Bow Limb Collapse

[Si]

I am making a set of limbs for a takedown bow and when I did my initial stringing, the top limb was bent properly but the bottom limb collapses instead of staying bent. I know the limbs were made too heavy. I guess there is too much reflex in the limbs. I had this happen on a factory bow years ago. Any quick thoughts?

[onetone]

Are pictures possible? But aced and unbraced maybe? Photos help a bunch.

[4 point]

If I understand what your saying I’d guess you bottom limb is way to strong

[kennym]

I know on one of mine with quite a bit oof reflex, if the tiller is off and brace height too low, this can happen , so like Travis said, check out the tiller and try a bit more brace height.

[Si]

Here are photos of the bow:
1. unstrung
2. strung with upper limb collapsed
3. strung with lower limb collapsed
You can see it happens on both limbs( one strung and one collapsed)
Also, if you pull up this photo of a Habu bow you will see my profile is very similar.( https://habubows.com/vk_largephoto.htm) The lam recipe is the same as the recurve limbs I make for this bow which means I have to reduce the thickness considerably to get the same weight. My outside limbs are .040 and I have sanded them down by .010 and no change. I guess I need to make lighter limbs? There is a first time in 20years but I learn something every time I make a bow. Beats me.

[Mad Max]

Brace it Higher 8/9"
If one limb is a good bit stronger it can do that on some designs.
After you get the limbs tillered closer then go back to normal brace height

[Si]

Thanks, I will give it a try

[Crooked Stic]

That is a stability issue. Check the thickness and compare on each limb. If one is way thicker it may do that. I say the reflex starts too soon after the wedge.

[Si]

Thanks for the reply. I was worried about the limb design also. I took it from a pair of real nice ILF carbon limbs I have. Also, the ILF limbs are back-mounted vs mounting on the front of the riser. Live and learn.

[onetone]

Si - getting your limb design from carbon limbs might explain part of the situation, as carbon composite is much stiffer than glass. Also there is a lot of reflex in those limbs which can make for longitudinal instability and collapse when the bow is braced. Don’t know what you have done since first posting this thread, but I think you should try what Max suggested, use a shorter string, until the collapse goes away. Then check tiller and compare limb thickness and arcs on the tiller board when drawn. Go from there with corrective sanding if and where needed.

Bows with lotsa reflex do tend to have some end-to-end “wobbly wobbly” goin on, and with a particular string length tend to collapse, back and forth like this bow is doing. However, they also have a bunch of preload on the string, which with careful tillering, tweaking and correct string length can render a fine shooting, fast bow. So all may not be lost!

[Si]

Thanks for the reply. Everything you have pointed out are good comments and anything we learn day to day is helpful for the next experience. I put on a shorter string and braced it up real high and it still did it. One of the things I look at on a reflex/deflex bow is where the string sets above the riser when you run a string end-to-end with bow unstrung tip-to-tip above back of riser.  On a lot of bows, the string will touch the top of the riser or a little above like mine does. Even with me removing the material, the bow is still quite heavy so I know my formula is  wrong and my material is too weak. My formula is the same as a set of recurve limbs I made for the bow. They are: 2 .040 glass,.090 wood taper (.002 / inch), .032 uniweft, and a .040 parallel. Maybe a piece of carbon would help? In all my junk, I might have a riser to do a top mount and see what that would do. Anyway, I am going to make my mold more conservative and try again but not right away because I have a nice Yew stave to make an English longbow. Thanks again.
Si

[Roy from Pa]

Si, I sent you a PM.

[4 point]

Si, I would double check your stack thickness in both limbs. There’s something off. Doesn’t matter the design of the limbs if they’re equal it will brace even. Check your limb pad angle and limb wedge length and placement as well

[Si]

Thanks again guys for your ideas.

[Kirkll]

Question….. when it’s strung like that can your grab the string and pull it vertical up and  and have  the other limb flip over ?  If so, I think the shape of your limb has too much deflex sweep to it….. it’s called vertical instability.

Check this video out…. I made this for a buddy of mine years ago. Both Dave and the bowyer that built this,  who I’ll leave unmentioned, have passed away. So there is no harm showing it now.    Kirk

[Crooked Stic]

Dont you mean reflex sweep Kirk If one limb is not way thicker than the other I say a bit more deflex before the reflex.

[Kirkll]

Dont you mean reflex sweep Kirk If one limb is not way thicker than the other I say a bit more deflex before the reflex.

The terminology is kind of used loosely Mike. Sorry about that….

I’m referring to just the limb shape. The reflex in the tip, vs the deflex in the belly of the limb, or what I call deflex sweep.

What’s happening is …..the more bend you put in that unbraced limb, the stronger it gets. Theoretically this can provide much higher performance using less mass weight in the limb. And that theory proves true….The same concept was used in the ACX system OL AdCock came up with years ago. Only it was done in the cross section of the limb.

The down side to using limb shape geometry to reduce mass limb weight for better performance is the stability loss. There is a trade off there…. There are a lot of bowyers that still do this and use carbon to maintain torsional stability, but they are still floppy vertically. I guess some folks don’t mind having floppy limbs.


 I’ve had much better luck using a bit flatter limb pad angle with less bend in the mid section of the limb, and push the reflex portion towards the tips more using almost a flat section in the working limb area……. This is nothing new either….

 Take a  Look at the old “Severn Balistic” design that was built years ago…. Those bows were hot! And…. Rock solid.

I don’t believe there is a right or wrong way to do this stuff, but if you tinker with tweaking the limb shape long enough you’ll find something you like and run with it….  My .02 cents…

 Kirk

[Si]

Thanks again for all the input. Based on my gut feel and especially the video, I got a hunch if they were mounted on the back of a riser, they would still collapse. I received another good suggestion from another bowyer to make some quick 2x4 risers with different limb-pad angles and see if the collapse goes away. The first one I will make will be to simulate these limbs on an ILF riser and if it works, maybe I  could change the wedge to ILF but that won't be easy and probably a waste of time.

[Tim Finley]

I had the same thing happen on a bow I designed I solved the problem by using one reverse taper on one of the lams and got one of the best shooting bows I have ever shot very fast and smooth completely stabil .

[Crooked Stic]

So Tim your limb was parallel?