I’ve spent about 2 hours watching videos on YouTube to understand how bows work but I’m getting more videos that are pitched at comparisons between different modern bows or how to shoot videos.
What I don’t understand is how a compound bow or springed bow is able to make a small person shoot at higher power. In my physics mind, I understand that cams and springs provide leverage. But the force applied to build potential energy is the same direction as the arrow being fired, so how does the arrow have more power?
if I’m using a seesaw analogy, I’m charging potential energy with the end of the seesaw, when I jump off, the far end of the seesaw still has the same distance to the fulcrum
In: 4
The cams on a compound bow are not circles, the radius change. That makes it possible to control how large the force it at different part of the draw
A classical log bow will have a higher fore the more you pull back the string, so you need max force when you have the hand just beside your head. That is not the point you can pull with max force. The max force point is also where you need to keep the hand still to aim.
A compound bow can have can initially have a fast increasing force that then remains constant until you reach a full draw where it drops. So you can spread out the work over a longer distance. The peak force can be at a location you can pull harder. It is also lower when you need to hold your hand still and aim. You can have other force curves too like a slow buildup and then a drop.
So a draw vs force curve can look [like this](https://www.brown.edu/Departments/Engineering/Courses/En4/Homework/2014/HW2_data/Force-v-draw.jpg)
It is not the peek forces that determine the energy of an arrow, The work is force x distance so the area below the force-distance graph.
That is not exactly the energy the arrow gets because part of the bow is accelerated too and that energy is wasted. The energy that is wasted to move parts of the bow is less for a compound bow.
Ok, let me try again.
A compound bow, unlike a recurved bow, uses pulleys (or eccentric cams) to wind the bow string on a spring. So, they take less strength, using mechanical advantage to Increas power, versus a recurve bow which doesn’t have mechanical advantage.
So, for example rather than a seesaw analogy, think lever in lifting a weight off the ground. recurve bows are always middle fulcrum, 1 pound down moves the other end 1 pound up. But, compound bows use pulleys and can get different mechanical advantage to pull down with 1 pound but get more leverage in lifting up, in the same way block and tackle can provide greater lifting power
>if I’m using a seesaw analogy, I’m charging potential energy with the end of the seesaw, when I jump off, the far end of the seesaw still has the same distance to the fulcrum
The seesaw analogy doesn’t work because the whole point of the gears and cams in a compound bow is to turn the seesaw into not a seesaw
A standard bow has a linear draw weight. It’s lowest when you start to pull the string and highest at maximum pull, if it’s 40 pounds of force at 20 inches then it’s 20 at 10. This means it’s hard to hold a draw bow (you’re at max force) and most of the travel is at low force. Work=force x distance and since you plot of force vs distance is a triangle work=1/2 x max force x draw distance
A compound bow doesn’t have a linear draw, it looks more like an upside down U. They can tweak the shape of the cams so max draw force is in the middle of the range. If you’ve got a 20 inch draw you can rig it so it quickly ramps up in force, 5-15 inches are at max weight then it drops down. Now it’s easier to hold since you’re not at max force and you’ve got more energy stored in the bow because more is at the max force. The middle half of the draw being max force holds as much energy as the other bow at max draw plus you get additional energy from the 0-5 and 15-20 sections
The big thing here is weird shaped cams so you tension the bow aggressively at the beginning, slow that rate in the middle, and really drop off at the end
>if I’m using a seesaw analogy, I’m charging potential energy with the end of the seesaw, when I jump off, the far end of the seesaw still has the same distance to the fulcrum
The seesaw analogy doesn’t work because the whole point of the gears and cams in a compound bow is to turn the seesaw into not a seesaw
A standard bow has a linear draw weight. It’s lowest when you start to pull the string and highest at maximum pull, if it’s 40 pounds of force at 20 inches then it’s 20 at 10. This means it’s hard to hold a draw bow (you’re at max force) and most of the travel is at low force. Work=force x distance and since you plot of force vs distance is a triangle work=1/2 x max force x draw distance
A compound bow doesn’t have a linear draw, it looks more like an upside down U. They can tweak the shape of the cams so max draw force is in the middle of the range. If you’ve got a 20 inch draw you can rig it so it quickly ramps up in force, 5-15 inches are at max weight then it drops down. Now it’s easier to hold since you’re not at max force and you’ve got more energy stored in the bow because more is at the max force. The middle half of the draw being max force holds as much energy as the other bow at max draw plus you get additional energy from the 0-5 and 15-20 sections
The big thing here is weird shaped cams so you tension the bow aggressively at the beginning, slow that rate in the middle, and really drop off at the end
>if I’m using a seesaw analogy, I’m charging potential energy with the end of the seesaw, when I jump off, the far end of the seesaw still has the same distance to the fulcrum
The seesaw analogy doesn’t work because the whole point of the gears and cams in a compound bow is to turn the seesaw into not a seesaw
A standard bow has a linear draw weight. It’s lowest when you start to pull the string and highest at maximum pull, if it’s 40 pounds of force at 20 inches then it’s 20 at 10. This means it’s hard to hold a draw bow (you’re at max force) and most of the travel is at low force. Work=force x distance and since you plot of force vs distance is a triangle work=1/2 x max force x draw distance
A compound bow doesn’t have a linear draw, it looks more like an upside down U. They can tweak the shape of the cams so max draw force is in the middle of the range. If you’ve got a 20 inch draw you can rig it so it quickly ramps up in force, 5-15 inches are at max weight then it drops down. Now it’s easier to hold since you’re not at max force and you’ve got more energy stored in the bow because more is at the max force. The middle half of the draw being max force holds as much energy as the other bow at max draw plus you get additional energy from the 0-5 and 15-20 sections
The big thing here is weird shaped cams so you tension the bow aggressively at the beginning, slow that rate in the middle, and really drop off at the end
So the thing about a bows limbs is that they have to accelerate their own weight as well as the arrow. And decelerate too which shocks the limbs and can damage them if they are undersized, or if you dry fire a bow and the arrow isn’t there to absorb some energy.
The cams on a bow allow for the use of higher stiffness materials that only need to bend a short distance for the same draw length compared to a recurve. Less distance the limb accelerates means less energy wasted in the limbs. You can get over this somewhat with just using a really tall bow with high stiffness as the limbs don’t have to bend as much for the same draw, but the compound bow is much more compact.
This also ties into the weight of an arrow. A lighter weight arrow travels faster and flatter however using lighter weight arrows means that the bow limbs use more energy accelerating themselves than the arrow. The overall kinetic energy in a lightweight arrow is less compared to a heavier weight arrow. Since the compound bows limbs are lighter with a short travel distance they work more efficiently at transferring energy to the arrow. You’ll also see compound bow arrow weights tend to be lighter. You are trading off some of the increase in power for a slightly more inefficient arrow that travels faster.
The cams also allow the limbs to be placed more horizontal instead of vertical. That means instead of accelerating forward and yoinking the whole bow forward out of your hands the force is mostly up and down, cancelling. There’s shock on the frame, but the frame is built to handle it and there’s much less force trying to propel the bow forward after a shot. You’ll notice that the more powerful a compound is the more horizontal the limbs that bend tend to get.
Lastly the cams are shaped so that instead of force building up to a peak, the force lets off at full draw. This doesn’t add more power directly, but it means that after the initial hard pull it’s much easier to hold back for aiming and makes it much more usable since you aren’t shaking trying to keep it drawn.
So the thing about a bows limbs is that they have to accelerate their own weight as well as the arrow. And decelerate too which shocks the limbs and can damage them if they are undersized, or if you dry fire a bow and the arrow isn’t there to absorb some energy.
The cams on a bow allow for the use of higher stiffness materials that only need to bend a short distance for the same draw length compared to a recurve. Less distance the limb accelerates means less energy wasted in the limbs. You can get over this somewhat with just using a really tall bow with high stiffness as the limbs don’t have to bend as much for the same draw, but the compound bow is much more compact.
This also ties into the weight of an arrow. A lighter weight arrow travels faster and flatter however using lighter weight arrows means that the bow limbs use more energy accelerating themselves than the arrow. The overall kinetic energy in a lightweight arrow is less compared to a heavier weight arrow. Since the compound bows limbs are lighter with a short travel distance they work more efficiently at transferring energy to the arrow. You’ll also see compound bow arrow weights tend to be lighter. You are trading off some of the increase in power for a slightly more inefficient arrow that travels faster.
The cams also allow the limbs to be placed more horizontal instead of vertical. That means instead of accelerating forward and yoinking the whole bow forward out of your hands the force is mostly up and down, cancelling. There’s shock on the frame, but the frame is built to handle it and there’s much less force trying to propel the bow forward after a shot. You’ll notice that the more powerful a compound is the more horizontal the limbs that bend tend to get.
Lastly the cams are shaped so that instead of force building up to a peak, the force lets off at full draw. This doesn’t add more power directly, but it means that after the initial hard pull it’s much easier to hold back for aiming and makes it much more usable since you aren’t shaking trying to keep it drawn.
So the thing about a bows limbs is that they have to accelerate their own weight as well as the arrow. And decelerate too which shocks the limbs and can damage them if they are undersized, or if you dry fire a bow and the arrow isn’t there to absorb some energy.
The cams on a bow allow for the use of higher stiffness materials that only need to bend a short distance for the same draw length compared to a recurve. Less distance the limb accelerates means less energy wasted in the limbs. You can get over this somewhat with just using a really tall bow with high stiffness as the limbs don’t have to bend as much for the same draw, but the compound bow is much more compact.
This also ties into the weight of an arrow. A lighter weight arrow travels faster and flatter however using lighter weight arrows means that the bow limbs use more energy accelerating themselves than the arrow. The overall kinetic energy in a lightweight arrow is less compared to a heavier weight arrow. Since the compound bows limbs are lighter with a short travel distance they work more efficiently at transferring energy to the arrow. You’ll also see compound bow arrow weights tend to be lighter. You are trading off some of the increase in power for a slightly more inefficient arrow that travels faster.
The cams also allow the limbs to be placed more horizontal instead of vertical. That means instead of accelerating forward and yoinking the whole bow forward out of your hands the force is mostly up and down, cancelling. There’s shock on the frame, but the frame is built to handle it and there’s much less force trying to propel the bow forward after a shot. You’ll notice that the more powerful a compound is the more horizontal the limbs that bend tend to get.
Lastly the cams are shaped so that instead of force building up to a peak, the force lets off at full draw. This doesn’t add more power directly, but it means that after the initial hard pull it’s much easier to hold back for aiming and makes it much more usable since you aren’t shaking trying to keep it drawn.
The long bow’s draw force increases and is released linearly. The compound bow’s draw force is initially much higher, but subsequently increases much more slowly. The result is that the force needed to hold a compound bow drawn is far less than a long bow.
So for the same holding force, a compound bow delivers much more energy to the arrow.
The long bow’s draw force increases and is released linearly. The compound bow’s draw force is initially much higher, but subsequently increases much more slowly. The result is that the force needed to hold a compound bow drawn is far less than a long bow.
So for the same holding force, a compound bow delivers much more energy to the arrow.
Latest Answers