# Elastic Powered Car

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

1. ### kingwinnerRegistered Senior Member

Messages:
796
Hi, I am in intro physics and have to build an elastic powered car that will be as small as possible, can build quickly and will travel as far as possible. I need a box with all the parts in it. I can use the wheels and axis that are pre-made. After the timer starts timing, I have to re-build my car as fast as possible, setting up the elastic system, and then release the car. Once the car STOPS, the timer will stop timing. The score is calculated using the formula Score = [(volume of box) * (time taken)] / (distance travelled) with the lowest score being the goal. So I think I need a car that can move quickly and stop quickly (since the teacher will continue timing even after I released the car) while travelling a very long distance.

I have already spent a day to make a rubber band car, this is how it looks like:

Top view:
http://www.geocities.com/asdfasdf23135/DSC00707.JPG

Bottom view and the wheels:
http://www.geocities.com/asdfasdf23135/DSC00708.JPG

I used 4 CD wheels, 2 thin axes and a thick rubber band. (I cut the rubber band so it's like a single stretch.) I used tape to stick one end of my thick rubber band on the rear axis and the other end in the front part of my car. It runs quite well on certain surfaces but when I run it on carpet and some smooth surfaces, it is NOT moving, the back wheels are rolling but on the same spot (i.e. not moving). I tried to adjust many different things but I still can't figure out why...and also, I have to roll my car backward for a very long time for it to travel only a short distance. Why? Would a thicker axis improve this? Furthermore, my car is not travelling in a straight line, how can I improve this?

Will it get a better performance overall based on the “score formula” if I use:
-a wider rubber band?
-a long rubber band?
-smaller wheels?
-larger axis(in diameter)?
-2 or more rubber bands?

I really appreciate for your help! Thanks a million!

Last edited: Mar 20, 2006

3. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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23,198
If must run on carpet, try "wheel rims" of coarse sand paper, only about 1mm wide, glued to CD disks. If you can find way to do it, holes in CD disks will reduce both weight and rotational moment of inertia and that should help. While winding up your car, say by rolling backwards on carpet strip glued to board, initially pull the rubber band away from car as it winds/wraps up to get full energy storage use of the first wraps around the axle. (Car "standing up on "hind wheels" only to let you do this. Perhaps a central "keel" bar will be required for strenth with depth greater than width, to get rigidity with least added weight.) I could not see your car. - site was down. good luck.

Last edited by a moderator: Mar 19, 2006

5. ### kingwinnerRegistered Senior Member

Messages:
796

I have decided to use CD's as wheels! I wrapped rubber bands around all of the CD wheels and now my car can run on any surface (carpet, smooth surface, rough surface) fairly well.
But would wrapping rubber bands around the wheels decrease the speed of my car? and would it decrease the distance travlled? In school my car will be moving in the hallway and the surface is fairly smooth.

Besides, how can I construct a car that travels a far distance with only little "rolling backward action"? My car currently has to roll like crazy, say 10 meters for it to travel only 7 meters. This rolling action really takes a lot of time which is to my disadvantage when doing the competition. How can I improve this?

And also, would my car travel faster and further if I stretch the rubber band more (i.e. attach one end of the rubber band to a point further away from the other end on the axle...so the band stretches more tightly)?

For this rubber band car to work, one end of the rubber band must move WITH the rear axle, so I must stick that end of the band to the axle! I am currently using tape to do this, but the band always fall off the axle, what can I do to stick it firmly on the axle so that it won't fall off? (remember that I must re-build all of this on the day of competition)

"...initially pull the rubber band away from car as it winds/wraps up to get full energy storage use of the first wraps around the axle."

I don't quite understand what you mean here...can you please explain more on how to do this? How should I tighten the rubber band before rolling the wheels backward and how would this make a better performance? If the rubber band winds around the axle by overlapping, would that be better than non-overlapping?

Thank you again!

7. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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23,198
Most important in all things relatd to the ruber band is to store as much energy in it as you can (with little risk of it breaking) Thus, make sure all parts are streched very tight - to near the breaking point. As I understood your "wind up method" there would be very little energy stored in the first turns wound up. I was only trying to tell you to make them very tightly streached also.

8. ### PeteIt's not rocket surgeryRegistered Senior Member

Messages:
10,166
I suppose it would be cheating to build a tiny car with no moving parts, then launch it from a rubber band between your fingers?

Does the rubber band have to be attached to the car, or can it be a separate launching mechanism?

9. ### kingwinnerRegistered Senior Member

Messages:
796
The whole elastic system must be self-contained, I can't use any external launcher to do something like shooting the car out

10. ### kingwinnerRegistered Senior Member

Messages:
796
How can I store more energy during the first winding of my wheel? Would this work by grabbing the rubber band and pull it further away from the drive (back) wheel while winding?

11. ### spidergoatValued Senior Member

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52,973
Those CD's are heavy for that size car. Weight, especially rolling weight, will decrease performance. Try cutting out the CD's to make spokes.

12. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

Messages:
23,198
As I understand your car, yes. That was what I was suggesting. Again: stand car front end up and with left hand roll it backwards on suitable surface. (Perhaps school hallway.) With the right hand, pull big loop in rubber band out from car initially so all of it is streached tight. (Rember I said a "keel" may be reqired to mke car able to resist force of your right hand pulling the loop.) Also a "fifth wheel" on mid line may be able to wind up faster and worth its weight. Perhaps a "tinker toy" diameter for CD main wheels.

One thing very well know by everybody 200 years ago, when water power was distributed in factories by belts, is that a belt (or your rubber band) will automatically center itself on the center of the wind up drum if the circumfirence of the center is a little greater than that of either edge. -If you do have a section of your "drive axle" that is fat to make quick wind up possible and do not have the central circumference greater than the edge of the wind up drum, there is a good chance, (especailly when you are excited and trying to be quick) that the rubber will fall over the drum edge - not "run true" on the wind up drum section, but start winding on the small diameter axle again, as now. A "bulging drum" should solve this problem as it did 200 years ago. If you do not understand try to look at any old belt driven power system and you will see the wind up surface is NOT exactly flat - that bulging center keeps the belt on the wheel instead of eventually falling off to one side.

As an execise, after the race is over, you can try to tell me how this works / why this is true. Answer is probably harder to put into words than to understand.

Last edited by a moderator: Mar 20, 2006
13. ### CANGASRegistered Senior Member

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1,612
There are experts in the field of using rubber as a power source. I am not one, but I will try to be helpful.

Experts acquire a batch of rubber manufactured at the "same" time, so that there will hopefully be great uniformity.

There are a few manufacturers of rubber strips intended especially for hobbyists who use such things. Experts have their own opinions about which manufacturer is superior, but it is certain that a source not intended for hobby use will be inferior in hobby use.

The most prominent hobby use of rubber power is serious rubber powered model aircraft. Contests are multi national and very serious and competetive.

A number of strips, or, "bands", are cut.

One or more strips are tested by lubricating, then winding or stretching to breakage while instrumentation records the breaking strain. Each expert has their own opinion about winding or stretching or both while testing.

A lubricant is used, both to minimize wear and to maximize delivered power. Each expert may have an individual opinion about the best concoction. There are commercial lubricants available.

The strips are broken in. This is done by stretching and/or winding to a strain somewhat less than the tested breaking strain. Each expert has their own preferred method regarding the percent of maximum stress and the number of times the rubber is stressed. The break in strain, the number of times the strip is stressed, etc. comprise the arcane art.

On game day, when the contestant goes for their attempt, the strips are wound to a high percent of the tested breaking strain. Again, each expert has their own opinion of what percent is the right power loading.

There is uniformity in some aspects of winding for the attempt. First, the rubber is stretched to a certain degree, the strain being judged either by tension instrumentation or by hobbyist "feel". Then, the rubber is wound coincident with the stretch being reduced. This is apparently to provide the early winds with the same potential energy as the later winds. Winding is often done with a torque gauge, although some hobbyists rely on "feel". Many hobbyists wind with both a tension gauge and a torque gauge.

Winding is done immediately prior to the attempt. Experts do not like to leave rubber under stress any longer than possible.

Between contests, rubber is stored unstressed.

Experts have opinions about how many serious uses an individual strip can suffer before it declines in power. Again, each expert has their own specific opinion.

Last edited: Mar 22, 2006
14. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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Can your car have any width you like? Only three wheels? If yes, I think I know how you can win.

15. ### kingwinnerRegistered Senior Member

Messages:
796
Hi, "weight will decrease performance". Would weight decrease the speed or the distance of the car?

And what do you mean by cutting out the CD's? It is already very thin, how can I cut it?

16. ### kingwinnerRegistered Senior Member

Messages:
796
Hi,

What is a "wind up drum"? I haven't heard of something like this. How does it look like and what is the function of it? Would it improve both the speed and distance of my car?

17. ### kingwinnerRegistered Senior Member

Messages:
796
Yes, my car can have ANY volume, and the smaller volume, the better score! Currently, I am having a problem on how to make a really small car. If I use CD wheels, they are big and can run very far. But those CDs are big so the volume would be big also.

I can use 3 wheels, by the way.

18. ### kingwinnerRegistered Senior Member

Messages:
796
There is one very important question that I would like to ask to continue working on my car:

Provided there are 2 identical cars, but the first car has CD wheels WITHOUT rubber bands warpping around it and the second car has CD wheels WITH rubber bands warpping around all wheels.

The second car has better traction. My question is, would higher traction (frction of wheels with the ground) increase or decrease the speed and travel distance of my car? Higher traction means higher friction, right? (and friction tends to stop things) What are the advantages and disvantages of having high traction wheels in such a competition like mine.

Thank you!

19. ### NeonBlackRegistered Member

Messages:
28
Hey Kingwinner:
Rolling friction is actually your friend. I would definately go with the rubber bands around the CDs. The more friction you have between your wheels and the floor, the less chance you have of losing energy to slipping as high accelerations.
The larger axle the rubber band is wrapped around, the more torque you will get from the same tension.
I would try attaching a bobin thread spool or something similar around your axle and wrapping the rubber band around that.
Your cardboard box has low density and high surface area, which increases drag force. I would make your car out of a small wood block or even better, a wooden dowel. Just drill 2 holes through the side.
What I wouldn't try is making 'spokes' out of your CDs. They're just too fragile and it's more trouble than it's worth. Since you are going for maximum distance, it may be better to leave them as they are, since they will have more angular momentum, which means after the rubber band stops supplying energy, your car will roll for longer.
The last thing that I can think of: Come up with a better way to attach your rubber band to the car. I can see that tape coming off during your race.

20. ### kingwinnerRegistered Senior Member

Messages:
796

"I would definately go with the rubber bands around the CDs."
But if there are rubber bands wrapped around the CD wheels, that means more friction with the ground, wouldn't that decrease both the travel distance and speed?

"The larger axle the rubber band is wrapped around, the more torque you will get from the same tension."
Is there any disadvantage of using a larger drive-axle (in which the rubber band is wrapped around) ? Would a larger one always be better than a smaller one?

"I would try attaching a bobin thread spool or something similar around your axle and wrapping the rubber band around that."
Attach the spool and axle using glue? The thing is, I have to re-built the whole thing on the day of competition. And the longer the time, the worse the score.

"Your cardboard box has low density and high surface area, which increases drag force. I would make your car out of a small wood block or even better, a wooden dowel. Just drill 2 holes through the side."
Is there a way to drill 2 holes that are just the right size (not too big, not too small) for an axle to pass through?

"Since you are going for maximum distance, it may be better to leave them as they are, since they will have more angular momentum, which means after the rubber band stops supplying energy, your car will roll for longer."
Speed is also a very important factor in my project because the timer will stop timing when the car STOPS, and the longer the time, the worse score I get. So the faster the car, the less time it will take to travel the same distance.
Another problem of my car is that when it travels to the end, the car suddenly stops and start moving backward for a while, how can it deal with this problem?

"Come up with a better way to attach your rubber band to the car. I can see that tape coming off during your race."
You're right! The tape came off many times during my test run and that really bothers me! But the thing is, I must stick the rubber band to the axle so that if the wheels wind backward, so does the rubber band. I tried making knots for the rubber band around the axle, but this doesn't work. When I roll the wheels backward, the rubber band isn't winding. What else can I do to ensure that the wheels and the rubber bands are winding at the same time?

Last edited: Mar 24, 2006
21. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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Ok - here is idea you might want to consider:
Two big CD wheels (as I suggested earlier with holes drilled in them to make them lighter) at front of car, but widely separated (Perhaps 20 inches, but this depends on the rubber band(s) you are using.) and only a small wheel at read on center line, perhaps 10 inches back on stick, that is forked only at extreme rear to accomidate he small wheel.

The two CD wheels are held apart by small diameter alumminum tube, which has a small partical cut in it. The rubber bands pass thru this tube and terminate their ends on the two CD wheels, pass thru a few well greased washers between end of tube and CDs to reduce friction at "hub".

That is, you store energy by twisting, not wrapping, the rubber bands, as in a model airplane. The center of he rubber bands is of course under the small partical cut of the Al tube and a drop of glue introduced secures the center of the rubber bands against rotation. You said that the wheel assbley could be already assembled before the clock starts. To assemsble your car, all you do is place the rear wheel stick partically in the midline cut of he Al tube and clip it in place. - 2 seconds or less. Pehaps a very tiny "seat" is also part of this clip. the volume of your car is very small if measured by imersion in water. If "volume" is defined by (Max width)x(max height)x(max length) this is not a good idea, but if it is the "true voume" (by water immersion or computation of space enclosed by car), it will be very small. - get straight exacly how "volume" is determined.

There will be some problems with making it "run straight" but if you wind up with straight run down hall, instead of rotating about your body (More distant wheel will make more turns as it travels larger circle and its half of rubber band will be fully wound before the other is.) In advance of contest, deterimine how far in hall you must roll backwards the CD wheels to wind them up "fully." Also you need to keep them in contact with floor or some thing to prevent premature wheel spinning.)

As I undestand contest rules. You assemble car far behind starting line, place CD wheels in contact with hall floor, but car is facing backwards, run to starting line, winding rubber bannds as you do so, spin car around to face forward in center of hall, just behind starting line, and let her go.

PS - I think the small wheel is out in front, not trailing, to provide counter torque needed. - It will be obvious when you try and flip to be correct.

Last edited by a moderator: Mar 24, 2006
22. ### sleeper555Guest

I think this is heading in the wrong direction. You guys are taking way too theoretical of an approach. Consider the rules, the materials, and the skills available.

Based on the scoring formula volume [(volume of box) * (time taken)] / (distance travelled) i would make the following recommendations.

- Use better pre-made wheels and axles. Try something like rollerblade wheels or disassemble a toy car for its parts. Makeshift wheels and axles will wobble and your car will not run straight.

- Come up with a way to maximize rolling distance after the potential energy of the rubber band is spent. If the floor is smooth, this will work greatly to your benefit. If you can get the rubber band to detach and keep the momentum of the wheels going forward that would be good (in this case, having some weight to the wheels might be beneficial).

- Cutting the body of your card from balsa wood is a good idea. It is strong, lightweight, and easy to work with.

- Don't worry too much about stoppage time.

Incendentally, this is a stupid exercise. I could create a really really really really small car with like 3 parts, very small volume, no assembly time and let it run two inches forward and stop and get a great score.

Trying manage all the variables is too much for a intro to physics class, imo.

If I were in the contest, I would do something like the following:

Step 1: Cut a block of balsa wood that is 6-8 inches long, 3-4 inches wide and 1/2 inch high (thinner is better, but be careful not to make it so thin that you can't drill a hole for the axle).

Step 2: Find 4 flat wheels with decent rubber gripping on outside and two axels. It would be ideal if both wheels and axles came from a pre-assembled unit that you took apart (and can easily be put back together). Each wheel should be identical and about 1/8 - 1/4 inch wide. I'm not sure what diameter wheel I would choose, but I guess about 2-3 inches. I would have it so that 4 weeks can fit flat atop the body of the car when stored to minimize volume.

Step 3: Drill a nice hole in the balsa wood for the axel to go through. Try to make it balanced (perpendicular to the sides). You may want to consider making the axle hole just wide enough to snugly insert a straw or a metal tube so that your axle is not turning against the wood, which will cause friction and slow you down. You can grease the axle later, which should turn nicely inside a tube.

Note: It is unclear to me whether you are only allowed to use one rubber band and if there is a limit to the size or quality of the rubber band. If you can use two bands, that would be a big advantage. If you can use surgical rubber, even better.

Step 4: If you can use two rubber bands, I would place one winding mechanism on either wheel. You can run each directly to the front of the car by hammering a nail into either side directly behind the front axle. The bands can be fixed here. If you can only use one band, I would recommend carving a channel in the middle of the car from the front axel to the back.

Step 5: In constructing the winding mechanism, try to find a way to have your band detach from the winding assembly once the majoriy of the forward energy has been spent. If you are on a smooth flat surface, you should be able to get a decent roll (assuming your wheels and axels are on straight).

Last edited by a moderator: Mar 24, 2006
23. ### DaleSpamTANSTAAFLRegistered Senior Member

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1,723
Does your car have to have an axle and does the rubber band need to remain attached? If not then just slingshot a marble. Close to zero assembly time, small volume, and a lot of distance.

-Dale