(..and what on earth are those people doing in that
Some very simple counter-weight trebuchet
This is definitely not intended to be an in-depth study of the
counter-weight trebuchet (you can find people on the web who can give you
the heavy-duty mathematics.. see the Virtual
Hurling page for some links).
Instead, it is a simple look at
a few points that would-be trebuchet builders are likely to encounter
Much of this info is derived from models. Many people will tell
you that trebuchets don't scale up from models very well. This is
certainly true about some proportions (like
the size of the weight bucket), the material strengths and the ranges
you can expect to achieve - but the basic mechanics remain the
Let's get started and look at what
happens after you've put on your helmet, moved well off to the side of the
trebuchet and pulled the rope tied to the trigger mechanism...
This diagram shows a
trebuchet shortly after the trigger has been released. The shot is in the sling
and is beginning to slide backwards along a launch trough. The trough is put
there to guide the sling and prevent it from getting caught up in the
trebuchet's framework. In the early part of the launch all the shot's motion is
horizontal and this speed will contribute to the rate at which the sling is
going to be whipped around the end of the treb's beam later. The trebuchet is
designed so that the beam is pulled down as nearly vertical as is practical.
This gives two benefits: 1. the weight has the longest distance to fall this way
and 2. the first movement of the beam gives the most horizontal pull to the
sling. The sling has to be picked up by the beam, so it can't be too long. (You
wouldn't want the treb to be standing with its beam in the air and the shot
still in the trough) Generally, this means a sling length something less than
the beam's throwing arm length, although some medieval illustrations show longer
Here the trebuchet beam has
rotated and of course the end holding the sling has risen. The shot has been
pulled down the trough and is now speeding backwards, but it has also been
lifted up and clear.
Now, any weight which is tied by a length of rope to
the end of a rotating beam is going to swing out - the so-called centrifugal
force (okay, it's actually just inertia in action, but you get the picture). Our
shot's motion has this effect plus the speed it has already acquired. The result
is that the sling will rotate around the end of the beam.
If your trebuchet's release
mechanism is the usual ring over a prong or hook, it is going to release the
sling as soon as the angle between the sling ropes and the arm is straight
enough for the ring to slip off the prong.
You can adjust when the sling releases in a number of ways:
By setting the angle of the prong - a more hooked prong will hold
the sling loop longer than a straighter one.
ie a prong less hooked or in line with beam = earlier release = higher
... a prong more hooked or forward-pointing = later release =
By changing the length of the cords that hold the sling pouch..
If the sling is rotating around the end of the beam slowly, the beam will
have time to swing through a bigger arc before the sling catches up to it. If
the sling is rotating quickly, the release angle will happen earlier.
shorter sling will rotate faster than a long sling.
ie short sling cords =
fast sling rotation = earlier release = higher trajectory
... long sling
cords = slow sling rotation = later release = flatter trajectory
By choosing the size of your shot..
Another thing that influences when a sling releases is the force on it - a
heavier projectile tends to pull the loop off the prong earlier than a lighter
ie heavy projectile = earlier release = higher trajectory
.. light projectile = later release = flatter trajectory
Finally, the follow-through ... A
bit disappointing, really. It's not as much as you might imagine.
If you had
the weight fixed rigidly to the end of your treb's beam (like Huw Kennedy's huge
piano and car throwing beast in Britain) you would have a simple pendulum and it
might well oscillate majestically until it eventually came to a stop. The design
shown in figures 1 to 4 uses a free-swinging weight and the interfering motions
pull up the beam in a series of jerks and starts.
(Note that this is more
noticible in a small model than a large machine.)