Gravity Vehicle C

[sachleen]

Here's something that SJ posted a while back:

On another note, I was thinking about whether a long car or a short car would be better, and while getting the longer car to go straight is easier, a shorter car can have a higher center of mass. My basis for these assertions are that if the mass of the wheels at either end are m and we add an additional 3m on the back wheels then the center of mass shifts back to 4/5 L. Therefore the CM for a 1meter car is 20 cm from the back while it is only 10cm for a 50 cm car.

How does having a longer car allow the vehicle to travel straighter than a shorter car?

I like to think of that in terms of turning radius. Imagine two cars, one long and one short. Neither are perfectly aligned, say the front wheels are slightly turned. Which car will make a shorter turn? The short one! At 10m, the longer car will deviate less from the center line. Think of a car with just one axle, how difficult is it to make that go straight? Very! The slightest error gets magnified by a lot

[fleet130]

Some discussion on turning radius here.

[Balsa Man]

fleet and sachleen have it pretty much nailed.

To go one more step, the ratio of wheelbase to track (width between the wheels on an axle) can help you here; without changing the wheelbase, you can improve ....linearity by narrowing up the track...

Fishman, you are correct - given even mass distribution along the wheelbase - that a longer wheelbase means the center of mass drops less distance (and therefore you will have less speed coming off the ramp). If a good proportion of the mass is positioned closer to the back, center of mass drops further- better speed. But if you're running too much rear weight bias (front wheels too lightly loaded), linearity goes down. Its figuring out and optimizing all the tradeoffs, like these, that I like about this event.
There is, btw, a way to have the center of mass fall....oh, on the order of 94, 95 cm, and have like a 60/40 rear/front weight distribution. That gets you good speed, and good linearity.....

[illusionist]

And in order to get something like that, you need to have a really small wheelbase, right?

[Balsa Man]

And in order to get something like that, you need to have a really small wheelbase, right?

No, actually; we're at about 38cm....

[fishman100]

I see; thanks to everyone who replied!

[luckisdedication]

What is everybody doing to ensure that their vehicle comes off the ramp perfectly straight?

[Balsa Man]

What is everybody doing to ensure that their vehicle comes off the ramp perfectly straight?

We're using a guide rail mounted on the ramp. Vehicle has a pair of hangy-on thingies-front and rear that ride the rail. What matters is not so much "perfectly straight", but the same every time- so on second run you know it will run the exact same line, and you can adjust your paper clip to nail the target point. Seems to work quite nicely....

A few things to consider in using this approach-
Friction- low friction materials, minimum contact area, setting the hangy-on thingies so that they are a hair wider than the rail, but only a hair, straightness, so rail/hangy-on thingy contact time is minimized.
Geometry- two aspects; first, distance to the ramp surface. If you look in side view, and drop vertical lines to the ramp, where the ramp is flat, the "drop distance" is constant. When you go thru the transition curve at the bottom, middle of the chassis lifts up from the ramp surface (how much depends on wheelbase and radius of the curve). At two points along the front-rear line, that distance remains constant. That 's where the hangy-on thingies go. Second is angle of the chassis to the ramp surface- parallel when on the flat part of the ramp. In the transition curve, you develop quite an angle (the angle of the chassis to the tangent of the transition curve). Let's say your hangy-down fingers on your hangy-on thingies go down the sides of the guide rail 1/8th inch. With your chassis at a 40 degree angle to the ramp surface, they will have rotated enough they no longer reach the rail. You have to figure out how to maintain finger engagement with the rail up to the angle your chassis gets going thru the transition curve. There are at least a couple ways that can be done.
So, some things to figure out, and a bit of careful work to make it happen, but done right, it works.....

[fishman100]

Agreed with Balsa Man; I've only seen a few teams without a "guide rail" system of some sort. Some use tape, others have the guide rails as part of the ramp, etc.

[illusionist]

I'm guessing some sort of plastic, but I can't think of any material that would be able to bend in order to fit the curve of the ramp... Or BalsaMan, if you dont mind, are you guys using straight rails (I don't want to be too nosy into your design, as you've already shared quite a bit with us)?

Edit: Just kidding, I forgot you can just cut a material to the curved shape...