Lam Bow Perry Reflex Experiment #2 - 67" Red Oak, 50# @ 28"

[mmattockx]

The first try was documented in this thread:  https://www.tradgang.com/tgsmf/index.php?topic=174761.msg2948328#msg2948328

After much time spent with building a house, moving and other upheaval I have finally finished the second bow. This time I glued up the lams into essentially a fancy board and added handle pieces to the outside of that instead of trying to run the lams up the riser ramps. The bow was glued up in 2 steps. The first glue up was the core and belly lams glued together with ~6" of deflex:




The second glue up was the above assembly glued to the back lam while pulled into ~2.5" of reflex. This pic shows the first assembly sitting on the second glue up form, showing how far it has to be pulled into the form for the second glue up:




Now the finished trilam assembly (with a 0.100" thick power lam added in the riser area). You can see the final ~1/2" of reflex after coming off the form. The weird bump in the middle is the power lam:




The riser pieces before glue up:




The bow was designed using software and I did no tillering on this, just cut and sanded the back profile to shape and then worked it out to 28" on the tree. Here it is just short of full draw:




The initial 1/2" of reflex pulled out on the tree and it has taken another ~1/2" of set after several hundred arrows, which is pretty good for a 50# bow using a fairly mediocre bow wood. This was the point of the experiment and it has worked out pretty well, IMO. I know for sure I never could have made the same bow tillering from a board and not had 2+" of set in the end.

While it was pretty well balanced on the tree, the bottom limb has come up a bit weak and final tiller is a bit under -1/8", so I should have done a bit of adjustment on the top limb to really nail it. I wasn't too worried about that on this one as the experiment was the point, not absolute tillering perfection. As I learn and fine tune this method I will care more about the fine tweaks at the end.

The final product. It draws 50# @ 28" (maybe a touch less after final sanding, I never checked again), is 67" NTN and 2" wide. Tips are 5/16" for the last several inches. The tip overlays are walnut and maple. Finish is tung oil.
















Mark

[Flem]

So you got no reflex out of this?
Tiller looks pretty good, considering the amount of torture that wood has been thru

[mmattockx]

So you got no reflex out of this?
Tiller looks pretty good, considering the amount of torture that wood has been thru

There was only 1/2" of reflex (measured as nocks in front of the handle) after everything was glued up and off the form, but you have to remember that the first glue up was pulled forward from a heavily deflexed starting point into the form to glue the back on. Being my first try at this I didn't know how much spring back to account for, so I just picked some numbers and went with it as a blind starting point.

Looking at my notes, the first glue up had the nocks at 6.25" of deflex on the form. That glue up kept 4.75" of deflex off the form (it retained 76% of the form deflex) . The second glue up had the nocks with 2.5" of reflex (measured from the back of the bow) on the form and kept 1/2". That doesn't sound like much but you have to measure from the amount of deflex that the first glue up had to get the true numbers. Measured from the deflex starting point it was glued up with 7.25" of reflex and ended up with 5.25" for a total of 72% retained reflex.

Now that I have a test run under my belt I can adjust the deflex/reflex amounts for the glue ups and be more certain of my final result.

I was most happy that hardly any reflex pulled out. In total I lost about 1" of the 5.25" it came off the form with.


Mark

[Flem]

Ok, what am I missing here? It's supposed to be a reflexed bow according to the numbers, but the unbraced pic you posted looks like it has taken about 3/4" of "Set"?

[mmattockx]

Ok, what am I missing here? It's supposed to be a reflexed bow according to the numbers, but the unbraced pic you posted looks like it has taken about 3/4" of "Set"?

I see I did a poor job of explaining this.

Perry reflex works by preloading the belly surface into tension so that it sees less compression at full draw. What I did was pull the core and belly into deflex first and then pull that assembly forward to glue the back on. This gives me the preload on the belly of a bow that has 5.25" of Perry reflex but in a flat bow that doesn't have the high stresses of a bow that has the tips 5.25" forward of the handle. My goal was to have the tips 1.5" forward of the handle in the end, but I missed that because I didn't know how much spring back to account for. The next one should end up much closer to my target now that I have some experience with it.

Reducing the stresses in this fashion lets me get decent performance out of a lesser bow wood, which is what I am stuck with in my area.

Does that help?


Mark

[Roy from Pa]

Doesn't look too shabby, nice tiller job.

Only thing I would have done differently is cut the sight window out more.

Bow's with slight string follow shoot very smooth.

[Shredd]

Nice bow...  Agree with Roy on the window thing...

[Flem]

You did a dandy job of explaining. I built a "Perry reflex" bow many years ago. It was a lot of work for marginal results. Once it becomes a string follow bow, the internal preload is negated and the backing is essentially pre-stretched, loosing strength potential in tension.
I understand this is an experiment for you and I am assuming you posted it up here for critique
 

[Shredd]

  Curious....  Did you get better performance from this bow??

[Longcruise]

My criteria would be:  would the  bow have more string follow and therefore lose more performance if it were built with the same lams as a simple flat bow or even with a smaller reflex glued in all at once.

[mmattockx]

Only thing I would have done differently is cut the sight window out more.

That is something I will think about for the next one. I'm not totally happy with the riser shape and will be thinking about how to make it less bulky next time. A longer sight window might help with that.

You did a dandy job of explaining. I built a "Perry reflex" bow many years ago. It was a lot of work for marginal results. Once it becomes a string follow bow, the internal preload is negated and the backing is essentially pre-stretched, loosing strength potential in tension.
I understand this is an experiment for you and I am assuming you posted it up here for critique

The preload doesn't magically all go away at one spot, but you are correct that it loses preload as it takes set. This is why I started with a heavily deflexed core/belly lam glue up, to provide as much preload as possible to start with.

Yes, I posted it for others to look at and discuss. There are so many years of experience here that it would be stupid not to share and learn from the group.

  Curious....  Did you get better performance from this bow??

I don't have a chronograph, but I can say I wasn't planning on a 50# bow, I was aiming more for 43-45#. I am a touch thicker than the design thickness called out by my software but that change doesn't account for all the extra weight. That indicates to me that I gained some weight with the preloading in the glue ups. That is the other thing that Perry reflex should do, getting some extra work from the core of the bow.

The thickness thing is another lesson learned. I kept all my lams 0.001-0.002" thicker than the nominal design thinking I would need a bit extra for sanding, but that clearly was not required. From now on I am taking them right down to the design thickness.

My criteria would be:  would the  bow have more string follow and therefore lose more performance if it were built with the same lams as a simple flat bow or even with a smaller reflex glued in all at once.

This is exactly what my main criteria was and what the experiment was really all about. I can say with certainty there is no way I could have made a 2" wide 50# flat bow out of a red oak board without way more set than this has. I would expect at least 2" of set, maybe much more depending on the quality of the specific board.


While my initial goal was to be able to make decent bows out of marginal bow woods, this will eventually be extended to getting higher performance out of good bow woods. In the meantime I plan to keep using red oak (it's cheap and available) and make a static recurve version along with an R/D version and see how the technique transfers over to those design types.


Mark

[Flem]

Pretty sure its lost all it's preload when it has taken a set, well maybe not in the handle area.
I believe at that point what has been created is enhanced hysteresis at the juncture of the lams, which are not in a neutral, unstressed state.

[willi]

Nice work!

I don't have a chronograph, but I can say I wasn't planning on a 50# bow, I was aiming more for 43-45#. I am a touch thicker than the design thickness called out by my software but that change doesn't account for all the extra weight. That indicates to me that I gained some weight with the preloading in the glue ups. That is the other thing that Perry reflex should do, getting some extra work from the core of the bow.

The thickness thing is another lesson learned. I kept all my lams 0.001-0.002" thicker than the nominal design thinking I would need a bit extra for sanding, but that clearly was not required. From now on I am taking them right down to the design thickness.

I would agree that the touch of extra thickness most likely caused the reflex pullout you obtained. Getting a 12% improvement in draw weight without losing more to set, would seem to verify the perry effect.
Performance gains of this order of magnitude are not all that easy to obtain when designing this close to the working limit of the material, in my opinion.

Pretty sure its lost all it's preload when it has taken a set, well maybe not in the handle area.

Flem,
your thought about stretching out the back fibers may well prove to be the limitation for utilizing the principle of perry reflex. I would like to see some other species tested as backings. As we know, many woods have been compared as bow woods, primarily in compression.  Stretching the back out is a failure mode not often seen. What little was lost to pullout most likely was only a few FPS at most.

Thanks for the follow up, Mark

[Pat B]

 How's this, Mark?   
The tiller looks very good. I'm assuming this is all red oak, back, core and belly, is that right? Red oak is a marginal bow wood and especially as a belly wood. This could be why it took some set. Also, I think you may have over stressed the red oak at each stage of the glue up. Again, that being said, it is a well tillered bow. How does it shoot?
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[Mad Max]

 

[mmattockx]

How's this, Mark?   
The tiller looks very good. I'm assuming this is all red oak, back, core and belly, is that right? Red oak is a marginal bow wood and especially as a belly wood. This could be why it took some set. Also, I think you may have over stressed the red oak at each stage of the glue up. Again, that being said, it is a well tillered bow. How does it shoot?

Yes, all red oak. I used it specifically because it is a mediocre bow wood. The goal was to see if I could reduce the belly stresses enough to make it work well. In the end it didn't eliminate set, but it did reduce it significantly and that is what I was testing. After a few more experiments I will move on to using better woods to improve performance. The next bow up will be a recurve to see how this technique works there.

The oak should not have been over stressed during either glue up. The maximum strain seen during glue up was in the belly/core assembly when it was pulled into the form for the second glue up and that was a bit over half the strain the bow sees at brace.

It shoots very nicely with pretty close to zero hand shock (I'd say zero, but a more experienced archer may notice something I missed). The weight is a bit heavy for me so I may be missing some of the nuances of its behaviour in having to work harder than usual to draw and hold the weight but I have noticed nothing bad so far.

Overall I am happy with how it turned out (aside from the riser shape) and the information I gained from the experiment. There were a lot of firsts for me in this one and most of them worked out, which is all I figure I can ask for.


Mark

[Mad Max]

Hickory or Maple vertical grain backing and Osage belly should work good.
I would not use Oak anymore unless it was a board bow for fun.

[mmattockx]

Hickory or Maple vertical grain backing and Osage belly should work good.
I would not use Oak anymore unless it was a board bow for fun.

I would happily use osage but there isn't any available within 2000 miles of me for less than $$$. I have both hickory and maple in the shed but the oak doesn't need a backing and I'm saving them for use with better compression wood. The red oak is cheap and available locally so I use it for experiments where things may or may not work out. Once I have a better handle on this technique I will start thinking about spending the cash to get some osage to work with.


Mark

[Pat B]

Osage, mulberry, ipe or yew would be good belly woods.

[Stagmitis]

Hey mark i would love to see more experimention but I would highly suggest investing in a chrony-About $130 usd- My brain has been always been able to process up to a 5 fps difference in speed but still no substitute for a chrony. Nice to know exactly what the changes you make in design exactly account for-