Thickness vs width questions

[RyanY]

Years ago I made a 2.5” wide poplar bow and even that was tough being a soft wood. I need a custom 3” wide spokeshave for the super wide ones.

[mmattockx]

I am wondering if there are two advantages to wide thin bows. The first being that thin limbs will be under less strain and are less likely to take set. The second being that a higher percentage of working wood is closer to the surface with less non working wood in the middle/neutral plane of the limb possibly leading to lower mass.

In a rectangular cross section limb the percentage thickness that is contributing the most work is always the same. The gradient of stress/strain is a straight line in the rectangular cross section, from zero on the neutral axis to maximum at the surface of the limb.

I am going to tinker a bit with your idea about the wider limb having more wood at a higher strain level, there could be something to that.


Mark

[Selfbowman]

I should add Steve’s thinking (if I remember correctly) that even in low set bows there is the possibility of damaged wood that isn’t contributing to work but adds mass. Making a bow extra wide may help avoid this due to very low strain. The big issue is no one wants to make 3” wide bows. 
[/quote]
I’ll make them wide if it brings up the performance. That Osage bow that holds the fifty pound broadhead record was 2-3/8 wide at the fades but it was as a pyramid close to perfect diminishing mass. Where the mass is that’s the key. The less of it the more efficient the bow. Then set comes into play. Dead wood is dead mass. If your building replica bows none of this matters. If your building for speed and cast it all matters. Arvin

[RyanY]

Mark, I get that the relative percentage is the same. My mind goes to the surface area of wood that is doing work. My thinking is probably flawed but it seems like for a narrow/thick limb, there is more wood between the surfaces doing the work than in a wide/think. Saying it out loud I’m starting to think it doesn’t make sense.

Edit: I guess I’m thinking in absolute terms where if we say the back of the bow is 1/32” that’s doing the work, that would be a greater percentage of wood in a wide thin limb compared to a thick narrow limb.

Arvin, I appreciate your thoughts and real world experience. That width in Osage would equate to a 3+” wide for most other woods is my thinking.

[Selfbowman]

Marc was the bow probably over built on the inner limbs ? I would say yes but some how it worked. Mass on the inners must not be as critical.

[mmattockx]

Marc was the bow probably over built on the inner limbs ? I would say yes but some how it worked. Mass on the inners must not be as critical.

It probably was overbuilt for osage, but the weight penalty is minimal near the handle because the inner limbs hardly move. All the losses happen farther out where the speeds and distance moved are higher.


Mark

[Allyn T]

People say overbuilt a lot on here but in Arvin's case it clearly wasn't seeing how well the bow performed. It seems like if someone makes a bow that takes no set it's considered exceptional wood or tillering or both. On the other hand if it was made extra wide people say it's overbuilt. I'm starting to lean towards the idea of it doesn't matter how wide it is as long as it shoots hard.

[RyanY]

Could have been overbuilt in the inner limbs without being so in the mid/outer limbs. I recall one of Arvin’s record breaking Osage bows did break in the mid/outer limb but he can correct me on that.

If you subscribe to Klopsteg’s ideas on a convex limb width taper for an even thickness bow with even strain along the limb and circular tiller, pyramid bows would most often be overbuilt in the inner and outer limb and underbuilt mid limb where the straight taper would match a convex taper causing an area of higher strain. I’ve been wondering lately if this is a pattern we see in pyramid bow breaks. Still working on my conceptualization of this.

[mmattockx]

If you subscribe to Klopsteg’s ideas on a convex limb width taper for an even thickness bow with even strain along the limb and circular tiller, pyramid bows would most often be overbuilt in the inner and outer limb and underbuilt mid limb where the straight taper would match a convex taper causing an area of higher strain.

Any links to Klopsteg's stuff?

Regarding convex limb width taper, David Dewey's spreadsheet outputs this shape for a constant thickness limb that tapers to a point at the nock, which is the classic theoretical pyramid design. If you analyze a straight taper pyramid on the spreadsheet it does indeed give higher strains in the mid-limb area. This correlated well with a straight taper maple pyramid bow I did last year (designed on the spreadsheet) where much of the set happened in the same mid-limb area.

My current bow project is a 3 lam red oak bow using the convex shape directly from Dewey's spreadsheet. It is tillered and ready for some test shooting but I have been swamped with a house move and setting up my shop before winter arrives tomorrow afternoon so it remains untested at this point.

Picture of the back shape after being cut on the bandsaw:



Drawn to near full draw on the tree:



Mark

[RyanY]

Mark, I don’t think there’s anything online yet but I think archerylibrary.com is working on uploading archery the technical side chapters. I’ll PM you.

[mmattockx]

I found this a while ago and downloaded some of Hickman's work:

https://www.archerylibrary.com/books/hickman/archery-the-technical-side/


Mark

[Gimlis Ghost]

Just found this and thought it might be of interest.
The first bow displayed is an Iroquois War Bow. Very wide limbs. Ash and draw weight of 80#.
Very powerful and impressive bow not considered suited to hunting because of over penetration on creatures such as deer.
A true war bow intended to penetrate heavy clothing and light armor at long range.
https://www.youtube.com/watch?v=5CWCSpAlick

[bownarra]

Bottom limb looks hinged? It looks like it could lose quite some weight off those mid/outer limbs

[PatM]

That video crammed a lot of questionable facts in  a fairly short period. Delicate squirrels being one of the most glaring. 

[Don W]

I haven't watched the video yet, but "over penetration"