Generally (but not quite always) if something breaks, the scary bits stay roughly in the plane of rotation. If the arm breaks for example, the projectile could easily go straight up or backwards, along with chunks of the arm and sling. Its pretty unlikely for anything to go straight sideways, though things that go almost straight up can fall down next to the treb. I generally envision the danger zone as an hourglass shape: thin around the center of the machine (axle) but widening the further down range or behind it you go.

Thus generally I try and keep almost as much space free behind as in-front, always stay to the side when working on a machine with any stored energy (counterweight not at the bottom for trebs). When ever possible avoid putting anything in the path of the counterweight or arm except when absolutely necessary, and have a safety in place when doing so (ex: loading the sling) that physically blocks the arm from moving in case of a trigger or winch failure (this is usually required by the rules of competition, and for good reason: its great practice to have a redundant safety). When ever possible (and for sure when firing) stay far enough back to the side that there is no chance the sling could even come close if whipping around.

Another thing to watch our for are winches. If you have a home made winch, if the ratchet breaks, or gets set to free wheeling mode, or you slip off the handle, you can very easily get a winch handle spinning really fast breaking you hands. Having a high quality ratchet, and be ready to just pull your hands back and back away to the side should anything go wrong while winching is a good idea. Always remember where the stored energy is, and how it could break loose and get you so you are prepared to just back away and let what ever happens happen if something goes wrong instead of having to think about it when something starts failing.

Should something go wrong (or right when actually firing it), hopefully you are to the side. Generally back away further to the side if you can so that if anything went straight up, you have more clearance before it comes down.

For small trebs, safety glasses are all the PPE I used: shattering wood or metal could easily take out an eye. Similar to working with a lathe, I always felt gloves might be more of a risk to get caught in something that to protect me. For larger machines a hard hat is a great choice. While it likely won't save you from an errant projectile direct hit, it helps communicate to others that its dangerous and the area nearby is only for people who are protected and know what they are doing, and it can save you from some things (like a ring from the sling to the head, or a glancing blow from a projectile). I did have a metal pulley shatter once: safety and a hard hat are pretty good against that kind of debris.

For more specifics, its really hard to give clear safe answers. Starting with low counter weights, safer projectiles, no spectators and ideally smaller model machines and/or simulations can really help you get a sense for the risks while keeping safe. Work your way up to higher risk situations carefully so you will be able to evaluate them better. Hopefully such an approach can get you a better sense of how big of margins are needed. Most large trebuchets tend to have low enough mass ratios that 300 arm lengths is easily enough, but it really depends on the details (drop height, mass ratio, how much roll is possible etc), so in some extreme cases that might not be enough, and in some other extreme cases is far more than necessary. Looking at total stored energy in the treb and computing how far it could throw if 100 percent efficient could give you a rough upper bound (after accounting for rolling), though most designs won't even get close to that.

I personally haven't operated any really large trebuchets, and I can never really know how lucky I got to come out of almost everything safely (one assistant got a bloody finger when hit by a broken pulley once), so I can't really give any high confidence or athoriative advice, but hopefully this, and some careful thinking and planning, can help you operate safely. Good luck, and may the stored energy leave your systems in a safe manner!

1. Range needs to be 300?X the arm length deep. More, if the projectile can bounce and keeps going.
2. How wide?
3. Will people wander through the range?
4. How much room behind? Do projectiles or trebuchet parts ever get thrown that way? How far?
5. Block spectators out of the area the sling will whip through (traffic cones? Posts and cords? A moat?)
6. When trebuchets, er, rapidly disassemble themselves, how far and in what directions do bits generally go?
7. Any need for PPE (Personal Protective Equipment)?

From the description I can infer that if it achieves CW or arm stall it would have to be with the CW well in front of the axle (or it would hit the ground), and has a short arm rotation (angle wise), which explains why it only works well for low mass ratios.

It shouldn't be very hard to make one in a simulator if you want to see how they work. To help you get started I added a really horrible simulation of one to the baka page.

Thanks in advance!

Typically difficult to model factors like aerodynamic drag (on the sling and projectile) and projectile spin are actually far more important.

In an ideal friction-less vacuum when throwing on flat ground in uniform gravity, the optimal throw from ground level is at 45 degrees. If your throwing from higher, the optimal angle will be a bit lower.

Assuming you you have a low drag sling (so its less of an issue than projectile drag) you will typically want to throw below 45 degrees anyway, and ideally with top spin. This is because aerodynamic drag on the projectile becomes a large factor for most typical trebuchets sizes and projectiles.

Note that for a particular trebushet, especially a poorly tuned one (ex bad arm ratio, or sling length) you may be able to get far more energy into the projectile throwing at some otherwise horrible angle. Optimal throwing angles in terms of distance for the energy really only come into play once you have a well tuned machine that is getting all the available energy into the projectile.

Its also worth noting that rather extreme machines often have rather strange tuning effects. For example some very large trebushets throwing pumpkins are basically unaffected by projectile weight: their arm speed is not effected by the sling significantly. Thus the arm swings right through where you would want arm stall, and they benefit from longer slings and later releases.

Anyway, release height is almost always going to be a bit less than long arm length + axle height + sling length if the trebuchet is anywhere near well tuned. In an optimal case the arm is nearly stopped at release, and pointing almost vertically, so you can consider the projectile to be going in a circle around the end of the arm, and use a bit of trigonometry to get a more accurate height.

I recommend looking at videos of trebuchets and pausing them at release. Looking at the first two frames after release tell you the angle (and velocity) of the throw and release position. For similar and well tuned trebuchets, the positioning is always nearly the same.

Thanks

Streetta

I didn't use the site much during its final few years, so I can't say much more than that. I wish I had saved the farewell message Ron posted before it was taken down around thanksgiving last year.

Some time when I have more time, I may try and check whats up with that, since I don't see any updates on the issue in almost a year.

now as far as the hurl.org, i tried that but there didnt seem to be a way to register, and i crawled into the help section, seems to say new user registration is disabled… is there a secret trick?

I'd guess you could get some more range with a lower area sling. It looks like yours has quite a lot of drag. Tuning looks pretty good, but I haven't worked with an F2K personally. I can't say for sure, but maybe a higher arm ratio might have helped get some of the counter weight energy out before the very bottom, for perhaps better range and less stress on the counterweight bar. Thats mostly a guess on my part though.

Note that the energy that went into bending your counterweight bar (assuming a stronger one, and proper tuning) could have gone into the throw.

A forked SA (put some of the weights in the middle) would have reduced the stress on the counterweight bar quite a bit, which looked like it ended up being the limiting factor. Its also possible to spread the counterweight across multiple shorter bars, which can make finding strong enough bars easier.

Your trigger design is interesting. I haven't seen one like that before, but looks like it does its job. Well done and I'm glad you guys had fun! Thanks for sharing.

Its mostly just me on the forums here; there is a much larger and more active community of even more knowledgeable folks over at www.thehurl.org forums.

search this string in youtube, please.

the link above is a series of fires with a treb the Cal Lab built in our off-time built for a Subbase New London Morale boosting event.

We had a ton of fun doing it. Especially since we all work in the electronics field and got to wrap our heads around a mechanical, physical problem.

Throwing arm is 8 ft. 105" weight drop of 195 lbs (I think underpowered but out bar was 1/2" blackiron pipe, maxed out). axle is 2.75 feet from weighted end. After tuning (you can see a couple shots go not so well in the first part of vid) sling length was 6 ft., release pin at about a 50 degree angle. Max distance was 67 yards by laser rangefinder and spotter.

So it was the first time we did something like this, and we got outshot by only one of the other trebuchets, who had the full 15 ft throwing arm and 750 lbs of counterweight allowed in the rules, not too bad.

Wanted to share this beast and look for some constructive criticism. enjoy, and fire away.

Looks like your mass ratio is ~37.5:1. Thats pretty low (Normal is ~100:1, which would mean 3 pound ammo here), so I'd guess, especially given that you have wheels, that you will get better results with a fixed counterweight than a hinged one. Either way, you will want a pretty low arm ratio, maybe even as low as 2:1 ( A simulator should help greatly here).

Also, personally I've found fixed counterweight trebuchets simpler to tune (and to work better when poorly tuned), but its been a long time since I worked with either type.

Anyway, I strongly recommend checking out a simulator of some kind. It will let you get a good sense of what dimensions to choose, and what adjustments have what effects.

Sadly I haven't kept this site very up to date, so there might be some newer/better simulators around. If you find any, feel free to add them to the simulators page.

Lastly, its mostly just me here. You will get lots more great advice at the The Hurl Message Board. Especially if you want structural advice, you will get much better help there, since I haven't built anything very strong at that scale, and many of them have.

Good luck, and stay safe! A treb that size can be pretty dangerous (Remember, you are very likely to throw backwards, or trigger unexpectedly for example!)

Good luck!

Thanks for your help!

Regarding wheels, sure they effect the energy transfer involved, but they are no more of a form of energy transfer than addling more weight, a longer sling or any other tuning changes.

Personally, I think what happens to the energy after the throw is the most interesting and educational. We all like to teach conservation of energy, but eventually the projectile, and the trebuchet are both at rest, and with less energy than they started with. Where did it go (heat mostly), and why can't you get it back? Thats a real applicable and useful concept! Aside from being a fantastic introduction to entropy, information theory, and thermodynamics, it provides an understanding of efficiency in general, and is widely applicable. The topic of thermal losses in systems is perhaps the most generally applicable transfer of energy in engineering and physics, not a cop-out. In-fact, there is evidence it will be the limiting factor of the universe itself!

Also regarding the frictional and other "small" losses: Most trebuchets lose much more than half of the energy this way, so it is in-fact larger than the transfer to the projectile. And since they are basically the same as the transfer the projectile makes upon impact, (both are thermal) all the energy goes through this transfer eventually. Most of the sound ends up as heat too by the way.

One of my favorite application of this: Where does the energy go when you walk down a hill? (heat!) Since going up hill gets you hotter than down, this proves (after some minimal logic+math) that walking up hill is less than 50% efficient. Neat right?

Its pretty interesting (and quite practical) to compute how much kinetic energy was in the projectile and thus compute the losses/efficiency of the trebuchet. It might be a bit heavy on the algebra, but all the hard parts can be looked up instead of derived. My dad managed to teach me some of the basics of integral calculus doing that exact problem around that age (We derived most of the details ourselves), but I suspect thats not for everyone.

My name is Mal and my 13 year old son, Jared, is supposed to be doing a physics project that shows 3 types of energy transfer. He decided to choose to build a Trebuchet Kitset (Pathfinders) and he already knew that it used Gravitational Potential Energy and transferred into Kinetic Energy but the third transfer of energy was a bit more difficult to pin down. He asked his teacher and was told that the third could be the projectile impact causing heat. I think this is a bit of a cop-out as the projectile is not part of the Trebuchet proper. I know there is friction / heat / sound at the fulcrum, sling, pin etc but all these energies are small and not really a full transfer of energy.

Can you please enlighten me on a third transfer of energy within the closed or semi-closed system of a Trebuchet? I was thinking more along the lines of the ‘cocking’ of the Trebuchet prior to firing. I have heard that the wheels on the Trebuchet add 25% more to the firing of the projectile - can this be quantified as an Energy Transfer?

Thanks in advance

Regards
Mal

If you are looking to build something efficient and compact, thats very different from a trebuchet that may take the same amount of work to build, but is simpler and larger with the same range. Until recently (and maybe even currently), the longest ranged trebuchets were just very large HCWs. If you haven't built trebuchets before, a HCW or FCW is a good starting place if you want a design thats not too hard to get right and gets good range for the effort. All the other designs are more complex and mainly efforts to get more range without simply increasing size. I also recommend building a small model first. If that goes well, you can try building models of a few more complex designs if you wish, and scale up which ever you like best.

Most common mistakes I for school project trebuchets: really heavy untapped throwing arms, crummy slings, and axels that are longer then needed (and then end up bending a ton).

Also, "Full Sized". Full sized generally means one of 2 things: medieval scale and can throw 200+ pounds a few hundred yards, or 8-10 pounds competition pumpkins. That said, I mainly work with stuff that fits on my desk.

Anyway, thats what I have to offer. Feel free to browse around the site and ask some more specific questions if you have any. If you want a higher volume of advice, the majority of the hurling community hangs out at TheHurl Message Board. You will get a ton of fantastic advice from those folks!

Looks good. Hopefully I will have more information about them as my bigger one gets built.

BeardedOne

Thanks for the help,

BeardedOne

For each variable it would be good to list, the variable, effects, cause, some exceptions.

My tuning guide is very abstract, and not a practical set of instructions which is what most people are looking for, so I agree some sort of chart like that would be a very good idea. Feel free to add/edit pages. I take a look at all edits, but I don't really have time to do much myself at the moment. If you need help integrating new pages into the navigation (most of it uses the page categories (that thing before the ":" in the url, tags, and the page parent, settable at the bottom options), let me know.

The need for this chart would be during competitions. Say your machine was well optimized on your first throw. For your second throw you want to add more weight to try and increase distance. Should I make my sling longer or shorter to still have a good timing? or should I open or close the pin to keep the optimum release angle. The chart would not be able to tell you how much it should be changed, but would give you a starting direction for which way to change one variable when you have adjusted something else.

Long version: For small trebuchets, they are generally durable enough that if you strike the front of the track on the follow through or on a miss fire, it won't break anything, so you really only need to consider how much track it needs to throw properly. (For GBB, the track needed to deal with a miss fire is perhaps 4-5 times longer than used on a normal throw, which is the main reason I think FAKAs scale up to large machines poorly) How long that is depends on your secondary trigger timing, which (when properly tuned) depends a lot on mass ratios, friction etc. You are aiming for a 100:1 mass ratio (great choice, though FAKAs actually can work with higher ones pretty well unlike many designs), so I'd guess your tuning should be similar to my GBB at 10 pounds CW, which used less half the length of the upper arm (called the Short Arm (SA)) + hanger of motion on the track. If you don't have a proper secondary trigger, or you opt for tuning like j-buchet (which lacks an upper arm stop), you will need to allow for motion around or a bit more than your hanger+SA. With an upper arm stop, and a well tuned secondary trigger, you can get away with a track thats much shorter, around half the length, or maybe even 1/3.

Note: these lengths are the amount of displacement needed, so add the 1" carriage lengths to that. Getting the tuning correct is easier if you don't tend to hit the front, and depending on your specific machine, you might need a bit of extra length, so I'd go with an 8" total track minimum, but make sure your machine's base is bigger than that. GBB tended to rock on its square frame. If you don't have an upper arm stop+secondary trigger (not recommended, but people seem to do this anyway, and it does work) you will want something more than 9 inches (example applechunker)

Since you have made a FAT, you probably understand the carriages reasonably well, so this applies to you less than others, but I still recommend a KA on wheels like mini-ka. It is almost the same on a small scale since frames can be really light for small machines, except you don't need to worry about track length, and its quite a bit easier to make. That said, I would love to see a well made small FAKA. If it works, please make a machines page for it with some pictures and ideally video. It would make a great addition (so would your similar FAT).

I am designing a tiny FAKA trebuchet out of popsicle sticks and other assorted scraps of wood.
How do you calculate the proper track length?

My planned projectile is a 4.77 gram marble.
My planned counterweight is 16.8 ounces (about 470 grams).
The planned height of the frame is 4.5".
The height of the arm (hanger and upper arm) is 9".
The wheel carriages are 1" in length.

How should I calculate the track length?

I have built and tuned a FAT trebuchet on the same scale.

Thanks!

Slings can be very accurate. Trebuchets used to hit castle walls consistently from hundreds of yards away with slings, and onagers are no different (I beat several trebuchets in an accuracy competition with one of my onagers). Onagers can be pretty inaccurate if you aren't careful though. Its good to make sure the sling is releasing before the arm hits the stop, or you can get inconsistent releases, and be sure that you either re-aim very well every shot or weight it down really well because onagers tend to jump around a lot when throwing. I should note that is you don't get everything right, you can end up with an inconsistent sling, I've heard of it, and seen people have issues with it, but I personally have never had the issue, and you can copy my sling design :)

Be very careful when tensioning your onager up to its breaking point, they can hurt you. As long as you arn't in the plane of rotation of the arm, and have safety glasses, you will generally be quite safe, but it varies on the design. Onagers if they break the arm, it usually goes in the plane of rotation (I'm thrown chucks of the arm backwards, but I have heard of them going forwards as well). If the rope breaks, its generally not exciting at all, my little onager has had a broken strand it its bundle for years. I've broken ropes in every single one of my onagers. If the frame breaks, it will usually just cave in a bit (then there is no more load on it), I've never had that happen though.

Some random notes: Since you have a length limit, you probably want to get the bundle pretty close to the front to allow a longer arm. Mine has a 24 cm arm and throws further than you need though, so thats not critical. You also have a pretty big height limit, you might want legs (like this little guy I just put up a page for: Tiny Onager) so you can cock the arm pretty far back while still having a long sling. Make sure you are using a good type of rope. Nylon string is an excellent choice, and cheap too (I use it on all my torsion devices).

And by the way, an Onager is what I would recommend for this project. They tend to have the most power for their size. Good choice.

Maybe someone TheHurl Message Board will be of more help. There are a lot more people with broader experiences there.

I have a trebuchet with a 14ft arm and a 1ton+ counterweight at home so hopefully i can provide some insight. I wouldn't bother with bearing on the axle. Your counterweight will have enough leverage and mass to overcome that small amount of friction. The biggest problem i generally have with bearings is positioning both sides well enough to slide the axle all the way through both of them. I'm not sure what sort of meathod you are going to use to get the axle in place and the arm up 12', but i can tell you that it isn't easy slidding all the steel through the axle hole that high in the air. I would just drill a 1/8" to 1/4" oversized whole in the wood and slide your axle into that. If you are worried about friction, add some farm use grease into the whole, but i doubt you will see a noticable improvement.

I am concerned about the strenght of your axle though. My machine has a 2" soild steel axle. The arm covers two feet of it, and there is a 1ft gap between the arm and the legs. (the legs are 4ft apart total) With only 2,000lbs in the cw basket the axle has bent a little bit. (it bent a lot during a dry fire/crash but a was able to straighten it.) I am assuming that your 1-1/2" pipe is probably boiler pipe (black pipe) which probably has less than 1/4" sidewalls. These tend to kink at high stress points (where the axle meets the arm or leg) and then they fold and collapse. If you had 1-1/2" solid steel i think you would be ok, but it still might be near the edge of the materials strength at full machine capacity.

Hopefully this helps. I've got a fair amount or free time for the near future so i can check this forum a little more often so feel free to post more questions.

I'll also add, don't underestimate the importance of considering you ability to move the machine long distances (to competitions and such), the difficulty in assembling something that tall, how you are going to get the cw loaded and unloaded (your water idea will help that).

If you post over at [http://www.thehurl.org/tiki-view_forum.php?forumId=1] you will get better/more advice. I generally don't build stuff that large, and not many others are likely to post here.

Anyway, here's the plan so far. We are using pre-treated 4x4s for the frame, the base is 5' by 10', 12' high at the axle with a 16' sling arm. We found a good piece of 1 1/2 inch steel pipe that's 6', so we have enough to either cut down or keep some extra, depending, that we will be using for the axle. We will be using a 55 gallon drum filled with water for the counterweight.

What we are trying to figure out is the idea of using bearings for the axle to reduce the friction. Either by fitting a pipe around the axle at the sling arm and putting bearings in it, or fitting pipe around the ends of the axle and having the bearings there.

Our biggest question is, if we use bearings on the ends of the axle will the weight make it malfunction? Or if we put the bearings at the sling arm?

This is our first shot at making one of this size, we have made a small tabletop trebuchet but it doesn't compare to what this thing will be!

If anyone reads this, I really appreciate it.

Dominick

I don't generally like the idea of giving out specific plans though, as its both uninformative to people (nothing I make is hard to design if you understand the concepts behind it, and seeing plans helps little with that), but I would like to provide enough technical information and design suggestions to get someone started with their own designs.

Honestly though, I do like the FAKA (its very cool), I don't really think its worth the trouble in most cases (especially if you are rather new to trebuchets). A regular KA gets almost as good performance (range for a given CW drop and projectile), and is much simpler to build and use, has less parts, and a smaller frame. Basically, for the same amount of building cost and effort, (and less design work) you can make a taller KA and throw just as far as a FAKA. Mini KA is a nice example of an easy to make, high performance KA.