How Does A Crossbow Work?

A crossbow is a fascinating ranged weapon that dates back millennia. Many bow hunters prefer a crossbow due to its outstanding speed and power. But how does a crossbow work? If you’ve ever wondered how your favorite weapon functions, the answer lies in the physics of a spring.


How Does a Crossbow Work

Crossbows are similar to standard bows, and both work in essentially the same way.

These two weapons harness the power of stored energy. When you cock a crossbow, the string pulls the prod’s limbs closer together. This creates something called elastic potential energy. By releasing the string, the potential energy becomes kinetic energy and launches the bolt.

The design of both the crossbow and the standard bow maximizes the stored energy, ensuring that the bow powerfully propels the bolt toward its target.

Although modern crossbows are quite technologically sophisticated, they still operate on this basic principle of converting elastic potential energy into kinetic energy.

The amount of energy a bow can hold affects its range and power. The higher a bow’s draw weight and the longer its draw length, the more powerful the bow will be. The crossbow’s design allows arbalist, or the person using the crossbow, to draw the string back farther, thus increasing both the draw weight and length.

a man adjusting the cocking rope of a crossbow

What Makes a Crossbow a Crossbow?

It’s a good idea to brush up on the basics of the crossbow’s design before asking, “How does a crossbow work?”

So, what makes a crossbow a crossbow?

Much like a bow, the crossbow features a string that is drawn back to launch a projectile. However, the crossbow’s design is slightly more complicated, integrating several features to maximize the weapon’s power.

A crossbow looks a bit like a mix between a rifle and a bow. And like a rifle, the crossbow features a trigger and a stock. However, the crossbow originated long before the first rifle.

We call the bow part of a crossbow a prod or a lath. Unlike a standard bow, the crossbow’s prod is oriented horizontally. The prod attaches to a tiller. When shooting a crossbow, the arbalist grips the tiller as he or she would hold the stock of a rifle.

Most modern crossbows include a device called a stirrup at the front of the bow. Stirrups provide a means of bracing the crossbow when drawing back the string. After drawing, or cocking, the string, it fits into a lock. The lock holds the string until it the arbalest releases it via a trigger.

Related Read: How to Shoot A Crossbow for Beginners

The History of Crossbows

old crossbow

Crossbows are ancient weapons, with the oldest European crossbow dating back to late 5th century BC in ancient Greece. The crossbow has a significant history in both East Asia and Europe — and incredibly, scientists believe that the Asian and European varieties developed independently of one another.

No one knows for sure exactly where and when crossbows originated. However, the earliest archaeological evidence of their use comes from a tomb in Qufu, a city in East China’s Shandong province. There, archeologists found several bronze crossbow triggers that date back to the sixth century B.C.

Later, the first crossbows appeared in the Mediterranean region. The earliest mention of a European crossbow came in Heron of Alexandria’s first-century treatise “Belopoeica.” In it, he describes a weapon called a gastraphetes. The gastraphetes, whose name means “belly-releaser,” was a precursor to the Roman crossbow that was cocked via a slider mechanism and released with a trigger.

However, it was not until the Middle Ages that the crossbow became widespread in European warfare. The crossbow presented an advantage to the standard bow: It was relatively easy to use. Whereas becoming a skilled longbow archer took great strength and much skill, soldiers could quickly become proficient crossbow users.

Today, the military applications of the crossbows are limited. However, they remain perennially popular for competitive shooting sports and hunting.

The Crossbow: How Your Favorite Weapon Works

Crossbows are powerful weapons that could seriously injure or kill a person, so use them with extreme caution.

Although crossbows operate on the same spring principle as a standard bow, the method for shooting one is a bit different. This is because crossbows are more complex than standard bows and have a few extra parts.

Parts of a crossbow

A crossbow consists of a bow-like prod attached to a tiller, which is the weapon’s frame.

Another characteristic feature of the crossbow is the stirrup. Stirrups attach to the front of the weapon and provide a mechanism for bracing the crossbow when cocking the string.

Some crossbows come equipped with scopes. Because gravity affects bolts differently across different ranges, the best crossbow scopes will incorporate multiple crosshairs.

Recurve Bows

How does a crossbow work with a recurve bow mechanism?

Most modern crossbows use recurve-style prods. This type of bow is also the one that is used for archery events at the Olympics.

When unstrung, the tips of a recurve bow’s limbs curve away from the shooter. These curved tips shorten the distance between the bow and the string when not cocked. As a result, the draw length of a recurve bow is longer than that of a straight-limbed bow.

However, recurve bows are not as powerful as the newer compound bows.

Recurve crossbow

Compound bows

How does a crossbow work with a compound bow mechanism?

Compound bows are more sophisticated than simple recurve bows. A compound crossbow’s prod makes use of cams, pulleys, and cables to bend its limbs further. As a result, compound bows are capable of applying much more tension to the cocked string.

However, all those cams, pulleys, and cables add to the complexity of the weapon, increasing the chances of something going wrong. The simple recurve bow, while less powerful, is often more precise since the tension on the cocked string transfers directly to the limbs, pulling both sides equally.

compound crossbow

Crossbow bolts

Rather than shooting arrows, a crossbow shoots bolts. Bolts are very similar to arrows, but they are typically quite a bit shorter and heavier. The bolt’s greater heft encourages it to attain maximum kinetic energy when launched.

You can further distinguish bolts from arrows based on the absence or presence of stabilizing vanes. Bolts do not make use of stabilizing vanes, but arrows always do.Bolts consist of four primary parts. The shaft is the main body of the bolt. At the rear of the bolt, the nock flares out to keep the bolt in place as you line it up to shoot. Fletchings are the small wings that sit beside the nock and stabilize the bolt’s trajectory. Finally, the bolt head is the arrowhead that attaches to the front of the bolt.

Some people refer to a crossbow’s bolts as “quarrels,” but that’s not strictly correct. Quarrels are, in fact, a specific type of bolt that features a four-sided tip. Quarrels are the most common type of bolt, but there are other varieties.

How does a crossbow work to create maximum kinetic power? In addition to increased draw weight and length, crossbows also make use of heavy carbon bolts, like this one, to enhance their bone-crushing power.

To shoot a crossbow, you must first locate the stirrup. Lower the stirrup to the ground and slip your foot inside it. Ensure that you have fully inserted your foot and brace the stirrup against the ground.

If you don’t position your foot carefully, it could slip from the stirrup when you begin to cock it. If that happened, you would likely lose control of the crossbow, creating a dangerous situation.

Next, you will grasp the string with both hands and pull it up toward the cocking mechanism. Be careful to apply even pressure with either hand to avoid setting the string off-center. An off-center string cannot shoot with accuracy or precision. Lock the string into the cocking mechanism, and the safety will engage automatically.

Finally, load a bolt into the groove. You can keep your crossbow cocked and loaded almost indefinitely because the locking mechanism is doing all the hard work. When you are ready to shoot, simply release the safety and launch the bolt by pulling the trigger.

The mechanics of a crossbow

The mechanics of a crossbow are unique to the weapon.

Instead of simply drawing and releasing a bowstring, you cock and load the weapon. Therefore, you must check that your crossbow’s safety has engaged immediately after cocking. Like any mechanical device, your crossbow’s automatic safety can fail — and accidentally shooting a cocked and loaded crossbow could have catastrophic effects.

Furthermore, crossbows must be decocked. You can accomplish this by shooting the loaded bolt into a backstop, such as this crossbow target. You should never remove a bolt and dry fire your crossbow.

The kinetic energy generated by the cocked string is intended to launch a bolt. So, if there is no bolt to launch, you could end up breaking your crossbow’s limbs.

How Does a Crossbow Work? Now You Know

So, how does a crossbow work? They convert elastic potential energy into kinetic energy, making them extremely powerful projectile launchers.

In fact, crossbows can be so devastating on a target that the Catholic Church banned them in Christian-on-Christian battles in the Middle Ages out of concern peasants could become redutable warriors on their own. Take it as a gun control effort before it was cool.

Unlike standard bows, crossbows are relatively easy to learn to use. And now is a great time to learn to use a crossbow as many states have recently begun to permit crossbow hunting for big game.

However, with great power comes great responsibility. So, before you start shooting, it’s a smart idea to ask yourself, “How does a crossbow work?” Understanding your crossbow’s mechanics will help you become a safe and responsible arbalist.

Related Read: The Best Crossbow For The Money [2020]: Our Top 11 Picks Reviewed

Featured image credit: Luke Bastien on Flickr

This post was first published on Nov. 22nd, 2019 and was last updated in July 2020.

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