Recurve bows are relatively simple to understand. The limbs store the energy which is propelled into the arrow when you release the string. But how does a compound bow work? The let-off of a compound bow almost seems magical. How can the draw weight of a compound bow decrease at the near end? In this article, I will explain in detail how it works, but here is the short answer:
A compound bow works as a block and tackle system, relying on mechanical advantage to decrease the draw weight. By changing the distance from the axis, the cam and mod decrease the amount the limb is compressed per inch of draw length at the end of the draw.
Although this answer explains the basic principle of a compound bow, there is so much more to discuss. Therefore, I will first discuss the different parts of a compound bow in more detail. Next, I will discuss what makes a compound bow different from a recurve bow. Then I will discuss the let off in more detail. At the end of this article, we will also discuss how the configurable draw weight and draw length works.
The parts of a compound bow
Alright, I want to pick things up by the basics, so anyone can understand how it works. Even if you have never touched a compound bow. If you are familiar with these parts, go ahead and skip this part.
The limbs are the flexible part of the compound bow, which stores the energy of the bow. These limbs are often made from carbon fiber or other composite materials. When you draw back the bow, the limbs will compress, therefore all the draw weight comes from the limbs.
Compound bows have 1 string and 2 cables. The outermost wire is what we call the string. The string rolls over the cams and is attached to the inside of both cams. When an archer draws the bow, they pull the string backward. When the string shoots forward it takes the arrow with it and shoots it.
Most people think that cables merely guide the limbs and are there for extra stability. That is not true, however. The cables in conjunction with the string create the let-off effect, which we will discuss later.
The cables are attached on one side to the cam and on the other side to the limb. In contrary to the string, the cables do not run over the cam but next to it. It runs over the mod, which we will discuss next. Both cables are the same length and are attached to the bow similarly only mirrored.
The cams are the large wheels you see on the upper and lower limb. All essential parts that make the let-off effect work comes together to the cams. The cams have a grove in the top which helps guide the string.
The mod is attached to the side of the cam. Each cam has a mod and each cable runs over one mod. The name mods refer to modify since you can modify the placement of the mod to adjust the draw length. We discuss how this works in more detail later.
Compound vs recurve bow
You might already feel that you understand how a recurve bow works, but I want to go back to the basics to explain some principles. We can then discuss how a compound bow uses these principles to make the let-off work.
When you draw back the string of a recurve bow, you will start to bend the limbs backward. Limbs are giant springs that allow you to store energy. When you let go of the string, the limbs will launch the arrow due to all the energy is stored.
All the energy you transfer into the arrow is the energy you first used to draw back the bow. Therefore, we say that the limbs are just storing energy.
When you draw a bow back, the draw weight will increase linearly. Meaning that the draw weight increases an equal amount per inch you draw the bow back. Because the string itself is used to bend the limbs, these bows don’t have a let-off.
Therefore, you must hold the bow at the point where the draw weight is the highest. This makes it more difficult to aim and strains your muscles more.
The let-off is what makes compound bows special. At the near end of your draw, the draw weight gets reduced. The amount the draw weight is reduced to is often expressed in percentages. For example, a bow with 70% let-off has only 30% draw weight at full draw compared to the peak draw weight.
As I mentioned earlier, both the cables and the string work together to create the let-off effect. To explain how we must discuss them as separate entities first.
Let’s first discuss the string in combination with the cams. When you pull back the string, the cams will start to rotate. Since the string is only attached to the cams, the string merely rotates the cams. Without the cables, the limbs will not bend, if you draw the string back.
For the bending of the limbs, we need the cables and the mod. Since the mod is fixed to the cam when the cam rotates the mod will rotate as well. When the mod rotates it will pull in the cable, like a winch. The more the mod rotates, the shorter the other end of the cable will be. Since the cable is attached to the limb it will start to bend the limb. This creates the draw weight of the bow.
So, there are two systems at work. The string in combination with the cams creates the rotation. And the cables in combination with the cams exert the force on the limbs.
How let-off works
Alright, we now know that there are two systems: the cables, and the string. But we don’t know yet how these systems work together to create the let-off effect. To explain this, we need to look at a simple block and tackle system and the winch system
Block and tackle system
With a block and tackle system, you trade weight for distance. When you loop your rope through multiple pulleys you can pull much heavier objects. But the caveat is that you must pull in more rope to get the same amount of distance.
A block and tackle system spread the weight of the object over a longer distance. This means that you must pull in more rope, but it will be a lot easier to do so. We can see this principle in a lot of other applications. Think for example about the gears on your bike or in your car.
This principle is called mechanical advantage, and this is also used to make the let-off of a compound bow work.
Reducing draw weight with draw length
The let-off acts like you would add a pully on the last piece of the draw. In the slow-mo, you can also see that in the last 2 inches of the draw the bow gets barely flexed further. That is because the bow spreads the force of the limbs over a longer distance (the draw length).
With gears and pullies, it’s quite easy to see how the mechanical advantage works. But on a compound bow, this is more difficult to see. But it relies on the rotation caused by the string and the pulling force of the cables. We will discuss this in more detail in the next section.
The physics of the let-off
Alright, we now know all the basics to put it all together. The goal of the compound bow is to control the amount of mechanical advantage. Till the start of the let-off, we want as little mechanical advantage as possible. Because we need the draw weight to speed up the arrow. When we near the end of the draw we want to maximize the amount of mechanical advantage, to make it easier to hold the bow.
This happens by changing the amount of rotation and the amount of we pull on the limbs.
Changing the amount of rotation
As I mentioned earlier, the string and the cams will freely rotate without the string. With the cams we, therefore, control the amount of rotation of the assembly. But why would we?
Well as we mentioned before the cable and mod system works like a which. And what happens if we turn a winch? We will pull in our cord, in a compound bow that would be the cable. When we pull on the cables, the limbs will compress increasing the draw weight.
In the last few inches of the draw, we want as little cord pulled into the winch as possible. How we can do this on a winch is by making the lever on your winch larger. Although we don’t have a lever on a compound bow, we can change the distance from the central axis by making the radius of the cam.
Therefore, at the last few inches of the draw, the grove containing the string is far away from the central axis. Therefore, you get little rotation over a longer draw length.
Changing the amount of limb flex
Another way we can reduce the amount of cable we pull in is by doing the exact opposite as earlier. Instead of increasing the distance from the central axis, we want to minimize the distance. In the let-off phase, the mod is completely flat. Therefore, the cable is as close to the axis as possible.
In the first few inches of the draw, the exact opposite happens. The mod keeps the cable far away from the central axis which significantly increases the draw weight.
If you have a compound bow, you can even experience this yourself. When you hit the corner of the flat side of the mod, you will feel that the draw weight decreases. Because from that point, the string gets closer to the axis.
Creating the let-off
So, the two systems work exactly in opposite direction. You can compare this to a gear. If you want to have the most mechanical advantage, you want the small gear on the motor and the big gear on the wheel.
The thing that makes this system so unique is that the mechanical advantage changes in one straight pull. You don’t have to change gears or attach another pully. This is possible because compound bows have cams and mods that don’t have a uniform round shape. Therefore, the distance from the center axis changes when you draw the bow.
Sometimes you see ‘’compound bows’’ that have only two pullies. Because the pullies are perfectly round, this isn’t a real compound bow, because the mechanical advantage doesn’t change. Therefore, the draw weight increases till the end of a draw just like a recurve bow.
How the configurable draw length works
Although let-off is the most difficult one to explain, it would be a shame if we would stop here. Let’s discuss two other aspects of the compound bow, starting with the configurable draw length.
With a recurve bow, you can draw as far as you please. But on a compound bow, the let-off happens at a certain draw length. Therefore, you can’t use a compound bow that is tuned to 28 inches if your draw length is 25 inches. If you would do so, you must keep the bow at the peak draw weight, defeating the purpose of a compound bow.
But what happens when you do the exact opposite. Shoot a compound bow that is tuned to 25 inches while you have a 28-inch draw length. When you do that, you will notice that the let-off comes to early. Therefore, you either pull past the let-off, making it harder to keep the bow at full draw. Or you can pull keep it at the let-off, but then you must stand in an uncomfortable position.
To change the draw length, we change the position of the mod. Since the mod allows you to tune when the let-off kicks in. If we focus on the upper limb, we will turn it anti-clockwise to decrease the draw weight and clockwise to increase the draw weight. By changing the position of the mod on the cam, we reach the flat section of the cam either earlier or later in the draw.
How the configurable draw weight works
Some compound bows have insane configurable draw weights. My compound bow can for example be tuned from 30 to 70 LBS. But there are even some more insane examples available. This has of course a lot of benefits, but we won’t discuss these in this article. We are going to discuss how this system works.
Essentially, configurable draw weight works by increasing the amount of pre-tension on the limbs. If you look at the draw graph of a recurve bow, you can see that the draw weight at the first few inches is quite low. But the draw weight increases when you draw back further.
This happens because you are flexing the limbs further. If we would put the limbs under some pre-tension, the limbs would bend farther when we draw the bow. We can create this pre-tension by tightening the two nuts that retain the limbs. By tightening the nuts, the limbs will flex more, before the bow is even drawn, which increases the draw.
If we want to decrease the draw weight, we do the exact opposite, decrease the amount of pre-tension. To do this we have to loosen the nut retaining the limbs
Alright, you got through it, it was quite a though read wasn’t it? Well, it was also quite difficult to write since there is so much to explain. Sadly, I couldn’t even explain everything without complicating it even further. If you want to know more about compound bows, I would highly recommend the article below:
I discuss in more detail, how draw weight works some example:
- Let off percentages
- Draw profiles
- The wall (the part after the let-off)
You might also want to check out my other articles about compound bows, by following this link.
Before I started with compound archery, I was already into recurve archery for a few years. I never really understood how it worked. Even when I got my first compound bow, I was still amazed by how it worked. I couldn’t wrap my head around it.
Looking up information online wasn’t helpful either. I found some articles, but they explained it really short, like: ‘’it’s a block and tackle system’’. And for me, that just wasn’t enough. It doesn’t look like a block and tackle system, so I wanted to know how it works.
After reading a ton of articles and experimenting I finally understood how it worked. It’s a winch existing out of two systems, the cables, and the string.
I think it’s helpful to know this stuff since it helps you troubleshoot issues with your bow. If have any feedback or questions, please let me know in the comment section down below. I want to make this article as helpful as possible and your feedback will help me improve it. I will send you an email when I replied to your comment!