Gravity Vehicle C

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Balsa Man
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Re: Gravity Vehicle C

Post by Balsa Man » November 20th, 2012, 3:47 pm

Sounds like it might be chassis flex and weight transfer- weight transfer flexes things up front, then it rebounds, re-loads/flexes, rebounds. Chassis stiffness is important.
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_HenryHscioly_
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Re: Gravity Vehicle C

Post by _HenryHscioly_ » November 20th, 2012, 7:14 pm

ahh....
how strong is super glue?
how sound is it to soak my 1/16" thick basswood chassis wth thn super glue?
i glued on weights and my car is going straght so dont need my adjusting mechansm anymore or any other modificatons, so I could just soak everything in glue...? :?

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Re: Gravity Vehicle C

Post by Flavorflav » November 21st, 2012, 4:33 am

Why use such a thin chassis? Weight is not your enemy in this event. Also, I might suggest throwing a rubber washer in between your wingnut and your frame. This slows the application of braking force, which should reduce skid and chassis flex.

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Re: Gravity Vehicle C

Post by illusionist » November 21st, 2012, 7:54 am

Flavorflav wrote:Why use such a thin chassis? Weight is not your enemy in this event. Also, I might suggest throwing a rubber washer in between your wingnut and your frame. This slows the application of braking force, which should reduce skid and chassis flex.
Not sure if I'm misunderstanding it, but won't a rubber washer create a huge increase in friction? The threads of the axle would catch the washer and force it against the bearing or wall, or whatever is on the other side. Eventually the bearing would start slipping off the axle, but that would create a lot of resistance. This would be happening for the entire duration of the run too.
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Re: Gravity Vehicle C

Post by Flavorflav » November 21st, 2012, 9:11 am

Not if you get one that fits tight to the threads and let it spin in the air a short distance away from the chassis, until it gets driven in by the wingnut. Alternatively, you could bore it out (so that the hole is bigger than the inner race but smaller than the body of the wingnut) and glue it to the chassis.

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Re: Gravity Vehicle C

Post by Balsa Man » November 21st, 2012, 11:58 am

_HenryHscioly_ wrote:ahh....
how strong is super glue?
how sound is it to soak my 1/16" thick basswood chassis wth thn super glue?
i glued on weights and my car is going straght so dont need my adjusting mechansm anymore or any other modificatons, so I could just soak everything in glue...? :?
First, same question as Flavorflav's - do you have a reason, other than that's what you have, to be going w/ 1/16 bass sheet? The thinness combined with it's flexibility (technically, cross-sectional moment of inertia combined with modulus of elasticity) doesn't provide much stiffness. Soaking w/ thin CA will stiffen it some; it's mod of e is quite a bit higher, but if thickness stays at 1/16th, not a big gain. You'd actually stiffen it more by spreading a thin "skin" of thick CA on both sides; those very stiff skin layers, separated by the thickness of the wood will get you significantly more stiffness. Better yet, go thicker, and skin with 1/64th birch plywood (or carbon fiber)

Second, (total) weight is indeed not your ememy, it is your friend, right up to the max weight limit. It's all about momentum - for a given "friction appetite", a vehicle right at max weight will get to a given distance faster than a lighter one. A lighter chassis platform (and other pieces) can be helpful in that allows you to locate mass where it does the most good.
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Re: Gravity Vehicle C

Post by _HenryHscioly_ » November 21st, 2012, 2:04 pm

I'm at 1.45kg with weights at the back
I was gonna use alumnum and make a frame, but i thought it'd be heavier and more difficult for me to cut and drill and assemble
wood+glue was more simple..and I only had 1/16" basswood -_-
For the back part of the chassis, I arrange the metal bars so that its pretty well supported, a couple metal bars glued across the entre wheel base, and a couple perpendicular to those
Further in front, I just glued 1/4"tall basswood walls, so its U-shaped
the car doesnt really bend up/down or side/side, but can twist really easily
Is there a certain way to add support to prevent torsion? or just increase cross-section as much as possible?

I have 4 rubber o-rings against my fixed nut. Since it is larger distance for the wing-nut to slow down, I'm guessng it'll be more gradual, but snce o-rngs have smaller cross section, it wont have as much resistance to slow down the car. I'll look for rubber washers when i go to the store next time.

Sorry, my "i" key is not working sometimes, so some are missng. frustrated and gave up on going back to everythng i typed :|

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Re: Gravity Vehicle C

Post by Flavorflav » November 21st, 2012, 6:06 pm

I've come around to Balsa Man's idea - it's flex in the chassis. You could try taking another sheet of that 1/16" and laminating it cross-grain, making your own little 2-ply plywood. That would increase the stiffness substantially, I would think.

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Re: Gravity Vehicle C

Post by Balsa Man » November 21st, 2012, 7:38 pm

Yup, laminating a second layer would stiffen. The 1/4" walls were a move in the right direction.
The lack of resistance to twisting could maybe be controlled by diagonal bracing- if it were 4-wheeled; a rectangle, X-bracing btw opposite corners. With a 3-wheel/triangle- without being able to see, a little hard to describe... You're looking for two.....bracing lines that cross; running from the back corners to the opposite side, up toward the front. If you hold the rear and do the twisting you describe, as you twist one way, one of those lines shortens, the other lengthens. Try taping pieces of thread. If you glue something that resists compression (shortening() and tension (lengthening) on both these lines, you brace against the twisting both ways. Carbon fiber bar (small, like 1/8th x 1/64th) would work really well; wood would work
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Re: Gravity Vehicle C

Post by _HenryHscioly_ » November 21st, 2012, 9:24 pm

OH MY GOODNESSSS........
i videoed the car breaking on my kitchen floor
i see the problem...

my axle is 1/4" all thread stainless steel, distance between bearings is about 40cm
last year was about 20cm...so, bends 4 times more easily....

The axle is bending from the force of the axle trying to turn, but stopped by the wingnut that cant move anymore
and...the wheel is actually able to turn a tiny bit even when the wingnut doesnt move
But, once the axle can't bend anymore, it bends back, and the wheel turns backwards...
and the axle bends the other direction....so wheel turns backwards a bit more than it should..and axle bends other direction, and wheel turns...
The wheels leave contact with the floor right when it start turning backwards
So it skips about 2-4 times....

hm..it looks like the chassis also bends opposite direction of the axle..probably adds more to the forward/backward wheel rotation....

i have the wing nut hitting a couple fixed nuts on the right side of the axle, so I would thnk that the bending/jumping might be uneven on both sides..but the car looks straight..

I either remake my car less wide, or make it decellerate more gradually..increasing my time score
haha...my car has 2mm clearnace between the nails on my ramp....

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