Thursday, 19 February 2015

First Flexing

The pic and video show it flexing to 28" at about 45#, nearly finished.
Hopefully I'll get the draw weight up to 50~ and tweak the tiller to get it a bit prettier.
Here's the story of how it got there.. it was a bit of a struggle!

I glued on some temporary nocks, adjusted my string to give a low brace height and tried to string it.
I could feel there was decent poundage there, but also the bow was trying to flex sideways, groan...
Lateral instability can be a problem with longbows or bows with thin tips. It's not helped by the way I tend to rough 'em out square e.g near the tips the bow is as wide as it is thick so it doesn't really have a preferred natural direction of flex.
I took the string off without letting it go sideways and took a good bit of wood off the corners where belly meets the sides, this removes a little poundage but makes it more inclined to flex towards the belly. It also gives the rounded belly expected on a longbow.
The irony is that at a high brace the bow is forced to bend the right way, or on a long string it is being pulled the right way. It's at a low brace it is most likely to try and flip sideways.

I increased the brace height and it was flexing ok but still trying to go slightly sideways. The upper limb also seemed much stiffer than the lower for no obvious reason.... damn!
Hmmm, all a bit stressful, but fortunately I have a host of tricks and techniques learned over the years which are applied one at a time to slowly remedy things.

Moving the nocks across by rasping away one side at the tip helped improve the string line but the bow was still biased a little to one side. I clamped it up with a slight sideways bend to compensate and heated the belly a little. This worked in getting it drawing nice and straight.
The stiff upper limb is probably due to some knotty areas in the Yew wood belly which tend to stiffen it. Rasping a little extra off got it nicely balanced... Whew!
But now the bad news... groan... the draw weight is lower than I wanted, it's about 40# rather than the 50# I want. Mind a little below 50# will probably be ok if it's a good fast bow, after all it's performance that matters not brute poundage.
Still, no need for panic, there are still a few trick remaining!

I always make bows a couple of inches longer than I really want and cutting an inch off each limb will give me another couple of pounds. This has been done and the video taken... the still is a frame from the video.
I can also heat treat the belly a tad (maybe just on the lower limb).
Heat treating will need doing with great care to avoid heating the glue line of the backing. I have a technique for this where I clamp thin slats of wood to the sides of the bow, this confines the heat along the belly and protects the back. This can give another few extra pounds an stiffen up a couple af areas that are flexing too much.

video




No comments:

Post a Comment