Primitive Archer

Main Discussion Area => Arrows => Topic started by: agd68 on August 24, 2010, 10:54:36 am

Title: Spine Testers
Post by: agd68 on August 24, 2010, 10:54:36 am

 shafts are spined for a 28" shaft with the posts of the tester 26" apart, then you add or subtract 5lbs for every inch greater or lower than 28 " of shaft lenght So would'nt increasing / decreasing the distance between the posts do the samthing as adding or subracting to compensate for length ?

Title: Re: Spine Testers
Post by: CraigMBeckett on August 24, 2010, 11:31:02 am

Hi agd68,

The answer to your question is no, the central deflection of a simply supported a beam subject to a load at its centre is defined by the formula y = (WL3/48EI) the 3 is meant to indicate the cube of L that is L x L x L where L is the length between supports. As the equation is based on the cube of the length between supports simply moving the support to increase or decrease the span length will not give you the same answer as that obtained using the standard AMO method plus modifiers.

You could of course use the old GNAS (British) method and tables these were meant for use with ELB's. In this method the arrow is supported just behind the pile and in front of the nock and a weight of 1.5 lb is applied. The deflection (in 1/100 parts of an inch) gives the GNAS spine unit and the corresponding spine comes from a table as below:

GNAS Unit          Spine lbs

81                      30
61                      40
49                      50
40                      60
35                      70


Don't seem to be able to find out much more about the GNAS system it no longer seems to be used and has not really made it onto the internet I have calculated out the curve and extended the table as below.

GNAS Unit          Spine lbs
25                    100
16.5                 150

Craig.

Title: Re: Spine Testers
Post by: agd68 on August 24, 2010, 12:28:33 pm

Thanks that is very helpful.

Title: Re: Spine Testers
Post by: Asafan on October 11, 2010, 07:24:45 pm

Here my scale for 26 inches and 2 lb of cargo. It is convenient to calibrate a tester any tubes. Sorry for literacy  :) . Added full size pic.

[attachment deleted by admin]

Title: Re: Spine Testers
Post by: jeff halfrack on October 11, 2010, 07:57:34 pm

  I'm  nobody,   but  could  this  be  saved  in  the  read  only  thread,,,after  it  runs  awhile???   this  is  good  stuff  thanks  JEFFW

Title: Re: Spine Testers
Post by: Thwackaddict on October 11, 2010, 11:38:12 pm

Asa- is that the exact size it needs to be?

Title: Re: Spine Testers
Post by: Michael C. on October 11, 2010, 11:40:52 pm

it looks like you can scale it according to the 40 mm legend down at the bottom

Title: Re: Spine Testers
Post by: Asafan on October 12, 2010, 03:19:34 am

Quote from: Thwackaddict on October 11, 2010, 11:38:12 pm

exact size it needs to be?

This plan is made for the print on A4, on the majority of printers the length of the lever turns out 38,5 millimetres. The line 0 - is horizontal. An execution example - on a photo:


[attachment deleted by admin]

Title: Re: Spine Testers
Post by: Thwackaddict on October 12, 2010, 11:35:38 am

thanks!!I been tryin to get mine to work but cant seem to get an accurate readin and i think this is just what i been lookin for!!