1. 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!

2.

3. Houm...

I would suggest you to use:

Thicker wheels (CD are too thin and so the contact surface is too small, that's why the car won't run on surfaces offering less traction like carpet)

A thicker axis with smaller wheels, ideally you should be rolling the rubber band on an axis larger (more diameter) than the wheels.

Also consider if the rules would allow you to tighten the rubber before rolling it; if you tighten it up as you roll, once it overlaps itself the pressure o the tight band on the band already rolled will prevent it from tighten, so actually most of the rubber band won't be properly tighten. This can be solved by reducing overlap with a thicker axis, or just tight little sections of the rubber band before rolling them.

Some suggerence for the wheels and the rolling axis would be to use pieces of the carboard pipe where toilet paper is rolled as wheels; a plastic pipe could be the axis and then you could fill the gap between them with "papier mache". It's as easy as add layers of newspaper cut in little bands, soaked with a mix of 50% water-based contact glue (like the white one usually used to glue wood) and 50% water; you shall paint the mix on the plastic pipe, add a layer of paper bands, then paint again, add another layer, always being careful to not trap air bubbles; once it is thick enough, you can let it dry out for 24 hours and glue it to the inside of the "toilet paper cardboard pipe" wheels. Cut the box so its sides can rest on the axis, add the rubber band, and that should make it. (I suggest you to use "papier mache" to fill the gap between the palstic pipe axis and the carboard pipe wheels as that way you can control the thickness of the stuffing material; also, if you used cello tape or similar then you wouldn't be able to glue the axis to the wheels...)

4. Hello, thanks for your help!

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)

"Also consider if the rules would allow you to tighten the rubber before rolling it; if you tighten it up as you roll, once it overlaps itself the pressure o the tight band on the band already rolled will prevent it from tighten, so actually most of the rubber band won't be properly tighten. This can be solved by reducing overlap with a thicker axis, or just tight little sections of the rubber band before rolling them."

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? If the rubber band winds around the axle by overlapping, would that be better than non-overlapping?

Thank you again!

5. Originally Posted by kingwinner

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.
Rubber bands do it great! What you need is as much traction as possible; traction is the soruc efo enrgy for winding and stetchign the rubber band, and is also how the enrgy stored is turne dinto movement. Aslo, the rubber bands on the wheels will quickly stop the car once the rubber band isn't tight. So you get easier winding up, better transmisison fo the enrgy from the wheels to the ground, and a quick stop.

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?
Well, I can't think of a better idea now... you actually need to roll the rubber band as many times as possible so the axis spins as many times as possible. As long as you spin the axis via the wheels, you wil need to roll back a lot to wind up the rubber band.

The problem here is that you need to wind up the band as fast as possible and unwind it as slowly as possible; a thick axis woud ehlp oyu tot eh first and ahmr you to the seoncd, wheras your thin axis harms you in the fist (needs alot for oll to wind up) and benefits you in the second case (spins a lot of times so rolls the wheels a lot thsu the car make a longer distance).

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)?
That wouldn't help much, as you actually will only stretch the part of the band that doesnt' touches the edge of the box; as soona s oyu start stretchign, the pressure on the "turn around the corner" will eventually block the rubber band and the length between the edge of th box and the place where it is fixed wouldn't tight enough.

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)
I have an idea on how do this, but it's a bit hard to explain with words (specially as englishh is not my language). What I am thinking off is about a sort of "clip" that "clipped" the rubber band to the axis; maybe you could glue a paper clip to the axis and catch the rubber band in it (as if it was some sheets of paper you clip together). When you wind up the band, it will press on the clip and help to hold the tip of the band in place.

(...)

OK, I've made a sketch so it is clearer... You can use any glue able to glue metal (Loctite or alike), and if it fails you may look for a bi-component epoxy-based glue.

"Also consider if the rules would allow you to tighten the rubber before rolling it; if you tighten it up as you roll, once it overlaps itself the pressure o the tight band on the band already rolled will prevent it from tighten, so actually most of the rubber band won't be properly tighten. This can be solved by reducing overlap with a thicker axis, or just tight little sections of the rubber band before rolling them."

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? If the rubber band winds around the axle by overlapping, would that be better than non-overlapping?

Thank you again!
Well, it's as simple as stretch the band with your hand and roll it whiel ti's already stretched... thsi means you can't "roll back" the car, rather stretch he band wih a hand and rll it with naother. I don't knwo if rules allow this, but it owdl be more proficent as when you start winding the band it isn't stretched enough. and it doens't stretchs as much as possible once it starts overlapping itself.

6. Thanks for your suggestions! :wink:

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.

7. Originally Posted by kingwinner

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.
Well...

Traction is what allows the car to move when the rubber band applies a force to the axis. So you need a lot of traction while the rubber band is moving the car. This mnas thta the better the traction, the most proficnet use fo enrgy and the higher speed. Think about a car with worn tires on a oily pavement. Tracion is minimal and so it won't go nowhere.

So, your car will work better with rubber bands on the wheel. It will be able to speed up more efficently than without them, and the additional traction won't have much effect as the wheels actally are rolling, not slipping, on the ground.

Anyway, you may want to try to reduce friction of the movign parts; as you can use a built axis, try to lubricate the axis where it contacts the box with some wax from a candle. It is cheap and works wonders in the contact surfaces with rough matherials as cardboard or wood. Also you may consider to improve the rubber band with a simple device that prevents it from "rolling back" once it has stretched out; youc old use an ordinary cramp (the little U-shaped pieces used to cramp papers together). The crmap should be unused, still with "U" shape, an so one "leg" of the "U" would pierce the rubber band and the other leg would hold the band to the clip on the axis; ideally, this union would keep together while you winded the rubber band, and would be easy that some of the "legs" released itself after the rubebr abdn inwinded, disconnecting the rubber band from the axis so the axis could turn without winding back the rubber band... alternatively, oyu could add the cramp tot eh axis (so ti sitll was a 2made axis" and jsut peirce the rubber with the cramps leg. The point is to find a way so the rubber band is disconnected from the axis once it no longer is of use.

8. 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:

http://www.docfizzix.com/shop/supplies/s300df.shtml

http://www.docfizzix.com/shop/supplies/s350df.shtml

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???

9. Houm...

Traction is good while the car is powered (the rubber band is still stretched).

Traction is less good when the car runs on inertia (the rubber band is unwinded but the car still rolls for a while from the "push" given by the rubber band)

The wheels you have found are for vehicles with a very quick acceleration (mouestrap device), whereas I think your rubber band will allow a longer acceleration. If all you got is a short push and then the car will run mosltly on inertia, the DVD are fine.btu thsi si for mousetrap devices. If you have a long push, then the better the traction the higher speed you will reach. Actually, I think the car will run powered by the rubber band a longer time than it will run through inertia, so then you need the better acceleration and so the better traction.

I would stick to the cd wheels with rubber band on the edge. They have a slim cross section and the rubber will offer better traction while the rubber band unwinds, and that's what your car will do most of the time. But you may give a try to the high traction wheels on the website, too.

10. Originally Posted by kingwinner

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)

"Also consider if the rules would allow you to tighten the rubber before rolling it; if you tighten it up as you roll, once it overlaps itself the pressure o the tight band on the band already rolled will prevent it from tighten, so actually most of the rubber band won't be properly tighten. This can be solved by reducing overlap with a thicker axis, or just tight little sections of the rubber band before rolling them."

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? If the rubber band winds around the axle by overlapping, would that be better than non-overlapping?

Thank you again!

just make sure the rubber bands on the wheel dont come off during a run. the bands could be superglued. and also wider bands on the wheels would give more traction and make it go faster. :wink:

11. Originally Posted by kingwinner

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?
the tighter the band is, the less it will wind up after it unwound, therefore making less rolling back action

12. I have an idea on how do this, but it's a bit hard to explain with words (specially as englishh is not my language). What I am thinking off is about a sort of "clip" that "clipped" the rubber band to the axis; maybe you could glue a paper clip to the axis and catch the rubber band in it (as if it was some sheets of paper you clip together). When you wind up the band, it will press on the clip and help to hold the tip of the band in place.

(...)

OK, I've made a sketch so it is clearer... You can use any glue able to glue metal (Loctite or alike), and if it fails you may look for a bi-component epoxy-based glue.

You idea is great, but I have to re-build everything on the day of the competition, so I can't use glue, right? 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?

13. seeing as you will be on a tile floor, make it heavy enough so that the wheels dont "skip", but if you DO use rubber bands in the CDs that might do it.

14. 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?

15. Originally Posted by kingwinner
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?

For the hook problem:

Instead of pass the axle through holes on the sides of the box, just make a couple of U-shaped cuts on the box sides and let the box rest on the axle, rather than be pierced by it. Just make sure that the cuts are deep enough so the axle is firmly hold inside of them.

Something like:

I----___--------___---I
I__/ O \____/ O \__I

For stretching/winding up, what you do by winding up is to stretch the rubber around the axle... so tha'ts already the most porficent way to go in a car. Actually the most proficent way to use a rubber band is to twist it, but then you have the problem of linking the un-twist movement to the wheels. Twisted rubber work fine for light airplane models, where they move the propeller.

16. Originally Posted by Lucifer
Originally Posted by kingwinner
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?

For the hook problem:

Instead of pass the axle through holes on the sides of the box, just make a couple of U-shaped cuts on the box sides and let the box rest on the axle, rather than be pierced by it. Just make sure that the cuts are deep enough so the axle is firmly hold inside of them.

Something like:

I----___--------___---I
I__/ O \____/ O \__I

For stretching/winding up, what you do by winding up is to stretch the rubber around the axle... so tha'ts already the most porficent way to go in a car. Actually the most proficent way to use a rubber band is to twist it, but then you have the problem of linking the un-twist movement to the wheels. Twisted rubber work fine for light airplane models, where they move the propeller.
Oh, you have tons of good ideas! :-D

But would the box fly out as the car moves? (with such great elastic forces)

In your diagram, the bottom is wider than the top...Would it be better if I cut the U-shape like this? (the top and bottom have the same width)

I----___--------___---I
I__| O |____| O |__I

17. Originally Posted by kingwinner
Originally Posted by Lucifer
Originally Posted by kingwinner
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?

For the hook problem:

Instead of pass the axle through holes on the sides of the box, just make a couple of U-shaped cuts on the box sides and let the box rest on the axle, rather than be pierced by it. Just make sure that the cuts are deep enough so the axle is firmly hold inside of them.

Something like:

I----___--------___---I
I__/ O \____/ O \__I

For stretching/winding up, what you do by winding up is to stretch the rubber around the axle... so tha'ts already the most porficent way to go in a car. Actually the most proficent way to use a rubber band is to twist it, but then you have the problem of linking the un-twist movement to the wheels. Twisted rubber work fine for light airplane models, where they move the propeller.
Oh, you have tons of good ideas! :-D

But would the box fly out as the car moves? (with such great elastic forces)

In your diagram, the bottom is wider than the top...Would it be better if I cut the U-shape like this? (the top and bottom have the same width)

I----___--------___---I
I__| O |____| O |__I
LOL!

I guess your "character-drawing" is better than mine, I hadn't figured how to draw the vertical lines & used "I" instead....

To take in account the elastic forces (which, as you say, could perfectly fly away the box), the cuts should be very deep, so the rubber band was in as narrower angle as possible. You may also want to put the front axle in the usual way (piercing) and let the U cuts for the back "powered" axle. You could even make the holes for the front axle in such way that they where closer to the bottom of the box than the back axle, so the box was titled forward and had a wedge-like shape, so the front of the bottom of the box, where the band "turns the corner", was at the same height from the ground as the back axle, that is, the box was tilted in such way as the rubber band ran horizontally from the front of the box to the back axle. A refinement would be to cut the U cuts as so that they where vertical & perpendicular to the rubber and the ground, rather than be perpendicular to the bottom of the box. That would make more difficult for the box to fly away. And, well, a wedge-shapped car also looks cooler, LOL! :wink:

18. 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 My car can travel around 8m for now! 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! How can I make a short, small volume rubber band car that can travel more than 15 metres and moves fast?

19. Originally Posted by kingwinner
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 My car can travel around 8m for now! 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! How can I make a short, small volume rubber band car that can travel more than 15 metres and moves fast?

Well, a way to avoid the car from turning upside down would be to have two "power axles", that is, two rubber bands, one on the back axle and another on the front axle. You could fix the rubber int he middle of the car so that half of it winded up on the front axle and the other half on the back axle (the band should start winding on the upper side of the back axle, and on the lower side of the front axle). Also could be used a design with two rubber bands, but then maybe the tension would crush/fold the frame of the car.

The rpoblem with a smaller car si that it implies a shorter rubber band, and the shorter the rubebr band is, the less itc an be stretched thsue the shroter distnce will make the car. The only way for improvement would be to twist the rubber band rather than stretch it, as a rubber band can store far more energy being twisted than being stretched. Yet twisting the rubber band means that both tips must be twisted in opposite directions, or one be twisted while the other is fixed... And this calls for a far more complicated car design, where the tip fo a rubebr band is fixed to an axle holding a single wheel (so there's a rubber band and a independent axle for each powered wheel). The problem is to make an axle that can hold firmly the tip of the rubber band and can spin without going away; plus a way to stick the middle of the rubber band to the box (so both tips can be twisted in the same direction, one by each wheel+axis set), and all be made in such fashion that can be built at the contest time...

It's a complicated thing... :?

20. Thanks to you, Lucifer!

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!

21. Originally Posted by kingwinner
Thanks to you, Lucifer!

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!
WOW!

Can we see a pic of the 2nd winner? :-D

22. 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 :-D

23. Originally Posted by kingwinner
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 :-D
Simplicity rulez, aye!

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