How does a compound bow work – the full explanation

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.

Limbs

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.

Cams

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.

Mod

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 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.

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

Learn more

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:

How draw weight works on a compound bow

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.

Final words

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!