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Basic Question: Why do compound bows shoot fast?


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Old 02-13-2014, 12:51 PM   #1
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Question Basic Question: Why do compound bows shoot fast?

A moveable pulley allows one to pull with less force than without it.

Less force comes with increased pulling distance.

Compound bow pulleys move as their strings are drawn.

I accept the fact that on release the power stroke is longer for a compound bow than it is for a conventional bow.

Why is the speed of the compound bow string faster than the speed of a conventional bow string?


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Old 02-13-2014, 01:03 PM   #2
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A it isn't just a pulley, it is a cam.

B the Cam is the reason for the "let off" making it easier to hold.

C Why is the speed of the compound bow string faster than the speed of a conventional bow string?

I suspect because they are typically of much higher power. Conventional bow would be 40-50 lbs for hunting large game. Compounds are I suspect considerably higher, even double. But because of the letoff, most people can shoot them.

I suspect that if you matched an 80lb longbow vs an 80lb compound string speeds would be pretty close.


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Old 02-13-2014, 02:16 PM   #3
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Compound bows are faster for two reasons.

1. They use a block and tackle system, enhancing the speed of the limbs
2. You can manage higher draw weights because of the "let-off" effect

It is just easier to overcome the heavy draw weight only for an instant and then fall back into a much lower "let off" draw weight for aiming.

I recommend this article. Start reading at "Understanding the draw cycle".

http://www.huntersfriend.com/bow-review-400-fps-bow/400-fps-compound-bow.htm

Basically, a bow with NO let-off, with a constant draw weight of as much as you can pull over the whole duration of the draw will shoot the hardest. Why? Because you have invested the most energy.

Shooters that can manage a 70 lb compound with a soft draw cycle won't be able to get much over 50 lb without any kind of let-off effect, but an evenly strong 50 lb for the whole cycle. But both bows would probably shoot at the same speed. Yet the 70 lb soft cycle bow would be much more comfortable.
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Old 02-13-2014, 02:50 PM   #4
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Quote:
Originally Posted by JoergS View Post
Compound bows are faster for two reasons.

1. They use a block and tackle system, enhancing the speed of the limbs
2. You can manage higher draw weights because of the "let-off" effect
A block and tackle is a moving pulley system.

A 100 lb bow can be drawn with 50 pounds,

When you draw 30" the bow is being drawn 15". Correct?

Thinking backwards, when you draw a 1 pulley block and tackle you pull twice as fast an twice as far than the load moves.

Reverse the order, and as the load pulls the string, the string moves faster than the load retracts.

Perhaps it is just this simple?
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Old 02-13-2014, 03:26 PM   #5
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Well, not quiet.

The bow limbs won't move as much as they would without that block and tackle system. This allows for much shorter limbs that can be mounted more horizontally than vertically.

If you would attach a bow string to those limbs without the block and tackle, then your draw would be 50% of he length (shorter power stroke) but twice as hard. A 70 lb bow with a 22" power stroke would then be a 140 lb bow with an 11" power stroke.

The absense of the block and tackle would also reduce the speed of the retracting string. Again this reduction would not be 50% as there is no friction without block and tackle, but it'd be noticeable.
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Old 02-14-2014, 12:08 AM   #6
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The bow limbs won't move as much as they would without that block and tackle system. Agree.

This allows for much shorter limbs that can be mounted more horizontally than vertically. (Short limbs can be mounted vertically)

If you would attach a bow string to those limbs without the block and tackle, then your draw would be 50% of he length (shorter power stroke) but twice as hard. A 70 lb bow with a 22" power stroke would then be a 140 lb bow with an 11" power stroke.

I agree if there is no stacking and draw length vs force is linear.


The absence of the block and tackle would also reduce the speed of the retracting string. This statement is unclear.

Again this reduction would not be 50% as there is no friction without block and tackle, but it'd be noticeable. ???? You lost me.


The rule of thumb is that a 150 lb compound bow is equal in performance to a 75 lb bow. 2:1

Since a bow is hand held when drawn, and only the strength of one arm is used to input all the potential energy,

A compound bow with a stirrup and drawn with both your arms and the other body muscles can input much more than twice the potential energy than a hand held bow.

The frictional drag on the compound system is more than compensated for with the extra stored energy.
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Old 02-14-2014, 04:59 AM   #7
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Hi just adding my 2 cents worth, cams started out as round /circle wheels with an off centre axle, and the off centre configuration added gradual acceleration hence the let off ,think how the arrow leaves the bow. slowly accelerates then really accelerates, because the draw force picks up, I think this way its more linear acceleration.
Then someone realise if they make them egg shaped eccentric wheels the acceleration will accelerate, what I mean by this the acceleration will change as cams turn (high school physics { ms-2} metres per sec squared) there by speeding the arrow more.
Your still talking about a short span of distance/time here, but hence the compounding effect.
So imagine the arrow being released from a straightbow (F=MxA force =mass times Acceleration) acceleration doesn't change much very much like straight bands. But on a compound I sure the acceleration component changes as the cam wheels turns.
I have tried to hold back a compound from a few inches back near impossible the levering effect was too much the distance out from the axle times the force, but as the cams turn the easier it was I think tapered bands have a similar effect the just keep accelerating to the end hence faster projectile.
sorry I miscalculated that should be 20 cents worth
ps I did see production bow and crossbow(u-tube) where the limbs have been reversed and are almost parallel to where your shooting to. so the direction off the limbs doesn't matter, they are just stored energy when cables and pulley/cams are involved.
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Old 02-14-2014, 07:06 AM   #8
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Quote:
Originally Posted by BoyntonStu View Post
Quote:
The absence of the block and tackle would also reduce the speed of the retracting string.
This statement is unclear.

Quote:
Again this reduction would not be 50% as there is no friction without block and tackle, but it'd be noticeable.
???? You lost me.
Block and tackle reduce the draw weight by increasing the draw length. The same physical work is achieved that way. But this also means that the end of the string (where the arrow is nocked in) moves TWICE as fast (at release) as it has to travel the doubled distance within the same time frame.

It is basically the same as a lever.

Block and tackle add friction and mass. Therefore some of the muscle energy is lost in comparison with a straight bow string. This means that the speed of the string won't be doubled, just increased some. How much? That depends on the quality of the mechanism. Really bad mechanisms with lots of friction and huge rollers can be SLOWER than the direct string. But factor 2 vs. the straight line can't be exceeded (with a 2:1 block and tackle).

Quote:
The rule of thumb is that a 150 lb compound bow is equal in performance to a 75 lb bow. 2:1
Where does that rule come from? I disagree with it. Compound bows are faster than normal bows at the same draw weight.

Or do you mean a 150 lb CROSSBOW vs. a 75 lb bow?

That would make more sense. But in that case, the explanation is simple: The crossbow has only 50% of the draw length ("power stroke"). It needs twice the draw weight to achieve the same energy.

Quote:
A compound bow with a stirrup and drawn with both your arms and the other body muscles can input much more than twice the potential energy than a hand held bow.

The frictional drag on the compound system is more than compensated for with the extra stored energy.
You clearly mix up "compound bow" and "crossbow".

From wikipedia:
Quote:
A compound bow is a modern bow that uses a levering system, usually of cables and pulleys, to bend the limbs.
Otherwise you are correct of course. But again, crossbows usually have a much shorter draw length (they also shoot much shorter arrows).

So can we build a crossbow that has the full draw length of a bow and the draw force of a crossbow?

Well... not really. You would need to go up MUCH in arrow weight. The 5-grain-per-lb-of-draw has to be halved for conventional crossbows because of the shorter power stroke. But that changes when we go to the full power stroke of a bow again.

A 150 lb crossbow with a full bow like power stroke would have to shoot 750 gr arrows or else the bow limbs might break. It would shoot that heavy arrow with tremendous force. 350 fps are possible, a whopping 280 Joule.

But it would also be a huge weapon, awkward to hold and carry, and even more awkward to cock. You'd want a tripod for it, or a carriage. Basically a ballista.
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Old 02-14-2014, 02:09 PM   #9
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Joerg,

I agree with your comments.

Sorry for confusing compound bows with compound crossbows.

I have learned a lot in this thread.

I never realized that comparing a one hand draw weapon to a full body draw weapon is an unfair comparison.

If we consider a stirrup weapon, we may consider pulley and lever designs with their inherent friction losses, because of the tremendous energy we can store.

You foot assisted slingshot that you drew sitting down comes to mind when I think of storing tremendous power.

BTW I don't believe that spine consideration is necessary if you pull the arrow.

Last edited by BoyntonStu; 02-14-2014 at 02:12 PM.
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Old 02-14-2014, 02:14 PM   #10
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Just keep in mind that anything over 150 lb usually needs a cocking aid because it is very painful for the hands otherwise. I hate cocking aids because they are awkward and slow.


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