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Thread: Chrono scores

  1. #49
    It's an X I've taken part in an AIUK Ironman Shoot.The Fonz Award.AIUK subscriber. Timid Toad's Avatar
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    Yes, but unless you're going to allow for a particular limb - your judgement - needing a heavier arrow and then calculating that back somehow to equity with one happy with a lighter arrow you're in the same boat. It will always be subjective in one aspect or more, no matter how you frame your experiment.
    "Time is an illusion. Lunchtime doubly so." Douglas Adams

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  3. #50
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    HI Bolerus, Yes measure speed with a chrono. Use the speed to calculate the efficiency of the limbs.Input and output, as you say.
    I think the reasoning is that efficiency can be measured and it can then be used to compare limbs. I guess it removes the variables created by differences in archers form and their chin to eye distances.
    My next question is, can efficiency be connected in some way to extra speed? Does 10% extra efficiency equate to 10 fps extra speed? Or is there some other correlation that can be worked out if you know how???
    If that is possible, then the actual speed of the original set up doesn't need to be measured.

  4. #51
    It's an X Whitehart's Avatar
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    I can change the speed of a pair of limbs by just by having a less than efficient bracing height still "within the range"

    You can have two pairs of limbs with the same stored energy yet one will be faster than the other down to design, materials used aerodynamics and mass weight of the working part of the limb.

    At the end of the day it is the size of your groups that matter and as I have said before Dennis Parker had great groups at 70m with 28lb on her fingers were these super fast limbs that we have now lost the ability to make or just great form.

    The potential for miss information is vast and IMO this should be the caveat on any results as an archer could dismiss a pair of limbs to the detriment of their archery.

  5. #52
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    You can have two pairs of limbs with the same stored energy yet one will be faster than the other down to design, materials used aerodynamics and mass weight of the working part of the limb.
    I think that is one reason why Bolerus is asking about the data.
    I understand your concerns regarding archers and misinformation. I think there is a huge potential for adverts to give archers a wrong sense of importance.( arrow rests costing 200 for example)
    Faster limbs are not necessarily going to benefit an archer; but faster limbs could get an archer reaching a longer distance. That could be heavier or more efficient limbs. Shooting a longer distance could be just the confidence boost the archer needs. Equally, it could be the last straw, if the new draw weight is too high.
    It is difficult to control how an archer's progress is managed. Who they meet and take advice from is very important.

  6. #53
    In the Blue

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    Quote Originally Posted by geoffretired View Post
    This sounds like a good idea. Can I just ask, how easy is it to measure the output?
    I believe that input can be calculated, using data and an equation??? Is that right?
    I think output has to be measured, as opposed to calculated? Is that right?
    The limbs have to shoot an arrow and speed has to be measured??
    With Rik's ideas and those of Bolerus and 4d4m put together, would efficiency as a % bring about some figures that would be meaningful?
    Yes, by output energy I mean the kinetic energy imparted to the arrow, based on its velocity and mass. It's a simple equation ke = 0.5 * mv^2 (energy = mass times velocity squared divided by 2). If mass is in kg and velocity is metres per second, the energy figure is in Joules. SI is a great system for simplicity, although from 35 years of playing with airguns I'm used to the Imperial units of grains, feet-per-second and ft.lbs for ballistics.

    Input energy is based on the force transmitted to the limbs during the draw (work done), calculated by force times distance the force was exerted over. Limbs of a recurve bow increase the resistance with draw distance, so you can't just multiply the peak draw weight by the length of the power stroke. But with a draw force curve you can calculate the total work done (it's actually the area under the curve).

    Quote Originally Posted by geoffretired View Post
    Rik mentioned a loss of 9 to 10 fps is a drop below the boss at 70m. Can efficiency figures be used to indirectly calculate an increase in speed? For example. limb x is shooting below the gold at 70m. Limb y at same draw weight is 5% more efficient. Can that data be used to show a likely increase in speed?
    Yes, in theory. Output energy is alsways less than input energy, due to losses. If we can assume these losses are fairly constant proportionally we can calculate how much faster the arrow might be launched. The amount of losses depends on a host of factors including the archer, the arrow spine, contact, how well "tuned" (sub-optimal brace height = more vibration) But I'm speculating that for the same riser, archer and everything else bar the limbs, a reasonable comparitive figure should be possible.

    With lots of data it would be possible to identify trends.

  7. #54
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    This way, you can easily calculate/measure the energy stored in the limbs.
    But the amount of energy delivered to the arrow is a different thing. Materials and shape of the limbs play an important role.

    Historical examples are the Turkish bow, the Korean bow, the Manchu bow, and the ELB.
    While the former two had been verifiably shot with a gpp of 5 or even below, the latter two shatter at such low arrow weights.

    The bow design limits the maximal velocity with which the limbs can move.
    Below a certain gpp value, arrow energy will plateau, any extra energy remains in the bow, causing oscillation/handshock/breakage.
    Modern recurve limbs are usually operated near this plateau limit.

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