Back to the draw board

The path taken by the nocking point of a compound bow during the bow’s power stroke has a very strong impact on the behaviour of the arrow. In the vertical plane the path is mainly determined by the cam design and the cam timing. In the horizontal plane it is mainly determined by the lateral forces on the cable guard, and by the manner in which the archer holds the bow.

The static path taken by the nocking point in the vertical plane can be measured using a draw board. The bow’s riser is held in a fixed position and the nocking point path can be followed as the bow is drawn using a winch.

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Note that it is very important to use such a draw board carefully. The full draw force of the bow is pulling on the cable from the winch, and if you are not careful the winch handle can turn very quickly indeed and give you a hard hit. It is best to use a ratchet on the winch and to not let go of the handle at all. Also, be very careful to not stand in front of the bow limbs – you always need to remember that something could break and you need to be out of the path of any debris. It is important not to have an arrow on the string – again, if something should break it would be dangerous. If you are not very comfortable about all this, I’d advise against using one.

You also need to be very careful to not over- draw the bow while it is on the draw board. The winches are very powerful, so take care to not draw the bow past full draw. Again, if you are not sure, don’t do it.

The path taken by the nocking point over most of the draw length as you draw the bow is determined by the cam profiles selected by the bow designer, by the cam type, and by the bow’s geometry. Consequently, while it is interesting and does have a significant impact on the arrow behaviour, there is little you can do to change it. The path does not necessarily need to be flat or straight, but generally flat or rising as the string nears brace height will help get the rear of the arrow over the launcher. One of my published papers covers the nocking point path in more detail.

A much more interesting, and important, part of the nocking point path relates to the draw length around full draw. If the nocking point moves vertically as the draw length is slightly changed at full draw we will lose accuracy.

03_Draw Board 002The archer holds the bow against the bow’s wall while aiming. If the archer always holds against the wall with the same force then the vertical position of the nocking point relative to the bow’s riser will remain constant and the bow will be accurate. However, most of us do vary a little in how hard we pull the bow into the wall from shot to shot. For example, if we hold a little longer than usual, most of us will change how hard we are pulling into the wall. If the nocking point path around full draw is not flat, we will then have the bow’s riser tilting backwards and forwards slightly as that force varies, the sight will move up and down, and we will lose accuracy.

If the nocking point path around full draw is not flat and we vary the holding force slightly as we aim, the sight will move up and down on the target and the bow will not aim well. If the nocking point path around full draw is flat the bow will aim better, we will not get high or low shots from pulling into the wall by differing amounts, and the wall will be firmer. We can adjust this by setting the optimum cam timing.

The first step I take is to get the archer to draw the bow while I watch the cables come onto the draw stops on the cams and the location of the arrow’s point. I am interested in seeing how much the archer pulls into the wall as they pull harder or softer. That is, how much does the draw length change as the archer pulls more or less into the wall. Usually it is up to about 5mm or so.

Then I put the bow on the draw board and draw it to full draw using the winch. I can then watch how the nocking point moves as I vary the draw length at full draw by the amount the archer’s draw length varies as they pull harder or softer into the wall. Over that interval I want the nocking point path to be flat – that is, straight back and not tilted.

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If the nocking point path around full draw is not flat and straight, then I change the cam timing slightly to try to get it as flat and as straight as I can by twisting one of the cables.

I usually find that with a hybrid cam (such as on Hoyt compound bows) I get the best results when the top cam reaches its stop just prior to the bottom cam doing so. Single cams may or may not be flat and straight – if they are not there is not much you can do about it. Binary and twin cams are usually better when they have two stops rather than one.

If you don’t have access to a draw board, what is known as ‘creep tuning’ aims to achieve the same result by having the archer vary the draw length slightly from shot to shot and looking to see if the arrows then hit the target at different heights. It is much more accurate to do this with a draw board, but if you can’t get to one, it’s a useful way to see if you are having variation in shot caused by your nocking point moving vertically at full draw.

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One comment on “Back to the draw board
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