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Actually, I was going to have tabs on the outer housing. I didn't put them in that drawing because they would have gotten in the way. There will be two tabs and those tabs will lock to the frame of the cart. The housing won't move at all in relation to the frame of the cart.

Resistance would be tuned with a blend of gear oil. Simply replace thicker oil with thin oil until I get where I want.

Is the idea sound? Will a fluid system like this actually limit top-speed of a 2lb cart going down a ramp?

I don't see why not. Like I said, it's equivalent to using a sail, except that you're using gear oil instead of air.

Any particular reason you want to do it this way instead of using a sail?

I think rapid prototyping is the correct term. "3D printers" are called rapid protoypers because you don't generally use them to mass-produce things, as that would be very expensive compared to casting/machining/what-have-you.

Ahh, learn something new every day.

Don't want to use a sail because of the feeling I get from our professor. I think he'd try to put holes in the sail idea much more quickly than he'd try to deflate the fluid idea.

What if it was windy? Is what he'd say.

Durability is also a factor. I'll probably coat all of the printed parts with some kind of protective coating. Not sure what yet, maybe just spray-glue. I'll manufacture in a bit of tolerance to allow for that.

Ah. Yeah, I hadn't thought of that -- the fluid setup would put things much more in your control.

The parts whose longevity would concern me most are the seals.

And in real life you'd probably want to use some kind of non-toxic viscous fluid. Maybe honey, although when it's watered down enough to work it might end up being a good growth medium for bacteria.