The little experiment I mentioned yesterday involved this rough and ready method of shortening the limbs to see if  different string angles have any effect on performance.  Clearly the potential weight savings from having shorter limbs will not factor into this equation.

The first of six shots can be seen in the video below.

Click for video, 20100628143846

The first two shots of this string of six was done with the string in its normal position placed in the nocks at the ends of the limbs.  Velocities were 231.9 fps and 238.3 fps.  The next three shots were all done with the string in this new position modeling shorter limbs.  Velocities were 227.8,  225,3  and 220.6 fps respectively.  The draw weight in this new position climbed to 4800 lbs, up 200 lbs  from the 4600 lbs registered with the string in its normal position. While it is true that the new string position gave slower velocities, they were not slower by much, and so it remains an open question as to whether truly shorter limbs with a couple of inches removed would provide a net boost in velocity by virtue  of being lighter in weight, especially out on the ends where unnecessary weight has a more pronounced effect.  More grist for the mill.  Always more grist.

The photo below shows the splats lined up in the order they were performed.

The final sixth shot of the day, seen on the far right, is noticeably more deformed than the first five.  This is because its velocity was boosted up to 270 fps by another major change that was  introduced into the limb set up.  I removed the bronze hardstops from the curved stanchion, and that allowed the limbs to rotate an extra 15 degrees past the usual 90 degree position they have always had relative to the string.  This allowed for a longer string and a longer draw before a 4800lb draw weight was achieved.  That represents a 50 fps increase basically for free.  One may wonder why this 270 fps is remarkable, given that 285 fps was achieved the other day with the limbs in their normal 90 degree at rest position.  Probably the pretensioning in the bundles was much higher the other day and that pretty well overwhelms any efficiencies that might be apparent from other factors.  (Stupid me didn’t record the draw weight from then because the dynanometer was not zeroed properly , so now there’s no way to know for sure)  Today’s testing was done with a deliberate attempt to keep the pretensioning of the bundles consistent throughout the experiment.  Not maxed out,  just consistent.  That being the case, it really does seem that having the limbs rotate past 90 degrees boosts the velocity quite a bit.  This is all something that Cap’t Harpoon advised me of some time ago,  and only now am I  getting to test it.  More on the good Captain later.

4 Responses to “The little experiment”

  1. Captn Harpoon says:

    Thanx for the great report!!

    I dont know how you measured positon for the shorter limb shots, but if you used the rachet stops as a measure, remember you would have to move the trigger down two inches also.

    For the velocity gain or loss, give a rough estimate of the weight of the two inches at the end of the limb and multiply by two. Should be about a thousand grains give or take.

    Shoot a few shots with the string in the normal position,why not add 500 grains onto the bolt (only 50% of weight savings) and see what velocities you get. On second thought it would be a lot easier to shave off 500 – 1000 grains off an existing bolt and retry.

    I betting a total of four inches of limb tip far exceeds a thousand grains though…

    On the extra 15 degrees – thats really super news. Im betting that elusive 400fps mark can be hammered again.
    Looking forward to next report and thanx again! The pieces are coming together slowly.

    Scorpyd has a new 425 fps crossbow – now the fastest production model in the world. I like to think we can better that – for a non compounding model and without cams!

  2. Captn Harpoon says:

    In giving the test results some more thought, I am not altogether unsure the first five shots velocities would not be duplicated by leaving the string in its normal slot for all five shots, given the same time period in which they were shot. Degradation of velocity with rapid succession shooting is normal (or should be) with unrelaxed or stretched ropes (meaning they havent “settled” fully yet).

    Or, do the experiment backward, starting with string in the second (shorter position). Do the first two with the string in the second and shorter limbed position, and the rest with the string in its normal position.

    In any case velocities aside for now, there are other secondary considerations that are not insignificant and perhaps worthy of note.

    Without major reworking you have just increased the machine maximum capacity without further stress on the machine. I suspect you might have forgotten to take into account that the draw would also be two inches shorter?

    In the case that you did not miss that, there is also the consideration of total mass that must be accelerated and must play a part in the acceleration potental numbers and formula’s.

    Whatever extra mass the extra two inches of limb provided, must be added to the total mass that must be accelerated. I suspect this effeciency increase in projectile weight might tip the scales in favor of the shorter limbs slightly? (LOL). In effect, IMHO) this machine performs like one that is much larger.

    In addition to the bolt weight, I would hazard a guess that the extra two inches of extra limb length(hardwood) for both limbs at five ounces representing about 2200 grains for approximately a 30% increase.

    Bottom line is you can add a foot to those those stubby looking little darts you throw outta that machine and use a real harpoon and kick em out at about the same speed as your 7100 grain.

    Utilizing the extra 15 degrees by removing the curved brass stanchion inserts will give you the option of trading the extra velcity for KE by using an even heavier projectile while still maintaining current or same velocity.

    Using the combination of shorter limbs and the added rotation in the “velocity zone” should be a very good thing for Firefly when she undergoes metamorphesis once again.

    As always, look forward to next installment of shooting and may the CG’s be kind.

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