Pushing the limits a bit

[Longcruise]

I'm taking this R/D down to a new low on width at the fades.  Just trying something new.

[Crooked Stic]

Narrow deep core works well.
I go 1.375 on R/Ds

[Jeff tech]

Go for it 👍

[Longcruise]

Started at  1.44, then went to 1.375 and called it good.  Then about the time the first finish coat was dry I thought "why not  get eve crazier an go even narrower.   It's tempting to go for 1.125 but just finished a couple hours of battling twist out of it.  Gonna call it done.

Rough shaped at 1.44 it was 51.2#.  At 1.375 it and with a serious trapping of the back it was 44.3#, now it will probably be 41 ish.  Going trap the back jus a bit  probably end at 39 ish.

Glad I didn't have a target weight. 

[Kirkll]

You can get away with the deep core narrow limbs to a point…but … once you pass that point it gets ugly real quick.

 My experiments with narrow profile have mostly resulted from missing draw weight really high, and figuring I got nothing to loose. Some of These crazy adventures have produced some seriously fast bows too.

One thing I did find out was that your torsional stability requires a certain amount of depth, or limb thickness, in the outer working portion of the limb. Narrowing the limb at the fades doesn’t have a serious effect on the limbs stability. This magic stack thickness is different for each limb design.

What I found helpful on those borderline draw weights between hunting and target. Say 32-42 @ 28” …. Is that changing your forward taper from a .004 to .003 will give you better stability. Or going from .003 down to .0025 FT can give you another .020 in thickness in the working section and add stability.

This is really important on recurve limbs. Especially target weight bows under 40# @28.  Same theory applies. On my static tip design, my magic # is .190 using an .001 FT.   Once I get under 40#,  I go to a par to keep it tracking straight, and under 35 # I use a revers taper and a longer working limb. This thickness trumps width 8:1.

My .02 worth…. 

Kirk

[Longcruise]

Thanks Kirk.  That's good stuff right there.

I shot this bow at 1.375" with a 520 +/-  44#.  No chrono numbers but it seemed a bit pokey.  Just shot it again at this new width (scaled 41.3#) and it seemed actually quite a bit faster.

So, narrowed at the fades, more side taper added (not a plan. The end result getting the twist out) with .003 taper and 4.25" tip wedge.  This seems to be a pretty good formula for this design.

I think on my wider versions I'm going to go to  .002 in the taper.  I think Kenny might have mentioned that he uses  .002.  Might be wrong on that 🤔

[Kirkll]

You got a photo of that bow unstrung?

I'm a bit curious on the limb geometry coming out of the form.

looking at the strung bow and the shape of the limbs at brace....... it  makes me want to ask about your brace height too.  I'm just curious....   Kirk

[Shredd]

 When I started making bows one of the first tools I purchased was a chronograph...  One of the most valuable tools I own when it comes to achieving performance...

  When I was a Greenhorn Kirk used to tell me, "Out of one form you could make 25 different kind of limbs"... I found this to be true...  First it takes a decent design... After that you may have 15 bows that are slow and a few that shoot like crap...  Make 5 that are moderately fast and 1 or 2 that shoot really well... About now you are zeroing in on a good recipe for that design.... 3 that are pretty fast and shoot good too... 2 that are very fast and shoot well also... 
     I learned a lot of tricks on slinging an arrow fast and look at me now... I'm still a Greenhorn...    So much to learn out there...

  Tip... If you change one thing and the bow shoots faster... It probably wasn't that one thing that made the bow faster... It was 1 to 4 things that were out of balance and the one thing you changed brought one or more of the others in better balance for better performance... 

   Just a few cents I'm tossing out there...

[Longcruise]

You got a photo of that bow unstrung?

I'm a bit curious on the limb geometry coming out of the form.

looking at the strung bow and the shape of the limbs at brace....... it  makes me want to ask about your brace height too.  I'm just curious....   Kirk

Brace is 8" from the back and 6 1/8" AMO.  Not what I'll be settling on in the end.  Just a handy string that was suitable to the build process.

[Kirkll]

You got a photo of that bow unstrung?

I'm a bit curious on the limb geometry coming out of the form.

looking at the strung bow and the shape of the limbs at brace....... it  makes me want to ask about your brace height too.  I'm just curious....   Kirk

Brace is 8" from the back and 6 1/8" AMO.  Not what I'll be settling on in the end.  Just a handy string that was suitable to the build process.

Thanks for the photo…. That explains the strung limb shape. I played with something similar in shape years ago when I was first getting started. As a matter of fact…. It was crooked stick that send me a pattern.  I built a few using that form  including my wife’s first long bow…. Then I moved on to a different shape that allowed a deeper core and better energy storage with much better vertical stability.   You can definitely squeeze some good numbers out of that design by shifting wedges around, but the ones I built had poor vertical stability I didn’t care for.     Kirk 

Here are a few pictures of that design. The photo of my wife is a full 32” draw. I had to have special long arrows built to put broad-heads those shafts. 

https://photos.app.goo.gl/8JyQW2uhFUpkLrh96

[Longcruise]

You got a photo of that bow unstrung?

I'm a bit curious on the limb geometry coming out of the form.

looking at the strung bow and the shape of the limbs at brace....... it  makes me want to ask about your brace height too.  I'm just curious....   Kirk

Brace is 8" from the back and 6 1/8" AMO.  Not what I'll be settling on in the end.  Just a handy string that was suitable to the build process.

Thanks for the photo…. That explains the strung limb shape. I played with something similar in shape years ago when I was first getting started. As a matter of fact…. It was crooked stick that send me a pattern.  I built a few using that form  including my wife’s first long bow…. Then I moved on to a different shape that allowed a deeper core and better energy storage with much better vertical stability.   You can definitely squeeze some good numbers out of that design by shifting wedges around, but the ones I built had poor vertical stability I didn’t care for.     Kirk 

Here are a few pictures of that design. The photo of my wife is a full 32” draw. I had to have special long arrows built to put broad-heads those shafts. 

https://photos.app.goo.gl/8JyQW2uhFUpkLrh96

How long was your bow?

[Crooked Stic]

I remember that design. Was not the best Mrs. Kirk made it look good tho. 

[Kirkll]

I remember that design. Was not the best Mrs. Kirk made it look good tho. 

Seeing this bow that Longcruise has going reminded me a lot of that old design we played with. Hell….. that was 15 years ago Mike…. Time flies…

That bow that Kat was drawing was 64” in length, and pretty light draw weight.
I think is close to 40 @ 32” . I shifted the wedges on her bow to get a longer draw and it did ok….. but that design is borderline vertically unstable. Too much reflex sweep in the outer portion of the limb.  I did figure out that going down to a .002 FT helped that design a lot.

I figured out that doing a more moderate R/D shape with a little more reflex pushed out closer to the tips works much better . It allows a deeper core in the working portion and totally eliminated the vertical stability issues…..

 the down side is that 62” is as short as I can go with this Flatliner design and still get it to 30” without stacking. I build a lot of 64” and 66” lengths that go to 32” draw.

Look at the shape of this limb design and keep in mind I use 8” tip wedges.

https://photos.app.goo.gl/xYs1QFHSwS6XwoBz7

https://photos.app.goo.gl/6chkz7wRRkjjq8zo9

Kirk

 

[Longcruise]

Kirk,  how do define vertical instability?  You've got me thinking about it and I need thoughts on how it manifests.

[Kirkll]

I’ve got a video I did years ago that I’ll see if I can find once I get out to the shop.
I’m drinking coffee in my bathrobe right now using my IPad.

But a simple explanation is…. If you grab the string and move it up and down vertically, the limbs move. In some cases a lot.

Or set the tip of the bow on the ground, and lightly bounce it…. The limbs flex too much with pressure vertically on the tips. They are floppy…..  can be damaged easily, and even throw a string occasionally if the tracking isn’t perfect.

The funny thing about these borderline unstable designs is they can be in the upper end of a performance test on the chronograph.

There was a software developed years ago by an engineer called “super tiller” that calculated energy storage capabilities on different geometrical limb shapes. The limb shape with highest energy storage capability was very similar to the design in question here, and considered vertically unstable by most experienced archers and bowyers alike.

Years ago Zipper Bows built a hybrid R/D design that was pretty hot on the chronograph, but had floppy limbs at brace. But …..he sold a lot of those because they were good shooting bows. They may even still produce that design. I don’t know…. I met the owner at ETAR about 10 years ago. He was a very nice guy.

Sooooo….. It’s not the end of the world if your bow is a bit floppy at the tips. That design can be a joy to shoot. It’s just a bit too fragile for my liking. Btw…. The heavier draw weights are much better in the same design. But this is true for stability on most limb designs too…..  the true test of your limb design will tell the story  rather quickly building 32- 35# bows.

When I get to building target weight bows like this I change my taper rate to keep them from going sideways to easily.    Kirk

[Shredd]

Kirk,  how do define vertical instability?  You've got me thinking about it and I need thoughts on how it manifests.

  How it Manifests.?? I would like to know some others view on this also... 

I believe Stic says, If you put too much reflex right off the fades it will cause instability... If my memory serves me correct too shallow of an angle coming off the fades with certain limb types can cause instability... Too long of a limb for certain designs can be too unstable... I took a 64" r/d with a decent amount of reflex out to 68" and it was highly unstable...

  My theory for vertical instability is when you have two limbs and they are pulling the string at a shallow angle it is easier for one limb to over power the other because of a mechanical advantage...  A tighter radius in your limb shape (agressive R/D) somewhat adds to a shallow limb angle to the string...

Brace height can be a factor also... Too little of a brace height can cause things to go floppy... But mainly because you have a floppy design to begin with and your limbs could be riding on that semi unstable/very unstable balance point...

   So if you are gonna have reflex in your bows it's probably important to know how much that you want to add and the angle of the limbs...  There is gonna be a balancing point... Tip the scales too much and you have got instability...

   If you are concerned about Performance in relation to Stability and Limb Angle off the Fades...  I have found with certain limb designs that there was no change in performance with changing the angle of your limbs...

[mmattockx]

There was a software developed years ago by an engineer called “super tiller” that calculated energy storage capabilities on different geometrical limb shapes.

Super Tiller is Alan Case's baby. I use it and a couple other pieces of software in my bow design work. It does much more than calculate energy storage for a given design and is very useful, but not very user friendly.

I believe this is the video about limb stability you mentioned:




Mark

[Kirkll]

Yup…. That is a super tiller design in the video too….. I know Alan fairly well, but we’ve never actually met. He’s been trying to get me into flight shooting for years, but my wife wants nothing to do with going to Bonnieville salt flats where they hold the national competition.

 last year I sent him a set of seriously heavy limbs to play with, and never heard whether he actually build a bow with them or not…..He is a seriously busy guy….

Kirk

[Longcruise]

Put the final finish on this little bow and did a check for the vertical stability and found it to be slightly "floppy", but not nearly as much as in the video that Mattock posted.  Still at 6.125 in BH (AMO) with a B55 12 strand string.  I didn't try it with a higher brace height but maybe tomorrow.

Definitely not a speed demon, but OTOH, it doesn't give me any stability sensations or problems when shooting it.  Room for improvement for sure.  One thing I'm satisfied with is that this design and be brought down to this 1.154 width without any problems.  It does have stabilcore under the belly glass.

---------------------------------------------------

6 1/8” BH, B55 12 strand string,
445 grain arrow = 10.18 GPP

Average of five shots.

1. 163.1
2. 165.7
3. 164.7
4. 163.4
5. 162.4
-----------
Total:   819.30 / 5 = 163.8

Dynamic efficiency = 61.82%

-------------------------------------------------

6 1/8' BH, B55 12 strand string,
538 grain arrow = 11.37 GPP

1. 151.7
2. 151.2
3. 153.1
4. 154.7
5. 148.3
----------
Total:   759.00 / 5 = 151.8

Dynamic efficiency = 66.27%

[Longcruise]

And the FDC