I really need some "mass principle" help

[Don W]

I need some "mass principle" help before i drive myself further into the ibis of insanity than I already wander.

A little background: I have about 10 self bows I've made in the past year and a half or so between 45 and 65 pounds I shoot regularly. But recently I made the mistake of buying a chronograph. All of the bows I've made chronograph right around 130fps. I've tried multiple arrows weights but the typical 10gpp is were I am at, but don't really see a substantial difference either.

So my curiosity went on a research bender and came to volume 4 of the bibles and "mass principle". Seems easy enough right. Just build the bow keeping the mass in mind.

My first attempt (elm)is now down to about 45#. Its 70 inches ntn and well under 1 1/2 wide, tillered decent, shots ok, it is 28 ozs and shooting 113fps. Getting it down to 14.5 ozes would leave me with kindling.

My second attempt (also elm) is my cry for help. It's still about 50# at 20" draw or so. It's 1 3/4" off the fades about half way and tapers to 1/2" at the tips, it's 3/4" thick off the fades  and tapers to a 1/2" thick at the tips.

Moisture content is just under 12% and both bends through the handle.

The second bow is just over 27 ozs and according to chart on page 92 of V-4 it should be about 14.5ozs. Now I know there is no way I'm going to loose almost 1/2 the mass of that bow and still have something to shoot.

I'm not doubting the principle, but my interpretation of it seems to be highly suspect, or I am totally missing something my head hasn't wrapped around yet.


Edit to add, My question wasn't so much about speed but weight. I understand my bows are overbuilt, the weight tells me that assuming the chart is correct, and I have to assume it is. I guess to simplify my question, or get back to basics, am I reading the chart right? Could a bow built like this really get down to 14.5 ozs?

[Digital Caveman]

You have fades on a bend through the handle bow
More dimensions would be appreciated, like the front profile, string follow, and length of the handle.
12 seems to be the damper end for elm.
What kind of string do you have?
No offence, but I considered my first to be 'decently tillered' to.

[Don W]

hopefully this is readable. Top number is width, bottom current thickness.

[RyanY]

Could be a number of things but based on your stats the moisture content is definitely high. 7-9% seems to be a good sweet spot for me. Your bows also sound overbuilt. For a bend through the handle bow of that length made of elm it could be quite a bit narrower. It’s possible your tiller doesn’t correspond to the profile or design. Photos and stats would be helpful. Also consider that the shooter is a huge factor for measuring speed. If you hold at full draw before releasing the arrow or short draw or don’t get a clean release, those can all result in lower speeds.

[willie]

maybe a little of all of the above things that have been pointed out, but can you confirm your drawlength when you are shooting thru the chrono? maybe have someone mark an arrow with a sharpie just before you release? have them make the mark at the back of the bow.

[Don W]

My question wasn't so much about speed but weight. I understand my bows are overbuilt, the weight tells me that If the chart is correct, and I have to assume it is.

If all my bows are shooting the same, I'm obviously doing something wrong consistently, but let's not think arrow speed but bow weight. It doesn't seem logical to me that I could get that bow down to 1/2 it's current mass and still be at 50# or even close.

I know most of the bows I've built were around 9% mc. The second floor of my shop keeps everything about 8-9%. These particular now were not stored there.

I guess to simplify my question, or get back to basics, am I reading the chart right? Could a bow built like this really get down to 14.5 ozs?

Jim Hann wrote in his book an elm now 67", 2" wide would be 45#. Are we saying this wouldn't be following the mass principle and wouldn't shot 140fps?

[Don W]

maybe a little of all of the above things that have been pointed out, but can you confirm your drawlength when you are shooting thru the chrono? maybe have someone mark an arrow with a sharpie just before you release? have them make the mark at the back of the bow.

I make my own arrows. My hunting arrows are 33" long. I've played around with a lot of different lengths and weights just playing around. If anything, I draw a tad over 28".

[RyanY]

In TBB Vol 1 I think they say a 66” bow that’s 2” wide to mid limb and tapering to the tips will pull a draw weight about the same as the woods‘a specific gravity. I have no doubt that bows can be built to those reported weights but, if my understanding is correct, Steve has since reported that he is getting higher numbers for mass depending on the design with equal or better performance due to lower set/hysteresis.

[willie]

maybe a little of all of the above things that have been pointed out, but can you confirm your drawlength when you are shooting thru the chrono? maybe have someone mark an arrow with a sharpie just before you release? have them make the mark at the back of the bow.

I make my own arrows. My hunting arrows are 33" long. I've played around with a lot of different lengths and weights just playing around. If anything, I draw a tad over 28".

I was asking if your form might be compromised when you are shooting thru the chrono.  if you are short drawing during the testing, low readings might suggest erronous assumptions about all your bows 

[Don W]

In TBB Vol 1 I think they say a 66” bow that’s 2” wide to mid limb and tapering to the tips will pull a draw weight about the same as the woods‘a specific gravity. I have no doubt that bows can be built to those reported weights but, if my understanding is correct, Steve has since reported that he is getting higher numbers for mass depending on the design with equal or better performance due to lower set/hysteresis.

I've got that written in my notes. That doesn't mention the weight of the bow or the speed of the arrow. I can't seem to find anything that correlates all that information together in a single flat bow design/build.

I've read a couple threads (like this one http://www.primitivearcher.com/smf/index.php/topic,17218.0.html)

[Selfbowman]

The moisture and length is making it heavy. Don’t worry about it not weighing more than the Bible says. It’s where the mass is in the bow that robs speed. Keep the tips and outer limbs light. Green wood is down doggy. Arvin

[RyanY]

Looking at your original question, I think you’d need to start narrower and thicker to get that low mass. It’s not as if you could remove mass from your finished bows and maintain draw weight. It’s that your current bows aren’t built optimally including having the lowest mass cross section possible to get there without taking much set.

[bradsmith2010]

sometimes if the bow is too long,, you have to cut it shorter and re tiller to get the mass down,,and keep the weight up,,

[scp]

130 FPS is not that bad. I am so terrible at shooting through the chrono by hands as to shoot about 20 FPS slower than using my crude homemade shooting machine.

Your bows are almost twice as heavy as expected. And you say they still shoot 130 fps. That's probably not impossible but rather suspicious. First, you need to check your scale.

Second, you need to check your humidity meter. Your bows might be even wetter than you think.

Third, you must be building your bows way too wider than necessary for the wood and design used.

You can put your bows in a heat box, or even heat treat them. You can also narrow them and spike them if necessary to keep the draw weight.  Good luck.

[Eric Krewson]

What is your full draw length? If you have a short draw length you can't expect to have stellar arrow speed from any bow, self or laminated.