Trebuchets

When I send out an invitation to game night, I typically include an easter egg somewhere in the list of things to bring:  “Please bring a snack, easter egg, and/or a beverage to share, and bring any games you’d like to play.”  Past eggs have included bowling balls, light sabres, and tam-o-shanters, but this months hidden directive was “trebuchet.”

I got several responses asking (a) what a trebuchet was and (b) where they could find one, so I sent out a follow-up email with a link to youtube (http://www.youtube.com/watch?v=-wVADKznOhY) and a link to instructables (http://www.instructables.com/id/The-Curiously-Strong-Trebuchet-A-Pocket-Sized-Me/).

(For the relatively incurious, a trebuchet is essentially a catapult with a sling on the end to give it a bit more oomph.  Wikipedia has a very nice overview.)

I wasn’t thinking anyone would actually go to the trouble of making one, but I got a couple emails suggesting that a trebuchet was in the works.  This got me to thinking, and I decided that it might be fun to build one myself.

I knew basically what the concept was, but I was still a little unclear as to what released the payload.  As far as I can tell, there are several different ways of doing it.  There’s the simple way, where one end of the sling has a loop in it that slips over a pin on the end of the swinging arm, but then there are more complicated ways, too:  ropes in various configurations that trip a release mechanism when the arm gets to a specific point.  The advantage of the former is simplicity, while the advantage of the latter is greater control over the angle of release.

I opted for simplicity.

I did a bit more research to get some idea as to the design principles and considerations involved:

One problem was that I didn’t have a whole lot of time — I needed to get going, and I didn’t really know where to start.

So I turned to the construction material of choice for would-be modelers of the slacker generation, Legos.  And not just any old Legos, but the real deal:  Lego Technics.

I happened to have a couple small sets on hand (quel coincidence!), so I started building right away.  I didn’t have any specific plans in mind, but I knew I needed a base, two sturdy towers for support, and a pivoting swingarm.  I figured I could iron out the details later.

I didn’t know exactly how big I wanted to make the thing, but I knew I was restricted by the available materials — I opted for a swingarm that was 8.75 inches long and had a pivot roughly 1.5 inches from one end.  When the swingarm hung straight down, the end of it was just slightly above the base.

Once I had the basic structure down, I still needed four things:  a counterweight (and a good way to attach it to the swingarm),a pin in the end of the swingarm, a sling of some sort, and a smooth track for the payload to travel along (the bumpy tops of Legos would not do).

I fashioned the counterweight out of a little plastic box by cutting the top off, trimming the sides, and punching two holes in it (for the pivot pin) with a paper punch.  I figured I could then fill it with as many coins as necessary to get the right amount of weight.  I hung it off the short end of the swingarm and put about $8 worth of quarters in it.

The pivot pin was a little trickier, but only because I was overthinking it.  I originally thought I would need something both fairly rigid and yet bendable (because of the way they were bending the pin to adjust for distance in that second youtube video), so I started with a paperclip that I bent into shape.  All the while I was bending and tweaking the clip, I was making a conscious effort to try to channel MacGyver.

As it turns out, though, it didn’t need to be so complicated.  I switched to a simple plastic Lego pin at some point to try to ensure a smooth release, and that works just fine.  A longer pin seems to make for a later release, and a shorter pin allows the sling to slip off earlier.

The track was easy:  I started off with a bit of Hot Wheels track and then, when everything seemed to be working well enough, switched to a dedicated bit of masonite.  The original Hot Wheels track was too long, and I couldn’t bring myself to cut it up.  (Yes, yes, I know — I do appear to have kept a number of my childhood toys.  My wife occasionally makes the same observation.)

Far and away the hardest part (and the part I’m still playing with) is the sling.  I started with some heavy cotton string and a bit of plastic bag (for the pocket) but then needed something a bit lighter and more flexible.  I’m currently using some black thread and a bit of nylon.  In order to get the pocket to have a bit of depth, I bunched up the ends, but I bunched them up a bit too much:  sometimes the payload gets stuck and doesn’t come out at all.

The difference between success and failure in the trebuchet business is a very fine line.  Small tweaks to any of these details (weight of counterweight / weight, size, and shape of payload / materials in sling / length of sling / length of pin / &c.) will mean that the payload either flies across the room in a satisfying way, gets stuck in the pocket, shoots straight up, falls out of the pocket, shoots backward, zips around in a circle (when the sling doesn’t slip off the pin), or just doesn’t go all that far.  If the payload is too light, it won’t cause the sling to slide off the pin soon enough; if the payload is too heavy, it’ll cause the sling to slide off way too soon.  Everything has to be balanced.

This makes it sound like a pain to get it dialed in, and I’ll admit it’s a little tricky.  But when  you get it to work, and specifically when you’re able to fling a small d-20 across the room and into the side of a cardboard castle you’re wife built (because you’re a very lucky guy), it’s very satisfying.

Here’s the castle S built:

Here’s the trebuchet against the same background:

And here’s the trebuchet sitting on our table:

You can see the release mechanism in that last picture, it’s the black plastic rod there on the left:  push it in to hold the swingarm in place, pull it out to let ‘er go.  The counterweight falls down, pulling the swingarm around; the swingarm pulls the sling around with its payload safely in the pocket; the payload swings out as it comes around; the sling releases from the pin in the end of the swingarm; the payload (hopefully) flies across the room.

So how did game night go?  As it turns out, this was the only trebuchet there.  It worked well, didn’t hurt anybody, and even hit the castle once.  I’d call it a success.

I was hoping to have several trebuchets present, so we could have a contest of some sort, but that’ll have to wait until next month.

I’m thinking it would be fun to build a bigger trebuchet (I may have caught the bug), maybe one that could launch a baseball.  It wouldn’t be that hard to do, and I don’t think the counterweight would have to be all that large, either.  A few free weights, a few 2x4s, a bit of nylon cord, and some leather for the pocket, and you could have a pretty decent baseball chucker.  I’m hoping to get a few of the other guys from game night interested, too.

And of course I’m inspired to try to come up with a board game about trebuchets, but so far I haven’t had any luck.  It seems to me the real appeal of this kind of tabletop trebuchet is its physicality, and the fact that you have to play with it to get it dialed in.  I’m just not sure how that would scale up (or down) to a board game:  you could have a game where multiple trebuchets attack a castle, or you could have a game where you’re building a trebuchet, but in the end you’ll still be left with a bunch of plastic bits and some cardboard.

Some things can be easily abstracted (building a railroad, creating a financial empire, &c.), but some things can’t (riding a motorcycle, firing off a trebuchet, &c.).

So you could make and sell a trebuchet toy, but I fail to see how you could make a good trebuchet game.

If anyone has any thoughts they’d care to share, I’m all ears.  :-)

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  1. May 14, 2012 at 4:04 am

    Great work! Your hardwork emerged out in some results. Although it’s difficult to make a good trebuchet game but I feel you can make it. Give a another try, Good luck!

  2. June 7, 2012 at 6:47 pm

    I’m thinking it would be fun to build a bigger trebuchet (I may have caught the bug), maybe one that could launch a [insert larger than previous projectile]. THE BUG.

    My friend and I both suffer greatly from this BUG. It has caused us to work on four trebuchet projects and has left the cannibalized remains of 3 of them in my back yard….. I like this addiction. :)

    • June 7, 2012 at 8:02 pm

      There’s definitely something addictive about it. A friend and I are talking now about making one that will launch baseballs. :-)

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