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Lexan for bow/crossbow?

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Lexan for bow/crossbow?

Unread postAuthor: Alster370 » Sun Jun 26, 2011 12:28 pm

Would strips of lexan be suitable for the prod of a crossbow? Ive heard its very strong yet flexible, which would be ideal in this case.
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Unread postAuthor: inonickname » Mon Jun 27, 2011 7:08 am

No. I certainly wouldn't describe lexan as being suitable. Fiberglasses are laminates of fiberglass/carbon fiber and wood/foam are much better choices.
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Unread postAuthor: Heimo » Mon Jun 27, 2011 7:26 am

I am going to build myself a horseback bow soon, the material I am going to use will be a laminate of bamboo and fiberglass.
I have build 2 bows in the past, and I agree with inonickname lexan wont work very well, my guess would be that a bow made from lexan would develop string-follow(not sure if that is the correct term) very quickly
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Unread postAuthor: inonickname » Mon Jun 27, 2011 8:22 am

Heimo wrote:I am going to build myself a horseback bow soon, the material I am going to use will be a laminate of bamboo and fiberglass.
I have build 2 bows in the past, and I agree with inonickname lexan wont work very well, my guess would be that a bow made from lexan would develop string-follow(not sure if that is the correct term) very quickly


I think you used the right term. You mean when the limbs fatigue, and start bending in the direction of the string (at rest). I.e. loss of poundage, limb efficiency, etc. Limbs eventually end up useless.

I'd also be hesitant of it breaking.

A bamboo fiberglass should be good. Bamboo is fairly strong and springy, and will act as a good spacer (btw, in a lot of bows with fiberglass and wood laminates, the fiberglass stores a high proportion of the energy, circa 90%, the wood or foam layers simply act as a spacer, but not always). The combo of light fiberglass and bamboo will make a light, quick bow.

If you want to DIY a crossbow prod, a good option is to form the prod from spring steel, properly tempered. Take care with this as it would be dangerous if done poorly. Built up wood and fiberglass laminates, or solid wood for a lighter duty prod could work.
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Unread postAuthor: Fnord » Mon Jun 27, 2011 6:33 pm

If you have nothing else availible, you could use lexan, though it may be a bit heavy for the energy you can potentially store in it.

To optimise such a design, you don't want to layer strips together if you don't have to. When you double the thickness, the force required to bend it factors up by 8, but most of that force applied to a thin layer of the surface which will cause a large amount of deformation.

Instead, make wider limbs to spread out the tension and compression load, then trim them by width to adjust your draw poundage. Also keep in mind you should let most of the bending occur in the near-handle parts, while the outer half should be light and stiff- acting more like levers.
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Unread postAuthor: Zeus » Mon Jun 27, 2011 6:39 pm

Why would you want a light bow, do what I'm doing. 50mm wide 10mm deep spring steel strap. I'm only using 1300mm of it.

More a man portable ballista than a crossbow.

I've used 10mm fibreglass round stock as a bow with great success, and there's much thicker available.
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Unread postAuthor: inonickname » Tue Jun 28, 2011 3:43 am

Zeus wrote:Why would you want a light bow, do what I'm doing. 50mm wide 10mm deep spring steel strap. I'm only using 1300mm of it.

More a man portable ballista than a crossbow.

I've used 10mm fibreglass round stock as a bow with great success, and there's much thicker available.


Because heavy limbs will have a huge amount of inertia, will decrease speed and also efficiency.

BTW, a round section is a terrible section for a bow limb.
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Unread postAuthor: Zeus » Tue Jun 28, 2011 4:26 am

I'm not suggesting round stock persay, there's strap available here, so I can't see it being an uncommon item.

And who said velocity was everything, huge limbs mean heavy projectiles which means high (muzzle?) energy.
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Unread postAuthor: inonickname » Tue Jun 28, 2011 4:37 am

Zeus wrote:I'm not suggesting round stock persay, there's strap available here, so I can't see it being an uncommon item.

And who said velocity was everything, huge limbs mean heavy projectiles which means high (muzzle?) energy.


Heavy limbs means low velocity, which means very low muzzle energy. Qualities you want in limbs are flexibility and to a degree, stiffness, energy retention, and durability.
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Unread postAuthor: jackssmirkingrevenge » Tue Jun 28, 2011 4:37 am

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Unread postAuthor: Zeus » Tue Jun 28, 2011 4:50 am

But I understand that there's many other factors involved with bows.

Anyway, I'm not game to work with materials that splinter, I can deal with a kilo of steel getting thrown at me, but I've seen what splintering things do when applied at great force to flesh, not my cup of tea whatsoever.

Edit: Indeed, would help if I remembered basic physics instead of filling my brain with obscenely basic chem and physics. Relying on calculators doesn't help either.
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Last edited by Zeus on Tue Jun 28, 2011 5:11 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Tue Jun 28, 2011 4:57 am

Zeus wrote:I'm thinking that a 1 gram pellet at 300m/s has the same energy as a 10 gram projectile at 30m/s.


Same momentum, but when it comes to kinetic energy the pellet carries 45 joules while the 10 gram projectile carries just 4 joules. Remember energy increases <s>exponentially</s> quadratically with velocity, but in direct proportion with mass.
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Last edited by jackssmirkingrevenge on Tue Jun 28, 2011 5:42 am, edited 2 times in total.
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Unread postAuthor: inonickname » Tue Jun 28, 2011 5:30 am

jackssmirkingrevenge wrote:
Zeus wrote:I'm thinking that a 1 gram pellet at 300m/s has the same energy as a 10 gram projectile at 30m/s.


Same momentum, but when it comes to kinetic energy the pellet carries 45 joules while the 10 gram projectile carries just 4 joules. Remember energy increases exponentially with velocity, but proportionally with mass.


Do you mean quadratically? :lol:
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Unread postAuthor: jackssmirkingrevenge » Tue Jun 28, 2011 5:39 am

:oops:

I had just used "exponentially" on a work document :D
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Unread postAuthor: Alster370 » Tue Jun 28, 2011 5:46 am

My current setup is a leafspring, but its a bit too resistant for hand cocking and i dont want to have to use a winch. So i though about ways to reduce its resistance to bending so I can cock it with less force. Would drilling holes or making cuts at the centre allow it to bend more easily?
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