failure with a fiberglass tension bar loose laminate prod

This is more a question directed at kenh or anyone familiar with building a loose laminate bow from the fiberglass tension bars used for chain fences.

So basically, I was shooting with my new crossbow with said prod type I built from 1/2" tension bars, when I heard the sound of cracking near the prods after cocking the prod. I knew right away what was happening, and managed to decock the bow before anything exploded.

My prod consisted of three bars - 30", 26", and 22" in lengths. Inspection of the prod showed delamination cracks right on the center of the prod with the 30" and 26" bar. The last 22" bar appears fine. My powerstroke is 13" with a brace height of 1".

I know that for a bundle or laminate bow, the middle part of the prod is not suppose to bend to reduce strain at the middle. For my prod I know I did have a slight bend when cocked. Seeing how the cracks are isolated at the center, could it be that I was I putting too much strain there because the gap between my bars too close together? How can I prevent a second failure?

Pictures of the damage and the bow in question cocked: https://imgur.com/a/2WlMy

Hello War,I done the exact same thing . I firmly believe your length is at least 1" too long. Maybe more. Seems to me Ken is around eight " draw . I could be wrong . I broke the same setup  as you're shooting at 13". dropped down to 12", so far hasn't let go.I do believe 12  would be max though. For that matter, 11" would be safer still.The way the lames are held together has a lot to do with how it all bends.Also I've since heat shrink wrapped the whole bow in an attempt to make it bend more all over instead of just the center.

c sitas wrote:Hello War,I done the exact same thing . I firmly believe your length is at least 1" too long. Maybe more. Seems to me Ken is around eight " draw . I could be wrong . I broke the same setup  as you're shooting at 13". dropped down to 12", so far hasn't let go.I do believe 12  would be max though. For that matter, 11" would be safer still.The way the lames are held together has a lot to do with how it all bends.Also I've since heat shrink wrapped the whole bow in an attempt to make it bend more all over instead of just the center.

Thanks for the reply! I just realize with the way my nocks are attached I'm losing 1 inch of prod width on each side, making my prod a 28 inch prod :/. I'm thinking of making a wider replacement prod with a 32 inch/22 inch (the old surviving bar)/16 inch configuration. You think that would be enough to maintain my 13 inch power stroke?

War, that sounds a lot better.What is your fistmle,or brace height?

c sitas wrote:War, that sounds a lot better.What is your fistmle,or brace height?

It was 1 inch, with powerstroke measured from string to back of nut being 13", which I guess would be a 14" draw? I like using a low brace height since from what I read on the forum, it would give me the most KE out of my prod and I don't mind shooting heavy, slow bolts.

Just curious, what sort of bolt speeds are you getting with your tension bar loose laminates? I shot mine through a chrono with a 338 gr bolt, and it was only getting 124 fps max. That works out to a measly KE of around 16 J :/. I don't know the exact poundage of my prod (since my FiL took my luggage scale that I was using before ), but I know it's at least 60+ pounds. Can I assume that because my laminates were so close together that it resulted in heavy tips, hence nerfed my potential bolt speed?

I'm drawing 14" with  28"/18.5"/10" lath at 3" brace

I broke lots getting there!  Tight bindings holding the laminates together were much more important than I thought.

I'm getting 168fps with 355gn arrow for 22.25 ftlb but the 255gn gives more energy at 203fps... small pool of data though as the arrows kept getting more and more damaged.

Anatine Duo wrote:I'm drawing 14" with  28"/18.5"/10" lath at 3" brace

I broke lots getting there!  Tight bindings holding the laminates together were much more important than I thought.

I'm getting 168fps with 355gn arrow for 22.25 ftlb but the 255gn gives more energy at 203fps... small pool of data though as the arrows kept getting more and more damaged.

 
I checked out your thread a few days ago and it was then that I realize my prod was shooting way too slow . I don't have access to heat shrink wrapping, so I been cheating with a wrappings of duct tape (non-sticky side down to allow laminate movement). New prod I'm going to bind the laminates together with cotton twine, which I partially did for the old prod. It's surprising how tight a several turns of string binding can be.

Have you tried adding a fourth bar to up the poundage?

The laminates are supposed to slip against each other as the prod is cocked and released.  I suspect with a 13" draw that you are simply exceeding the material's structural capabilities.

I used non-stretchy string for bindings but fiberglass packing tape seemed to work.

It is in the plan to try 5 laminates for 125 lb.  I think your inner lams are too long.  BTW I was crunching some numbers and my tension bars are .250 to .251 thick.  The boat canopy bars are .246-.247.  It looks like the those 4 thousandths (1.6%) make almost 5% difference in allowable draw (using the 2 by 8 rule)

Might be worth getting precise measurements.

Guys, I'll tell you one way to know if your exceeding your bendabilty .After you have flexded your prod a few times, VERY carefully slide a finger back and forth on the back side of your prod. If you start to get a picky in your finger,QUIT, your bending toooo much and cracking the glass. That is also why I heat shrinked mine. The shrink also makes it look a lot more like a bow, instead of what it is.When you measure your draw, measure from the back face of the bow to the string catch. My fistmle is 2.25 inches .That is measured from the back face of the bow to the string edge.  I have had great luck using fiberglass fence posts. 3/8" by 48" long .cut as you like.A little bundle of 4 will top over 200#.Also  very light. Any farm store and most hardware stores have them . 1 buck apiece.I'm using three right now.

c sitas wrote:Guys, I'll tell you one way to know if your exceeding your bendabilty .After you have flexded your prod a few times, VERY carefully slide a finger back and forth on the back side of your prod. If you start to get a picky in your finger,QUIT, your bending toooo much and cracking the glass. That is also why I heat shrinked mine. The shrink also makes it look a lot more like a bow, instead of what it is.When you measure your draw, measure from the back face of the bow to the string catch. My fistmle is 2.25 inches .That is measured from the back face of the bow to the string edge.  I have had great luck using fiberglass fence posts. 3/8" by 48" long .cut as you like.A little bundle of 4 will top over 200#.Also  very light. Any farm store and most hardware stores have them . 1 buck apiece.I'm using three right now.

Will try!

Wish I can find these thicker and wider bars you guys are using :/. All I see at the hardware stores are the small 1/2" x 1/4".

@kenh - won't widening the prods increase the max safe draw?

Look on ebay for the bars. I just bought two --54" long, 7/8 by 1/2 for twenty bucks , shipped.  You know after I posted before ,it hit me that instead of using your fingers,take nylon stocking. Does the same thing ONLY it will feel better

The new replacement prod:



Going back with a measuring tape on the old prod, I realize I had my previous measurements wrong (I had made the prod a year ago, this crossbow had taken me a year to build ). The laminates were actually 32"/28"/24".... I can see how that would result in excessive strain on the middle of the prod and an especially slow bolt speed of 120 fps (not to mention the excessive recoil kick!).

The new prod is 32"/24"/14". I should have made the outer laminate a bit wider, but I only had a 46" bar at hand. The last prod lasted two dozen shots - I'm hoping the new one would last longer with its more balanced taper and curve, but if not, I guess I'll have to go with a wider prod (36"?) to keep my 13" (actually 13.5" :/ ) power stroke.