Elastic Powered Car

Discussion in 'Physics & Math' started by kingwinner, Mar 19, 2006.

  1. kingwinner Registered Senior Member

    Yes, the car must have an axle and wheels for it to be called a "car". The elastic system must be self-contained and I can't do something external like shooting the car with a rubber band from the outside.


    Back to the problem of traction, I thought that CD's are great wheels because the contact surface is small and thus less friction (therefore can travel faster and further). If, indeed, more traction = better travel distance, wouldn't thicker wheels and wheels made of rubber be better than wheels thin like CDs? After all, why are CDs great wheels?

    Let's consider two examples of wheels that I found on a website:



    Which one would be more suitable for my project? (this 2 examples of wheels tackle the issue of traction) It seems to me that the second one (the high traction wheels) is only good for maximizing the speed but not the distance.......so high traction tends to decrease the travel distance???
    Last edited: Mar 26, 2006
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  3. kingwinner Registered Senior Member

    Since I have to re-build everything on the day of the competition, so I can't really use glue? I need something that can be detached and assambled quickly......How can I make the rubber band and the axle to stick together and roll together? A hook may do this, but there is no way I can pierce the metal axle with a hook and if there is a hook on the axle, I can't take the axle out of my car frame quickly...

    And for a rubber band car, would a heavier or lighter car be better to increase the speed and travel distance?
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  5. spidergoat Venued Serial Membership Valued Senior Member

    It will decrease acceleration as well as total distance traveled. I realize the CD's are thin, but from the side they are solid. You could draw spokes on it like a bike, and cut out the space between them with a dremel. Keep the minimum amount of material you can get away with.
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  7. kingwinner Registered Senior Member

    I have tried to glue a hook to the rear axle to catch and release the rubber band to maximize the travel distance, it works really well, the rubber band is released after unwinding...

    But I have to re-assemble the axle, wheels, and the elastic system on the competition day and will be evaluated on (the less assemble time, the better the score), so if I glue the hook to the axle, I wouldn't be able to slide the axle into the frame becuase the hook prevents to do so. How can I deal with this problem?

    Besides, would it be better to make the rubber band car by stretching the rubber band to store energy instead of winding the rubber band around the axle? Would it work (stores energy by stretching) if I tie one end of a string to the rubber band and the other end to the rear axle?

    Thank you for your reply!

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  8. How about a using a something that clips to the axle, or can be tightened around it like a vice? Alternatively, you could notch a hole in the center of the axle and put a peg directly through it (fitting snuggly) that forms the base of the hook.
  9. kingwinner Registered Senior Member

    I have developed another simpler way of releasing the rubber band after unwinding, I can simply hold the rubber band and wind the first few wraps around the axle using my hand, and then keep winding the rear wheels, after that, the car goes and the rubber band will fly out (released from the rear axle) and I can see a great increase in travel distance

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    My car can travel around 8m for now! I have tried my very best already, but I want it to go further

    I have made a car with the frame size around 10cm x 8cm x 4cm, this is a short car (not long), and when I stretch the rubber band, the wheels are spinning off sometimes and goes upsidedown, should I add some heavy weight to prevent this from happening? Would this heavy weight affect the car's performance in speed and distance?

    I feel that small volume car can't go as far and as fast as a larger volume car! Is there a reason for this? How can I make a short, small volume rubber band car that can travel more than 15 metres and moves fast?
  10. Lensman Registered Senior Member

    You want to maximize the friction between the driving wheels and the ground, because that's where you get your traction. Lack of friction between drive wheels and ground means your wheels spin, which is why automobiles slide off the road when it's icy. And that's why your car works better with rubber bands or sandpaper wrapped around the drive wheels.

    Otherwise you want to minimize the friction between anything on your car that's turning and anything that's not. Anything that rubs is a source of friction. Eliminate it, reduce the surface area as much as possible, and/ or lubricate with something like powdered graphite.

    Something generally not appreciated by the general public is that wheel weight makes a great deal of difference in starting and stopping quickly. Exactly why has to do with angular momentum. It's not easy to explain; ask your physics teacher. Anyway, the lighter your wheels are the better. That's why the front wheels on drag racers are small and have wire spokes instead of being solid. (The rear wheels have to be a lot stronger, for several reasons.) And that's why expensive bicycles have expensive, lightweight alloy rims, for reduced wheel weight.

    CDs are very lightweight compared to their strength. So that's good. But they may not be the best diameter for your car.


    Try taking the rubber bands or sandpaper off the non-driving wheels. The driving wheels definitely need the friction, but the others may not and it will save a bit of weight where it will do the most good. But this may cause the car to veer off course. If it hits a wall before it goes as far as it can, then that's a bad idea. But if you can try it out in the actual place the race will be run, try it and see if it helps.

    Are you limited to a specific rubber band? If not, I'd say dump the cut-apart rubber band. Drill a small hole thru the axle, put a pin thru it. Depending on how small a hole you can get drilled, you may need to use a sewing-type straight pin, or else a very small nail. In either case, cut it off to the right length to hold the rubber band. Assembling that is easily done, requires no glue, and the head on the pin/nail will prevent it from coming loose in one direction. Loop the rubber band around the pin before winding. The rubber band will prevent the pin from falling out even if the pin is smaller than the hole. Using a whole (loop) rubber band will give you more initial power and lessen the time required for winding. If the car is heavy enuff, its inertia should carry it nearly as far as with a single rubber band. Try using more than one rubber band; you can "tie" two together by looping one around the other and pulling tight. See if two or more rubber bands will give you enuff extra distance to make up for the increased winding and travel time. The other end of the rubber band can be held in place with a metal hook inserted into the balsa wood body. That will take less time than taping the end down.

    Experiment with putting weights inside the car. See if that makes it go farther. A weight will give you more traction at the start, and will give it inertia to coast farther. This should be especially effective with a stronger, shorter rubber band, as it will help keep the drive wheels from spinning at the start. If weights work, carve a shallow hole in the bottom of the car body to hold it/them in place. Don't waste assembly time by fastening them down.

    It is extremely important to reduce the friction between the axle and the car body as much as possible. Use graphite powder for lubrication. You might try buying nylon sleeves (from a large hardware store, in the nuts & bolts section) and inserting those into the body, then putting the axles thru those. Experiment to see if the distance gained with reducing axle friction is worth the additional time necessary to insert the sleeves. It probably will be.

    To make an extremely tiny car, you'll need some hobby car wheels. I know from my own experience as a kid that Hot Wheels wheels and axles have much lower friction than anything I could build myself. Hot Wheels made some cars that were self-propelled and NiCad battery powered. Dunno if you can figure out a way to use those. Theoretically you could cut the top off, remove the motor, and attach a rubber band to the axle for power. The problem is of course those weren't made to be quickly or easily taken apart or put together. Moving to a slightly larger car, slot cars (electric powered model race cars) can be custom-built, so some hobby stores should carry wheels and other parts for those. Dunno if those are still as popular as when I was a kid, so they may be harder to find, but I see they do sell parts online.

    Looking at your picture, it appears you're using tape to hold the wheels on. No wonder they're falling off! Drill a hole in each end of the axle and use cotter pins, or wires bent to act as cotter pins. Experiment and see what you can put on the quickest. Cotter pins may be too hard to bend with your fingers; wire in a hairpin shape may work better for you.

    Reducing friction between the wheels and the body is also very important. If the wheels are rubbing against the body, that will cut down on your speed a lot. That can be fixed if you carve a car from balsa wood; you can carve away the sides and leave just a projection where the axle slides thru, like the front end of an Indy-style race car. Nylon sleeves will also help in preventing wheels from rubbing against the body; get the kind with a lip on it, and use the lip to act as a spacer between the body and the wheel. If that's not possible, use nylon washers between the wheel and the body.

    You need to do something about those CDs. Sure they're nice and light, but the holes in the middle are just too big, and taping cardboard over the holes is not a good solution. You'll need someone with access to a machine shop who can cut a perfect circle out of lightweight plastic for you, and drill a hole in the center of exactly the right size. CDs are made of polycarbonate, but any plastic light and stiff enuff should do the trick. Get a thin sheet of plastic at the hardware store or the hobby store. The driving wheels should fit tightly over the axle, so they won't slip. The other wheels can be a bit looser to get them on and off fast. If the driving wheels fit tightly enuff on their axle, you won't need cotter pins to hold them on.

    If you must use CDs, instead of taping pieces of carboard over the holes, cut out the thing in CD or DVD cases that actually holds the CD or DVD, and drill a hole thru that which will fit over the axle; again very tightly for the wheels on the driving axle and loosely for the others. That will take longer to assemble than one-piece wheels, but should still be faster to assemble, and more secure, than taping cardboard to the CDs.

    The front wheels should be smaller than the rear (drive) wheels, just as with a drag racer. If possible, get rear wheels of several different sizes and experiment to see what works best. The size of the front (non-powered) wheels aren't as important, but I'd hazard a guess that about 1/2 the size of the rear wheels should be right, so long as that will keep the car from dragging on the ground. If you must stick with CDs, see if you can find a couple of CD singles, which are smaller than regular CDs. Try them on the rear to see if they improve performance. If not, at least those for the front wheels, where they will reduce weight.

    Lastly, you might experiment with using a variable-diameter spindle. I went to a race with cars powered by mousetrap springs, and the car which won had a string wrapped around a wooden spindle which was about 3 times as large at one end as at the other. The car's owner wrapped the string around the spindle starting at the small (diameter) end, and ending at the large (diameter) end. So, when the car started it was using power from the large part of the spindle-- giving it high torque (power) for a quick start. As the car continued, the string unwound along the spindle towards the small end. That gave the car more speed near the end of its run. I dunno if that sort of thing will be as effective with an elastic driving force, but it couldn't hurt to try it. If it works, you'll have quite a leg up on your competitors! Presuming you've stuck a metal pin thru the axle, it should be easy to fix a wooden spindle in place (so it won't rotate on the axle) by carving out a slot on the inside of the spindle, so it can slide over the pin. See if you can find an old sewing thread spool that's made of wood. If you know someone with a woodworking lathe (got any friends in shop class?) they can easily taper it down for you. Otherwise you're gonna need a file, sandpaper and a LOT of elbow grease. Be sure to leave a lip at the small end so the rubber band won't slide all the way off. Use of such a wooden spindle will also reduce your wind-up time by quite a bit.

    I see you mentioned the possibility of winding a string around the axle and just using the rubber band for a "spring". That should work nicely with the tapering wooden spindle.

    I'm missing something here. I thought everything had to be in separate pieces? If it's possible to glue everything together before hand, then glue your wheels and axles together, and construct the body so you can just drop them into slots on the underside of the car. Gravity should keep everything together. A hole properly placed in the bottom of the car would allow attachment of the rubber band or string to the axle or spindle. If you can glue everything together beforehand, then you'd only have 4-5 parts: 1) body (including a hook for rubber band attachment, and perhaps weights); 2) front axle/wheel assembly; 3) rear axle/wheel assembly (including pin/hook for attaching the string or rubbber band to the axle, and perhaps wooden spindle); 4) rubber band; 5) string. In fact, you may be able to superglue the string and rubber band together; experiment and see if it works.

    But somehow I doubt that's what your teacher had in mind.

    Anyway, good luck on race day!
    Last edited: Apr 7, 2006
  11. kingwinner Registered Senior Member

    Oh Lensman, you are an expert! Thanks for your time!

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    Yes, some of the parts in my car can be pre-assembled in the box, but everything must be within the box (nothing sticking out)! But does that help? If I glue the wheels and axles together, I can't fit them into the small box, and the volume of BOX is what counts (what needs to be minimized).

    I have currently put 4 washers on each side of the car on the axle, do you think that it is worth doing this? The washers are tiny and would take up quite a bit of time putting 4 of those in the axle which is not a good idea based on the marking system.

    In order to save time and complexity to attach a hook/pin to the drive axle, I have developed a simpler way of releasing the rubber band after unwinding, I can simply hold the rubber band and wind the first few wraps around the axle using my hand, and then keep winding the rear wheels, after that, the car goes and the rubber band will fly out (released from the rear axle). My car can travel around 8m for now! I have tried my very best already, but I want it to go further

    I have made a car with the frame size around 10cm x 8cm x 2.5cm using 4 small-sized CDs (for the drive-wheels, rubber bands are wrapped around the wheels). this is a short car (not long), and when I stretch the rubber band, the wheels are spinning off sometimes and goes upsidedown, should I add some heavy weight to prevent this from happening? Would this heavy weight affect the car's performance in speed and distance? (because higher normal force means more friction, right?)

    I feel that small volume car can't go as far and as fast as a larger volume car! Is there a reason for this? How can I make a short, small volume rubber band car that can travel more than 15 metres and moves fast? (I have made another small car around 6cmx3cmx2cm, with small-diamter axles and small rubber wheels, I used the same winding method (use my hand to wind the rubber band on the axle for the first few wraps, and continue winding and let go the car, but the car can't even move (0 distance)...

    My Score = [(volume of box) * (time taken)] / (distance travelled) with the lowest score being the goal, but for a car that goes far requires longer winding time and longer travelling time and prehaps a larger volume, do you think that a very tiny car with little assemble time that travels only 0.5meters would actually be the winner over a distance-traveller?
  12. Lensman Registered Senior Member

    I'm not an expert, I'm just throwing out some common sense ideas. A lot of your questions are better answered by you trying it to see what happens.

    What are the 4 washers for? Just a spacer? You can replace them with a hollow cylinder. Just one piece is obviously better than four, for assembly time. Try cutting pieces off a plastic straw, see if that works. Altho you may need something stiffer. I'm sure you can find a hollow plastic cylinder if you look around at home, and you can cut some pieces off that.

    As I suggested earlier, try putting weights in the car and experiment to see if it travels farther. More weight gives you more traction and more inertia, which is good. But it also means more friction on the axles and a slower start, which is bad. I can't tell you if it will help; try it and see!

    A tiny car will have smaller diameter wheels, and for that reason probably won't travel as far. But as you've already said, making the box smaller will give a higher score.

    Again I can't possibly tell you if a smaller car will work. You should be able to figure out your own score; you have the formula, measurements and distance. Do the math and see how far your smaller car would have to travel to give you a higher score. Like I said, experiment. Isn't that the point of the school project? To experiment and find out what works the best in trying for a specific goal?

    My thought on your wrapping the rubber band around the bare axle was that at some point the rubber band will let go, before the tension is entirely gone, so you're losing some power that way. Also you complained about the time it took to wind it up, and if you went back to the whole (loop) rubber band you'd save winding time.

    If you built a car that doesn't move at all, either there's too much friction or else the rubber band isn't providing enuff power. Do the wheels spin freely? If not, go back over my previous post for suggestions on reducing friction.
    Last edited: Apr 9, 2006
  13. kingwinner Registered Senior Member

    Thanks to you all!

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    I won 2nd place out of about 30 classmates using a tiny car that can be assembled quickly and ran 3 meters! I have spent more than 40 hours on this project, and it all paid off at the end! Thank you!
  14. Physics Monkey Snow Monkey and Physicist Registered Senior Member

  15. Nice work KW! .. Glad to be of help.

    I used to love these projects. My shining accomplishment in high school physics was building a bridge out of only cardboard (no glue, staples or fasteners of any kind). The span had to be at least 50 cm and 10 cm off the ground and the whole thing had to weigh less than 400 grams (if my memory serves). The contest was to see whose bridge could support the most weight applied directly to 5cm in center of the span. I believe that my record of 235 lbs still holds from 15 years ago. The span never even bent, the supports collapsed to one side (probably because the teacher didn't apply the weight evenly). I actually caught the teacher taking my bridge apart after class to see if I had cheated by inserting metal rods through corregation.

    How I ended up working in finance, I'll never know.
  16. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    Probably more than you realize now.

    It is like life. Winning is nice (or being second) but not nearly as important as the process of doing your best while working at something you enjoy and learn from.

  17. DaleSpam TANSTAAFL Registered Senior Member

    Excellent! Why don't you post a photo of your winner?

  18. kingwinner Registered Senior Member

    Too bad I don't have a digital cam! But I will tell you how my car looks like! It's very simple and very tiny: a box (volume=90cm^3) with 4 wheels, 2 axles, a nail and a rubber band, that's all! My method of winding it to store elastic energy is to use my hand to wrap the rubber band around the axle instead of using a hook! (that saves a lot of time for building my car...and the rubber band is able to release from the axle at the end, so that the car can continue moving by inertia) I can build the car and set up the rubber band in 12 seconds.
    I have build more than 10 cars before this final design, and it works

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  19. kingwinner Registered Senior Member

    Wow, that's amazing!

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    235 lbs...that's unbelievable...

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