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# Engineering Fair project

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Im in the 7th grade doing an engineering fair project! Im making a rubber band powered car.. I have sort of an idea of how I want to make it. I think I'm going to make it out of legos for less weight then put 2 straws for the wheels to spin to reduce friction. Then large wheels with a lot of grip for max acceleration.. But the thing I can't get is how to get 1 rubber band to turn it all and move it.. please help! Thanks! if you want more info, the accual rules and regulations are at --- http://www.ospe.org/associations/7506/files/2008%20Official%20Rubber%20Band%20Powered%20Vehicle%20Rules.pdf ---

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Im in the 7th grade doing an engineering fair project! Im making a rubber band powered car.. I have sort of an idea of how I want to make it. I think I'm going to make it out of legos for less weight then put 2 straws for the wheels to spin to reduce friction. Then large wheels with a lot of grip for max acceleration.. But the thing I can't get is how to get 1 rubber band to turn it all and move it.. please help! Thanks! if you want more info, the accual rules and regulations are at --- http://www.ospe.org/associations/7506/files/2008%20Official%20Rubber%20Band%20Powered%20Vehicle%20Rules.pdf ---

Good for you, it sounds fun! :confused:

I would maybe try to think of something other than straws as the axles as they are not very slippery. Although, the rules don't mention anything about using lubrication, so it might work with the right lubrication, though weight could easily become an issue as the car must support a 1 pound load (ie box of baking soda). Make sure you consider the box size when designing your chassis. :hyper:

Something else to consider:

Objective

To design a wheel-driven vehicle, powered only by one rubber band (size specified above), which will transport a one-pound "load" (specified above) the farthest distance (on a level, industrial type, carpeted floor) while remaining within the boundaries of a 3 ft. wide parallel track (marked off with masking tape). Speed is not a criteria.

The quicker the vehicle accelerates, the more drag it will incur. For this reason, I would suggest working on a design that slowly transfers the energy of the stretched rubber band to the axles/wheels. This way you will get maximum distance.

What ideas have you come up with so far?

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But the thing I can't get is how to get 1 rubber band to turn it all and move it.
On of the simplest ways to turn a wheel with a rubber band is to attach a spool wheel to the rolling wheels axle, wrap a strong thread around it, attach the other end of the thread to the rubber band (with a dowel or padding to assure that the thread doesn’t cut the band), stretch the band as far as possible, and attach it to a peg or hook on the car’s chassis.

The contest rules state “Transferring the energy of the rubber band to wheels via gears, fly wheels, etc., is encouraged.”. This is a strong hint. While a spool-on-axle transmission is very simple, it isn’t the most efficient possible scheme, especially for a fairly heavy car on a carpeted floor with an inconstant power supply (a rubber band). A more effective transmission could use an eccentric spool.

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On of the simplest ways to turn a wheel with a rubber band is to attach a spool wheel to the rolling wheels axle, wrap a strong thread around it, attach the other end of the thread to the rubber band (with a dowel or padding to assure that the thread doesn’t cut the band), stretch the band as far as possible, and attach it to a peg or hook on the car’s chassis.

The contest rules state “Transferring the energy of the rubber band to wheels via gears, fly wheels, etc., is encouraged.”. This is a strong hint. While a spool-on-axle transmission is very simple, it isn’t the most efficient possible scheme, especially for a fairly heavy car on a carpeted floor with an inconstant power supply (a rubber band). A more effective transmission could use an eccentric spool.

I really appreciate doing this for me, but could you please tell me with more detail about what a spool wheel, or eccentric spool is and how to use the thread.. I've tried googling it but i had little out come. Thanks!

What I think you're saying is put a circly thing with little hooks on the axle (the metal thing that goes across), then put a hook on the chassi, hook then stretch the rubberband and wind it up.. am I correct?

Also, could you tell me the MOST effective way to get the energy of the rubberband to go to the car?

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I really appreciate doing this for me, but could you please tell me with more detail about what a spool wheel, or eccentric spool is and how to use the thread.. I've tried googling it but i had little out come. Thanks!

I think Craig was referring to this:

Eccentric (mechanism) - Wikipedia, the free encyclopedia

What I think you're saying is put a circly thing with little hooks on the axle (the metal thing that goes across), then put a hook on the chassi, hook then stretch the rubberband and wind it up.. am I correct?

That would work.

Have you tried googling "rubber band car". I got 429,000 hits. Looking at other designs can give you some ideas about how to build your car.

This one looks pretty neat. I wonder what makes it go so fast. ;)

Rubber Band Powered Racers - The Sling Shot

I recommend you don't simply copy a design though. It's not as fun that way and takes away from the spirit of the competition.

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That would work.

Have you tried googling "rubber band car". I got 429,000 hits. Looking at other designs can give you some ideas about how to build your car.

This one looks pretty neat. I wonder what makes it go so fast. ;)

I recommend you don't simply copy a design though. It's not as fun that way and takes away from the spirit of the competition.

Alright, thanks.. also, i saw the spool, but i dont get how it works...

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Alright, thanks.. also, i saw the spool, but i dont get how it works...

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Im in the 7th grade doing an engineering fair project! Im making a rubber band powered car.. I have sort of an idea of how I want to make it. I think I'm going to make it out of legos for less weight then put 2 straws for the wheels to spin to reduce friction. Then large wheels with a lot of grip for max acceleration.. But the thing I can't get is how to get 1 rubber band to turn it all and move it.. please help! Thanks! if you want more info, the accual rules and regulations are at --- http://www.ospe.org/associations/7506/files/2008%20Official%20Rubber%20Band%20Powered%20Vehicle%20Rules.pdf ---

How about using a ducted fan, rather than driving the wheels? :) Ducted fan - Wikipedia, the free encyclopedia

If you make the ducting removeable, you can run the car with and without to show what effect the ducting has. :eek: :)

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How about using a ducted fan, rather than driving the wheels? :) Ducted fan - Wikipedia, the free encyclopedia

If you make the ducting removeable, you can run the car with and without to show what effect the ducting has. :eek: :)

Rule #5 states:

Propulsion of the vehicle must be through the wheels (i.e., no propellers, no launching mechanism).

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Rule #5 states:

Propulsion of the vehicle must be through the wheels (i.e., no propellers, no launching mechanism).

:)

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Alright, thanks.. also, i saw the spool, but i dont get how it works...
Pardon the crappy mouse-sketch, but hopefully even a badly drawn picture can communicate the idea better than words.
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It's too bad you can't cut the rubberband. :)

Does anyone know if there's a certain spool size that is the "sweet spot"? It seems that you'd want it as small and light as possible. Maybe a wine cork?

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I read the rules. :lol: Flywheels encouraged I hear. Substitute the stretched rubber band wound on a spool for the metal spring in this design. :doh:

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hmm.. Im starting to think legos are heavy.. what would happen if I could make the car out of like a plastic bottle or somethin..?

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Or balsa wood...

Just remember that the car must be able to carry 1lb of weight and it needs rails or some other form of containment to keep the baking soda box from falling off.

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A unusual feature of this contest is that it is to take place on “on a level, industrial type, carpeted floor”, contradicting the usual approach of minimizing rolling resistance by using very thin wheels. Another is that the car must carry a 1 lb (0.45 kg) load, contradicting the usual approach of making it very light. As with most contests of this sort, all cars have the same amount of available energy (possibly – see below), using the same type of fairly small rubber band.

Let’s look at the basic mechanics of this.

The amount of energy the car takes to move a distance $X$ over the carpet is given by the formula $W = X \cdot F_{\mbox{car}}$, where $F_{\mbox{car}}$ is the force required to move keep the car moving, and is the opposite force to the force of friction. As a general rule, rolling friction is proportional to speed, so the faster the car goes, the shorter the distance $X$ it will go.

So the design goal should be to have the car move as slowly as possible, without stalling (stopping) due to some small obstacle or imperfection in the carpeted floor or its wheels or bearings.

This force will come, of course, from the rubber band (or possibly some more complicated scheme where energy from the rubber band is stored in another form, such as a spinning flywheel, though since every transformation of energy in a real machine involved some loss due to friction, I’d be inclined to Keep It Simple). The rubber band, however, provides a varying amount of force at different stretched lengths, making things more complicated.

Good first steps in the design process are to measure the forces involved

For $F_{\mbox{car}}$, this means building the car without the rubber band drive, putting a 1 pound mass on it, and pulling or pushing it across a carpeted floor as identical to the one to be used in the competition as possible with some sort of force-measuring device – a scale, in other words, the more sensitive, the better. Since most scales are made to measure vertical, not horizontal force, getting a working test rig may involve a bit of a construction itself.

For the rubber band’s force $F_{\mbox{band}}$, you need to get a plot of it at different stretched lengths. You could use a scale for this, too, of reverse the process and hang weights of known mass from the band and measure how far it stretches.

Once you know the force of the rubber band and the force required by the car, you can calculate the necessary size of the transmission spool $R_{\mbox{spool}}$ and the wheels $R_{\mbox{spool}}$. For the car to move, $F_{\mbox{band}} \cdot R_{\mbox{spool}} > F_{\mbox{car}} \cdot R_{\mbox{wheel}}$. How much greater? Make a guess, then test it.

Here’s the fun and complicated part: remember that, as the rubber band pulls the string to turn the spool, axle, and wheels, if gets shorter, and the force it exerts decreases. So, to get the most efficient transmission, $R_{\mbox{spool}}$ needs to vary as the thread is pulled from it.

There are several ways to do this, some involving gears, some not. You could go with a fixed-ratio transmission, avoiding this complication, but if you do, you’re likely to lose to a more efficient design.

Another primary design goal is for the car to minimize $F_{\mbox{car}}$, by making the axle bearings as well as you can (ball bearings, etc), large wheels, etc. A truly weird configuration might do well – “thinking outside the box”, combined with making and testing lots or prototype models could allow you to hone-in on a winning design.

Last, keep in mind a rubber bands can store energy other than by being stretched lengthwise – they can be twisted. Figuring out the best way to use the band will take imagination and experimentation. It’s possible, if the rules permit it, that conditioning the rubber band with pre-stretching, oils or balms can increase the energy it can store. Other tricks are likely possible, and within the rules.

I imagine the winning car will be an interesting, and possibly surprisingly weird machine. Best of luck to you, hongher – may the winning car be yours (or your team’s)! :thumbs_up

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A unusual feature of this contest is that it is to take place on “on a level, industrial type, carpeted floor”, ...

Here’s the fun and complicated part: remember that, as the rubber band pulls the string to turn the spool, axle, and wheels, if gets shorter, and the force it exerts decreases. So, to get the most efficient transmission, $R_{mbox{spool}}$ needs to vary as the thread is pulled from it.

There are several ways to do this, some involving gears, some not. You could go with a fixed-ratio transmission, avoiding this complication, but if you do, you’re likely to lose to a more efficient design

....

I imagine the winning car will be an interesting, and possibly surprisingly weird machine. Best of luck to you, hongher – may the winning car be yours (or your team’s)! :thumbs_up

:D Aye, there's the rub! The carpet. The long & the short of it is that the vehicle needs a granny gear. Speed is not a component of the contest, rather it is distance that wins.

So, how to accomodate that ever-changing pull of the rubber band with a granny gear? Why with a flywheel of course, which evens out that irregularity. Is making it hard? Of course! But we don't do these things because they are easy, we do them because they are hard.

Virtually all wind-up action toys use a flywheel and gears arrangement, and for one simple reason; it is the most effective and efficient. I can't find any other drawings but the one I posted earlier, but take any little wind-up toy you have and take it apart to see how it works. Measure the parts, count the gear-teeth, wind it up and let it go as you watch how it works. One you have internalized the system, apply the principles to your car.

Since the rule writers put 'flywheel' in, then that's what they expect to see. :cup:

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