# Floating Arm Trebuchet (F2K)

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Ladies and gentlemen, I present to you:
The FIRST EVER (Free) COMPLETE GUIDE FOR CREATING YOUR VERY OWN FLOATING ARM TREBUCHET

Of this size, anyway. When i was looking for a good set of plans for a floating arm trebuchet, i couldn't find a single free one. I know that a lot of people share my frustration in not being able to find free plans- so I wanted to create the first one. This is a guide that will show you exactly how to build one.  As always, let me know if there is anything I can do to improve it-and I will add a video after I figure out how to.

I had a school science project (not fun) so I wanted to make it fun by building something cool. My friend and I previously made a catapult that was 7 feet high, and threw a baseball 40 ft. It was pretty bad. So this is a way to make up for that. The height of the channels is 8 feet, But the arm rises higher than that in its resting position. The arm is 7 feet. The short arm/ long arm ratio is what I was testing, and the different ratios are 2:1, 2.5:1, 3:1 and 3.5:1.  I won't tell you which one was the best yet, although you can probably guess. The counterweight is about 36 pounds, but the big, wide wood piece (the short arm) was probably around 20 lbs. I'm still not quite sure...  The total cost for me was around 130\$.  I don't think it was more than that.

If you want more information on how a floating arm trebuchet works, there are plenty of videos on the internet.
I would visit Ron Toms's site, he's the inventor of this type of trebuchet.

I want to apologize for not having very many pictures of the building process, but I made sketchup models, so hopefully they will help. There are several things that could be improved, which are listed on the last page. The first good throw was about 206 feet, although the trajectory was way too steep, to the point where we could not see the baseball. So, I'm guessing that a baseball would go in excess of 300 feet! And that's with only about 50 pounds of counterweight. A 1 lb projectile would probably go about the same distance. Trust me, it's powerful. We just need to fix the pin and find the best sling length. The sling length is also probably not perfect. I'm still tweaking. Most throws went either straight down or backwards. The backwards throws were caused by a glitch in the guide system for the arm as it falls. The pin was also previously a double-headed nail and a keychain ring. The ridges in that ring would sometimes get caught on the two heads of the nail, which we think is the reason for the downward throws. So, you should include in this project the turnbuckle pin system, which is based off of this trebuchet: http://www.trebuchet.com/10101
.
The entire thing took me about 2 months to build, but I wasn't working on it every day.
The diagonal 2 X 6s that attach to the longer drop shannel diagonals were not included in this instructable because they were a lot of trouble to add to it and I don't think they were really needed or worth it.

Always be careful; never fire it around windows or people.
Also, be very careful with the trigger. When you pull it out, it tends to fly towards the person pulling the trigger very quickly. Always hold the rope at the very end and jump out of the way after you pull it.
Let's start...
Oh but wait before you do...Remember to look at EVERY SINGLE ONE of the picture notes. I'm sure they'll help you a lot...

Credits: (Yes I just had to)
My mom and dad for paying for (almost) all of it
and for putting up with a messy cluttered garage
My friend, Michael, for always helping me with the loading and firing.
My neighbor and.. older friend, Joe, (He's an adult) for giving me tips on building the thing, especially the trigger design. He always believed it would work, unlike my dad. But my dad payed for most of it so I can't complain.
Coby helped move the trebuchet back from school (His grandpa has a pickup truck) and also helped me load and fire on one of the testing days.

Thank you to everyone that helped.

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## Step 1: Materials

This list of materials is copied directly from my science fair project. It includes what I used at the time. The lumber list is really long, but I think I bought somewhere around 20 8' long 2 X 4s, I'm not sure... This still does not include the 2 X 6 pieces or any other pieces of plywood. I would still recommend that you read through the list, and buy the lumber as you need it to avoid wasting wood. Buy 15 2 X 4s to start off with, then buy more later. Also, if I mention anything else in the instructable that I didn't mention here, let me know.

Materials

Lumber

2 segments of 6 ft. (1.8288 m) long 2 X 4
These are the horizontal beams that are used as the track.

2 segments of 6 ft. (1.8288 m) long 1 X 6
Long sides of the base

2 segments of 2 ft (0.6096 m) 1 X 6
Short sides of the base
*It would be better if the two previous 1 X 6 pieces were 2 X 6, I'll explain later.

2 segments of 6 ft., 2 in. (1.8796 m) long 2 X 4
2 segments of 5 ft., 10 in (1.7272 m) long 2 X 4
The two previous segments are diagonal supports. Cut at least 3 inches longer unless you are very good at cutting angles accurately (A miter saw would help)\

2 segments of 8 ft (2.4384 m) long 2 X 4
These will be the drop channels
*look at the last step: improvements. This may need to be changed.

8 segments of 3 ft., 6 in (1.0668 m) long 2 X 4
Main track supports
look in improvements, may need to be changed

4 segments of 4 ft., 5 in. (1.3462 m) long 2 X 4
These are the pieces of wood directly on either side of the drop channels.

2 segments of 2 ft., 11 in. (0.889 m) long 2 X 6
3 segments of 3 ft (0.9144 m) long 2 X 6
More diagonal supports. These Are not included in the instructable because I did not think they were necessary. I might include them later. If you really want to include them, they should also be cut about 3 inches longer, until you know what to cut.

2 segments of 1 ft., 0.5 in. (0.3175 m) long 2 X 4
lateral supports

2 segments of 2 ft. (0.6096 m) long 2 X 4
extra lateral support

4 segments of 1 ft., 5 in. (0.4318 m) long 2 X 4
4 segments of 1 ft., 5 in. (0.4318 m) long 2 X 6
diagonal supports, lateral and track-supporting

2 segments of 3 ft. (0.9144 m) long 2 X 4

6 segments of 2 ft., 5 in. (0.7112 m) long 2 X 4
These make up the short arm
May need to be shortened, look at improvements

2 rectangles of 3/4"  in. (2.54 cm.) thick plywood, 2 ft., 5 in. X 3.5 in. (0.7112 X 0.0889 m)
Plywood also included in the arm

1 segment of 7 ft. long 2 X 4
the arm itself

2 rectangles of 3/4 inch (1.905 cm) thick wood, 1.5 in. by 2 in (0.381 X 0.0508 m)
The two bits at the open top of the cut channel..keeps the drop channel from "opening"
*look at the last step, improvements. This may need to be changed

4 rectangles of 3/4 inch (1.905 cm) thick wood 9.5 X 5.5 in (0.2413 X 0.1397 m)
4 rectangles of 3/4 inch (1.905 cm) thick wood 9 X 4 in. (0.2286 X 0.1016 m)
4 rectangles of 3/4 inch (1.905 cm) thick wood 9.5 X 9 in. (0.2413 X 0.2286m)
Threee previous sections are for the counterweight boxes. This might change depending on your weights setup. This is what fit for me. Also, 1 inch thick wood would would be stronger.

Hardware

about 250 2.5 in. (6.35 cm.) wood screws, phillips head drive, #8
I would get more in case some of their heads strip...
if you buy pt lumber make sure they are approved for use in it..

8 .5 in. (1.27 cm) wood screws, phillips head drive, # 8

16 1.5 in (3.81 cm.) wood screws, phillips head drive, #8

3 ft. (0.9144 m.) long 5/8 in. (1.5875 cm.) diameter threaded rod, coarse thread
NEEDS TO BE MADE THICKER: LOOK AT IMPROVEMENTS
2 segments of 2 ft. (0.6096 m.) long 5/8 in. (1.5875 cm.) diameter threaded rod, coarse thread
May need to be made thicker

20 5/8 in. (1.5875 cm.) nuts, coarse thread

24 5/8 in. (1.5875 cm.) washers

10 1/4 in. (0.635 cm) diameter, 5.5 in. (15.24 cm.) long carriage bolts
For the arm assembly

6 1/4 in. (0.635 cm) bolts, matching thread
for the carriage bolts
6 1/4 in. (0.635 cm) washers, matching carriage bolts
for the carriage bolts
3/8” X 4” eye bolt (Including bolts)
8 matching cut washers
these will be used in the trigger

2 1/2” X 10” eye bolts
these are the trigger pins

Wheels, weights

3 3 in. X 1.5 in. (7.62 X 3.81 cm) diameter wheels, 5/8 id bearing load rating of 200 lbs (90.71 kilos) or more
May need to be hanged if the diameter of the threaded rod is changed
excellent site for cheap, strong wheels http://www.thecasterguy.com/

2 hand weights, 10 lbs (4.54 kilos)*
2 hand weights, 8 lbs (3.63 kilos)*

*must be able to fit inside counterweight boxes

Sling
Approx. 50 ft. (15.34 m) of 3/16” (0.47625 cm) braided polyester cord
The sling lines, I bough 50 ft because i needed to cut the right lengths for each ratio, you probably wont really need more than 15 ft.

Approx 25 ft. (7.62 m)1/2” braided nylon rope
(This is actually for the trigger)

15 in. X 6.5 in (38.1 X 16.51 cm.) denim fabric
for the sling
Just find a pair of old jeans

Approx. 10 ft. (3.048 m ) of gorilla tape
for the sling

1 keychain ring
A solid metal ring of any kind would work better.

One thick nail.
This is the pin.
I don't know what gauge mine was, I found it in the trash Buy one that is at least 4 inches. The thicker the better to prevent it from bending. You should also cut off the head if you keep getting downward throws no matter how you adjust it.
If you want a better pin system, look at the turnbuckle pin system here.
(third picture on the right) RLT industries trebuchet.com

Tools
erm...
common tools

hacksaw if you don't have a METAL CUTTING BLADE FOR YOUR JIGSAW,
jigsaw the one I used here

circular saw (The one I used here) OR miter saw AND table saw
you need to miter, rip, and crosscut.

drill (The one I used here came in a kit here) with a varied drill bit set, for pilot holes, etc. One drill bit MUST be the size of your threaded rod (Mine was 5/8", it should be thicker) Ideally, the drill bit would be a foot long, or 6 inches, so that the holes in the arm don't become misaligned. I'll explain later...
also screwdriver bit driving in your screws.

coping saw and orbital sander (The one I used here)

speed square
...I'm kinda adding these as I think of them..

Tape measure, ruler
rubber mallet
claw hammer
um..

I think that's it
OK onto step 1
the base...
the beginning...

## Step 2: Base

Ok, so this is what the entire bulk of the trebuchet rests on. It's 6 feet long and 2 feet wide.
And it is constructed with 1 X 6s
That's its fault.
1 X 6s flex alot.
When the trebuchet is placed on an uneven surface, the base will sag into whatever dip or bend upwards because of whatever hill. This, in turn, changes the width of the drop channels to narrow (BAD) or widen (usually better). Although 1 X 6s did end up working, 2 X 6s would have been more sufficient. Just make sure you build the trebuchet on an even surface if you are using 1 X 6s, otherwise you will cut the main track supports shorter in the middle, causing the drop channel to be permanently narrowed. If this happens, use it on a hill or place two very thin pieces of scrap wood under the base, directly under the drop channels. That should bend it back upward. Yeah, this happened to me.

Ok, now, the base is constructed using half- lap joinery. For this you will need your circular saw, sander and the claw hammer if you don't have a router. A wide chisel would also work.

You will need the two 2- foot lengths of 1 X 6 and the two 6- foot lengths of the 1 X 6.

On each piece, mark 5.5 inches (The real width of a 1 X 6) from each cut edge.
Make a square line across the flat side of the board. (Should be square to the length of the board, like a crosscut.) From this line and out to the edge, cut 3/8" deep (Half the depth of a 1 x 6) with your circular saw every 16th of an inch or so, all the way across the flat side. Then, using the claw hammer, hammer out these slivers of wood. They should come out easily if they were cut narrowly enough. Then, use the sander to sand it down flat and smooth. The first sketchup picture (Sorry only have real pictures of the finished thing) shows what it should look like when your done. Male sure you always do these cuts on the same side.

If you have a router, it would be easier to do this by just cutting away the material.

Ok, now you need to flip over the long (6-ft.) pieces and slide the two foot pieces underneath. Use a forstner bit or any other drill bit and drill holes parallel to the length of the long 1 X 6 and near the inside corner. This is only to coutersink the screws, do not drill the hole all the way through. 1/4" at the very most. Look at the pictures to see this; it's kind of hard to explain the placement in words.

Then, obviously, drive in the screws. Before you do that, though, make sure that you are driving them in from the bottom. The bottom is the side where the 2 ft 1 X 6 overlaps the long 1 X 6. Lastly, make sure everything is square.

You will later put more screws into the half-lap area, so the joint is not only held together by these screws.

If you want to, you can glue the joint, but only if you are sure you won't need to take it apart later.

The base is finished.

## Step 3: Tracks and Main Track Supports.

For this step you will need four of the eight 3.5 ft pieces, and the two 6 ft. long pieces. This step is fairly simple.

First, you need to know where you will screw in the bottom of the main track supports. Line them up so that their 1.5 inch edge is adjacent to the edge of the 2 ft 1 X 6, and so that their 3.5 inch edge is on the outside cut edge of the long 1 X 6. The first picture shows this.
Ideally, you would drill pilot holes for three screws to avoid cracking the wood. For each track support, you should use 3 of the 2.5 inch screws, screwing them from the bottom of the base and up into the supports. Make sure they are placed correctly, and make them as square as possible to the base. The second picture shows what it should look like after they are screwed in.
Now you simply lay the 6 ft pieces across the supports, lining up the edges, and drive three of the 2.5 inch screws into through the track and into the supports. Mortise and tenon joints here would be best, but I didn't feel like they were necessary. I would still recommend doing one, though. Although they need to be lined up, don't worry too much about the squareness of the main track supports to the base, since that will be corrected by diagonal pieces, from the track supports to the track itself.

## Step 4: Extra Supports and Braces

So now we're going to add some extra supports that will prevent the 2 ft 1 X 6 from bending, Lateral support and extra track support.

1. Bending Braces.
These are the 2  2 ft. long 2 X 4s. They have cutouts on them so that they can be screwed directly onto the base with their 2 ft. long edge flush with the outside 2 ft. edge of the short 1 X 6. The cutouts go around the main track supports. It is later screwed through the base and through the main track supports.  The first through fifth sketchup picture will show this.

2. Diagonal Track supports. These are the four sections of 1 ft, 5 in 2 X 6. If you want the diagonal to go exactly 1 foot out and 1 foot down from the corner, you need a more precise measurement, it's something like 1.41 feet. Luckily, the exact size of these supports doesn't matter too much. Simply miter off each edge at a 45 degree angle, so that the piece is the shape of a trapezoid. Again, the exact size of the support does not matter, but the long edge should be under 1 ft, 6 in.
The placement of these pieces is shown in the sixth picture. Basically, put them in the corner where the main track supports and the track meet. Put them in that corner, towards the inside. Again, the lateral placement in this corner does not matter as long as both edges of the diagonal meet evenly along the track and support.

Use three screws in every place that the diagonal touches the track or support.

3. And lastly, for the extra supports, we have the lateral diagonal supports. These keep the distance between the two tracks from narrowing or widening. Unfortunately, there were some problems in the sketchup base that I made (and could not be easily corrected) that threw off a couple of measurements. So, the lateral diagonal supports did not come out too well... But I'm sure you'll get the idea. They are basically the same things as the 2 x 6 diagonals except they are 2 X 4 and they have more half-laps cut in them. So, again, cut off the ends of all the 1 ft, 5 in. 2 X 4s at a 45 degree miter.
Position them in between the two main track supports and trace a line as best as you can along where they overlap; this is where you will cut the half-lap. This time, It's half the depth of 1   2 x 4, which is 0.75 (3/4) inches. To cut the half- lap, refer to step 1. It's the same process of cutting slits, hammering out the wood and sanding it smooth.
Follow the measurements given of the depth and width of the cut. Remember the length really is only 1 ft 5 in.

For now you are done with the extra supports...
Although later there are more.

## Step 5: Drop Channels

Beside the arm when it's at rest, these two channels are the tallest point of the trebuchet. When you look at the picture, they are the big, tall, straight pieces. They are fairly simple to make. First, grab two of the 8 foot long pieces. Find the center of the board. Then, measure 3/8" from either side of this mark. Scribe the line all the way down the length of the board. The two marks should be 6/8" apart. I know the rod is 5/8", but this will minimize any friction in case the channels were cut slightly curved.
*(One note, this rod is probably too thin. You can cut the gap wider up to a certain pint, but the trigger design might limit the amount you can cut it wider. For the weight I used ( 36 pounds) a 5/8" HARDENED STEEL rod MIGHT work. Still, I would make it a bit thicker. I'll write more about this in the improvements step. The CW bolt  MUST  be made thicker and stronger, otherwise it will bend and cause problems. )
Now, cut very carefully and very straightly down the lines you made. Leave about 5" of uncut material at one end. The other end should be open. It would be better if both ends were one solid piece of wood. The first picture shows what this looks like.

Next, grab two of the 4 ft 5 in pieces. Again, this is not a critical dimension, something near that would work. Miter off one end of each at a 30 degree angle (with respect to the length of the board) This angle just makes it look cooler.

Place the tips of these boards 1 foot from the end with the tiny rectangle of wood. This will be the top. Drill holes every 6 inches into the mitered pieces deep enough so that the wood screws can penetrate the drop guides itself. For me, this was a little more than halfway through the side pieces. Then, drive in the screws.
Remember you need to make two of these. Just in case you didn't know...

Ok, now to attach the drop channels.
First, pick one end of the trebuchet to be the back. It does not matter. From this side, measure 2.5 feet (2 ft, 6 inches) from one end of the tracks. The lines shown in the 9th picture show 2.5 feet.

Now, measure that same distance on the base. Line up one edge of the bottom of the track with the 2.5 foot mark. (Look at the picture)
Its 3.5 inch edge should also be 1/4" from the long edge of the 6 ft. long 1 x 6. Look at the pictures for alignment.
Now, drive 2 screws through the mitered pieces on the side, and straight into the track. 2 screws for each mitered channel support. Then drive 2 screws through the board with the actual channel on either side of the gap, diagonal from each other so that you can fit two into the narrow width of the wood on either side of the gap. Just get several throught the drop channel assembly and into the track.
Then drive 3 through the base and into the bottom of the drop channel.
The trigger will be added  later.
Next we have the drop channel supports.

## Step 6: Drop Channel Diagonal Supports

These should be fairly simple-they're just diagonal pieces. The hardest part, really, is getting the angles correct and then cutting them accurately. For the diagonals you will need the
2 segments of 6 ft., 2 in. (1.8796 m) long 2 X 4
2 segments of 5 ft., 10 in (1.7272 m) long 2 X 4
i just copied that...
anyways
erm

Ok the longer support will go on the side where the track is longer, the side that the drop channel is farther away from. It's a little bit tricky to get the angles perfect.
*A few notes: the angles were based off of the sketchup model which had a few errors. It may have thrown some little thing off so just use your judgment about the angles. They're probably correct, I'm just letting you know. Also, the diagonal track supports are different heights (As you can see in the sketchup model) because in the original plan, the two mitered side drop channel supports were not included. I had to add them later. For them to fit, I had to cut different lengths off of the diagonal supports. Since one is steeper than the other, this meant that the vertical height ended up different. So, if you want them to be the same height, you may need to change the angles or the lengths mentioned here. Now, let's get started actually building and attaching them.

As I said before, the longer diagonal support will be on the side of the drop channel that is farther away from the end of the tracks. Mark the height that you want the diagonal support to rise to. For me, for the side with the longer support, this was about 5 ft, 7 inches from the very bottom of the drop channel. Mark this height. Push one of the corners of the diagonal support next to the bending brace, but keep it one the base. Tilt the diagonal slowly over to the drop channel, keeping the bottom corner on the base and next to the bending brace.  Once the higher edge is at the height mark you made, trace a line on the diagonal down the mitered side support. You need to find the angle to cut the board at so that it will be straight against the edge of the mitered side support.
Do the same with the bottom of the board, the part that you held against the bending brace. Lower it below the base, just rest it on the floor. Keep the top part aligned, and carefully trace a line on the diagonal along the edge of the base. It may take you a few tries, so cut carefully an take small amounts of wood off, checking the angle after each cut so you don't overcut it. The angles shown in the pictures are based off of the sketchup model, but they seem to be correct. They would at the very least be a good guideline.
drive 3 screws through the base and into the bottom of the diagonal, and 3 more through the diagonal and into the mitered side support pieces.

Ok, now repeat the entire process mentioned above for the "short" side. leaning, tilting marking, etc. Or you could use the angles I give you.
The first 5 pictures (except the first and second) specifically show the dimensions for the "long" side.

the next 3 specifically show the dimensions for the short side.

## Step 7: Other Main Track Supports

simplest step.
grab the last 3 ft, 6 in 2 X 4s. Line the short edge up with the 8 ft piece of the drop channel. They have the same orientation as the main track supports. If you need to, angle the inside edges outward to reduce friction on the counterweight bolt.  You also will need to push them out slightly, so that the gap is 3/4" at least. Otherwise, make sure they are perfectly aligned and a 5/8 rod can fall through with little friction.

Push them up against the drop channel. Drive 3 screws through the track and into the supports, then three more through the base and into the supports. Drive 2 screws through the drop channels and into the supports. Again, make sure a 5/8" bolt can easily fall through.

You done...
with that.
Now comes the moment  of truth. Hopefully everything has been built straight and solid. Because we are about to cut the tracks in half. Otherwise, the counterweights would just stop like the safety makes it do (Second clip in the video in the first step)
To cut the tracks, mark a 3/4" gap that is aligned with the drop channel gap and the support directly below. Mark the line directly across the flat face of the track, perpendicular to the long edge.
Take off the drop channels completely so you can cut through here.
Cut through here.
Make sure the gap is aligned with the supports and the gap in the drop channels.
Hopefully, your frame is not super wobbly...
If it is, that will be solved later in the finishing touches step.
Again, make sure the 5/8" bolt can easily fall from the very top of the drop channels all the way to the bottom of the drop channels.

Now you done...

## Step 8: Arm

This is it. The part of the war machine that means business. The Arm.
Luckily, It's also pretty simple to  make. You need:

6 segments of 2 ft., 5 in. (0.7112 m) long 2 X 4

2 rectangles of 3/4" in. (2.54 cm.) thick wood, 2 ft., 5 in. X 3.5 in. (0.7112 X 0.0889 m)

1 segment of 7 ft. long 2 X 4 However, if you want your arm to be longer, just buy this piece longer. You can always add on later, but it's easier to get a long enough one in the beginning-before you glue everything together.

3 ft. (0.9144 m.) long 5/8 in. (1.5875 cm.) diameter threaded rod, coarse thread

10 1/4 in. (0.635 cm) diameter, 6 in. (15.24 cm.) long carriage bolts
For the arm assembly

6 1/4 in. (0.635 cm) bolts, matching thread
for the carriage bolts
6 1/4 in. (0.635 cm) washers, matching carriage bolts
for the carriage bolts
....and several of the bolts for the 5/8" threaded rod.

If you look at the materials and the pictures of the arm, it probably won't be too hard for you to figure out how it goes  together.

Before we start, there were a few problems.
1. I did not have a drill bit long enough to go through all of the 2 x 4s of the short arm, so I had to drill individual holes in each individual 2 X 4 and hoped that they lined up. Well, they didn't becasue I did not have a drill press. It took a lot of force to wrench the bolts through because they did not line up. Luckily, I eventually lined up the holes. although I'm still not quite sure how that happened. Maybe the wood "stretched" and then was kept in place as the glue dried. I'm still not sure.
Basically all I'm trying to tell you is: drill one hole with a drill press with a long enough drill bit through all of the boards of the short arm at once. Use a drill press if you are going to drill one hole because one hole needs to be better aligned than a bunch of smaller ones. Then just screw the bolt through when you need to.

2. The pin...is crap. It was an old rusty double- headed nail. We think that the downward throws were caused by these two heads. The ridges would sometimes catch on the heads and sometimes not. It would also bend out of position because of the force of the swinging projectile. So, use the turnbuckle system I mentioned in the introduction. Also use a continuous, smooth metal ring

3. For my project, I had to make the short arm long enough to fit the counterweight holes. If you are going to use a shorter ratio, such as 3.5 : 1, make the short arm pieces shorter. This will take more weight off of the long arm, which should be as light as possible.

4. Make the long arm lighter. Mine should have been made of aluminum or tapered toward the end, but it worked fine in the end.

Ok lets get started. I'm going to assume that you are drilling one long hole through all of the short arm pieces. If not, just measure the distances on each piece as accurately as you can.

1. pretty simple step. Remember to shorten the short arm pieces if you are using a larger ratio that will need to make it shorter. otherwise the long arm will be too heavy.
Ok, find a flat surface and 3 scrap pieces of 2 X 4. Grab 3 of the 2.5 foot pieces. Put a generous amount of glue on the flatter side, the 3.5 inch side. Lay them on top of one another, making sure the edges line up.

2. Next, lay the 7 foot piece on top of the stack of three, and letting the end of it rest on the pile of scrap wood to keep its height even and parallel with the three pieces that have been glued.

3. Lay the last three pieces of the 2.5 foot sections on top of the arm. Just as a reminder...remember to add glue...

4. Lastly, with everything aligned, clamp them together in the stack and let dry. Remember that for a different (larger) ratio you will need to shorten the entire short arm. 3.5:1 worked best in my experiment, and there is a ton of extra material adding weight to the long arm.

6. Drill through the entire thing horizontally for the bolt holes. Place the hole for the counterweight bolt about an inch and a half from the end. Put the ratio hole wherever it goes. It will differentiate depending on your ratio.
If you have a certain ratio in mind but you do not know the lengths, use this example:

*I want a 3.5:1 ratio. This means that 3 and 1/2 parts of the short arm plus the short arm itself equal the total length of my entire beam. the short arm is the 1 (In the ratio)  and the long arm is the 3.5 (in the ratio) -3.5 times longer than the short arm.
so:
I want a 3.5:1 ratio and 7 ft long total beam.
3.5+1 represents the total length (Short plus long arm). Add the two numbers of your ratio together.
4.5...
then divide your total beam length by that number.
this number is your short arm length in feet(If you kept everything in feet)
so then the long arm would be 3.5 times that...
1.6 X 3.5 = 5.6
then, to check we add the short arm and long arm length together:
5.6 + 1.6 = 7.2.
Thats a bit longer than 7 feet because i rounded 7 ÷ 4.5 up, it was really 1.55555555
so, the placement doesnt have to be exact, but the numbers help if you can follow that.

So for a 3.5:1 ratio and 7 ft beam, my short arm would be 1.55 feet and my long arm would be 5.6 feet, more or less. The axle bolt hole would be drilled in the short arm 1.5 feet from the end.
Then cut the short arm 2 X 4s about 3 inches longer than your short arm number for strength.

NOW after that math lesson..back to building the arm

5. Line up the pieces of plywood and drill holes through everything for the carriage bolts. Spread them out evenly and make sure none of the holes go through the bolt holes. Then just bolt it all up...You can look at and copy my bolt arrangement if you don't really know what to do. Remember, though, that if your short arm is shorter you will need fewer bolts.

6. Lastly, cut through the plywood rectangle so the wheel does not bump over the edge of the plywood as the arm falls. cut about 6 inches from the edge, along the 7 ft. arm.
Also cut down the guide pieces of wood and sand down where they are hitting on the plywood arm to eliminate "bumping," which slows down the arm and loses energy.

7. Use a wrench to thread the threaded rod through the holes you made..make sure it fits and is level. To get it in, you need some sort of head on it for the wrench. Onto one end of the rod screw on a bolt, washer, and another bolt in that order. Tighten the bolts together, sandwiching the washer. Use the top bolt for screwing in, and the bottom one for unscrewing. ...yes, it works

8. Drill a hole somewhere near the end of the arm...it doesn't matter, but it should be at least 3/4" from any edge. It should be drilled all the way through the flatter 3.5 inch edge side.  Lastly, hammer a nail anywhere into the end of the arm. This is the pin.

The arm is done.

## Step 9: Counterweight Boxes

I won't go into huge detail here because everybody will use different counterweights. Basically make a box that can contain your counterweights, whatever they are.
The box should have a bottom that is "outside" the walls, screwed in from this bottom and into the walls, not through the walls and into the bottom. This way, the screws take force from top to bottom, not sideways. It will be harder to split or break the wood.
Also, the box needs to have a top so the weights don't come flying out when it reaches the bottom of the drop. Lastly, drill a hole through both the walls of each box at least two inches from the top edge for the counterweight bolt.

-Use screws! nails will slide out of the wood, and will not keep the box together.

-Make sure it is strong enough to take the force of your falling counterweight. (The counterweight bolt has to be strong enough, too. )

-The box needs to be made narrow enough to fit on the outside of the drop channel and still have room to be loose..that's room for other bolts and washers. It needs to fit completely on the bolt.

## Step 10: Sling

This is the only part of the trebcuhet that can vary a lot. You will need the denim and the monkey-  no, sorry, gorilla tape. You also need the braided polyester cord and the keychain ring. As long as it's a strong, flexible cloth pouch that is big enough to wrap around your projectile, It's fine.

1. cut off about 8 inches from the leg of the jeans.

2. Parallel to the seams, cut all the way through so that you can open it into one sheet.

Different jeans vary, so this next step might be different for you.
3. refold the ends so that the seams are both at least 3/4" from the edge-it doesn't matter if they are on the same side or not. There should be two layers at the outside and 1 layer inside, in the middle. fourth through fifth picture will show this.

4. Next tape all outside edges so that the perimeter is completely taped. This will stop fraying and make it more solid.  On the fraying edges, use only one strip and fold it over. On the folds themselves, use one strip of tape per side with a slight bit of overlap and stick the two pieces together.

5.Next find the center of the folded edges. Stick a few inches of tape over that area, making sure you get it on the inside and outside of the seam. This tape will further reinforce the areas where the string will go through.

6. Punch two holes. This will take some effort. I recommend using a pin, then a thick nail, then a screw and finally a pencil to widen the hole for the string. Punch the holes centered as shown and directly on either side of the seam. The seam will help prevent the string from pulling through and takes all the force of the swinging pouch.

7. Cut to the edge of the tape that you folded over the fraying edge. Then fold in on top of the other one to crinkle the pouch into a more pouchy shape. (8TH picture) Then tape them in place. The more you pull the edge over the other one, the deeper the pouch will be.

8. Finally, thread your sling lines through the hole on the outside of the seam to the back, and then through the back to the front. Tie a figure 8 knot. The knot should be on the inside of the pouch.
*Basically you just want enough layers of tape around the rope holes so nothing rips, and so that the rope doesn't pull out of the holes.

I'm sure this pouch design could be improved. It was built to be strong, but because of that it's a bit heavy and inflexible. The ball did slip out a few times. There are plenty of places you can go on the internet to look for better ideas. It worked fine for me, though.

Finally, tie the keychain ring to the slightly shorter rope. Use a bowline knot. This ring will be what loops around the pin. Tie the longer end of the rope through the hole in the end of the arm. Again, use a bowline knot.

## Step 11: Trigger

The trigger is a fairly simple design. For the trigger you will need the:
Approx 25 ft. (7.62 m)1/2” braided nylon rope
3/8” X 4” eye bolt (Including bolts)
8 matching cut washers
these will be used in the trigger

2 1/2” X 10” eye bolts
these are the trigger pins

The 3/8" eye bolts go at the top of the drop channels. The eyes face inward. when the counterweight is raised, the trigger pins are slipped through one eye bolt, under the counterweight bolt, and through the other eye bolt. The trigger pins keep the weight from falling. Once they are pulled, they do not interfere with the motion of the arm. I have real pictures, but no sketchup ones. It should be easy enough to follow.

1. First, grab your counterweight bolt (hopefully thicker than 5/8") and use some sort of clamp to keep it pushed up against the top piece of wood, as if it was being loaded.

2. Next, grab you trigger pin and just hold it beneath the CW bolt.

3. Try to figure out where to put the 3/8" eye bolts so that you can raise the CW bolt all the way to the top, But make sure there is enough room for the trigger pin to go through and under easily.

4. Drill the holes for the eye bolts. Use a washer wherever metal is coming in contact with wood. Tighten the bolt as much as you can.

Make sure the washers do not interfere with the drop channel. If they protrude into the channel, cut it small enough to fit only on the wood on the side.
Remember, if you make your drop channel wider for a wider CW bolt, You may need to cut the washers down even more and move the bolts farther to the outside.

If you make the channel a lot wider, I would suggest using a wider piece of wood or strength. This would, however, change the lengths of the diagonal supports. Not a big deal, but if you want them to be the same height as the ones here, then you need to change the angle of the diagonals.

5. Cut the threads off of the long eye bolts. These are the trigger pins. Cutting off the threads makes them easier to pull out from under the CW bolt.
Make sure all the hardware can support whatever CW you are using.

6. Grab your nylon rope and cut 4 feet off the end. You should have 21 feet left. A four foot piece and 21 foot piece. Again, the lengths can be changed slightly, but you want at least 22 feet of length away from the trigger pins when you fire.

7. Tie the each end of the four foot piece to one of the trigger pins. Use a bowline. Next use a sheet bend (There might be a better knot for this) and tie the 21 foot piece to the middle of the four foot piece. You should have a " Y " shape rope with a trigger pin on each end.

8. Tighten all the knots and make sure none will slip.

9... Fairly simple trigger design.

Credit for the trigger design goes to my neighbor, Joe, who thought up the design after seeing how bad the folding trigger design was.

Otherwise, I'd probably have a concussion because the folding trigger design released before we wanted it to. Luckily we caught it and injuries were avoided.
...I would probably actually be dead. This thing is powerful, which is why you need a trigger. Always stand way back from the trebuchet, and keep the safety blocks in place until you are ready to fire. (Safety blocks explained later)

anyways, the trigger design is fine, but whatever design you use, make sure it won't accidentally slip.

Now we're finally going to add the last few lateral supports and the guide wheel.

## Step 12: Last Finishing Touches Before We Put It Together....

Ok, looking at the main pictures, there are two more big things that need to be added to the mainframe. Without one of them , the arm would simply fall into the ground. Without the other, the tracks may not be aligned or the width of the drop channels might narrow or widen (Not staying even= bad)
So, the first one is  more of those charming "Extra supports." They keep the side-to side motion of the tracks to a minimum. The lateral diagonals keep the track supports from moving in or out, but the tracks will still pivot around the supports. These are really simple. Just cut a length of 2 X 4 To fit in the spots shown in the pictures. These go in the front.

Now, I believe that the width between the bottom of the main track supports is somewhere around 1 ft, 1 inch. As a starting point, cut these two
um...
Twisting braces ? ya that sounds good

Cut these two twisting braces at around 1 ft 2 inches because the length of these will affect the width between the tops of the two drop channels. Measure the distance from the outside of one track support to the outside of another; do the same at the top. You want the drop channels to be the SAME DISTANCE APART THE ENTIRE WAY up and down their length. Otherwise, the washers in between the CW box and the channel will be smooshed up against the drop channel, creating friction, which is bad. If it's too narrow at the top then the smooshing will happen as they fall closer to the bottom.
So, cut the first twisting brace, on the very front,  as wide or narrow as you need it to keep the drop channels straight.
You also want to keep the track in a straight line, otherwise the wheels will fall off the edge.
Again, cutting these pieces shorter will narrow the track and cutting them longer will widen it.
Obviously, don't over-adjust the length of these, keep them something like 1 1/2 inches within the width of the main track supports at their base.
You can use a clamp and a tape measure in combination to figure out the right lengths before you cut them.
Change the lengths as you need to to keep the drop channels and tracks aligned.

Remember that these adjust the front half of the tracks, the next bit adjusts the back.

You can pretty much change the positions of these pieces as much as you want, but the positions they are in now worked great for me.

Next, going to the back of the frame, we have the guide wheel and the guide blocks. These keep the arm straight as it is being loaded and dropped. Otherwise, the arm would derail a lot more.

A note about the guide blocks:
Make them small as possible, but make sure they can still guide the arm until its wheels hit the track. Otherwise, The arm will get puched over and on top of them, slowing down the arm a lot. If you see or hear bumping, look carefully at this back area w/ a video camera or just your eyes to see why it's doing that.
Remember, smooth transitions.
Everything moving should always be on a wheel, and only touching a wheel.
If you need more help with the guide blocks let me know. I had no idea what the problem was at first.

The third clip in the video shows this problem well- the arm kind of rolled over and on top of the guide blocks because they were too high. The arm slowed down a lot, and the ball spit out the back.

Drill a hole horizontally and squarely directly through the track. Keep the hole in between the main track supports and the diagonal track supports.
...That should be about 2 inches from the end of the track. Again, not critical.
The guide blocks can be any shape you want, but look at the picture notes below for shaping. Avoid contact with the arm as much as possible, they are only there to guide.
Cut them low but long out backwards.
Drill the hole accordingly, so that their position is similar to the one pictured below.
Slide the bolt through one hole, screw everything on in this order:
nut, washer, guide block, wheel and bearings (Go together) , guide block, washer, nut.

slide the bolt the rest of the way through the other hole. Center the wheel in the middle of the guide blocks and the wheel with the guide blocks in the middle of the tracks.
Everything should be centered. Finally, put a nut and washer on the outside bits of the bolt that protrude from the tracks.

By tightening these outside bolts, you can change the alignment of the tracks and narrow or widen the distance between the drop channels.

Okay now you need to cut the axle. Measure the amount of space in between the drop channels. The axle will have to easily pass in between them.
Then cut the axle about 3 inches shorter than the width.
This may change depending on the width of your wheels. You only want enough of the bolt so that after it comes out of the arm there is only enough room on either side of the bolt for a 3 washers, 2 bolts and the entire wheel assembly.
Hole on to the axle for the next step..

Lastly,
Once everything is square, true, straight aligned or whatever you feel like saying, We can put this beast together.

## Step 13: Put Everything Together

After looking at the dozens of picture I took, It should be pretty easy to figure out how to put everything together.
All you are really doing is putting the arm in the drop channels.
First, obviously, just lay the arm in between the drop channels (Just the middle of the treb)
Line up the holes in the arm with the drop gap.
Use a wrench or hopefully a driver of some sort to drive the CW bolt all the way throughone drop channel, through the arm and  out the other drop channel. Center the bolt in the arm, with the same length of bolt coming out from either side. Next center the arm itself in the middle of the drop channels, so there is the same amount of space between the arm and the drop channels on either side.

Bolt on the CW boxes. You may want to keep the weights out while you are doing this.
Use the order of bolts and washers on either side shown in the first picture (From inside to outside the order is) : drop channel, washer, bolt, washer, CW box wall, Washer, bolt, Bolt, washer, wall of CW box, washer, bolt.
Just make sure there is a washer on any side of a bolt that will be touching wood.

Place the weights in the box and screw on the top. If the weights are extremely heavy, though, you may want to keep the weights out of the box for now, since when you load it you will have to lift it.
The sling and pouch should already be tied together and to the arm...
If not look at step 10.

Finally, thread the Axle through the other hole in the middle of the arm. Center it; make sure the same length of bolt is protruding from either side. Then bolt on the wheels. (Order from inside to outside.) Short arm, washer, bolt, washer, wheels and bearings, washer, bolt.
The tighten the bolt against the short arm and the bolt on the very outside. Keep everything tight.
Make sure the axle bolt is short enough to clear the frame easily.
Unless I've forgotten something, I think we're done. Now I'll tell you how to lock load and FIRE!
...look out for flying cats
jk

## Step 14: Guide Chute-for the Sling

Well, for me, it wasn't really a chute.
I simply layed a fairly long board over the diagonal supports in the back, extending out a bit to give support to the sling until it was lifted up into the air.
It wasn't the best design for very straight shots, but for me it worked very well.
The distance that it extends out from the diagonals affects the distance a lot. At least i think.
If you wanted to simply use a board, it might be good to attach two long pieces of wood to the length of the board. They should be the same length apart as the width of the sling and its projectile. This way, it's easy to angle the board to get angled shots. (Left or right)

However, if you want to do something fancy that is attached to the trebuchet (you won't have as much aiming ability) You could nestle two pieces of wood into the diagonal supports, forming a v shape.  Then just screw them in. they would be horizontal, not angled. They would extend outward, just like the board chute.
Really, you just want something to support your sling during the first part of the arm's swinging so the sling is not dragging on the rough ground.

Well, I don't have many pictures of this...
But I'll try.
First of all you need two people
(Hopefully your friends haven't deserted you over this trebuchet obsession. Hopefully they're willing to help. Even though IT'S VERY DANGEROUS IF YOU DO IT INCORRECTLY!!)
There are two main safety mechanisms: The safety blocks, which prevent the arm from falling,  and the second is common sense. NEVER, NEVER LEAN OVER THE ARM!!! And back up at least 20 feet when firing (The sides are safest)

Make sure everyone knows you are firing.
Okay step one.
1. Have someone stand on the front half of the trebuchet. Just on top of the tracks. (Hopefully a strong someone)
2. Have someone stand at the back of the trebuchet, next to the arm (To the side)
3. The person on the trebuchet needs to push back on the arm so the person on the ground can reach it.
4. The person on the ground should pull back and down on the arm like a lever, using the guide wheel as a fulcrum. Keep pulling back on the arm until the counterweight bolt is about 3 inches over the topside of the tracks.
5. Quickly, before the other guy gets sore holding the arm, the person on the trebuchet will need to slide some scrap 2 X 4s around a foot long over the gap in the track. This will prevent the counterweights from falling.
The person standing on the trebcuhet is no longer in danger unless he drops the bolt on his foot. The arm will not swing up. Look at the second clip of the video for the safety "demonstration" (We accidentally left it in)

6. The person holding the arm up can relax. He can back away. Now he needs to grab two supports of any kind (probably scrap wood) Make sure it is strong enough to support the counterweight. They should be about 2.5 feet tall.

7. This is the hardest part of loading. You need to lift the counterweight to the height of the supports so your friend can get them just underneath the CW bar. Lean the supports in towards the drop channel as they go up so they don't slip out. Remember, even if you drop the CW, the safety blocks will prevent the arm from swinging up.
Take a break.

8. Grab the trigger pins. I keep them in one of the mitered side support screw holes so I can easily reach them once I get the CW to the top.

9. Raise the CW to the top and hold it there with your thigh (raise your leg to support it) and one hand.

10. Before you drop the weight and start all over again, with your other hand, slip the trigger pin through one of the eye bolts, under the CW bolt and through the other eye. MAKE SURE THE EYE OF THE TRIGGER PIN IS FACING THE FRONT OF THE TREBUCHET otherwise you will have to switch it.

11. Unless one eye bolt cannot support the weight, relax.

12. Whenever you feel like it, slip in the other trigger pin, with the eye toward the front.

13. Make sure that the washer in between the CW box and the drop channel is not caught on the bolts protruding from the eye bolts (Trigger pin... holder.. things) Make sure it will be able to fall freely, But do not remove the safety yet.

Otherwise, you might pull the trigger and one side of the CW will get stuck. Then very carefully you will have to put in the safety blocks again and reset the trigger.

14. Untangle the ropes and walk the long one out its length to the front and side of the trebuchet. (Diagonally).

15. Place your projectile in the sling. Slip the ring over the pin, and stretch the sling lines to get rid of as much slack as possible.

16. Double- check everything. Make sure your projectile is in the sling (Yes we do forget that a lot) the ring is on the pin, the CW bolt won't get stuck on the eye bolts coming out of the drop channels and that the safety blocks are still in, along with the weights (Obviously)

17. NOW AND ONLY NOW take out the safety blocks.

18. Get everyone away from the back and front of the trebuchet.

19. Position yourself diagonally to the front of the trebuchet.

20 yell heads up, countdown , just make sure everyone know you are firing.

21. I forget what you are supposed to do next...

22. still thinking.

23. Thinking *this is not funny only annoying*

24. Okay yes you fire.
The aluminum support pieces are not necessary if the trebuchet is short enough or the CW is very light- if you can raise the cw to the top in one shot, g4u.
I only barely can, with only about 60 lbs.
If you can't get it to the top in one shot, use the supports.

Record the next shot if the first one does not go well.
Try to determine the problem.
If the shot releases at the wrong time, adjust the pin angle. (Forward to the direction of motion makes it release later, backwards= release sooner. Heavier projectile= earlier, lighter projectile = later. )
Remember the bumping problem.

If you have built it up till now I'm pretty sure you will be able to figure out whatever the problem is. But if you can't...
Ask me if you have a question.
I speak Spanish and English, along with English and Spanish. Yes i am proud to be
multilingual.

I'm still working on the "improvements" page but everything I was going to mention on it was kind of already mentioned throughout the instructable.
So I'm still not sure if i should add it.
and if everyone wants the step I'll add it.
Lemme know how to improve the instructable
Have a nice day
(I had no idea what else to finish it off with.
Don't judge me)
Have fun and be safe.
Hey I just made that up right now.
Cool ring
...
have a nice day
be safe
COMMENT RATE

Okay, now I'm not kidding. Really, be careful.

Thank you for reading through all this, and let me know if you actually build this monster!!

Yes, I decided to add this step after awhile.
After the first 25 shots or so with the trebuchet, there were two things apparent: the arm could easily be made longer and could greatly increase the range, and the the CW bolt needed to be made stronger.
The arm wasn't too hard to extend, BUT whatever you use to join the parts MUST be strong enough. Or you could buy a longer beam to begin with.
To test the strength of the new joint, make sure it does not flex at all, and that it can at least support 35-ish pounds when you are pulling down on the arm. Basically, lift the weight into the air using the arm  as a lever and the seam in the arm close to the fulcrum. I used the trebuchet frame as a fulcrum to lift it in the air.
Make sure that there are no cracking noises or flexing from the joint.
I attached the arm extension to the rest of the arm with some wood pices, dowels and some leftover aluminum channeling. more on that later.
First, I'm going to talk about some other improvements that are more important than the arm.

FIRST PROBLEM

GoochesVFD said that there was some cracking around the base of the long 2 X 4 used as the drop channel.
He was right. The uncut section at the bottom should either be left longer, reinforced or improved.
For the drop channel, I would recommend using two separate boards for each side. This would eliminate cracking and make the drop channel much stronger.
ALSO, the base boards (I used 1 X 6) should be thickened. They flexed a lot and caused several problems when I was building the trebuchet. You should use 2 X 6s, or something stronger than a 1 x 6. Flexing will be reduced and you will have a stronger base to work with.
DRILL PILOT HOLES FOR EVERYTHING!!!!!!
there are small cracks throughout the trebuchet now because I was lazy. None seem to affect it structurally, but we'll see.
Other than those, there haven't really been any problems with the frame.

SECOND PROBLEM

This one is pretty bad. The 5/8" CW bolt bent like a taco with only 16 pounds on either side. That was bad. The CW boxes tilted, and would rub on the drop channel when it dropped. Additionally, the bending of the CW bolt meant that it was not perfectly aligned with the drop gap, so it would slow down and bump on the way down.
Basically, THE CW BOLT MUST BE MADE STRONGER!! It will prevent frame strikes, jams and other problems later on.
To fix this, you get a thicker CW bolt, and hopefully a hardened one. Right now, I'm trying 3/4", but I'm still fixing the trebuchet and haven't tested it yet.
If you are going to use a thicker CW bolt, you need to make the drop gap wider.
Hopefully you'll use a wider bolt to begin with...
but if you didn't (my bad) You need to recut the drop gap to 1/8" wider than the CW bar, cut the gap in the track slightly wider, and spread apart the track supports directly underneath the drop gap.
After a certain point, the drop channel 2 X 4s will have to be made thicker so they can still hold up the CW. (remember the CW is supported by the trigger at the beginning, and the the trigger is screwed directly into the drop channels. )

You also might want to improve the pin system-at least make sure your pin is smooth.
The pouch also isn't the best. There are countless designs that are probably better.
This one worked, but it was just a bit heavy, small and inflexible. Still, it was strong enough with all the gorilla tape.

## Step 17: Arm Extension

The arm extension is added onto the rest of the arm with three different methods. First, the two pieces are drilled and doweled. A mortise and tenon joint would have also probably worked.
To dowel the pieces, simply drill two holes into both the arm and the extension. They must be perfectly straight and lined up. drip some wood glue into the holes, and put glue all over the dowels. Then just hammer the dowels all the way into the holes. I'm pretty sure I made my dowels 1/2" wide and 8 inches long. So, you need to drill the holes four inches deep into both the arm and the extension. Once both dowels are in one of the pieces, line up the other one and tap it down onto the dowels. Make sure there is glue all over them. To do this, I stood the arm vertically on the ground, then climbed to the top of a ladder with a mallet. I lined up the holes with the dowels and hammered home. The seam should be as narrow as possible.

...*Still working on this step... more to come soon!*...

Runner Up in the
Craftsman Tools Contest

Participated in the
Epilog Challenge

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## 75 Discussions

How would this work at half the size? I am making one for a school project and hope to have it done in a week or so.

Awesome! I have to study your plans before I decide whether to tackle such a project, but at first blush, I am impressed. Thank you for your generous gift to the DIY community.

Hey, I definitely encourage you to do this! It's been 4 years since I built this in 8th grade (graduated high school by now), but its still lots of fun to set up and launch! Really cheap, really fun. Could be built in probably a few weekends. (if you do it, let me know; I'll tell you how to make fireball projectiles)

I have to make a Trebuchet for my Engineering product, and it must be no more in height than 80cm, so what do you think are the best dimensions and ratios for it?

Tbh I'm not sure exactly how well the trebuchet would work when scaled down , but I imagine that it should be fairly similar in terms of arm length and drop height etc.
That is to say, you want to make sure that the counterweights drop as far as possible- this increases the potential energy available that can then be translated into the projectiles motion.
That is to say, you should make sure your drop towers reach the full length of the 80 meter height.
Additionally stresses on the arm will be (exponentially?) less than on a longer, heavier arm, so you have more of an opportunity to make it extremely light, if that makes any sense. Additionally, I would make the arm as long as possible as well. Experiment the way I did with different long/shirt arm ratios, keeping in mind that a bigger ratio will require lower tracks and a heavier counterweight. Mess around you'll figure it out.

Awesome trebuchet! I like the adjustable arm ratio. I just posted an instructible of our own design, it's a lot beefier, we threw 10-15 lb pumpkins around 300ft, and we actually got 720 lbs in counterweight on it, not including the weight bar or short arm. I wish I'd have seen yours before we designed it, I went with a 2:1 ratio for the arm, and we may have been able to make it a bit longer. Take a look at our trigger design if you ever want something that works for heavy payloads.

This is stellar. I really appreciate your work in building this instructable.

We just built an hinged counterweight trebuchet for our annual group camp of friends from highschool (we're all in our 30s now....).

We're all a bit nerdy and geeky and figured a weekend engineering challenge was a good way to make camping and drinking more awesome. We built a trebuchet that we call the medieval stuff chucker (MSC). I have included a link to a hack-up vid I made on youtube in case you're interested.

I really want to build a floating arm trebuchet, so maybe I'll do that between this year and next and we'll have stuff chucking competitions. Here's some vid of ours. http://youtu.be/MS9OepeTELc

I would love to check out the Medieval Stuff Chucker, but the link to the youtube video doesn't work.
If you do end up building a floating arm trebuchet, please let me know. I would love to see it while being constructed and finished.

I'm nearly done with the trebuchet but I'm having trouble understanding the sling. I don't know how long to make the rope or how to attach the sling or how it is even supposed to work? I've made the pouch and strung one side through but from then on I'm lost. Please help because this project is due the 20th.

The length of the end of the arm to the center of the sling should be 1/2- 2/3 the length of the entire arm. A shorter length sling will work better for lighter projectiles, while a longer length will be better for heavier projectiles. You're going to have to do some testing and fiddling to find out what works best.
Now, to attach it:
Imagine the pouch itself. It has two ropes coming off it, one in either direction.

-----------(pouch)-----------

One end is tied through a hole that's drilled in the end of the arm. You do not want this side to come undone, so make sure it's tied tightly to the end of the arm however you can.
The second length of rope is tied to a small ring- like a loose keychain ring.
On the end of the arm, there will also be a nail, or some sort of strong metal pin. This metal loop goes around the pin, and as the centrifugal force increases as the arm swings, the ring slides off the pin to allow the pouch to open.
so its looks like this... if you can use some imagination :P
and if it doesnt get messed up once i post this

(======arm=======--)(----------rope------------)(sling)(----------rope------------)0
^that's the pin tied to sling ring tied to end of rope^

I realize that wasn't an awesome explanation, let me know if you still don't get it.
And send me some pictures :D

I tried to reply but it wouldnt let me so this is my other option. So, This project is not due until sometime in may, but I will be glad to post pictures. Two last things...

How much will this cost?

And, I'm not looking to chuck this 200yds. I will be launching this in a range 200yds and I'm looking to toss under that because I will be demonstrating my trebuchet about 50yds or so from a football field and I will be tossing the weight towards the field and at the other end is a parking lot. So if I could toss the weight a 100yds would be perfect.

It cost me about \$130 not including the weights.
If you need to beef things up, like the counterweight bolt and wheels, it might cost more. But make sure you go to the website for wheels that I recommended in the Instructable to avoid paying too much for wheels.
And as for tossing a weight that far, you would need to change the ratios on the arm yourself and work with different sling lengths. If you could find a simulator that would be great, but I never could. It takes awhile to calibrate the different lengths and ratios to reach optimal firing capabilities.

I may or may not fget a response considering it is 2013 and not 2010 but I need to build a trebuchet for my physics class and i came across your model and had a few questions. I will be modeling my trebuchet after your and would like to know...
Can the trebuchet launch a 2-lb dumbell?
Can the trebuchet launch a 6-lb shot?
If I need to bring the scale down, what would the dimensions be?

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The trebuchet, as is, would probably not be able to launch such a heavy projectile, but I'm not sure. To launch such a heavy projectile will require much more weight than the approx. 50 pounds that was used here.
The main problems would be the extra weight needed to really make the 6 pound weight fly, which would put extra strain on the wheels on the arm and the bolt holding them, the counterweight bolt, the counterweight boxes, the sling, the sling lines and the trigger assembly. Oh, and the arm itself. I haven't noticed any cracking with a 1 pound weight being fired at full speed, but a 6 lb weight will exert much more force on the arm because of its higher inertia..
Essentially, beef up the arm assembly. And do lots of testing, with a realllllly long trigger string.

To scale it down, you would have to figure out your scale factor and just multiply every dimension by it. Alternatively, you could download the sketchup model, highlight the entire thing, scale it down and get the dimensions directly from there.

do you have the exact dimensions of the final product. it would be much appreciated!! i am building a trebuchet for my physics class. i need to make mine 3x3x3. so i need to cut length of everything down to size. will this still work that small? thank you

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Sorry I haven't responded until now. I don't remember the exact dimensions, but I think it was a little bit taller than 8 feet, and about 6 feet long. Width was around 2 feet, I think. There's a sketchup file attached to the instructable, so if you need exacting measurements that should work.
Although the trebuchet will work so small, it might be better to increase the arm ration. Give it more of a flicking action than a long drawn out flinging.
I don't really know what I'm talking about.
Do some experiments, that part is the most fun.

thank you very much. i will keep that in mind. if theres is any other advice that you can give me (especially for it being smaller) it would be a lot of help.

Pretty nice, but you could get a lot more power by lowering your tracks by about a foot. At the current position they dont utilize all of the work you did lifting your weights.

Here is a floating arm that two other students and I from FRC team 11 MORT designed for punkin chunkin in Delaware. As far as we know, its the largest floating arm ever built by youths, and the largest wooden floating arm trebuchet in existance.

The arm is 18 feet long, we use a 1.5 inch steel bar, and can load it up to about 800 pounds.

http://mort11.org/pumpkin

also be sure to check out the rest of our website, FIRST is a great program for anyone interested in engineering!