The Original Floating Arm Trebuchet!
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Here it is in the cocked position. The counterweights are steel plates that
sit inside two wooden boxes, one on either side. The fulcrum is the wheel on top
of two tracks.
Here's a picture from the back (the boxes are empty because the steel
plates are missing in this photo):
What keeps the wheels on the track? Very narrow tolerances. The CW boxes
and the channels ensure that the end of the arm stays centered, and there
is only a 1/4 inch clearance on either side of the arm between the tracks.
When the trigger is pulled, the CW drops straight down in the channels,
pushing the arm backwards on the wheels and simultaneously rotating it.
Once the weight has passed through the middle, it begins to jerk the arm
forward while continuing to rotate it. Notice how the axle is direcly over
the counterweight at the bottom of the drop.
In a dry-fire situation, there
is no stall or wobble at all and the momentum of the arm carries it through,
lifting the CW a bit in the process, and then it all comes back.
A normal oscillation.
When firing a 10-12 lb load however, the pull of the projectile on the arm
tends to stall it in exactly the vertical position, leaving zero residual
motion in the machine. A perfectly tuned machine!
Here's the front of the machine showing the trigger assembly (which happens to
be sprung open in this photo). The trigger is at the top, between the two CW
Here's a close-up of the trigger in the set position. It is two pieces of
angle iron. One piece supports the end of the arm, and the other piece
supports the first piece. A loose clamp holds them together. Pull the clamp
out, and the trap-door falls open releasing the arm.
(to see an animation of the trigger in action, go to
- Arm: 3 feet from CW to axle, 6.5 feet from axle to pin.
- Height: 5 feet to track, 8 feet to top of channels.
- Wheels: 10 inches
- Sling: 7 feet long.
- Total counterweight: 470 lbs.
- Total drop: approx. 5.5 feet.
- Range: slightly better than 300 feet using 10-12 lb projectiles.
To see some shots of this thing in action, check out the Sherman Oaks Skeet
Club and the videos link from the home page!
What's the point?
As you probably know, trebuchets traditionally consist of an arm resting
on an axle, which rests high on a base structure. The arm of the trebuchet is
like an off-center see-saw with a huge counterweight on the short end and a
sling attachment on the long end.
This arrangement worked great for the last 700 years or so, but recently
people have begun to study the mathematical efficiency of trebuchets and are
attempting to make them even more efficient. In this quest, there are great
debates about what should work best. This design is just one more experiment
in that spirit.
But what do we mean by "efficiency"? It's fairly easy to calculate the
energy generated by a dropped weight. It only depends on the amount of weight
and the distance dropped. You can measure the height difference of the
weight on a treb between the cocked and un-cocked resting positions, and from
that know how much energy you have to work with. Then calculate the maximum
possible distance that much energy can propel a projectile of a certain mass
(assuming a perfect trajectory). It's all very easy, but misleading.
The problem is that the ratio of the short arm to the long arm makes a
big difference. Since gravity always works at a constant acceleration (and
gravity is the only source of power for us) then people tend to amplify this
acceleration by making the short arm a small fraction of the long arm. If
the short arm is 2 feet long, and the long arm is 10 feet, then the velocity
of the tip of the long arm is 5 times that of the short arm. But, then the
counterweight can only drop a maximum of 4 feet! (from 2 feet above the axle
to 2 feet below it. In practice it would actually drop much less than this)
You could get much more power by shifting the ratio of the arm and letting the
counterweight drop further, but this reduces the amplification effect.
Basically what happens is that the machine becomes less efficient, but
due to the extra power from the longer drop, it can throw the projectile
further. With the same weight, a LESS "efficient" machine can potentially
So, what to do. One of the problems with a longer drop is that the
counterweight revolves around the pivot point of the axle. Some of the energy
is used in pushing the weight out and back horizontally as it moves around
the axle. This can be solved by hanging the weight from the end of the arm,
and propping it up so that when it falls, it tends to fall straighter. But this
creates a problem at the bottom of the drop. Since the counterweight will be
far out in front of the axle, it hits the bottom of its drop while the arm is
still leaning towards the back.
This causes the arm to stall, and if it happens too early it will rob energy
from the projectile. So with a fixed axle machine, the longer the counterweight
drops, the earlier the arm will stall, and energy is lost.
It seemed to me that a good machine would let the counterweight
fall absolutely vertically, the way gravity works best. The solution-
Rather than swing the counterweight around the axle, let the axle move out of
the way so the counterweight can fall straight down. Then the axle should
move BACK so that the counterweight doesn't stall the arm.
This machine does that by letting the axle roll freely on a track, and
keeping the counterweight in a vertical channel. As the counterweight falls,
it pushes the arm away, then jerks it back with the full force of the weights
dropping. The axle passes OVER the weights, and the momentum of the arm carries
through and pulls the weights back up a tiny bit before sliding back to the
rest position. Since there is no stall, the ratio of short arm to long arm
can be much smaller. In this case it's about 1 to 2 instead of the more common
1 to 5 or 1 to 4. I lose some amplification, but I more than make up for it
with increased power from a longer drop. It's a very efficient machine.