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Soap Box car - heavier better ? Part II

  1. Oct 30, 2013 #1
    After reading throught the original thread about soapbox cars, I have a better understanding of what is happening. The soapbox derby that I'm in has few rules. You must have a 6" or less wheel, steering and some sort of brake. The hill is a constant slope and there is a 10' push zone. From the last race, it became clear that the initial push seemed to have the most influence on the result. Those that got out first were hard to catch.

    However, there were a couple heavier cars that seemed to catch cars that were in front of them, but not too much. From the thread, it seems that heavier is better, but only if the wheels/bearings could take the extra weight without creating excess friction. The fastest cars used urethane scooter wheels. Skateboard/longboard wheels were a close second.

    So - I'm thinking of making a two-seater or a car capable of adding weight so it would be one of the heavier cars in the competition.

    My question: Would extra wheels be an advantage in spreading the weight out ? I was thinking of running 3-4 wheels on each (toward the rear) side spaced fairly close - similar to that of an inline skate. I could also add a couple additional wheels in the front in an attempt to keep the weight balanced.

    Is it as simple as I'd like it to be ?

    Does that make sense, or would the extra wheels create even more friction ?

  2. jcsd
  3. Oct 30, 2013 #2


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    This sounds fun.
    Letting the skateboard and scooter designers do the work for you seems to be a good idea. They use no more of those particular wheels than necessary so that would be best for this design philosophy too, I reckon. "Spreading the weight out" is not an inherent advantage with friction until things are so loaded that they distort. Basic friction theory says that the actual friction force does not depend upon contact area. This is true over a big range of areas in most circumstances (where the materials compress linearly). Too many wheels would probably cause more drag; not just simple friction in the bearings and with the road.
    The only time when more wheels might help could be when the road surface is very soft but ordinary tarmac would suit minimal wheel count and the wheels are designed for that sort of surface. Big diameter could help when there is grit on the surface - which is why the scooter wheels beat the skate wheels, perhaps.

    A high mass would probably be best for the 'downhill' phase because air resistance will be a smaller portion of the forces acting on a bigger mass. The limit here could well be in the force available in the 'pushing' phase, where there must be a limit to how fast you can launch off. (Basic Power / Weight ratio situation). Some experimenting could help here, to find the limit to the speed that the driver can push the car vs the mass.

    A good rigid frame could help reduce losses due to wheels scuffing if the frame distorts and pushes them apart (sideways). Cars and bikes all have good rigidity where it counts.
  4. Oct 31, 2013 #3
    So, do you think that "less is more" in this situation ? I was just thinking of inline skates and hockey skates - where they typically have 4 wheels on each shoe. I always thought that was to spread the weight out. Perhaps it is just to help stabilize things ?

    I could add more info here -

    The road is asphalt and while pretty smooth, is not in any way clean. Also, there is hay scattered about as well. So, dirt, grit and tiny pebbles are a factor.

    My car has a 1" square tubing frame and is fairly small so it is quite rigid. I did notice some rattling when others drove it. It could have come from the steering mechanism or perhaps the grade 8 bolts that I was using for axles. I could always put a tack weld on the bolts to stop any movement/vibration.

    Next year I will have fixed push bars similar to that of a bobsled. We will also practice our starts for better consistency and power.

    I could also take a pair of the wheels off and do some timed trials.

    Any more thoughts ?
    Last edited: Oct 31, 2013
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