CIRCULAR TILLER BOWS AND DRAWING AN ACCURATE BACKING BOARD
Posted: Thu Aug 27, 2015 6:26 am
I have written an article below intending to show the disparity between what actually often is and what is purported to be a circular tillered (compass) bow and how to effect a remedy with the use of an accurately plotted tiller backing board showing how, instead of a series of horizontal lines used to trace the progress of a bow's limbs, a board can be drawn which can quite accurately show a series of partial circles from very shallow to the maximum curvature of any bow which form true part-circular bends.
I will post it in parts because of its length.
PART 1. - HOW CIRCULAR IS CIRCULAR TILLERING?
The fact is, that such tillered bows are rarely ever correctly circular if that is the ideal tiller for an ELB.
On investigating this desirable phenomenon, I have always questioned the expression used by Ascham regarding ‘compass’ bows as not necessarily meaning circular at all, but referring to a well-rounded shape to the limbs – having no stiff areas nor areas which ‘come’ too much or create a hinge.
Nowhere in ‘Toxophilus’ does Ascham recommend or enjoin readers that a bow must bend through the handle. That is a derived assumption by today’s bowyers. The error arises from our misunderstanding of his use of the word ‘compass’ to mean circular when there is no actual evidence to support that belief.
Nobody seems to notice that he also describes the trajectory of an arrow as being ‘compass’ in shape. We know today that the trajectory of an arrow is definitely not circular. It is elliptical. In his day, he must have noticed from the side position that the path of arrows differed in shape to that of a bow. Yet he still used the same word for both arcs. So, did the word mean ‘circular’ or simply ‘curved’ or ‘rounded’?
Every ELB-styled bow, that I have ever seen, including all of my own, has had an elliptical tiller. Many bows I have seen progress from shallow elliptical at brace height to distinctly egg-shaped through the middle at full draw, with the outer limbs having arcs of much shallower bend than the middle. This shape of tiller is what Ascham referred to as ‘staffish’ because of the thickness of the outer bow limbs.
It was a surprise and a disappointment to me when I realised this. I then began to do some scale drawings to find out what kind of bend one would need to produce in a bow which, at every stage of the tillering process would take a distinctly circular bend throughout its draw.
What follows is an explanation of what I found.
As many of you have seen, I have used a technique by which I can analyse the tiller shape of a bow whilst on the tiller using three chords of equal length (from nock to bow centre) and having another line standing perpendicular to this chord at its exact middle. The purpose of this perpendicular line is to measure the distance from the base chord to the belly of the bent limb.
That same chord is then placed on the opposing limb at the same position and then finally, a third chord is positioned centrally on the bow with the perpendicular line touching the bow at its exact centre.
For a bow to have a correctly circular tiller, each of these chords must touch the bow’s belly at the end of the perpendicular line and each end of the chord must touch the bow’s nock and mid point as in Fig. 1 below. Fig. 1 – An example of a poorly tillered English Longbow. The upper limb is on the right
In Fig. 1 above, each of the red chords and their respective blue vertical depth indicators are of equal dimensions and are placed between both string nocks and then at the middle of the bow. If this bow were circularly tillered, these chords would all touch at exactly the same place and the depth of the bend indicated by the blue line would be also equal.
It can easily be seen from the Fig. 1 that the lower limb on the left has a much deeper bend than the bow’s upper limb and at the bow’s centre, it has a quite stiff section where the blue depth indicator is partially into the bow itself. The same occurs with the upper limb at right which is even worse.
Thus far, of the many bow pictures from eBay (which is a good source for this kind of research) which I have tested similarly, surprisingly few have accorded with this simple test of circularity. Almost all have purported to be circular in tiller, from the common assertion that to be circular in tiller, the bow must bend through the handle. That is not the sole criterion for circular tiller. Most of such bows have had outer limbs with very little bend.
The following is another example which is even worse than the bow in Fig. 1 above. This particular bow is an ELB format but is very short at close to 60 inches.
Fig. 2 – A very short ELB with far too much bend through the handle section.
In Fig. 2 above, I tried another technique of ascertaining the circularity of bend in bows using the principle of radians. This particular bow at 60 inches in length would have a radius of 30 inches. Based on that measurement, I drew the red line which is a true circle based on the radian principle. From the picture, it was easy to find where to place the focus to draw a circle of radius 30 inches.
It can easily be seen that this particular bow has far too little bend in its outer limbs which such a short bow really ought to have in order to spread the bending load over the whole length of its limbs.
The blue circle which contacts the nocks at the bow’s nocks graphically shows just how much bend there is through the handle section – almost two handle thicknesses too much - a dreadful load to put on a deep section bow of only 60 inches. Without those circles, this particular bow easily deceives the eye into thinking that it is truly circular in tiller. Indeed it is far from that.
The same maker of the bow in Fig 2 above has made another example of an out-of-circular limb tiller with this very heavy bow of 74 inches.
Fig. 3. – A 74 inch bow with too much bend in the middle and not enough in the outer limbs.
In this bow (Fig. 3), again we can see how little bend there is in the outer limbs compared to that in the handle section. I have drawn a red circle with a radius of 37 inches (one radian) around this bow to show just how far out of circular that this bow is.
Fig. 4. – Degree of out-of-circular tiller of bow in Fig. 3 above.
In Fig. 5 below, I edited this picture in Photoshop to bring more curvature into the outer limbs so that they matched the circle I had drawn around the bow above (Fig. 4). If you look closely, you can see the original horns peeping from behind the foreground layer where I have Photoshop-bent the bow’s limbs to follow the red line.
Fig. 5 – Fig. 3 image edited in Photoshop to show correct circular tiller.
The horizontal blue lines were added to estimate the amount of tip deflection needed to bring this bow’s limbs to circular tiller.
Surprisingly, as I have followed the bows this English bowyer lists for sale on eBay, he seems to have a penchant for bows which bend far too much in the middle.
As we know, limbs which bend circularly have the benefit of evenness of load distribution along the limb length which tends to limit the amount of inevitable set and minimises the chance of frets and pinches forming between hard and soft spots along a bow’s belly.
At any rate, debates upon the meaning of the Tudor archery term ‘compass’ are academic and largely unverifiable. However, using an accurately made tillering backboard, it is both possible and practical to tiller an English Pattern Longbow to an accurately circular bend other than by doing it by eye alone. What follows is how to make such a board and the principles behind it.
I will post it in parts because of its length.
PART 1. - HOW CIRCULAR IS CIRCULAR TILLERING?
The fact is, that such tillered bows are rarely ever correctly circular if that is the ideal tiller for an ELB.
On investigating this desirable phenomenon, I have always questioned the expression used by Ascham regarding ‘compass’ bows as not necessarily meaning circular at all, but referring to a well-rounded shape to the limbs – having no stiff areas nor areas which ‘come’ too much or create a hinge.
Nowhere in ‘Toxophilus’ does Ascham recommend or enjoin readers that a bow must bend through the handle. That is a derived assumption by today’s bowyers. The error arises from our misunderstanding of his use of the word ‘compass’ to mean circular when there is no actual evidence to support that belief.
Nobody seems to notice that he also describes the trajectory of an arrow as being ‘compass’ in shape. We know today that the trajectory of an arrow is definitely not circular. It is elliptical. In his day, he must have noticed from the side position that the path of arrows differed in shape to that of a bow. Yet he still used the same word for both arcs. So, did the word mean ‘circular’ or simply ‘curved’ or ‘rounded’?
Every ELB-styled bow, that I have ever seen, including all of my own, has had an elliptical tiller. Many bows I have seen progress from shallow elliptical at brace height to distinctly egg-shaped through the middle at full draw, with the outer limbs having arcs of much shallower bend than the middle. This shape of tiller is what Ascham referred to as ‘staffish’ because of the thickness of the outer bow limbs.
It was a surprise and a disappointment to me when I realised this. I then began to do some scale drawings to find out what kind of bend one would need to produce in a bow which, at every stage of the tillering process would take a distinctly circular bend throughout its draw.
What follows is an explanation of what I found.
As many of you have seen, I have used a technique by which I can analyse the tiller shape of a bow whilst on the tiller using three chords of equal length (from nock to bow centre) and having another line standing perpendicular to this chord at its exact middle. The purpose of this perpendicular line is to measure the distance from the base chord to the belly of the bent limb.
That same chord is then placed on the opposing limb at the same position and then finally, a third chord is positioned centrally on the bow with the perpendicular line touching the bow at its exact centre.
For a bow to have a correctly circular tiller, each of these chords must touch the bow’s belly at the end of the perpendicular line and each end of the chord must touch the bow’s nock and mid point as in Fig. 1 below. Fig. 1 – An example of a poorly tillered English Longbow. The upper limb is on the right
In Fig. 1 above, each of the red chords and their respective blue vertical depth indicators are of equal dimensions and are placed between both string nocks and then at the middle of the bow. If this bow were circularly tillered, these chords would all touch at exactly the same place and the depth of the bend indicated by the blue line would be also equal.
It can easily be seen from the Fig. 1 that the lower limb on the left has a much deeper bend than the bow’s upper limb and at the bow’s centre, it has a quite stiff section where the blue depth indicator is partially into the bow itself. The same occurs with the upper limb at right which is even worse.
Thus far, of the many bow pictures from eBay (which is a good source for this kind of research) which I have tested similarly, surprisingly few have accorded with this simple test of circularity. Almost all have purported to be circular in tiller, from the common assertion that to be circular in tiller, the bow must bend through the handle. That is not the sole criterion for circular tiller. Most of such bows have had outer limbs with very little bend.
The following is another example which is even worse than the bow in Fig. 1 above. This particular bow is an ELB format but is very short at close to 60 inches.
Fig. 2 – A very short ELB with far too much bend through the handle section.
In Fig. 2 above, I tried another technique of ascertaining the circularity of bend in bows using the principle of radians. This particular bow at 60 inches in length would have a radius of 30 inches. Based on that measurement, I drew the red line which is a true circle based on the radian principle. From the picture, it was easy to find where to place the focus to draw a circle of radius 30 inches.
It can easily be seen that this particular bow has far too little bend in its outer limbs which such a short bow really ought to have in order to spread the bending load over the whole length of its limbs.
The blue circle which contacts the nocks at the bow’s nocks graphically shows just how much bend there is through the handle section – almost two handle thicknesses too much - a dreadful load to put on a deep section bow of only 60 inches. Without those circles, this particular bow easily deceives the eye into thinking that it is truly circular in tiller. Indeed it is far from that.
The same maker of the bow in Fig 2 above has made another example of an out-of-circular limb tiller with this very heavy bow of 74 inches.
Fig. 3. – A 74 inch bow with too much bend in the middle and not enough in the outer limbs.
In this bow (Fig. 3), again we can see how little bend there is in the outer limbs compared to that in the handle section. I have drawn a red circle with a radius of 37 inches (one radian) around this bow to show just how far out of circular that this bow is.
Fig. 4. – Degree of out-of-circular tiller of bow in Fig. 3 above.
In Fig. 5 below, I edited this picture in Photoshop to bring more curvature into the outer limbs so that they matched the circle I had drawn around the bow above (Fig. 4). If you look closely, you can see the original horns peeping from behind the foreground layer where I have Photoshop-bent the bow’s limbs to follow the red line.
Fig. 5 – Fig. 3 image edited in Photoshop to show correct circular tiller.
The horizontal blue lines were added to estimate the amount of tip deflection needed to bring this bow’s limbs to circular tiller.
Surprisingly, as I have followed the bows this English bowyer lists for sale on eBay, he seems to have a penchant for bows which bend far too much in the middle.
As we know, limbs which bend circularly have the benefit of evenness of load distribution along the limb length which tends to limit the amount of inevitable set and minimises the chance of frets and pinches forming between hard and soft spots along a bow’s belly.
At any rate, debates upon the meaning of the Tudor archery term ‘compass’ are academic and largely unverifiable. However, using an accurately made tillering backboard, it is both possible and practical to tiller an English Pattern Longbow to an accurately circular bend other than by doing it by eye alone. What follows is how to make such a board and the principles behind it.