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Rubber Band Car For Physics- Any Help?

by godzillafan868
Tags: band, physics, rubber
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godzillafan868
#1
Mar7-08, 02:58 PM
P: 7
I'm building a rubber band car for physics and need as much help as possible. I'm by myself this time because I chose (and regret it). But, so far I have found this much out:

Stuff so far:
*Records/CD's are good because of large Diameter

*Heating the rubber bands and then cooling before using helps with stretch (still questioning)

*I've been thinking of using three wheels, one in the front for support and two in back with the rubber band to wind up the back two wheel's axle then letting go, but I'm not sure if I'll hit 10m this way.

Guidelines:
*Can use anything except for Rubber Band kits like a K'Nex or Lego Rubber band kit, but I can still use K'Nex and Legos in my own design.

*Must travel 10 meters if more it's even better

*No weight limit or minimum

*Design can be anything except for what I mentioned in part 1
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Danger
#2
Mar8-08, 04:26 AM
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It seems that you have a pretty good handle on it already. We don't design things for you, but will comment upon your ideas.
You didn't mention anything about acceleration requirements or a time limit. If such don't exist, then the tallest wheels that you can use would be best. The trade-off is in acceleration vs. top-end or distance. You don't want the wheels so large that the stored energy in your rubber band can't start the thing moving at all.
Two things regarding the distance: use the longest rubber band that you can get away with, and make sure that it decouples from the drive axle as soon as it reaches full extension to that it won't then start to wind back up and decelerate the car.
RonL
#3
Mar8-08, 11:46 AM
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Quote Quote by godzillafan868 View Post
I'm building a rubber band car for physics and need as much help as possible. I'm by myself this time because I chose (and regret it). But, so far I have found this much out:

Stuff so far:
*Records/CD's are good because of large Diameter

*Heating the rubber bands and then cooling before using helps with stretch (still questioning)

*I've been thinking of using three wheels, one in the front for support and two in back with the rubber band to wind up the back two wheel's axle then letting go, but I'm not sure if I'll hit 10m this way.

Guidelines:
*Can use anything except for Rubber Band kits like a K'Nex or Lego Rubber band kit, but I can still use K'Nex and Legos in my own design.

*Must travel 10 meters if more it's even better

*No weight limit or minimum

*Design can be anything except for what I mentioned in part 1
10 years ago not much information was handy, about using rubber for energy storage, but things are changing.
I'll share a few things i have picked up along the way, maybe they will be of use.
First keep the "heat" away from rubber, and the less UV light the better.
Second, stretched rubber stores energy as well as twisted.

Third, a graph of energy stored in a rubber band, will look a lot like a graph of a ni-cad battery, office grade rubber bands will give a storage capacity of around 3,000 ft-lbs/per lb of rubber, while higher grade rubber will go closer to a 4,000 ft-lb value (and a much higher price).

Most rubber will stretch between 7-8:1, i find 6 to be most dependable.

To give the longest life, and the most useful amount of energy stored, a rubber motor is best cycled between 20% and 80% of its stretch capacity.

Don Ross wrote a book, "Rubber Powered model airplanes" in which he gives information about how to break in a rubber motor, and YES it does make a difference, also energizing a rubber band too often in a short time will cause a quicker breaking of the band.

There is a lot more information, but one last thing i have found is treating rubber with a protectorate such as "Black Magic" will extend the life of a rubber band to a great extent, and act as a lubricator.

Danger
#4
Mar8-08, 02:52 PM
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Rubber Band Car For Physics- Any Help?

Excellent post, Ron.
godzillafan868
#5
Mar8-08, 05:28 PM
P: 7
A few questions RonL, but great post thanks a lot.

What's "Black Magic"?
What's the 7-8:1 Ratio?
How can I better energize a rubberband without making it more breakable? I was thinking of stretching it and keep it stretched overnight, but if you have any better ideas please say.
Danger
#6
Mar8-08, 06:55 PM
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Since I am now officially off-duty and about to embark upon an unwanted birthday party (so I'm as pissed as a nit), I have to wonder whether or not a twisted condom might be better than an office-supply rubber band... Maybe your school nurse could supply somethin.... aw, never mind.
RonL
#7
Mar8-08, 08:56 PM
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Quote Quote by godzillafan868 View Post
A few questions RonL, but great post thanks a lot.

What's "Black Magic"?
What's the 7-8:1 Ratio?
How can I better energize a rubberband without making it more breakable? I was thinking of stretching it and keep it stretched overnight, but if you have any better ideas please say.
http://www.pennzoil-quakerstate.com/...lack_magic.htm

Black Magic , is the clear liquid you find in auto parts, and any retail outlet that sells car care products, it is designed to keep most rubber products, including tires, from aging too fast, due to heat and sunshine (UV).
The ratio mentioned, is the stretch of a rubber band, in relation to its at rest length. My favorite is the big red bands 7" long, they will break if you stretch them much beyond 56" inches, they are 1/8"sq. in thickness.
One test i did several years ago, was to drill 1/4" holes in a board, spaced 1" apart, the board was a 2X4 clamped to the work bench, using a dial indicator fish scale attached to one end, ( make sure the hook that hold the bands have a very smooth surface, the slightest rough spot or scratch will sever a rubber band under tension) i stretched 13 bands (one at a time) to 42" inches, with a results of 50 Pounds of pull, they were secured around a 1/4" bolt in the board, at the 42" hole. Leaving them in this condition, and covered with a cloth to keep dust off, i let them set for three months, during this time they they lost tension down to 45 pounds. I removed them from the test stand, let them set overnight, and the next day they had returned to the 7" length. These bands were treated with the Black Magic.
Of interest is the fact that my shop is not insulated, and gets quite hot in the summer heat here in Texas, I have left untreated bands stretched around things, and the heat causes a drying, and vulcanizing condition that will deteriorate a bit of rubber in just a few weeks.
I have a refrigerator in the shop that holds around 50 pounds of rubber and o-ring products, keeping them cool and out of the light.

From Don Ross's book, Breaking in the rubber,
" Here's all you need for a good break-in. Stretch your motor to three times it's at rest length and hold for two minutes. While stretched, work some lube into the rubber with your fingers. Allow to rest for 10-15 minutes then stretch again to 4-5 times the rest length and hold for 4 minutes. Thats all you need to do."

Maybe some of our chemistry people can shed some light on the bond structure of rubber, and the effects of break-in, or is this a myth-busters project?

A couple of personnel views, one is that oils from your hands especially when under tension will be forced into the pores of the rubber and cause it to break down early. ( i use the nitrel gloves when handling the rubber, same as the gloves we used in the Fab at AMD.) Another is that any dust and especially grit, will very quickly cause a slit in the bands when under tension, causing them to fail.

As stated before, rapid stretch and release of a motor ("i think") causes an internal heat buildup on a micro scale that when not allowed to equalize will produce weaker spots in the bond structures.

The reason for me having such a large amount of rubber, is the plan to build a rubber energy storage for a 15-1/2 foot canoe that i built about 8 years ago. 10-20 pounds of rubber will fit inside a 4" PVC pipe, and using stainless steel cable and a gear reduction to eliminate too fast of a release of energy to the prop, and by using clutch bearings, energy can be levered into the rubber with a push-pull action. Hum too much info.

Thanks

RonL
Danger
#8
Mar9-08, 01:04 AM
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Quote Quote by RonL View Post
the heat causes a drying, and vulcanizing condition that will deteriorate a bit of rubber in just a few weeks.
Maybe that's the reason for a problem that I've been running into for years. I keep finding rubber bands, either performing a function or just lying around on a table, that have 'glued' themselves to their surroundings. The appearance is that something bled out of the elastic and then solidified to bond the two surfaces together. Those same elastics (our term up here for rubber bands) almost shatter when stretching force is applied. It's as if the material has crystalized. When I try to stretch one, I can get it to about 110% of its rest length and then find myself holding a couple of separate pieces with a few lying on the floor beside me. Is that from the same cause?
RonL
#9
Mar9-08, 08:45 AM
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Quote Quote by Danger View Post
Maybe that's the reason for a problem that I've been running into for years. I keep finding rubber bands, either performing a function or just lying around on a table, that have 'glued' themselves to their surroundings. The appearance is that something bled out of the elastic and then solidified to bond the two surfaces together. Those same elastics (our term up here for rubber bands) almost shatter when stretching force is applied. It's as if the material has crystallized. When I try to stretch one, I can get it to about 110% of its rest length and then find myself holding a couple of separate pieces with a few lying on the floor beside me. Is that from the same cause?
Thats exactly what I'm talking about. Over the last few years I have bought bags of bands from different stores (most are the quarter pound bags). Office Max sells 7" big reds, made in Taiwan, these seem OK in quality and the last ones i bought were .99 cents per bag.

http://alliance-rubber.com/products/...crepegold.html

These seem to be the best in, less expensive rubber. I did order a sample pack of larger, and different color bands, and was disappointed in the quality of thickness of the larger bands.

I have so many test going on in my shop that i have lost tract of what each pile is doing, and what treatment was applied (my ADD thing in action)
So far the Black Magic really shows good results, but there is a messy transfer of the silicon (even when wiped fairly clean), so be careful where they are used, especially if something needs to be painted afterward.

The advantage of using a lot of individual bands, in a larger energy storage design, is that if one band breaks, only that % of storage is lost.
godzillafan868
#10
Mar9-08, 10:22 AM
P: 7
The rubber bands I have are of some age and I was curious with all the dust and particles in their bonds according to you should I wash them first with cold water?

I'll start using gloves when I handle the bands just in case now.

So if I used let's say 4 Rubber bands on my car would that give me more distance than just 1?


Off of the rubber band subject for a moment:
I Currently have two records secured to a pencil this is the axel wherethe rubber band will wrap around to make it go, but I don't know what to do about the front wheel. I currently have only CD's that I could use for a front wheel, but I'm not sure how I'm going to secure it.
RonL
#11
Mar9-08, 05:56 PM
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Quote Quote by Danger View Post
It seems that you have a pretty good handle on it already. We don't design things for you, but will comment upon your ideas.
godzillafan868!
Based on dangers comment, does this fall under homework ?
I have a pretty basic idea of what you are doing. You say the Axel is a pencil, and the back wheels are records. Are they 45's or 78's, and what are you using for a body, and how long is it ?? do you have bearings to let the Axel spin in ?

Give as much information as you can, and if there is no problem in helping I'll through out an idea or two.
Also what kind of tools do you have to work with, and what supplies?

Ron
godzillafan868
#12
Mar9-08, 07:43 PM
P: 7
its due Mon. I have the basic idea i was just wondering if 2 bands or 4 are better than 1
RonL
#13
Mar9-08, 08:27 PM
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Quote Quote by godzillafan868 View Post
its due Mon. I have the basic idea i was just wondering if 2 bands or 4 are better than 1
Yes each band adds its own energy, so 4 would be 4 times as strong as one
As mentioned in my test, 13 added together, pulled 50 Lbs on a fish scale, so each band was contributing 3.8 pounds of pull.
These were 7" bands stretched to 42"

If you buy a bag, try stretching a few till they break, and make your own decision where to call it quits. Do practice your safety, "glasses", "gloves"(they can sting a little), and make sure no one is close enough to get hurt.

Good luck on your project.
godzillafan868
#14
Mar9-08, 08:59 PM
P: 7
ok thanks a lot then I lubed up two with the Black Magic stuff and two are normal which i'll probably just stretch out a bit before school and they'll have time to relax by 6th period. But, THANKS! A lot you are a great help to me!


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