As a first step, it was important to find out what was already available. A search on the internet revealed numerous examples of rubberband powered cars. I chose one of these designs to build in order to ascertain what was involved in the process, identify limitations and other issues.
The design chosen came from the following internet site: http://pbskids.org/designsquad/projects/rubber_band_car.htm
It was a simple two wheel design that could be put together in about 20 minutes using materials lying around my house. I also thought this design would highlight many issues to consider because of its simplicity.
Although I improvised with materials and took some liberties as to how it was put together, the principles of the overall design are consistent with that specified on the website. The picture below shows the final build.
The axel is a thin wooden skewer. The wheels are CDs. The CD holes are too wide for the axels so I placed duct tape on one side of the CD and dropped a small washer onto the tape from the other side. The CDs and axel are held together with â€œbultakâ€ . The rubberband is held using electrical wire holders (not sure what they are called but I have a lot of these). This also gives me flexibility to try different rubberbands and test with more than one.
Momentum is achieved by winding the wheels (and hence the rubberband) and releasing the vehicle.
Seven windings appears to be optimal for this model. If you wind the rubberband any tighter, too much energy is released at the start and this flips the vehicle. Seven windings results in a distance of 2.6m.
Does adding a second rubberband equivalent to the first increase distance and acceleration? The answer is no and yes. In fact, adding a second rubberband and winding in the same way as for one rubberband (ie. seven times) acts in the same way as making too many windings. Too much energy is released at the start thereby flipping the vehicle. If you halve the number of windings to compensate, the travel is more stable but the distance achieved is only about 1m.
I thought further distance could be achieved if the front of the vehicle was raised off the ground thereby reducing drag. So I added two washers as front wheels (hence the skewer at the front in the picture). However, the extra weight (albeit only 24 grams) was sufficient to counteract the force in the rubberband and prevent the vehicle from moving.
I added a second rubberband to compensate for the extra weight. However, even with seven windings, the vehicle was sluggish and barely achieved half a meter.
With the single rubberband in place, I tested it out on a small ramp. As expected, the ramp acts in the same way as the additional weight and stops the vehicle dead in its track.
Choice of rubberband is an important factor. Not all rubberbands are made the same. Some stretch a long and promise distance. However, in practice, the weight of the test vehicle alone was sufficient to prevent the release of the potential energy. Others may not stretch much but promise great acceleration at the start; in practice too much for my test model. Getting the balance between weight (including ramp) and elasticity is going to be tricky. Something that works well on the flat may not have enough power to overcome a ramp. Something capable of overcoming a ramp may have too much power on the flat.
In addition, if our models have to carry weight, ie. the remote, a single rubberband is unlikely to be enough. My test vehicle performed poorly with a single rubberband (even with two) with additional weight of just 24 grams. A remote is likely to be between 120 â€“ 150 grams. This is a significant challenge even with more sophisticated designs.
What we need to do is test the rubberbands to determine how much force they exert; in particular, whether they are capable of greater force than the weight force. If not, the weight force will prevent our models from moving.
Continue to Choice of Materials
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