How does a compound bow work, particularly the function of the cams and the “let off”?
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A compound bow is basically a bow with a pulley system. The cams are essentially the wheels of the pulley. However, in an efficient pulley, the wheels are perfectly round. The cams are not perfect circles, or at least they shouldn’t be. The irregular shape is what generates the let off you feel a full draw, due to some fancy physics.
Compound bows are much stiffer than other bows, so for the same energy stored, the limbs move much less. This is more efficient. It is also much more difficult to draw, because a slight change in draw length makes a big difference in energy. A short draw length also flexes the arrow more, as more energy is imparted in a short amount of time. Bows with traditionally short draw lengths, like crossbows, tend to use bolts rather than arrows to deal with that.
So to lengthen the draw length, a pulley system is used. The trade-off is force against distance. You can pull with less force, but you have to pull back further to deflect the short limbs a certain amount (impart a certain amount of energy).
And if you are putting pulleys in your bow anyway, you can add another trick: eccentric pulleys. So when the pulley is small, you have less leverage against the limbs than if the pulley is big. A limb with a small pulley will impart more force on the string, and thus on the arrow; which is what you want. A big pulley will give you extra leverage against the limb, and allow you to have to impart less force on the bow; which you want too. But the first one is important when the bow is driving the arrow, the second is important when you are holding the bow to aim.
So making a pulley that has a big radius on one side and small on the other; for example if it has the shape of a snail shell, marries the two concepts: a bow imparting a lot of energy when firing, but with a ‘let down’ that needs much less force to keep drawn when drawn far enough so you are in the wide part of the cam.
Disclaimer, I am not a archer or any expert in the world of bows, but I do have enough approximate physic knowledge to some what understand. aka, take my world with a bowl of salt.
The goal of a bow is to transform the energy of a archer INTO the kinetic energy of a arrow.
When you draw bow, you are essentially storing energy INTO the bent limbs.
When you release the bow, the limbs return to their neutral state, and such, release the energy into the arrow via the string, producing a forward thrust.
Surprise surprise, Energy imput to draw bow > energy of the arrow.
To make the arrow go fast, you store more energy into the draw, and we know energy = work X distance.
by using pully, we essentially extends the length of the rope, creating more distance, thus more energy stored.
As for let off, the Pulley system when combined, creates a non linear draw cycle, meaning, instead of gradually increasing the effort to draw as you extend a traditional bow, the compound bow peaks during your draw, and then falls off, thus at the maximum of your draw, you are actually using less effort to extend the bow compared to a traditional bow. This nonlinear makes perfect sense, because the total amount of work is conserved, its just the peak that was moved.
Because a compound bow differs from a recurve bow in that it has shorter limbs (the part that extends past the grip), the power to drive the arrow forward into flight comes from a system of pulleys, or “cams” on the ends of the limbs.
Essentially, a regular recurve bow works by putting force into the limbs by pulling back on a string. The force is transferred to the limbs so that they are drawn back, and when the string is released, the limbs snap forward, pulling the string and the arrow along with it. This produces the force that propels the arrow.
Compound bows, on the other hand, instead of putting the force of the string at a perpendicular angle to the limbs, use their pulleys (cams) to essentially try to pull the limbs together instead of back during draw. There is the part of the string that the arrow rests on, but there is an additional length that is doing the actual “pulling”, and it pulls straight up and down on the limbs, pulling them together. When the string is released, the limbs snap back both up and down, instead of forward, making it easier to control. This reduces the strain on the archer, and allows for higher accuracy in a more compact frame, since you’re not fighting against the bow trying to fly forward out of your hand. this design comes with the trade-off of needing stronger materials to function properly, but that isn’t much of an issue with modern materials.
As for “let-off”, the cams can be designed and positioned in such a way that the strain of holding the bow at full draw (all the way back) is reduced by a certain percentage, usually anywhere from 70-85%. If you look at a modern compound bow, the cams are not circular, but rather a strange ovoid shape; this is the design that allows for let-off.
Let-off allows trophy hunters or casual archers to hold a drawn shot for longer while they sight the target or wait for their prey to come into range. If the let-off weren’t there, you would have to hold a full 80 lbs or so of draw, which is difficult to do for any length of time; a let-off of 80 means that you’re dropping 80% of full draw weight at the very end of your draw, so you only have to hold 20% of what you did to draw it back. An 80lb bow with a let-off of 80 only takes 16lbs to hold at full draw, which can be done relatively easily.
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