do this, the twisted balloon would sort of weld to itself and would not unwind as far.
It is very important to do a lot of testing. I really left it too late to realize I had a problem with attaching the balloons. The first thing I tried worked really well ... until I tried to do it again on another set of balloons. Then it failed miserably. I went through about six different approaches until I found something reasonable. Even then, the balloon broke on race day the first time I wound it. So I had to install a new engine and try again.
The best I did in testing on horizontal smooth concrete was close to 50 feet. Going uphill on race day, and perhaps underwinding a bit so as not to blow my second engine, I barely beat 20 feet. My hat is off to those vehicles that went 70-100+ feet. They had some real engineering in them.
Thanks Paul for a neat activity.