I've been working once a week mentoring my mate JT while he produces a warbow from a stave of Austrian Yew. He has 2 staves so, as is my wont we worked the worst one first, it had some sideways bend, one big knot and was a tad thin at the grip.It's progressed nicely and is managing to pull 100# at about 24" . It needs the outers to come round now, but that will happen automatically to some extent when the nocks are fitted and blended in. Video here:-
Mean while the crossbow prod is ready for test... I'll update this post later.... fingers crossed.
The pic shows the string catchers / string bridges, they serve several purposes.
1. The stop the string riding over the top of the prod at the end of the power stroke. This can easily happen with a low brace height, string vibration and stretch.
2. They are made of horn mounted on sheet rubber which will hopefully kill some of the vibration and shock as the string hits home.
3. It improves the force draw curve, increasing early poundage as the bow is effectively shorter until the string lifts off the bridge. Another way of looking at this is that it is effectively allowing a longer string at the brace height. A longer string will pull back further (e.g. In the extreme limit a 30" string pulled back to double up on itself would be 15" long, but a 32" string would come back 16" ).
The Eagle eyed amongst you will notice the aluminium mounting plate has no hole for the bolt to travel through! That's just a blanking plate to allow it to be assembled and tested on the tiller. The actual plate that will be used is mounted on the stock and is an inconvenient angled shape so it can't be used for tiller testing.
The ends of the prod have been bound with carbon fibre/epoxy to prevent the string splitting down the end of the bow. The nocks have been double served, so hopefully it should be ok this time.
I put a load of wax on the track to show how far the string overshoots the brace position when actually shot.
Update:- I took some slo-mo video later to see how the string catchers work. It looks like they do a good job!
https://www.youtube.com/watch?v=LzVH8sQQkVo
Do you know how your camera scans? I'm wondering if the wave travelling along the string is just the camera scanning and if that's the case then the string is just bouncing back and forth.
ReplyDeleteIt was filmed at 1200 frames per second and slowed further on with Windows Movie Maker. There may well be some artifacts due to the scan. It prob doesn't help that I'm at an angle and the lighting could be better. It does show enough to see that the string catchers are pretty much essential and you can certainly see the limbs bouncing back.
ReplyDeleteFaster cameras are prohibitively expensive.
Don, I've just found this high quality video that show how the string does become slack as the limbs bounce back... it's a compound but it's basically the same.
ReplyDeletehttps://www.youtube.com/watch?v=fz9V8EKXe74