07-11-2010, 06:05 PM
I know this is an old thread, but I got to ask... how come the cheiroballistra is so underpowered even though it's has a very high draw weight (500+ lbs)? Of course the draw weight increases dramatically at the end along with the string angle, but still... A few bow/crossbow statistics as a comparison:
30 lbs all-wood, straight handbow: 39 m/s avg using a 26 gram arrow
50 lbs all-wood, straight handbow: 47 m/s ang using a 36 gram arrow
150 lbs crossbow with steel bow: 55,7 m/s avg using a 28 gram bolt, 47,7 m/s avg using a 50 gram bolt
300 lbs crossbow with steel bow: 54,4 m/s avg using a 52 gram bolt, 46,5 m/s avg using a 81 gram bolt
Any of these weapons (except the 30lbs bow perhaps) can easily punch through 9mm plywood or 1mm aluminum, depending on design of the arrow/bolthead. The bolts from both crossbows go through 2cm (soft) plywood and punch holes (but don't fully penetrate) 1mm soft steel plate. The boltheads are not designed for maximum penetration as they're pretty thick: 12x12mm (50 gram bolts) and 15x15mm (81 gram bolts) in diameter. Penetration could be easily doubled by using thinner and longer hardened steel heads - similar in shape to those you use on your cheiroballistra. Also, handbows used for war are generally of very high draw weight (100 lbs and up) and hence more powerful than the ones listed above.
Anyways, what I'm getting at is that current performance levels are clearly not good enough to warrant the complexity of construction and maintenance in Roman times. Have you managed to improve the results lately? Also, why not just use a chronograph to measure the initial velocity of the bolt? Cheap ones cost around 90 euros and give accurate readings _very_ easily. If you also measure the weight of the bolt, you can really start collecting hard data and start improving your cheiroballistra.
I hope all of this does not sound too negative but I felt somebody had to do a reality check. Besides this, I think your cheiroballistra is awesome, even if I disagree with some construction details.
30 lbs all-wood, straight handbow: 39 m/s avg using a 26 gram arrow
50 lbs all-wood, straight handbow: 47 m/s ang using a 36 gram arrow
150 lbs crossbow with steel bow: 55,7 m/s avg using a 28 gram bolt, 47,7 m/s avg using a 50 gram bolt
300 lbs crossbow with steel bow: 54,4 m/s avg using a 52 gram bolt, 46,5 m/s avg using a 81 gram bolt
Any of these weapons (except the 30lbs bow perhaps) can easily punch through 9mm plywood or 1mm aluminum, depending on design of the arrow/bolthead. The bolts from both crossbows go through 2cm (soft) plywood and punch holes (but don't fully penetrate) 1mm soft steel plate. The boltheads are not designed for maximum penetration as they're pretty thick: 12x12mm (50 gram bolts) and 15x15mm (81 gram bolts) in diameter. Penetration could be easily doubled by using thinner and longer hardened steel heads - similar in shape to those you use on your cheiroballistra. Also, handbows used for war are generally of very high draw weight (100 lbs and up) and hence more powerful than the ones listed above.
Anyways, what I'm getting at is that current performance levels are clearly not good enough to warrant the complexity of construction and maintenance in Roman times. Have you managed to improve the results lately? Also, why not just use a chronograph to measure the initial velocity of the bolt? Cheap ones cost around 90 euros and give accurate readings _very_ easily. If you also measure the weight of the bolt, you can really start collecting hard data and start improving your cheiroballistra.
I hope all of this does not sound too negative but I felt somebody had to do a reality check. Besides this, I think your cheiroballistra is awesome, even if I disagree with some construction details.
