Gravity Vehicle C

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Re: Gravity Vehicle C

Postby Balsa Man » October 21st, 2011, 5:30 am

Hey guys. Thanks for all the quick responses! When I mentioned the 2 wheeled vehicle I thought of having some sort of frame with a wingnut braking system that would attach to the axles and hold some sort of weight on it. The reason for the 2 wheel idea was that I was thinking about ways to keep the center of gravity of the vehicle as high as possible. One thing in the rules that I am concerned about, however, is rule 3f: "The vehicle must have... a Measurement point on either side of the vehicle between the front and rear axles." Although the rules never say you have to have a front and rear axle specifically, it is implied that every vehicle will have a front and rear axle in 3f. Any ideas? Thanks again so much.

That's a really good observation. As usual, this isn't the place for official clarifications, but I think the wording does require 2 axles. Of course you could potentially make the device 2 wheeled like a bicycle (as opposed to a segway)
Sonac, it sounds like, but I can't tell for sure, you're thinking bicycle-arrangemend ("...the axles..."); if so, that's clearly consistent with the rules, no measurement point issues. If you're thinking single axle-segway arrangement, it does sound like a clarification would be needed to clear that. A challenge with that arrangement is going to be getting it to run a consistent path - precise alignment and release. Then once its rolling, it's going to be much more sensitive to imperfections in the floor than a 4, or 3-wheeler; one of the wheels getting deflected will turn the direction of motion. With 3 or 4, the contact of other wheels will act to keep it rolling on the path it's on. Something to think about.
Having the center of gravity/mass as high as possible at the start is, indeed important. As discussed earlier in this thread, the basic physics at work are:

Mass (M) x gravitational acceleration(g) x height (h) = potential energy at the top = kinetic energy at the bottom = 1/2 M x velocity (v)squared

Mass cancels out, so the final velocity would be the square root of (h x g)

So, getting as much of the mass as high as possible is the first part of the challenge of getting as much velocity off the ramp as possible. The second is maximizing how far the (center of) mass falls - maximizing h. If you have a "high" center of gravity/mass (CM), when it rolls off the ramp at the bottom, you have effectively given up the potential energy from the height of the CM above the floor down to the floor. To maximize h, you need to somehow start the CM as high as you can, and have it come off the ramp as low down as you can. Figuring out the various possiblities and tradeoffs to do this is a key part of the design challlenge on this event.
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Re: Gravity Vehicle C

Postby Faustina » October 22nd, 2011, 7:21 am

There's been a lot of talk about keeping the center of gravity as high as possible. Wouldn't an easy way to do this (while still having 4 wheels) be to make the distance between the axles just short enought to accomodate 4 wheels? Are there any negative effects that this would have? Specifically, I'm wondering if a car like this would have problems going straight. Does anyone have experience from other vehicle events like Scrambler where the distance between the axles was short?

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Re: Gravity Vehicle C

Postby fleet130 » October 22nd, 2011, 7:47 pm

Short wheelbase=lesser yaw stability
Long wheelbase=greater yaw stability

With a shorter wheelbase smaller errors in alignment between the axles/wheels increase the curvature of the vehicle's path. If you make the wheelbase too short, you can have problems making the vehicle travel a straight line.
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Re: Gravity Vehicle C

Postby gh » October 23rd, 2011, 7:57 am

Also, just as important as stability, remember to design for adjustments. I've started to see way too many "bolt some fixed bearing blocks onto rigid wood/metal/carbon fiber plates" vehicle designs which probably have terrific precision and repeatability, but can not be corrected for yaw tendencies even a bit.
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Re: Gravity Vehicle C

Postby illusionist » October 28th, 2011, 8:15 pm

I realize this isn't the place for official clarifications and all that, but does the release mechanism have to be triggered by a vertical movement of the actuating pencil? Or can the pencil be pushed horizontally to release the mechanism? I think that in Mousetrap C, you had to vertically push down to release the mousetrap. Any such limitations this year?

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Re: Gravity Vehicle C

Postby buzzbuzz » October 29th, 2011, 5:24 am

I was wondering that myself. I don't think it really matters as long as the pencil is not providing any energy to move the car forward. Also, does the actuation have to be a "push," or could the pencil "pull" something? Any ideas?

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Re: Gravity Vehicle C

Postby Balsa Man » October 29th, 2011, 6:07 am

All the rules say is "actuated" with a pencil- no other modifying/constraining language anywhere else. Vertical/horizontal/push/pull/twist/thump/flip/wave like a magic wand, insert in a hole and use as a lever, held at either end, or held in the middle, are all, clearly, ways of using it to actuate.
Purely, just imho, no need for a formal rules interpretation here. If someone sees a solid basis for thinking otherwise, do speak up, but this looks pretty cut and dried.....
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Re: Gravity Vehicle C

Postby illusionist » October 29th, 2011, 8:20 am

The reason I asked is just to make sure in case I have a really strict and misguided supervisor at regionals (this has happened before :/)
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Re: Gravity Vehicle C

Postby fishman100 » October 31st, 2011, 1:59 pm

Since you're trying to get the mass as low as possible (when it reaches the bottom of the ramp) would bearings actually help?
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Re: Gravity Vehicle C

Postby Littleboy » October 31st, 2011, 4:39 pm

But when on the ramp you need it heavier. There will be a tradeoff, so bearings still may help.


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