Trebuchet

[goose_man]

Okay, I've built a few and tested many of these variables. Wheels definitively help both kinds. And it's a fact that a swinging weight (or hinged weight) is more effective than one that's fixed to the arm. A simplified explanation is that it is becauce gravity is pulling the weight directly down instead of around a curve.

A 4:1 ratio is generally accepted as a throwing-side of arm to counterweight-side of arm ratio. I actually find that I get better distance with a 4.5:1 (or 9:2) ratio. This may be due to the fact that I used a 80:1 counterweight:projectile ratio. However IIRC the distance seems to converge about at a 100:1 ratio with a fixed 4:1 arm ratio.

Also, remember when placing the arm that anything past the first swivel-point (anything not fixed to the arm) does not contribute to the counterweight arm length.

With mine, I used ball bearings on both sides of the support structure's axel, on the arm's main swivel-point, and on all the counterweight swivel-points. I had two but the more the better just to reduce friction and therefore the time it takes the weight to be at it's lowest point for the arm's insatantaneous position.

Hope this was helpful!
-goose_man

[jimmy101]

Paaiyan: Don't worry about "free arm", that is a level of technology the treb'ers get into after building about 10 trebs! The ancients had no concept of the design (IIRC), it is strictly a 20th century design.

Structural integrity is always an issue with a treb. Might not be with your initial counter weight and ammo, but I can gaurantee you that if the treb works OK the first thing you'll do is try to boost the weight of both the counter weight and projectile. You will quickly get to the domain where the treb's structural integrity and strength is an issue.

Besides, a well designed treb is like a well designed anything else. Brute strength is not a characteristic of a good design. The structure should be no stronger than it needs to be (taking into account a little "just in case" extra strength.)

For trebs, it is easy to overdesign the throwing arm by using a beam that is much stronger than needed. This is not a good design. A more massive arm moves slower and will significantly underperform a lighter arm.

[jimmy101]

Goose_man: Have you any web pages on your treb studies? A well designed treb is a thing of beauty.

[paaiyan]

I've got a 12' 4x6 for the throwing arm, I'm actually going to cut some of it down on the long side to lighten it up. Trust me, I've given thought to pretty much everything.

[goose_man]

No, sorry. I did it all in high school and it's not on the net. During winter break here (college) I might go visit my old high school and record it all down just in case I ever need it.

EDIT: Also, remember to take the arm's weight into account if it's significant. Technically you need calculus to calculate how much the arm affects the effective weight (parts closer to the swivel-point have less effect), but you could simplify it by just dividing it into a couple sections and calculating each one. I hope someone could explain that better.

[paaiyan]

OK people, trying to come up with a name for this thing. My friend and I like Shenanigans at the moment.

[trae08]

for the axle use the steel pipe like you said an fill it with concrete an stick a few rods of rebar (sp) in there

[paaiyan]

Well, the name for now is Shenanigans.

But my friend and I have decided that if it launches past a certain distance, we are going to paint it pink and call it the Fairy Flyer.

[markieclarkie]

three words. Tapered throwing arm. Take a look @ most of the better ones thru history and they all have 'em ALSO swinging counterweight,wheels.

[paaiyan]

three words. Tapered throwing arm. Take a look @ most of the better ones thru history and they all have 'em ALSO swinging counterweight,wheels.

I've already planned the tapered arm, and the swinging counterweight. Wheels won't be entirely effective on this as I'll most likely be using it on ineven terrain, and that would risk damage to the trebuchet's infrastructure.