IGotScience Posted November 9, 2009 Report Share Posted November 9, 2009 Hello all, and yes, I am new to this forum. I have done much research on this but I have not quite got what I want.Basically, it's a project to build a rubber band car of any size and not use any materials designed or it's intentional use is for a car. And only utilize one rubber band for it's main power source. The Aim of this project is distance. It must travel at least 20 meters and best of all is above. Anything below 20 meters is a failing grade. What I got so far:Wooden Dowels for the structure of the car. (30cm for the length)2x smaller wooden dowels for the axles4x CDs for wheelsAnother smaller wooden dowel for a place holder near the front of the card for the rubber band to attach to.1x Gear made out of cardboard, attached to the rear axle to wind up the rubber band Essentially the idea is to tirl the back wheel to make the gear go backwards and stretch the rubber band. The course it runs on : Tiled flooring Restate of Goal : At least 20 meters What I help with is, whether or not this car will travel 20 meters. The car is not built yet, and have not collected the materials, they are quite expensive if it fails.. And the question of wrapping the CDs with rubber band(s). Thank you for your help. Any comments are also appreciated. Any additional ideas or helpful information are also welcome.Thank you again. Quote Link to comment Share on other sites More sharing options...

IGotScience Posted November 10, 2009 Author Report Share Posted November 10, 2009 No one can comment on this?100+ views :wub:. Quote Link to comment Share on other sites More sharing options...

CraigD Posted November 12, 2009 Report Share Posted November 12, 2009 Welcome to hypography, IGotScience! :) Here’s the basic calculation you need to design your project car: [math]D = (L_1 - L_0) \cdot \frac{r_{wheel}}{r_{spool}}[/math]Where [math]D[/math] is distance traveled, [math]L_1[/math] and [math]L_0[/math] are stretched length and relaxed length or rubber band, [math]r_{wheel}[/math] is the radius of the wheel (a CD in your list) and [math]r_{spool}[/math] is the radius of what the band or its extension winds around the axle on (a gear made of cardboard in your list). This gives the distance the car may travel under propulsive force. If the car coasts for some distance after the band is fully relaxed, it will go further than [math]D[/math], if it “stalls” before then, it will go less. It assumes that the rubber band is not itself wrapped around the axle spool (eg: a string attached to it is wrapped around the axle instead). If the band is wrapped around the axle, the effective [math](L_1 - L_0) [/math] will be less, so the distance traveled will be less. The best design should travel only a little more or less than [math]D[/math] – more, and the rubber bands stored energy is wasted in unnecessary friction due to excess speed, less, and all of its stored energy isn’t released. Your main design decision, then, is how large to make the spool (gear), [math]r_{spool} = ?[/math] So, manipulating the formula and substituting in some knowns, (for a CD, [math]r_{wheel} \dot= 0.06 \,\mbox{m}[/math], while I’m guessing that for a typical rubber band, [math](L_1 - L_0) \dot = 0.3 \,\mbox{m}[/math]) we get: [math]r_{spool} = (L_1 - L_0) \cdot \frac{r_{wheel}}{D} = 0.3 \cdot \frac{0.06}{20} = 0.0009 \,\mbox{m}[/math]. That’s pretty small (1.8 mm diameter), likely about the size of the dowel you plan to use as an axle, so you may not need a spool at all, but rather be able to wrap the rubber band string directly around the axle (finding some way to prevent it from slipping, of course). And, or course, you need to actually measure your rubber band, as my guess of its stretch length may be way off. Your main engineering challenges are to make good, low-friction axles with precisely attached CD wheels, and build a car chassis sturdy enough that the force of stretched band doesn’t break it. If you’re trying for a really optimal design, you may want to have a variable-diameter spool, so that as the band relaxes and the force it applies to the axle and wheels decreases, the spool’s diameter where the string touches it increases, keeping the force on the wheels constant. If you have difficulty getting the car to overcome its initial resistance and begin moving, you may want to rig it so the initial spool radius is larger, giving a higher initial force, then falls to a small radius to give a smaller, long-lasting force for the rest of its run. Best wishes on your project, and please post pictures, and, or course, questions or answers you come up with. Buffy 1 Quote Link to comment Share on other sites More sharing options...

IGotScience Posted November 13, 2009 Author Report Share Posted November 13, 2009 Welcome to hypography, IGotScience! :) Here’s the basic calculation you need to design your project car: [math]D = (L_1 - L_0) \cdot \frac{r_{wheel}}{r_{spool}}[/math]Where [math]D[/math] is distance traveled, [math]L_1[/math] and [math]L_0[/math] are stretched length and relaxed length or rubber band, [math]r_{wheel}[/math] is the radius of the wheel (a CD in your list) and [math]r_{spool}[/math] is the radius of what the band or its extension winds around the axle on (a gear made of cardboard in your list). This gives the distance the car may travel under propulsive force. If the car coasts for some distance after the band is fully relaxed, it will go further than [math]D[/math], if it “stalls” before then, it will go less. It assumes that the rubber band is not itself wrapped around the axle spool (eg: a string attached to it is wrapped around the axle instead). If the band is wrapped around the axle, the effective [math](L_1 - L_0) [/math] will be less, so the distance traveled will be less. The best design should travel only a little more or less than [math]D[/math] – more, and the rubber bands stored energy is wasted in unnecessary friction due to excess speed, less, and all of its stored energy isn’t released. Your main design decision, then, is how large to make the spool (gear), [math]r_{spool} = ?[/math] So, manipulating the formula and substituting in some knowns, (for a CD, [math]r_{wheel} \dot= 0.06 \,\mbox{m}[/math], while I’m guessing that for a typical rubber band, [math](L_1 - L_0) \dot = 0.3 \,\mbox{m}[/math]) we get: [math]r_{spool} = (L_1 - L_0) \cdot \frac{r_{wheel}}{D} = 0.3 \cdot \frac{0.06}{20} = 0.0009 \,\mbox{m}[/math]. That’s pretty small (1.8 mm diameter), likely about the size of the dowel you plan to use as an axle, so you may not need a spool at all, but rather be able to wrap the rubber band string directly around the axle (finding some way to prevent it from slipping, of course). And, or course, you need to actually measure your rubber band, as my guess of its stretch length may be way off. Your main engineering challenges are to make good, low-friction axles with precisely attached CD wheels, and build a car chassis sturdy enough that the force of stretched band doesn’t break it. If you’re trying for a really optimal design, you may want to have a variable-diameter spool, so that as the band relaxes and the force it applies to the axle and wheels decreases, the spool’s diameter where the string touches it increases, keeping the force on the wheels constant. If you have difficulty getting the car to overcome its initial resistance and begin moving, you may want to rig it so the initial spool radius is larger, giving a higher initial force, then falls to a small radius to give a smaller, long-lasting force for the rest of its run. Best wishes on your project, and please post pictures, and, or course, questions or answers you come up with. Thank you so much, yes I will indeed try that. Quote Link to comment Share on other sites More sharing options...

Experiment Garden Posted November 14, 2009 Report Share Posted November 14, 2009 Make the pull back wind motion geared up so that just pulling it back a little way charges a really long rubber band. With such a system you can probably meet the distance requirement quite easily. A flywheel might be good too, though on second though, charging the flywheel with a rubber band would probably be a waste of energy. Quote Link to comment Share on other sites More sharing options...

DFINITLYDISTRUBD Posted November 14, 2009 Report Share Posted November 14, 2009 Basically, it's a project to build a rubber band car of any size and not use any materials designed or it's intentional use is for a car. And only utilize one rubber band for it's main power source. Basically you can use anything ya? That’s pretty small (1.8 mm diameter), likely about the size of the dowel you plan to use as an axle, so you may not need a spool at all, but rather be able to wrap the rubber band string directly around the axle (finding some way to prevent it from slipping, of course). If you were to use a nail (a small one of course) on the axle to attach the rubber band when it ran out of length, the car could coast until friction brings things to a halt (the nail would allow the rubber band to release and the car could roll further via momentum). Ideally the nail should be bent to point towards the direction of travel to help ensure release.Wrapping the drive wheel cd's with either rubber bands, or applying some other grippy type material might do well to prevent wheel spin. Bearings are a must as well. Depending on your axle and chassis material choicefriction between the two are going to cost you big. Excellent free/super cheap bearing materials are pens, coffee stirs (the hollow type of course), and straws. Quote Link to comment Share on other sites More sharing options...

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