Wednesday, 27 March 2013

Tillering Symmetry

A while back, last year I reversed a bow on the tiller because it looked better.
I've been angsting (ok I may have made that word up) about my latest bow since the last post, I adopted my policy of 'when in doubt, step away from the bow' This is much easier once you are past 60 and the impetuousness of youth has waned a bit.

I had already reversed this current bow at about half way through the tillering as one limb seemed stronger.
Now the horn nocks are on and it's back to 28" I thought it was looking a tad weak on the left tip. So I tried reversing it back to the original way round.
Originally the slightly thicker, wider limb (we are talking maybe 0.5-1mm here) was to be the bottom one, this is in line with the measurements found on the Mary Rose bows and with current tillering practice. Even modern target bows are set up with the lower limb adjusted slightly stiff, by angling it away from the archer a tad more than the upper... they still call this adjustment of the limb in it's socket, (done by tightening or slackening a bolt) 'tillering', which amuses me rather.


To do this experiment, I marked the true centre of the bow and that's where it's supported on the tiller. I put masking tape at the true centre of the string and hooked the winch just above the tape (E.G about 1" towards the top of the bow so it's in line with the arrow pass which is 1" above centre)
I took a pic at brace then 27" draw (I didn't want to leave it at 28" on a freezing day while I messed about with the camera)
I then flipped the bow left to right and did exactly the same.
Now, to me it looks much better one way round.
I'm not going to say which, I'll be interested to see if you agree with me, comments please!
Here are the 2 pics at 27" draw (uncropped). You can see, in both pics the left tip is about 1 brick higher than the right*, so there isn't much difference, it's just the shape of the curve that's of interest.

The upshot of all this is, I have now decide which way the bow will sit, and I shall proceed with making a decent string and coaxing it back towards 32".

If you zoom in you'll see the bow is at about 63# at 27", this looks a bit low, but it's at a lowish brace height with a stretchy Dacron string. I'm still hoping for at least 65# at 28" after it fully tillered to 32"




*This is normal and is due in part to the way the bow is drawn from above centre. The braced pic below which shows how early in the draw when there is little tension on the bow it looks skewed (left tip is about 2 bricks higher than the right).
Some people make the mistake of having a bow dead centre on the tiller and pulling the string back from dead centre, this can make a huge differnece on a shorter bow, and when it's finally drawn by hand with an arrow on the string the tiller can look way off. It can be argued that maybe some of this is cosmetic, but it can cause the lower limb to be under undue strain and take a set.
In most bows the hand pressure is probably just below centre and the sting pulled from about an inch above centre.






2 comments:

  1. Looking at the pictures and comparing the curve using a coffee mug gauge(look across the top of the mug and tilt it to match the curve of the arc) I discovered a useful tip: make sure you drink the coffee first, otherwise it makes a hell of a mess all over the desk...

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  2. LOL.
    Good point about tilting to get an elipse rather than a circle.

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