Performance Evaluation - what are your thoughts?

[cunruhshoot]

Here is the situation...I recently built two 3 Piece Take-down longbows. Both from the same form - both 62". The first bow was for me at 57# @ 28" Red Elm core wood - Cocobolo veneers. The core wood was made up of two parallels and one taper lam.

The second was for my son 70# @ 28" Maple core wood -Grey action wood with some added Southern Red Cedar veneer on the belly. The core wood on this one came from Bingham and was the reverse taper layout. Ending up with Parallel limb design.

Ok now for the results. Today I borrowed a Chrono and my 57# bow is throwing a 630gn arrow 175 - 179 fps. This is where it gets interesting. We were expecting to see the 70# bow shoot the same arrow significantly faster. But to our dismay we were getting about the same fps out of the 70# bow with the same arrows.

Herein lies the question. Does the reverse taper layout create a slugish bow? What else would account for the lack of speed that we expected from the hgher poundage bow?

[savage1]

First I think there is a point of diminishing return and second I would look at the glass to wood ratio on the #70. Did you use any.050 glass?

Lou

[cunruhshoot]

I will check the wood to glass ratio - I believe we used .040 glass but I will see if I can figure it out.

[cunruhshoot]

I typically get my core wood from Troy but for this bow we just ordered the material from Bingham because my son wanted the grey action wood for the back of the bow. So this stack has one maple core and two grey action wood tapers reversed. And I believe the glass is .050 as best I can tell. So if that is the case the limb stack is about 30% glass.

I have not trapped the limbs yet on this one so that may help a bit.

On another note I am very excited to compare two other bows. I have a Tomahawk Diamond SS 55# @28 a 58" bow. I compared it to a 60" 50# @ 28" one piece Kenny M. bow design that I recently completed. I chrono'ed both bows using the same arrows and my 50# was matching the speed of the 55# Tomahawk.       It is just as smooth in the draw or smoother and equally as quiet as the Tomahawk.

[fujimo]

what is the mass of that laminated action wood per cubic foot compared to the other woods?

[mikkekeswick]

Yep my thoughts too.
Check the s.g. of all working woods. Elm is light 0.60 s.g. at a guess your actionwood will be higher.
If the bow is bending more near the fades (no taper #70) then yes it has a higher draw weight but it also has a lot of material that isn't storing much energy therefore it's dead weight in the worst place. This is assuming that the lighter bow is bending perfectly.
The best bow will store a lot of energy and have as  little weight as possible in the mid to outer limbs.
what weight are your arrows?
A golf ball can be thrown further than a pingpong ball.

[Bowjunkie]

Just curious... what weight are the arrows?

[cunruhshoot]

I am shooting 630gn arrows out of my 57# bow with 250gn field points. For the comparison we were shooting the same arrows out of the 70# bow.

[monterey]

The thing that confuses me is your reference to using the "reverse" tapers.  Are you saying you laid the bow up with the thin end of the tapers at the handle and the thick ends at the tips?

If so, then your limbs are working more in the insides and are too stiff towards the tips.  When the bow is drawn the outer limbs would be traveling way back and when released the inner limbs, in addition to propelling the string and arrow forward, they must propel the outer limbs further.  All that compounded by the extra weight of the outer limbs.  

That's all based on my interpretation of your lay up description.  If I'm reading it wrong, then disregard all of the above.  

[Dale in Pa]

I think every design has an optimum wood to glass ratio and taper rate.

When I built my first recurve I used all parallels and found it was noisier, had more stack and just didn't feel as smooth as using a taper of .002 or .003.

I just reread the original post and saw that you were referring to longbows. My statement above applies to them also. My hybrid design works best with .003 or .004 per inch taper.

[cunruhshoot]

Monterey - the way that Bingham lays out their limb design on their take-downs is one parallel lamination and two tapers - but the tapers are reversed...meaning one taper starts thick end at the butt end of the limb and the other taper starts thick end at the limb tip. You end up with a limb that has no taper.

I don't typically build my limbs in this fashion but since my son chose to buy his limb set up from Bingham we went with it. I don't plan on repeating this process with any of my bows that I build going forward.

[cunruhshoot]

Here is a picture of the 70# bow we are discussing at full draw...

 

[monterey]

Did you intend to build the bow to 70#?

[cunruhshoot]

Yes that was the desire of my son against my better judgement. He was committed to build a 70#er.

[Gundog68]

There are a lot of possible reason´s for this:
The 70# maybe do not loss speed with heavier arrows. It is always good to chrono arrows with 9, 10 and 11 gpp for compare.
Limb mass: parallel has a lot more limb mass to the end than tapers. I am always working on the best ratio with this on several bow forms. Such more mass can work as a lever, which is wanted with reverse tapers (see tip-wedges). But to much can be to much. If possible, i would taper the 70# bow. You will get less #, but better performance.

[bamboo]

like gundog68 says--you can lighten the outer limbs to get better performance-both trapping and tapering

[cunruhshoot]

Thanks for all the insight. I will to some trapping and tapering and get back to you with the results...

[LittleBen]

I tend to agree here. I think the issue is mass I the outer limb. A limb with all parallels (or reverse taper, which is essentially identical) needs to have narrower tips to have the same tip mass as the same bow build with tapered lams.

secondly, did one person shoot both bows? It's possible that the 70#er is not being drawn as far. 1" of draw will make a big difference. I'd try again with an arrow that's exactly 28" bop so you can confirm that the bow is at full draw.

Lastly, if you used thicker tip overlays to compensate for the higher weight, that also will obviously slow the bow down.

I assume that both bows are the same length and use the same string material and silencers etc.

I suspect it's a combination of shooting form and tip mass that's causing the issue. I know when I shoot 40-45, maybe even 50# I typically get 1-3fps variation between shots through a chrono,  but at 60-65# it could be 2-3x that cause my form is not as solid.

[cunruhshoot]

Thanks Ben - good insight. I was shooting both bows and attempting to pull back to the same draw each time but I was getting a variance in the readings as well - as much as 5 fps. Both bows are the same length and I twisted up both strings with d-97 and wool silencers.

So it makes sense to me that what you and others have pointed out as the issue is mass relating to the outer limb. So I will make some adjustments and see what happens.

My son loves the bow as is so I have to be careful to only make improvements and not mess it up for him. LOL