Self-propelled Car

  • #1
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Main Question or Discussion Point

For a science competition I have to construct a self-propelled car that doesn't use gas or electricity. It must accelerate horizontally and then go off a an incline and land 1 meter away from the end of the incline. I was completely prepared and I used a gear mechanism which worked beautifully but my Physics teacher told me that my design wasn't allowed. So yesterday I designed a car made of K'nex that that has a basic body and then two axles. One on the front and one on the back. There is a support beam in the middle and I clamped a hook onto it. I then clamped a hook onto the back axle and connected an elastic between them. I then wind the wheels and release them. My problem is I don't get enough speed. I have reduced the weight and put more elastics on but it can't even go off the ramp. I'm in serious trouble and I need a solution quickly!

Thanks.
 

Answers and Replies

  • #2
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It would be rather helpful to know why your original design was rejected, until we know the constraints on the building of the car it's impossible to help.

I mean if it was rejected becuase it was blue... make it red, red cars go faster anyway.

Also major hint: You need speed. How do you get speed? (think along the lines of gears, and how the rotational velocity is translated to linear velocity)

Adding more elastic and reducing wieght will help what? (Think of Mr Newton and his 2nd law) Why would this not necessarily help top speed? (in this case it probably will, but there is another factor that will help more)
 
  • #3
901
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The gear mechanism wasn't completely original, there are factory made components. For instant parts of the gear mechanism were already constructed - not by me.
 
  • #4
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The gear mechanism wasn't completely original, there are factory made components. For instant parts of the gear mechanism were already constructed - not by me.
That's a totally ******** reason for it to be rejected (im not saying by you, just by the teacher). You could make the argument that you didn't make your own k'nex plastic peices.

This is essentially an engineering design challenge, the key to design is to use as much of what is already there as possible. There's no sense in reinventing the wheel.

/rant
 
  • #5
901
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Maybe I wasn't clear. Most of the gear mechanism was pre-assembled. Oh well, it's tomorrow anyways and it's just for fun!
 
  • #6
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Still, you took a component (assembled or not) that you knew would do the job. That, to me, shows good design sense.

Anyway, back to the problem at hand.

How can you achieve what that gearbox did? (the obvious answer is to simply copy the design, but build it yourself) More fundamentally what is the gearbox doing?
 
  • #7
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It appears like it's compressing some coil which experiences some resistance and wants to return to its original position, by turning the gears you compress it and releasing it causes it to rotate the other way inducing motion.
 
  • #8
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I found a solution! I put a hook on a support beam in the middle and attached an elastic to it, I stretched the elastic the direction of motion with the elastics coiled on the axle and release it. The initial acceleration allows the car to go up the ramp and the coiled elastics force it over! Now I have to get it to land at 1 meter away specifically! Any solutions to this?
 
  • #9
Danger
Gold Member
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I would suggest either a calibrated tether or a capture net.
 
  • #10
2,017
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It appears like it's compressing some coil which experiences some resistance and wants to return to its original position, by turning the gears you compress it and releasing it causes it to rotate the other way inducing motion.
I mean more fundamentally what does a gearbox do? Forget about the actual mechanism.

Think about how you calculate the speed of a car from the engine rpm.
 
  • #11
Danger
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A gearbox is simply a structure to hold a bunch of gears together, although the term usually implies that the gears are part of it. There are different ways in which it can work. The simplest is like in a clock, where all of the gears are fixed in position and maintain a constant ratio from one to the next. This includes both speeding up and slowing down of several output shafts relative to the input shaft.
The second is like a manual transmission in a car, wherein straight-cut gears mesh according to how the linkage is set by the shift lever. This gives a variety of speeds, including reverse, of one output shaft relative to the input shaft.
An automatic transmission has the same result as a manual one, but generally uses a planetary gear train rather than a linear one.
There are a lot of other types as well, but those are the most popular.
 
  • #12
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Hmm this isn't taking the direction I would have hoped. Although this is a homework type question, i'll just give a solution and then we can discuss it.

If you want more speed, use bigger wheels. This may cause acceleration problems (it will certainly accelerate slower so you may need more elastic to compensate)

Your gearbox increases speed output by turning the wheels more quickly. For a normal engine the forward speed:
engine rpm * gear ratio * diff ratio * pi * tyre diameter.

(one rotation of the wheel means that the carmoves forward 1 wheel circumfrence)

Obviosly if you can't use a gear or diff you get:
rpm * pi * diameter

To make the car go faster yo could either increase the rpm or increase tyre diameter. As the rpm is fixed according to the elastic drive arrangement, we are left with the only option.


You can calcualte how fast the car needs to be moving to jump 1 metre. You can also then calcualte how many RPM the wheel needs to be doing to get this forward speed. Attaining this value is then just a bit of trail and error.
 
Last edited:
  • #13
Danger
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Jeez, Chris, I screwed up. Sorry, dude. I thought that I was responding to the OP asking about how a gearbox worked, and just now realized that it was you asking him what he thought about it. I should start paying attention to the poster's ID before responding. :redface:
 

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