would a leaf spring from a car be a good choice for the prod of an oversized crossbow?

Question

My physics project is something that has to launch a baseball and my idea was to make some sort of ballista/crossbow like weapon to launch it and instead of using a normal prod from a real crossbow as it would be too small in comparison to the size of crossbow/ballista i want to make, i was thinking of using a leaf spring from a car. Would this be a good alternative or are there any other materials i could use?

Answer 1

Ive seen them used before too but the efficiency and force they produce falls way short of compressed air. Making an air canon would be your best bet and its probably safer. It would definitely be easier to construct.

Answer 2

I would suggest that a car leaf spring is too much spring for the mass of a baseball. There are two issues, the compression of the ball as it is launched (you might have a bucket or something in mind, but the ball will deform and get "stuck" in it until the bucket stops accelerating and the ball can "relax" back to a round shape).

The other issues is that if the bow is too stiff, you have the same issues as exercising an empty bow - not enough enrgy dissipation and the risk of catetrophic failure due to excessive "through flex" or rebound.

I would suggest buying some pre-made fibergalss sheets in about an 1/8" thickness, and laminating these together with epoxy and clamping with 2 x 4 's while curing. By playing with the thickness, you could find a very optimal spring rate (you should probably try to calculate this by seeing how many pounds of force it takes to compress the ball, and seeing if that force over the distance you have is adequate to meet your launch distance)

If I were doing this - I would use a coil type car spring and a lever arm (2 X 2 X 1/8" steel tube). By mounting the base of the spring to a suitably heavy base, tilting the base to 45°, and then winding the arm in the direction of the spring, you would have a nice catapult. The advantage of this system is that it is tunable / adjustable; you can vary how far from the center of the axis of rotation of the arm (the spring coil center) to find the optimal force for launching. The less force you use, the longer the arc you are applying it with this device, so the amount of energy applied is constant and determined by the amount of "winding" you do.

Good luck.

Answer 3

A leafspring would make a very strong crossbow, but the problem would be drawing it back. That leafspring supports roughly 500-1000 lbs, depending on how much the car weighs. You're gonna need some sort of geared winch to draw that back, and you probably will need a good supply of baseballs, 'cause your crossbow will keep flinging 'em off into the weeds a quarter mile away.

Try making a bow instead. You can shape the bow to your size and needs. Or you can always try for a trebuchet. That's a real fun project.

Keep in mind that any of these machines will store a significant amount of energy, and a stray finger or arm or head in the wrong spot when it gets released will result in a chance for a local boy scout to earn his first aid merit badge.

Answer 4

I built a 150lb draw weight crossbow and the prod was the only thing I purchased. The stock, trigger, and head to hold the bow to the stock, I made myself. The plans available at the time only went up to 25lb draw weight, so I modified my own. I don't think we can discuss here, at length, all the factors that you (we) would like to cover. The pressure on the trigger will be incredible so it will have to be hardened. Shaping the prod (bow) will take consideration so that you don't heat it up to the point that you change the temper of the steel and lose its spring. Or worse, cause it to harden and risk snapping under load! The longer the bolt, the more flex it will have effecting its flight. A boat wench will work to cock it Most have keyhole slots on the bottom, making it possible to be removable after cocking. So many options...Is it possible to contact each other by email? Wish we lived nearby. I'd love to see this when finished. I think you answered on my 410 choke question?

Answer 5

I've seen it done on a show called 'Junk Yard Wars' but they were launching things much heavier than a baseball... Also, what they found was the design of a leaf-spring is to support tension, i.e. if you arrange it so it looks like a cross-bow when you pull back on the two ends, you are mostly only putting energy into the longest spring... They found it worked a lot better backwards..

Answer 6

I have seen crossbows made from leaf-springs before, but you have to be real careful. The spring can cause some serious damage if it comes loose, you know, lots of tension. Also make sure the body of your crossbow can stand the strain.

Answer 7

I think I would try to find a leaf spring off an old Ford Model A, they are thinner in width than today's leafs, and a bit easier to use. Make sure that your bow's body can withstand the strain of bending your bow, and can withstand the shock from release. Have fun!!!

Answer 8

Ballista Spring

Answer 9

you need to access a copy of The Boy Mechanic, a publication of Popular Mechanics that came out in the mid 1950-60's. It has detailed drawings of the leaf-cross bow fabrication procedure.

Answer 10

They are very stiff. You would need a heck of a mechanism to pull it back far enough.
A high tensile plastic (type of pvc?) might do better.

Why not try a butane propelled 'gun'. Much like a spud-gun. They are pretty advanced with burn-chambers and all.