noticed something interesting

[smokin joe]

I was tinkering around with my Centaur TD longbow, and I noticed something I had never noticed before.

I twisted up the brace height by 3/8" just to see what would happen. Pretty much no change in the shooting characteristics, except one thing. My bare shaft kept hitting nock low. Then I lowered the brace height back down to its normal height and the bare shaft was back to perfect. Repeating the experiment several times offered proof.

Well, that made me sit down and think. Here is my conclusion: Since the nock point is not in the exact middle of a bow string the nock point moves a tiny amount with any brace height change. I have known for a long time that nock height is sensitive to very small changes and getting it just right involves very small incremental adjustments. It stands to reason that brace height change will change nock height a tiny bit, but I had never put it to the test.

I learned something today that I had never thought through before.

[Roy from Pa]

 

[McDave]

I agree.  Everything we do in tuning is interrelated to some degree or another.  We look at nock height, spine, brace height, bowstring and silencers as being separate issues, but changing any one of them has an effect, sometimes subtle, on the others.

[Orion]

Agree that changing brace eight can change nock height minutely, but the changed brace height also changes when and how the arrow leaves the bow (and a tad of poundage).  The higher brace height might produce a slight porpoising, which shows up as nock low at close distance. Everything is related to everything else. 

[smokin joe]

I thought about porpoising and shot bare shaft at different distances to check that out. I had the same results. So in this case I believe it was probably the nock height change from the increased brace height.

Little things make a difference.

[McDave]

I've noticed that the relative finger pressure on the string also affects nock orientation. More pressure on the index finger = nock high. More pressure on the ring finger = nock low. At least that is my experience with 3 under. Don't know if it would be the same with split or not.

[smokin joe]

You are correct, Dave. I knew that and took it into account in my testing.

The results looked consistent in terms of the effect of brace height on bare shaft nock low versus perfect. I do think that the cause was the slight shift in the relationship of nock height caused by the brace height shift.

I wanted to post it because it was a pretty good reminder to me that if something changes, even something small, there is probably going to be a bit of a chain reaction in terms of the tune of the arrow to the bow.

[BAK]

Learning is a good thing.

[Hud]

When your bow is strung, hold it by the handle with one hand, and the string with the other, then try to move the string up and down, notice what the limbs do as you change the height of the string nocking point.  Next, try to twist, or torque the string side to side. If you can move the string, up or down, it means you need to avoid doing it unconsciously, by changing bow hand,  string finger pressure above, below,  or if you are string walking.  It can affect arrow flight and grouping. Check the string nocking point with a  bow square when you change the brace height. I haven't found it necessary to change the nocking point unless it slipped. If your using consistent bow hand pressure, and finger pressure you shouldn't have a problem. However some bows and especially light weight bows are easily affected, based on my own experience.

[smokin joe]

You are correct, Hud, that different torque and finger pressure can create different results. Plus, some bows are more susceptible to torque more than others. That is why I believe it is important to take great pains to run any tuning experiment a lot of times to only test one factor change. In the case of my experiment, I only changed the brace height and it threw the bare shaft tune off. That single change, plus shooting the bare shaft at a few different distances, verified the results over many shots.

The old "only change one thing at a time" tuning advice seems pretty good.