Request for those who are starting a new bow.

[Badger]

    Would like to get some feed back on this from those here with a chrono who are currently building a bow but have not yet drawn it past 24".

  Here is the experiment.  When you get to 24" draw but have never passed it  please take your current draw weight multiply by 10  say 44X10=440  then multiply that by 24 and then divide by 28.  In this case it would be 377 grains. So please shoot an arrow of the right weight through the chrono at 24" and then when you finally get to 28" shoot another arrow through the chrono at 10 grains per pound. Last request, go back to 24" recheck your weight and recalibrate your arrow weight accordingly and shoot through the chrono again and please report to us all 3 speeds. I already know how it works out for me but I would like for a few others to report on how it works out for them.

  Theoretically if the bow takes no set all 3 shots should be about the same.

[sleek]

This should be fun, and I agree with your conclusion.

[Badger]

  Here is what is interesting about this test. best case scenario all 3 shots the same. If the 28" draw ever exceeds the 24" draw it would invalidate the test for any number of reasons which could also include invalidating the formula.

  My prediction is this. First 24" shot will be the fastest in all cases. 2nd 24" shot will be the second fastest and 28" shot will always be the slowest. The 28" shot and the second 24" shot should be pretty close, if they are too far apart it would indicate a bad test for any number of reasons. Aside for the point I am trying to make this little test really drives home how important it is to avoid set.

[sleek]

As an added thought, the difference in energy can be calculated at the 24 inch draw before and after taking it to 28, and energy loss can be actually quantified and seen. Then you can also see the loss in efficiency, I love it.

[Badger]

As an added thought, the difference in energy can be calculated at the 24 inch draw before and after taking it to 28, and energy loss can be actually quantified and seen. Then you can also see the loss in efficiency, I love it.

  Sleek I think that is what I defined in the test. If not I may need to rewrite it. I have done this test myself countless times so I am very familiar with how it plays out.

[DC]

Will do. I've got a 68" Saskatoon(Serviceberry) close to that now.

[Badger]

Will do. I've got a 68" Saskatoon(Serviceberry) close to that now.

  Appreciate!!

[StickMark]

I will try to borrow a friend's chrono.  This and the 10gpp rule is fun to think and play with.  In 2012, buying a hickory bard, never thought I would have so much sheer fun doing this passion.

[Badger]

  I do my formula just slightly different than Sleeks. I use it as a percentage of the power stroke. So if you test the bow at 24"  you would use 8.2 grains per pound of draw weight.

[leonwood]

My osage is at 50@20 and has not been pulled further than that, will not be able to work on it for the next ten days so if you are not in a hurry I can do it when I continue working it

[DC]

I just realised that I can't draw 28". At least not accurately enough to get chrono results. This is the motivation I need to build a shooting machine but you'll have to wait a bit longer. But with better results.

[Badger]

My osage is at 50@20 and has not been pulled further than that, will not be able to work on it for the next ten days so if you are not in a hurry I can do it when I continue working it

  Thanks Leon, hopefully it will just filter in.

[Badger]

I just realised that I can't draw 28". At least not accurately enough to get chrono results. This is the motivation I need to build a shooting machine but you'll have to wait a bit longer. But with better results.

  DC, take your first reading at 22" or so if you haven't passed that yet. Use an arrow for 22" that is 7.2 grains per pound or sleeks method 7.8 grains per pound. Maybe compromise at 7.5.

[avcase]

Steve,
I am trying to understand why you state that a bow’s performance would fall off beyond a 24” draw length?  I could understand that this would be true if the bow is designed to the material limits for a 24” draw, but then drawn beyond this. It just means the bow design is not optimal for the longer draw length, correct?

There is something else that could come into play which may reduce efficiency which is not due to overstressing the materials. If the bow draw weight is light and the draw length is long, then this requires more working limb length. This reduces a bow limbs secondary natural frequencies which can rob efficiency.  It is a similar issue low draw weight horn bows have compared to high draw weight horn bows.

Yet another reason this can occur is a shorter draw length has a proportionally better force-draw energy storage since it is less likely to stay out of the stack-zone, and the hump in the beginning of the force-draw curve takes up a bigger proportion of the whole.

The results of this test are also swayed heavily based on how the test arrow mass is adjusted for draw length too.

Alan

[Badger]

  Allen, that is exactly right. In all most all cases the bow will show at a very minimum some signs of overstress usually past 26" on a well designed bow. I have yet to see one that showed zero sign of stress. But I haven't tested all of them obviously.

  Allen, how would you adjust for shorter draw. When I firt started doing it I based it on stored energy that I actually measured. Too much trouble so I switched to a method just using an equal percentage of power stroke ratio. I don't think any method is perfect and that is why I am against using it trying to level the playing field. I don't think it can be done with any real accuracy.