Any archers here ? Bow re-design math (Page 1/1)
MidEngineManiac JUN 26, 01:44 AM
Making my head hurt trying to figure this one out.

Picked up one of these for 5 bucks. I am re-purposing the limbs into a crossbow.

Ditching the riser and the inner ends of the limbs will be about 3 inches apart. Limb angle will flatten out a little, probably around 10-12 degrees. Dont want the string hitting the back of the limbs or nose-piece, but I do want it pushing on the bolt as long as possible.

So, the new overall length is around 48-50 inches

HOW do I calculate the new draw length and weight (weight probably wont change much, but draw will be a bunch shorter). It wont be firing regular bolts, I am designing it around these and a captive-track system of 4x 1/2" AL tubes.

I could go trial-and-error and make a bunch of nose pieces and get a fishscale and measuring tape, but if there is an easier way to do it, I'd rather not waste a roll of filament and a weeks print time. It's not MAJOR changes to the geometry, but enough that a couple inches either way will be underpowered or get into the range of cracking the fiberglass. These things arent exactly high-quality.

[This message has been edited by MidEngineManiac (edited 06-26-2022).]

hnthomps JUN 26, 08:33 AM

quote
Originally posted by MidEngineManiac:

Making my head hurt trying to figure this one out.

Picked up one of these for 5 bucks. I am re-purposing the limbs into a crossbow.

Ditching the riser and the inner ends of the limbs will be about 3 inches apart. Limb angle will flatten out a little, probably around 10-12 degrees. Dont want the string hitting the back of the limbs or nose-piece, but I do want it pushing on the bow as long as possible.

So, the new overall length is around 48-50 inches

HOW do I calculate the new draw length and weight (weight probably wont change much, but draw will be a bunch shorter). It wont be firing regular bolts, I am designing it around these and a captive-track system of 4x 1/2" AL tubes.

I could go trial-and-error and make a bunch of nose pieces and get a fishscale and measuring tape, but if there is an easier way to do it, I'd rather not waste a roll of filament and a weeks print time. It's not MAJOR changes to the geometry, but enough that a couple inches either way will be underpowered or get into the range of cracking the fiberglass. These things arent exactly high-quality.




Your starting bow is rated at 40 pounds draw and the crossbow bolts are 50-80 pounds according to the site info. You might want to consider a better match of the components to get a more effective or accurate weapon at the end of the game.

Nelson
MidEngineManiac JUN 26, 08:59 AM
Yeh, but those are pistol bolts, designed for maybe 6 inches of acceleration. I'm giving them around 2 feet.

Think the difference if firing a bullet from a pistol or rifle. Same round and caliber, is going one hell of a lot faster out of the rifle.

Same concept.

Meh, I just grabbed this thing at a yard sale on a whim. Maybe I can do something neat with it, maybe not.

If all else fails I can cut the limbs down in size a little to stiffen them up.
TinaBeyer AUG 25, 07:34 AM
So, please, how to calculate the draw's parameters? I couldn't get a conclusion. When I see some data about tubes and needed calculus, I get stressed because all these are hard for me. I am still a student, and the biggest challenge is mathematics, so I do it only with college algebra help, with solved and explained questions. https://plainma...dary/algebra is my salvation. Otherwise, I would fail the math courses since it is a science too difficult for my creative personality. Therefore, life math problems are also challenging me.

[This message has been edited by TinaBeyer (edited 08-29-2022).]

MidEngineManiac AUG 25, 04:14 PM
Never did calculate it, just trial-and-error with a chunk of wood and some c-clamps. Ended up around 25" draw.

Shot it a few times and got 200 bucks for it. Not bad for under 40 build cost.

[This message has been edited by MidEngineManiac (edited 08-25-2022).]