Axle length and shooting fingers

[scruffy]

I'm getting into shooting a compound bow (I only have very small amount of recurve experience...).  I picked up a PSE Nova L3 with then intention of shooting with a release.  Well, I haven't found one I've liked yet, but just starting the release search.  Anyway, I've thought about shooting fingers in the meantime and pulled the bow back, two fingers under the arrow, and not much pinch at all, not what I was expecting based on what I've read on the internet.  Being a numbers guy I pulled out the old geometry equations from the deep reaches of my brain and determined why I didn't get pinched.  It got me thinking that maybe axle to axle length isn't the only thing that determines finger pinch.

So, when you pull a bow back to full draw you have a right angle triangle.  The vertical axis runs from the arrow up to the top of the upper wheel ((axle length /2) + 1.5" assuming 3" wheel).  The horizontal line from where the string sits at rest to full draw (draw length - brace height).  To calculate the angle at the nock/arrow I used the equation Angle = 90 / ((horizontal axis / vertical axis) + 1).

Doing this a PSE Dakota (often referred to as a good fingers bow) with 41" axle to axle length, 30" draw length, and 7 1/4" brace height would have a vertical axis of 22", a horizontal axis of 22.75", which would give it an angle at the nock/arrow of 44.2 degrees.  Multiply by 2 for both upper and lower triangles (above an below the arrow) and you get an 88.4 degree angle to get your fingers and arrow into.

With that as my baseline I figured up my PSE Nova L3, 36 1/2" axle to axle, 28" draw length, and 7 1/2" brace height.  It has a vertical axis of 19.75" and a horizontal axis of 20.5", giving it an string angle of 44.2 degrees.  That would give me the same 88.4 degree angle as longer axle to axle, longer vertical axis Dakota.  But the longer draw length and short base height of the Dakota baseline gives it a longer horizontal axis as well, negating the longer axle to axle length advantage.

I also read in another post that howie got a pse bruin l3, 29" draw.  It would have a 86.6 degree angle to get his fingers into.  A 36" axle to axle, 29" draw, 8" brace height.  That would be a 19.5" vertical axis, 21" horizontal axis, giving a 43.3 degree angle at the arrow, times 2 for upper and lower triangles (above and below the arrow) and he will have 86.6 degree angle for his fingers, only a 1.8 degree difference.

To compare to a short axle bow, like a pse Firestorm with 30" axle to axle, 29" draw, 8" brace height, it would have a vert axis of 16.5", horizontal axis of 22", and an angle at the arrow nock of 38.6 degrees.  Times two for above and below the arrow an you're putting your fingers into 77.2 degrees, or 11.2 degree reduction from the baseline 30" draw 41" axle Dakota and my 28" draw 36 1/2" axle nova.  A real pincher!!!

So saying a finger bow has to have an axle length 40+ inches is only taking one side of the triangle into the equation.  To actually figure the angle of the string and therefore the pinch of a bow you have to figure the axle length, draw length, and base height.  

Atleast that's the what my old math brain cells are telling me.  What do you guys think?  

later,
scruffy

[Dalton]

Man you paid more attention in Geometry class then I.:?  I'll not question the formula laid forth, that's up to the mathematicians out there.  But I do question your conclusion.

I did this on a corkboard using pushpins and a rubber band.  Substitute whatever materials you have.  I took two pushpins and placed them 12 inches apart vertically.  Then I took another pushpin and placed it 8 inches to the right of the vertical pins, centered between the two.  Stretch the rubber band around all three pins.  Mark the angle at the pin on the right with a pencil.  Now move one of the vertical pins up or down simulating a decrease in the axle to axle length.  You will see that you now have a smaller angle on the inside of the triangle.  Meaning you have less room to put your fingers.

Axle to Axle Length
A bow with a higher axle to axle length won’t pinch your fingers as much. Does it mean you can’t use a bow shorter than 40”?  No.  In fact one thing we have not even mentioned that plays a factor in this is the size of your fingers.  A person with small fingers may not feel the pinch of a 38” bow.  I feel the pinch on anything less than 41”.

Brace Height
Brace height has more of an affect on the forgiveness of a bow then does the axle to axle length.  The longer that arrow stays on the string, the more exaggerated any mistakes in the shooters form will become.  This measurement does not play a role in finger pinch, but does contribute to the overall forgiveness of the bow.  Therefore it is in a finger shooters best interest to use a higher brace height bow.  That is why I don’t shoot the PSE Dakota.

Again I would reiterate that these are guidelines, not hard and fast rules. I don't see anybody on the internet saying you can't shoot a short bow with your fingers.  Some people do and are successful at it. :grin:

[scruffy]

Dalton, there's one major difference between your illustration and mine.  With yours your horizontal axis (pin 8" to the right of the vertical pins) is constant.  In my illustration the horizontal axis is variable, depending on draw length.  

For your pushpin example to be similar to my example when moving your vertical pushpins up and down you also have to move your horizontal push pin in and out.

With the vertical push pins set at 10" and the horizontal push pin set at 11.25" (actually 11.375, but 11 1/4 is close enough) to the right you have basically the dakota's 41" a-a, 30" draw dementions to a 1/2 scale.  Mark the angle.  Now move the vertical pins in to 9.75" (9.875" would be more accurate...) apart and also move the horizontal pin 10.25" to the right.  Mark the angle.  Now you basically have my 28" draw 36.5" a-a nova l3.

So not only does smaller fingers play a roll, but also shorter arms because of shorter draw length.  The farther you pull back the bow the tighter the angle.  The angle with the string pulled back 0" is 180 degree.  The farther you pull it back the tighter the angle gets, so us 28" draw shooters can shoot a shorter a-a and still have the same angle.  Where a longer draw shooter needs a longer a-a to get the same angle because he's pulling the bow back that much farther.

Make sense?

You illustration is much better than mine, mush simplier!!!!

I agree, the brace height have very little to do with the angle.  The brace height is just used in the equation for the horizontal axis (draw length - brace height).  It's just needed to plug in the equation to get the triangle dementions for the calculations but I agree, not a noticable effect on the angle.

I hope this makes more sense.  

I'd think (if my math is correct) that the guidlines for a-a and fingers would be better if they were 40+" a-a for 30" draw, 38+" a-a for 29 draw, 36+" a-a for a 28" draw.  

later,
scruffy

[Dalton]

Scruffy,

I'm understanding your point a little better now.  

Your absolutely right. If you decrease the draw length, you decrease the the axle to axle length necessary to avoid pinching the fingers.  You are basically decreasing the size of the triangle while keeping the proportions the same.

That would be a great chart to come up with.  You could have a finger pinch rating system based on your equation.

Have a good one,

Dalton