Archery Tech Questions

[Cousin Brown]

Didn't want to high jack another thread ( https://www.njwoodsandwater.com/topic/2040-shes-home-my-new-hoyt-compound-matrix ) but there is a lot of good info there.

 

After reading on arrow weight, speed & spine how about added length of broad heads ? For example if I shoot a 28" arrow at (X) amount of draw weight and shoot say a Slick Trick head, 100grains ..my overall length is say 28.5". Now I shoot the same arrow but with a Trophy Ridge Hammerhead 100 grains that's 2" long. I am now shooting a 30" overal length arrow. Even though the arrow weight is the same wouldn't this change the FOC & spine on certain setups?

[BowTechExperience]

Cousin, It could, especially if you are at or near your limit with arrow spine and length of arrow, because as the whole arrow becomes longer with a longer broad head it changes the spine of the arrow and also it's balance point ( FOC ).. In a sense as the whole becomes longer, it get weaker in spine. That does not mean though that the combo will not work, you could off set the spine by reducing draw weight a bit to accommadate for the length gained with the new head.

Edited August 1, 2013 by BowTechExperience

[Matty]

Everything BTE just said is on the money and the accepted thoughts on this.

 

However, I may take some heat for the following, but I've never seen the effects claimed.

 

In fact, my belief is as follows: The broadhead is not part of the shaft, so it doesn't lengthen the amount of shaft that can flex (which is what weakens spine). It's a separate piece that would experience it's own flex (if it even does). The fact that it is also spreading that weight out over a longer distance should slightly negate it's effects. If the broadhead was longer and all of it's weight in the most forward tip section, I would believe it could weaken the spine. But it's not. Most all broadheads have the majority of their weight near the insert.

 

Picture an arrow laid on a table, half of it sticking off, the other half held firmly to the table. Now add a heavy weight to the arrow at the very end of the portion sticking off the table. It's going to flex a lot more than if you place that same weight on the arrow much closer to the table. This is the same as what's happening with the broadheads. Picture that arrow as the actual broadhead now. If all of it's weight was forward, it would flex.

 

However, most of the weight of any bh is near the thickest part of the ferrule and threads where it meets the arrow. So all of them have the majority of the weight in the same place and shouldn't be adding additional flex.

 

The shaft is still the same length and will flex the same as long as the point weight is the same. Therefore spine should remain unaffected.

 

That's my theory anyway.

 

However, FOC will be ever so slightly affected because you're spreading the weight out over a further distance.

 

Someday I'd like to do some tests to see if my belief regarding spine/bh length is true, because as far as I know, the other theory has never been tested either. It's just what is believed to happen. I would love to know definitively.

 

BTW, excellent question!

Edited August 1, 2013 by Matty

[Haskell_Hunter]

I'm not a physicist, but there's a reason heads are done by weight and not length.  When you release the string on the bow, the energy is transferred through the shaft to the head.  Because the mass of the head is denser and in a smaller place, that causes some of the energy to go back into the shaft.

 

For example, if you had a balsa wood board that would breach at 100 grain and put an arrow shaft up to it, the shaft would bow to a certain extent before the board was breached.  It would flex more with a board that breached at 125 grain.

 

The flex occurs at the point where the head it attached to the shaft.  The force from the string would travel through the shaft and stop at the end, where the mass is most dense, and then travel back into the shaft.  The string and the shaft have to move the mass that's on the head of the arrow, and that's why we measure heads in weight.  The mass affects the flex, flight, and path of the arrow more so than the length.  Is the length of the head important?  Yes, but it's negligible because the ratio of the length of the head to the shaft.  You'd need a dagger sticking out of the shaft to make a significant difference.

 

So, IMHO, it makes a slight difference, but it's mostly academic.