Thursday, 12 October 2017

Re-Tillering "Twister"

Now re-working Twister is at first glance verging on the unthinkable, as it's been my fave' bow for years (made in 2011). On a good day I knew I could punch out my left hand at a target and I'd hit it.
Recently though I'd been toying with taking off a little draw weight, also my draw length has dropped a tad and is nearer 27" than 28".
Having been shooting in the tri-lam longbow at a good 28" draw, I realised that twister by comparison felt "stacky", that's to say I'd get to about 26" draw and it then felt stiff and unyielding instead of lively and supple.
Over the Years Twister has taken a little set mid limb (more on the lower) but is still good by my test of putting it belly down on the floor (unstrung) and seeing how many fingers you can get between grip and floor. One or less is good, which is how Twister is. Two is ok, 3 is poor. It was a superb bow, but maybe the two of us aren't quite what we were, so I thought I'd ease off the outer limbs a tad and maybe that would make it feel smoother, take some stress off the mid limbs and ease off the draw weight.
To get myself in shape too I've rasped a bit off my belly ... just kidding... I've re-instated the dozen push ups night and morning.
Unfortunately I didn't take a pic before doing any work, but this first pic shows the tiller after I'd eased off the tips a tad and lost a few pounds. It's at 40# at 28" where it probably started out and nearer 45#.
You can see the tips are stiff and it's bending quite hard mid limb almost in the style of a Mollegabet bow with stiff levers for the outer third. The two circles (or ellipses as they may not be exact circles) are quite tight radius and I'd like to see the stress spread more along the whole limb. It must have been worse than this before I eased off the outer limbs.

I've heat treated the mid section of each limb with the limbs strapped down and a slip of wood about 4mm thick under the back where the set was. That effectively pulls it into the merest hint of reflex so that when the heat treating is done and the strapping off, the limb is about straight.
It's now about 40# @ about 24" which gives me some room to re-tiller whilst keeping a decent draw weight. I think what I need is about 40# at 27" and I'd like the bow to have it's original speed and feel.
I've done a little more work now and actually shot it. Here's an after heat treating both pulled to just over 27"
The curve is looking much better and I've ended up pretty much where it was when first made about 45# @ nearly 28" , I've shot half a dozen arrows and it feels smoother and faster like it's old self.
The difference in curve is pretty subtle but if you look at the chalk marks on the wall behind the right limb you can see in the "after" picture the limb is nicely on the curved line whereas in the first pic the tip is pulled inside that line with more of the bend in the middle/inner limb.
I'll probably quit while I'm ahead now!
Maybe this last pic shows the difference better, I've done the 2 ellipses and this time you can see they are flatter and the tips conform to the curves better.
Finally another try to fit ellipses as well as I can, this one shows the right limb having a slightly tighter curve, which is fine as generally you want the lower limb a tad stiffer.
Right you can plays spot the difference now!

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