Gravity Vehicle C

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Re: Gravity Vehicle C

Post by Balsa Man » October 4th, 2012, 1:47 pm

Just personal opinion - the usual caveats.
"The pencil"- given to you by the ES can only touch the release mechanism. The second pencil is a part/component of your release mechanism; no restrictions on what, or how many parts the release mechanism can consist of; no language saying a(nother) pencil can't be part of the release mechanism- it could have 2 pencils, or 3. It's been noted a number of times, that if something is not expressly prohibited (and doesn't violate safety, or spirit of the rules concerns(, it's fine.

Glad the bearing disscussion provoked some thought.
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Re: Gravity Vehicle C

Post by illusionist » October 5th, 2012, 6:07 pm

While the rules certainly are an improvement from last year (especially the "leftmost face" part), the one thing that I still wish they had was a limitation as to what type of pencil could be used- hexagonal v. circular. In my own system, it'd be easier to use a round pencil, though a hexagonal one could also be made to work with a little modification.

Can one of the rulemakers explain why it wasn't defined? It's not like either one is difficult to obtain. Just curious to know.
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Re: Gravity Vehicle C

Post by chalker » October 5th, 2012, 7:25 pm

illusionist wrote: Can one of the rulemakers explain why it wasn't defined? It's not like either one is difficult to obtain. Just curious to know.
I don't think we even thought about it. We try not to limit things if we don't have to, and this is a case where I think it didn't even cross our minds.

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Re: Gravity Vehicle C

Post by bearasauras » October 6th, 2012, 12:04 pm

But it did. We had 1 team last year at national where the hexagonal pencil didn't fit the hole they made for the pencil to go in to push the trigger. They ended up filing down that hole a little bit for the pencil to fit. If one were to look up "pencil dimensions," you will see that the "standard" dimensions are exactly standard. Like Chalker said, we try not to limit things which we don't have to. Just as we always recommend that students should make sure the devices slightly smaller than the maximum dimensions, I also recommend making sure your device will accommodate pencils slightly bigger than what you normally use.

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Re: Gravity Vehicle C

Post by onyxflame » October 9th, 2012, 6:37 pm

iwonder wrote:Oh, no, no, no... I don't use zinc plated or galvanized steel for my axles, I used raw steel(It's actually O-1 Tool Steel),
I'm sorry but why would you bother using raw steel? As I know there are no downsides to using zinc plated steel (the standard stuff from home depot). Also I would think the only benefit of titanium would be that it is lighter. I used the standard home depot stuff for my axles and never experienced bending.

On another note, would using a threaded rod as part of the axles slow the vehicle down? (Last year my entire rear axel was a threaded rod, it also served as the breaking mechanism).

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Re: Gravity Vehicle C

Post by onyxflame » October 9th, 2012, 6:40 pm

illusionist wrote:This is about a specific scenario and how it relates to the rules. I realize all answers are simply opinions as they aren't posted on soinc.org, etc., etc.

The rules state that "competitors must release the vehicle by using any part of an unsharpened #2 pencil, with an unused eraser... to actuate teh release mechanism on the ramp... Competitors must not use the pencil to touch any part of the vehicle to start the run"

My plan is to use the ES-provided pencil to pull string that will in turn pull out another pencil on the ramp.

.
I think that you would be fine but if you wanted to be "Extra safe" you could substitute that pencil for a metal rod, wooden dowel, or anything of the same shape.

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Re: Gravity Vehicle C

Post by iwonder » October 9th, 2012, 6:51 pm

onyxflame wrote:
iwonder wrote:Oh, no, no, no... I don't use zinc plated or galvanized steel for my axles, I used raw steel(It's actually O-1 Tool Steel),
I'm sorry but why would you bother using raw steel? As I know there are no downsides to using zinc plated steel (the standard stuff from home depot). Also I would think the only benefit of titanium would be that it is lighter. I used the standard home depot stuff for my axles and never experienced bending.

On another note, would using a threaded rod as part of the axles slow the vehicle down? (Last year my entire rear axel was a threaded rod, it also served as the breaking mechanism).
Well, to clarify, the standard stuff from home depot(as in round bar, not threaded rod) is cold rolled, which isn't the same as zinc plated... maybe your home depot is different than mine though.

As I said, my back axle was a piece of raw 1/4" tool steel(unhardened), I did this for two reasons A) the only zinc plated steel I've found is threaded rod B) I've had a really hard time finding straight pieces of round bar at home depot(not to mention they're raw steel anyways) so I went to the local machine shop supplier and bought a piece of what's called 'drill rod', it's very carefully made and kept to tight tolerances and I can actually buy a straight piece of it. Bending is not an issue. The point of the titanium is to try and reduce the rotational energy stored in the back assembly, by using titanium it's not only lighter, but I can use 1/8" bar instead of 1/4" steel bar. It's also about the same price, if you know where to get it ;)

The front axle was machined by a local shop from a piece of 3/8" steel(not sure what they used, exactly). It has a stop, and threads cut into it, as well as ends that are 1/4" diameter for the bearings and wheels.

The answer to your question is both yes and no. It depends on what bearings you have, if you have bushings(like I had) where the rotational contact occurs between the axle and the bushing, then threaded rod will wear the bushings faster(which is why I didn't go with it), or if you use angle brackets(I've seen this...) then the threads will most certainly slow you down. However with ball bearings, where the shaft and the inside of the bearing don't move relative to each other, using a completely threaded axle is not a problem.
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Re: Gravity Vehicle C

Post by Balsa Man » October 10th, 2012, 9:22 am

A few quick thoughts/comments back-
Why does axle weight matter? The lighter you make the vehicle, the more mass you have left over to play with. Horizontal velocity coming off the ramp depends on how far the center of mass drops - you want as much mass as you can starting as close to the max height you're using, and falling to as close to the floor as you can. If, say, you can build a vehicle that weighs 500gr, then you have a kilo to play with- to position/distribute to get max drop of the center of mass.

A quarter inch steel rod, at the front of a vehicle, is a significant amount of mass starting well below your max height. 1/8th"- in either steel or titanium gets you plenty of stiffness.

At a given size, titanium gets you the stiffness of steel at around 1/2 the weight.

Threading titanium, though, if you're using a wingnut braking approach, is not something you want to take on- it's a bear to machine.

At a given diameter, threaded rod is a lot less stiff than smooth rod.
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Re: Gravity Vehicle C

Post by illusionist » October 10th, 2012, 12:17 pm

Balsa Man, earlier you said your team used the sliding-mass mechanism to get the best of both worlds by having the mass start as high as possible and yet end up in the middle of the vehicle for traction during the run. Will the "kick" that you get out of the mass hitting the vehicle upon launch give the vehicle an extra boost and decrease run time/increase velocity, or is it just a false natural belief?
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Re: Gravity Vehicle C

Post by Balsa Man » October 10th, 2012, 1:38 pm

illusionist wrote:Balsa Man, earlier you said your team used the sliding-mass mechanism to get the best of both worlds by having the mass start as high as possible and yet end up in the middle of the vehicle for traction during the run. Will the "kick" that you get out of the mass hitting the vehicle upon launch give the vehicle an extra boost and decrease run time/increase velocity, or is it just a false natural belief?
No, it works; it’s real; we were able to clearly measure additional speed running with and without it .
Having the mass end up in the middle of the chassis (and close to the ground) was not about traction – it’s about getting down-close to the ground as possible- falling as far as possible, and not being at the back of the vehicle (hence light load on front wheels, hence poor straight-line stability)
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