Since I started being interested in making a crossbow I have been interested in this kind of thing. I was wondering what I would need to do to about equal the speed of my longbows. right now I have a 75 pound longbow that will shoot 700 grain arros about 170 feet per second. How would this compare to crossbows you guys have built?
It shouldn't be too difficult to achieve 170 fps (50m/s) with 700 grain (45,5g) bolts, I've made two crossbows that roughly equal your longbow and one that's about twice as powerful. A good candidate (for a steel bow) is 150 pounds with 40cm (16") draw. This is not especially surprising, as "pounds times draw length" is roughly the same as in your longbow
. Anyways, a steel bow would have to be ~80cm (31,5") long, 0,5-0,6cm (~1/5-1/4") thick and 4-5cm (~2") wide at the middle, with width tapering to as near zero as possible (see here why) to achieve this performance.
Going heavier will increase bolt velocities; just how much depends on the bow material being used. In steel bows reducing bolt weight reduces efficiency (energy in -> energy out) pretty quickly. Glassfiber should be better in this regard, as it's lighter. Anyways, here are some statistics from my heaviest steel crossbow:
- Leaf-spring cut to an enhanced pyramid front-view profile
- Draw weight: ~315 lbs
- Power stroke: 27cm (10,6")
- 81 gram (1246 grain) bolts: 50 m/s (167 fps)
- 47 gram (723 grain) bolts: 62 m/s (207 fps)
- 30 gram (450 grain) bolts: 71 m/s (237 fps)
There are a few small tweaks I can still do for this beast, giving it perhaps 5% performance boost, with velocities rising at most 2-3 m/s (6-9 fps).
Hope this did not went too much off-topic
Last edited by basileus on Sat May 28, 2011 11:55 am; edited 1 time in total (Reason for editing : Fixed a typo)