Weight powered car

[Turtle]

Originally Posted by CraigD

It occurs to me that, if you’re allowed to stretch the definition of “car” to “something with wheels”, you could take advantage of this one’s need to have no mechanical attachments other than gravity (unlike a mousetrap car), to minimize mass, and eliminate bearing, and pulley friction by eliminating the chassis altogether.

Simply beautimous!!!

[Turtle]

Originally Posted by CraigD

The design sketched in the attached image is nothing but 2 wheels of about 30 cm diameter joined by an axle, a drive string connected to the weight and wrapped around the axle, and a support string looped loosely around the axle and attached to the weight to prevent it from descending more than the allowed 10 cm and touching the ground. The axle has a diameter less than 1/100th that of a wheel, so it should roll more than 10 m under power, coasting perhaps a bit further.
[ATTACH]http://hypography.com/forums/science-projects-and-homework/19557-weight-powered-car-6.html[/ATTACH]

I can't even figure out how to get the attachment to display ( turtle down!), but nonetheless something about this particular elegant design keeps nagging me. With 30cm wheels, the radius is 15cm, and subtracting the 10cm drop we have but 5cm left. Allowing say a 1mm clearance between ground and fully fallen weight, we have only a 4.9cm clearance between the top of the weight and the axle when the weight is fully up. The upshot is that these exact design dimensions constrain the physical dimensions of the 1kg weight to a degrree that a "standard" lab 1kg weight may not fit.

The thinner the weight in its vertical dimension, the smaller diameter wheels one can use down to a limit of maybe 10.2cm (20.4cm diameter with a 1mm clearance & 1mm thick weight), but making the weight thin, one must either use a longer axle and separate the wheels wider, or have the weight stick out front & back, or both. Given these new constraints, what might make the ideal situation now for a 2-wheel 1kg weight-powered vehicle? (Shall we specify a lead weight for arguments sake? Gold? )

PS just realized no reason the weight could not have a slot for the axle & string to pass through & actually start & remain partially above the level of the axle at the end of the fall.

[CraigD]

Originally Posted by Turtle

I can't even figure out how to get the attachment to display ( turtle down!),

In most browsers (IE, Firefox, etc) you should just be able to click on the thumbnail, and open a larger view of it. Rightclicking on that should allow you to save the full size image – and see just how sloppy a mouse sketch it is.

Originally Posted by Turtle

With 30cm wheels, the radius is 15cm, and subtracting the 10cm drop we have but 5cm left. Allowing say a 1mm clearance between ground and fully fallen weight, we have only a 4.9cm clearance between the top of the weight and the axle when the weight is fully up. The upshot is that these exact design dimensions constrain the physical dimensions of the 1kg weight to a degrree that a "standard" lab 1kg weight may not fit.

You’re right – with my quick estimated dimensions, a typical weight likely won’t fit.

A 1 kg lead sphere has a diameter of about , so even ignoring whatever’s used to attach it to the strings, it’ll hit the ground before descending 10 cm.

Rather than messing with odd-shaped weights, however, I think you could just use larger wheels – 40 cm diameter, for example. The axle, then, should be about 4 mm (or less, if possible) in diameter, and separate the wheels by 8 cm or so – enough to assure the mass doesn’t scrape either wheel if the vehicle rocks slightly, and as the string unwinds.

Reducing mass by cutting out as much wheel material as possible without compromising its strength is even more important with these bigger wheels.

The assignment/contest rules, I think, require the vehicle to have 4 wheels. I think you could satisfy this with a pair of small, light wheels attached to the axle with piano wire or similar, so that the vehicle remains effectively two-wheeled.

These school (10th grade, in my local public schools) tech assignment seem like great fun. There was not such class when I was in those grades. You could do stuff like this independently, but getting school credit for it would have been extra nifty.