Tillering advice please

[OTDEAN]

Hi all,

I am hoping someone can give me some tillering advice on what to do at the stage I am at with my elm pyramid bow that I am making.  The design is a flat bow with the width tapering as a pyramid.  The bow is symmetrical.  I am aiming for 50lb.  The bow at the moment tillers at 25lb @ 13" on a 6" brace.  The next 10lb increment of 35lb is 20".  The length of the bow nock to nock is approx 72" in length. 

What I need help with is this.  I am using a tillering stick.  Each bow limb is divided in 1/8ths so I can measure the distance between the back of the bow and the string at each 1/8th when the bow is being tillered.  I figured this will allow me to keep the limbs equal in bend and strength if each 8th is the same on each limb as I increase the inches I am pulling the bow on the tillering stick.

The problem is, at 35lbs at 20" the bow is fine in respect that each 1/8th on each limb is exactly equal, but, when you look at the bow on the tillering stick, one limb looks weaker than the other??  I really am confused, what should I do?

Sorry I have no pictures my camera is broken but I thought someone might know why one limb looks weaker than the other when all the 1/8ths are equal from the back of the bow to the string line, if that makes sense.

Look forward to reading what you all think.

Dean

[Del the cat]

A couple of points.
1. If you want to make a 50# bow, you have to pull it to 50# at some time before it reaches brace else it may well come in underweight.
2. Make sure you have it on the tillering stick supported as it will be in the hand and pull the string from wehre your fingers will pull it. Better still take it off the tiller now and then and try it in front of a morror (or a big glass door will do).
3. Don't rely too much on measurements... they are good for getting you roughed out ready to get it on the tiller and good for  checking an even thickness taper. BUT... Bottom line is, it's how it looks when being shot that matters... all the rest is just a means to an end.
As they say up North in the UK... you don't fatten a pig by weighing it.
Similarly you don't tiller a bow by measuring it!
A good deal of the skill is getting your eye in... taking pics, or video clips of it flexing can help, as can holding a plate or somesuch in front of it to help you see the curve. Reversing it left to right on the tiller or swapping the bow top limb to bottom can sometimes help. Generally the bottom limb is under greater strain and needs to be a tad stiffer to compensate.
Del
PS. This post on my blog illustrates some of what I'm saying. You may find the blog a good source of info, as I show mistakes, problems and fixes as well as the pretty stuff. Some video clips bows being pulled on the tiller ate various stages of tillering in there too
http://bowyersdiary.blogspot.co.uk/2013/03/tillering-symmetry.html

[OTDEAN]

Thanks Del.