On width profile and tiller shape.

[RyanY]

Since I haven't been doing as much bow building, I've been spending more time thinking of design concepts. Finally got around to playing with David Dewey's bow design spreadsheet to test some ideas. Many of us understand that tiller shape and width profile should match each other in some way. In the simplest terms this is for even strain along the bow limb. Certainly there is a wide variety of tiller shapes that we see so the question is which one is correct? I trust that David's model in the spreadsheet is accurate based on the physics of a bow limb but that is an assumption I am making going forward.

Here I compared to width profiles and their corresponding tiller shapes. Both bows have even strain along the whole bow limb. One is an even thickness taper that projects to a roughly straight line taper to the tips. The second is a pretty standard ALB shape with parallel (as close as I could get) width to mid limb and then a straight width taper to the tips.

To my surprise, the tiller shapes are not as different as I have been suspecting. In fact the are very subtly different to the point that I would say we could tiller most of our bows to a circular shape and be very close to even strain. I might have to take back everything I've said about the tillering gizmo. I also adjusted the draw length to 35" to see the extreme end of what the shape would look like and still not much difference.

I think this has definitely changed how I think about tiller shapes and there is more to learn from here with other designs. Wondering where to go next. Would love to hear your thoughts, arguments, jokes about this.

[RyanY]

More pictures.

[simk]

Thank you for taking your time and posting this Ryan! That is very interesting. Indeed less difference in shape than one would think.
Watching this, I would say, my goal when tillering is not even strain over the whole limb but best energy storage witout too much set. And it looks like those two definitly don't go together. I just made a slim alb-profile tillered my way in a quite extreme way; it makes a 7,7gpp arrow travel over 200fps....this one has paralell limbs only the 1st third, then tapers straight to the tips. Its the upper one on the first pic, with slight reflex towards the outers.
chheers

[RyanY]

Simk, certainly there can be other goals in tillering but I think the width profile and strain have a stronger relationship. For that tiller shape you could have drastically narrowed mollegabet style outer limbs to get low mass for the same tiller shape without sacrificing risk for strain and achieving the same energy storage.

[simk]

thank you for critics Ryan  theoretically you are probably right. practically this bow is 33mm at the widest part and 70" long. slimming more towards the outers I would be afraid the bow becoming unstable with 52#@28"... but according design and performance this bow was an eye opener to me  the bend for sure could be spread more towards the outers withouht loosing drawweight. Ihmho it would loose speed doing that. Decided not to touch it anymore.

[RyanY]

Certainly wouldn’t make adjustments at this point. But if the limbs were a bit thicker that change could have been made early without taking more set. The outer half of the limbs store so little energy, the inner limb wouldn’t be much more strained.

I plugged some mollegabet width profiles into the spreadsheet and was amazed how much they bent if we really strained them. I think limb vibration and energy storage make up for performance increases with that style but I’d be interested to see how a molly with totally even strain would perform.

[simk]

Ryan, I'm sure there is much to be learned from playing with software like you do. If you share your findings about differences of bow designs like recurves, d/r's alb's, mollys and bendys It would sure be a very interesting read for many. I often wanted to do this but was always stuck with parcitcal experiments. From this particular bow I learnt so much, but some things I still do not understand. E.g. why does a bow so long perform so well with low arrowweights? I was surprised.
I don't understand your statment: "But if the limbs were a bit thicker that change could have been made early without taking more set. The outer half of the limbs store so little energy, the inner limb wouldn’t be much more strained". (this bow since glueup only lost 1/2" of reflex).
How would a molly tillered to even strain look?
cheers

[RyanY]

I just mean that if you were able to leave the outer limbs thicker so you could make them narrower, the tiller shape wouldn’t change much. The assumption could be made that stiffening the outer limbs more by doing that could put more strain on the inner limbs and result in more set. I think it would be mild enough that it shouldn’t make much of a difference.

That is an impressive amount of retained reflex. Definitely explains the good performance.
If I decide to crunch more numbers I can add it to this thread. The software is pretty complicated and I have a lot to learn. I have a reflex deflex bow in mind I want to plug into it. The evenly strained mollegabets with levers 3/4” wide tapering to 1/4” tips actually had more bend than the outer limbs of your bow!

[simk]

I now understand  I found optimized theory often ends in practical problems. Slimmer and thicker limbs are less stable than thinner and wider. I had overstrained bows where sideways stability on the outers became a serious issue despite good alignement. I'm happy with physical weight of the bow also...think 540 gramms is within tolerance.

looking forward to your findings 

Now lets hear what others say about your initial topic. Didn't want to hijack your thread with my bow 

[mmattockx]

Ryan,

It is hard to tell much from the graphics on the spreadsheet. They are too small and not scaled equally on the x and y axes, which distorts things somewhat. I have found it instructive to take the coordinates from the spreadsheet for full draw and plot them accurately in autocad. It shows smaller differences better and you can see a bit more of what is going on. No matter what you do, the differences between designs are quite subtle. Another thing to look at is the nocks and where they end up at full draw in different designs. There is noticeable difference on that between designs.

My experience with the spreadsheet shows it is quite accurate in predicting the bend. You might see a bit more if you model a traditional longbow as it should show a more pronounced elliptical tiller shape.


Mark

[Del the cat]

I've been saying this ad nauseum....
Del

[bownarra]

Yes Ryan the differences would be 'subtle' between those two bows. But the subtle difference is still there and important. My comparison would have been a true pyramid and an elb design. These would have a less subtle difference.
Elbs (my speciality after having made hundreds of them!!) if tillered circularly will have a 'thump' (due to extra tip weight being moved then stopped) and if the balance one limb to the other is off a little that thump becomes more pronounced handshock.  It holds the same for other bows, its possible to tell how a bow will feel when shot by seeing the full draw bend and the width profile....but this is a subjective thing. A bow with a little handshock could be described in a variety of ways depending on who shot it.....we have to remember that feel is subjective. However set patterns are not and will show the same 'issues' just in a different manner.
A few tests limbs would be able to show this satisfactorily.

[RyanY]

Bownarra, definitely agree that the difference has some meaning. I think the small difference still wouldn’t make up for differences in mass distribution causing hand shock. We would likely need a more pronounced elliptical shape for proportional mass movement in a parallel tapered flat bow. I can’t speak to ELB’s yet but I imagine their longer working limb/working handle section relates to part of a more pronounced curve for some reason.

There are definitely other reasons tiller shape should be different like mass distribution, limb vibration/working limb length, and energy storage. I chose to focus on strain because set and achieving low hysteresis is such a big factor. If we can achieve low/set hysteresis then I think those other factors are more easily modifiable.

I’ll add that this is probably most relevant for average flatbows that beginners want to survive with low set. It seems very common for many bowyers to have too much inner limb bend which is a worst case scenario for set and mass movement.

Mark, appreciate your input and experience with the sheet. It is a very interesting tool coming back to it after nearly 10 years. I’ll have to try that more traditional longbow to see. I think I have some width tapers from a warbow I made years ago I could go off of.

[Selfbowman]

I have no experience with any spreadsheets but have built a few bows not as many as bownara. But the parallel bows could have more mass at mid limb causing the bows to have uneven return.  I don’t have any proof of this. The more working reflex at the last 9” of travel of the limb will give it a whip type recovery. Basically a recurve response in limb action. The trick is to not add unneeded mass at this last nine inches. Still keeping that portion of the limb stable. Free of vibration. It would be interesting to me because that’s what I’ve been trying to achieve for some years now. I must be getting close because of the flight distances. Probably not clear to you guys but that’s my best explanation. Arvin

[RyanY]

Thanks for the insight Arvin. Your real world experience is invaluable. We need someone to start taking high speed footage of bows so we can get a better idea of limb dynamics, return speed/characteristics, limb vibration. I definitely agree with your thoughts on limb mass distribution. Do you have more pictures of your tiller shapes? I recall from one of your record breaking bows slightly less inner limb bend. I think there is a combination of low inner limb strain, very light outer limbs, and shorter working limb for low limb vibration as reasoning for the outstanding performance of that bow. Also a great set of fingers pulling back the arrow.