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Helge B.





Joined: 06 Mar 2008

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PostPosted: Wed 11 Feb, 2009 4:19 pm    Post subject: Definition of draw weight and crossbow cocking devices         Reply with quote

How do you actually define the draw weight of a bow or crossbow? Is it the weight you need to pull back the string to the maximum power stroke?

How does this relate to the force needed to cock a crossbow with a mechanical aid? If you use a goatsfoot for example you should get a mechanical advantage of 1:5. Does this mean if you use this device to cock a 100 lbs. crossbow you would only have to lift 20 lbs.?

What are the mechanical advantages of the different cocking devices used for historical crossbows?
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Randall Moffett




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PostPosted: Wed 11 Feb, 2009 10:56 pm    Post subject:         Reply with quote

Helge,

It means you need apply the draw weight to ready it. For a crossbow this is to the nut or slot. So if it is 400lb draw you need exert 400lbs to get it to the full draw.

The windlass and cranaquin could handle the most power of a mobile system but both were slower than the belt hook, belt hook with pulleys and goats foot lever but these last ones could not pull the same amount of draw weights.

If I had to guess on the pull they could generate it'd be-
Windlass/Cranequin
Goatsfoot lever
Belt hook with pulley
Belt hook
unaided.

You do have spanning tables in castles for all but the biggest great crossbows that at times were manned by two men. These could likely reload these massive weapons fairly quickly but one could not move them very far as they were rather large and out on campaign it'd be a hinderance for mobility.

RPM
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Leo Todeschini
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PostPosted: Thu 12 Feb, 2009 12:53 am    Post subject:         Reply with quote

Quote:
Helge B wrote What are the mechanical advantages of the different cocking devices used for historical crossbows?


All were different so it depends on exact manufacture, but roughly

Windlass 140:1
Cranequin 45:1
goats foot 4 or 5:1
Belt hook and single pulley 2:1 (but legs/back are far more powerful than arms/back
belt hook 1:1 (uses back and legs
straight draw 1:1(uses back and arms

Tod

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Gary Teuscher





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PostPosted: Thu 12 Feb, 2009 8:25 am    Post subject:         Reply with quote

I have somewhat of an idea, but does anyone know of when these various spanning devices were first used and/or what time period they were used?
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Sean Manning




Location: Austria
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PostPosted: Thu 12 Feb, 2009 9:32 am    Post subject: Re: Definition of draw weight and crossbow cocking devices         Reply with quote

Helge B. wrote:
How do you actually define the draw weight of a bow or crossbow? Is it the weight you need to pull back the string to the maximum power stroke?

Its the force required to draw the bow a certain distance- that's all. The distance is fixed for crossbows and flexible for other bows, which is why you'll read things like "a draw of 650N at 70 cm".
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Jean Thibodeau




Location: Montreal,Quebec,Canada
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PostPosted: Thu 12 Feb, 2009 10:35 am    Post subject:         Reply with quote

Leo Todeschini wrote:
Quote:
Helge B wrote What are the mechanical advantages of the different cocking devices used for historical crossbows?


All were different so it depends on exact manufacture, but roughly

Windlass 140:1
Cranequin 45:1
goats foot 4 or 5:1
Belt hook and single pulley 2:1 (but legs/back are far more powerful than arms/back
belt hook 1:1 (uses back and legs
straight draw 1:1(uses back and arms

Tod


For the belt hook method I would gess that someone used to doing it would be able to do it more efficiently than one of us trying it the first time no matter how strong our leg muscles might be.

My guess, but the way I would set up the belt hook would be for it needing only the 5 or 6 inches of draw where my legs would be bent just enough for the hook to attach to the cord and would need to just straiten up to pull.

The point being that the legs are strongest in the upper part of leg extension, in other words it's easier to do it at the top end rather than using a short belt hook and squatting deeply.

I would also imagine that the way the belt is made and adjusted would make a great difference in how much weight one could handle with the belt hook ? In my case I can do 1000 pound leg presses so I think that a 350 to 500 pounds belt hook crossbow should be easy to pull with the right equipment and technique.

( Edited for spelling mistake )

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Last edited by Jean Thibodeau on Thu 12 Feb, 2009 2:39 pm; edited 1 time in total
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Gary Teuscher





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PostPosted: Thu 12 Feb, 2009 11:37 am    Post subject:         Reply with quote

Quote:
Is it the weight you need to pull back the string to the maximum power stroke?


To re-iterate what Sean said, the draw weight the peak weight on the draw.

One thing about crossbows though - for whatever reason many usually refer to the draw length is the nut to the braced position of the bow - many leave the brace out when factoring drawlength. When figuring a selfbows draweight, the draw + brace are figured in. For example, a 30" long draw on a longbow means a 22" draw or so with an 8" Brace.
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