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 Post subject: recoil pad wormhole - a technical perspective
PostPosted: Wed Apr 02, 2014 9:00 am 
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not sure this is the right place to post. I've been on a quest to find a comfortable recoil pad for my 12ga O/U. I tried a few pads with mixed results. In my search I've found a whole lot of opinions and not much data, so I decided I'd take the pads I have sitting around and do some measurements. Below is a graph comparing static pad stiffness of the following:
    1/ KickEez trap style 15/16" medium, part number 401-8-LB
    2/ Pachmayr D752B 1" medium, part number 10408
    3/ Limbsaver slip-on, part number 10545
    4/ factory pad that came on a 2012 CZ Canvasback, believed to be Pachmayr SC100 0.6" medium

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Details of measurement technique and further info can be found here: viewtopic.php?p=3164798#p3164798

I hope this data can help others make an informed decision about recoil pads. I'll update the graphs as I try more pads and collect more data. Stay tuned to the thread above for updates.

As always, questions/comments appreciated.




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 Post subject: Re: recoil pad wormhole - a technical perspective
PostPosted: Wed Apr 02, 2014 4:08 pm 
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What if your shoulder isn't shaped like that block? Seriously, I would like to see the results using a block bigger than the pad positioned longitudinally. I guess that may partly explain why I like Pachmyr. Heck, I even like the old factory Remington pads. I do not like my gun to move a lot with heavy loads, just need something not sharp edged and rock hard.

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 Post subject: Re: recoil pad wormhole - a technical perspective
PostPosted: Wed Apr 02, 2014 7:23 pm 
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That was exactly my first thought. I tried that too but found it was difficult to differentiate pad characteristics vs. contact area. There's no way I could measure the contact area during compression and convert it back into psi for fair and relative comparisons.

Every pad and gun manufacturer has a different idea what your shoulder pocket shape is, so what curve radius should I use? What is the shape of my shoulder pocket vs. someone else?

First I tried a relatively flat block (actually matched to the length, width and curve on the original CZ Canvasback stock before I cut it to change pitch), which was fine for the D752B and the Limbsaver, but on the SC100 and KickEez it gave skewed results. The ends of the SC100 are really stiff, the heel is made from hard plastic. The deflection was 0.032" with 30lbf applied load. The middle of the KickEez had a tighter radius so only the inside corner of the flat ends contacted the block. The deflection was 0.084" for 30lbf. For comparison, D752B and Limbsaver were 0.054" and 0.128" respectively for 30lbf applied load. But the KickEez doesn't feel significantly softer than the Pachmayr, something was amiss...

That doesn't seem like a fair comparison, so I cut a block that matched the width and curve of the KickEez, a much tighter radius than the factory stock on the CZ. The block made full contact throughout the curve, and was not contacting the flat ends of the KickEez, which is probably how it mounts to your shoulder. Here's the deflection of the 4pads with that block design.
    KickEez: 0.059", 30lbf
    SC100: 0.069", 30lbf
    D752B: 0.100", 30lbf
    Limbsaver: 0.224", 30lbf

Whadda ya know, that's the same relative order of stiffness as the flat block across the middle! Plus, I can only realistically drive 50lbf into the block due to limitations of the test setup (it's my puny arms providing the force, and the force gauge only goes to 50lbf), and 50lbf into a large surface area is far away from the contact pressure the pad experiences during recoil. Thus, I made the decision to use that 1.535" block as the control - it's consistent, repeatable, and a little more representative of the contact pressure during recoil.

BTW this property, force vs. deflection is exactly how Sorbothane engineers want you to input data for analysis of performance in the application. I couldn't get to 20% deflection (0.188" for this 15/16" pad) with 50lbf. I could do it with a smaller surface area, but then I'm more sensitive to internal geometric structure variation of the pads - stiffness would vary significantly if I move the pad a little - less averaging effect.
Sorbothane design guide wrote:
Load the given shape to a 20% deflection. Measure the static deflection (dst) and record the load (W) at this deflection.
Source: http://www.sorbothane.com/Sorbothane-De ... 101409.pdf

I can go back and repeat the test for any width, curvature and length of contact one could think of - anyone have an idea what a proper block should look like? Wish I had access to a tensile test machine (like an Instron)...

I can upload pics of the other block-to-pad interfaces and resultant stiffness graphs later if you're curious.

Glad this thread is generating some discussion. I would hope we can eventually talk about recoils pads in concrete terms vs. speculations, sales slogans, and personal opinions (although those are valuable too!). I was tired of shelling out cash for pads that had (IMHO) less than desirable performance.


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 Post subject: Re: recoil pad wormhole - a technical perspective
PostPosted: Wed Apr 02, 2014 7:37 pm 
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Good try, but this discounts a lot of factors.

Some pads seem to be great at first, but degrade relatively quickly. Some get rather hard quickly; some get sticky and gooey.

Others "feel good"; but don't seem to attenuate recoil or lead to more cheek slap.

Like the poster above; in prefer the Pachmayrs- specifically the SC100.

Others I've liked are the Remington Supercell and the new Browning Inflex pads.


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 Post subject: Re: recoil pad wormhole - a technical perspective
PostPosted: Thu Apr 03, 2014 11:42 am 
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that's fair, it does discount a lot of factors. It was an attempt a performance measurement only, and only one aspect of performance - deflection under load, i.e. stiffness/softness. I think this one data point is better than nothing right?

Pretty clear it's by no means a durability test - I have no personal experience how these pads respond to chemicals, UV exposure, heat/cold, ozone, cyclic or dynamic loads, abrasion, etc and I did not test this.

Maybe someone has some of that info to contribute and we can all benefit? Would love to hear experiences with long term usage. I have heard from many people that Pachmayr pads are particularly durable...


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 Post subject: Re: recoil pad wormhole - a technical perspective
PostPosted: Wed Dec 17, 2014 2:04 pm 
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I thought I'd take up the recoil pad topic once again. I never felt like I did it sufficient justice.

The recoil pad wormhole part2
For part1, see this post

Recoil pads, general principles:
The kinetic energy of the projectile and expelled gas creates an equal and opposite reaction on the gun. This energy is absorbed by the gun and the human who started the chain reaction. Part of the kinetic energy is used to accelerate the gun away from the projectile and the remaining kinetic energy, sometimes called recoil energy (RE), must be dispersed and absorbed by the recoil pad and human. This principle is the basis for the second rule of shotguns: the heavier the gun, the less RE the human has to deal with. The first rule is the lower the energy of the projectile, the less generated kinetic energy - hence 3/4oz loads @ 1150fps, etc.

The recoil pad does not magically absorb all the RE, it mostly translates the sharp, fast, high amplitude peak into a lower amplitude, but spread out (i.e. longer) pulse. A good pad will also absorb _some_ of the energy and generate a little tiny bit of heat by means of a process/trait called hysteresis. In general, the longer this energy is spread out, the lower the perceived recoil. I.e. thicker pads can spread the pulse out over longer distances and time, thus they can be softer. Both these factors make thicker pads generally more comfortable.

All recoil pads have some amount of hysteresis, some more than others. This is mostly a function of the rubber material used (and sometimes how air travels in/out of the pad). Hysteresis allows the pad to absorb and dissipate some RE and slowly return the gun to the resting position. It is a necessary and desirable feature.

Pad design strategies:
I've found recoil pads fall roughly into the following self-named categories. Some pads employ multiple strategies.
1/ NikeAir
2/ Column Buckling
3/ Rubber Foam
4/ Rubber Ball (with large hysteresis)

NikeAir refers to the recoil pad's internal structure, which is mostly hollow and sealed. You know those really comfy running shoes you love? yup, same idea here. Against loading, the recoil pad offers resistance in the form of compressing the captured air inside the pad.
    Examples include: Browning Inflex, Limbsaver

Column buckling is a commonly employed technique. It was first mathematically described (http://en.wikipedia.org/wiki/Buckling) by Euler in 1757. Rubber columns inside the pad elastically deform under the load thus spreading the time and distance of the energy pulse. Rubber and other elastomers are perfect for this because they can withstand massive deformation and return to their original shape. You could make a pad with steel or aluminum columns with the same stiffness and deformation characteristics, but one shot and it would be flat as a pancake never to return to its original shape.
    Examples include: Pachmayr (most of them)

Rubber foam is a specialized version of NikeAir and column buckling and really deserves its own category. Foam is really a collection of thin elastomer walls or cells in a low density random arrangement. Basically foam has millions of elastomer columns. It's low density because the empty space inside and around the foam elastomer is air (or other gas). To absorb energy, the cell walls within the foam buckle/collapse and the air within the cells is compressed (if closed cell) or dispersed (if open cell foam).
http://epubs.surrey.ac.uk/56944/2/Foam_SRI%20.pdf
http://www.ergaerospace.com/Energy-Absorbtion.html
    Examples include: Isis, Remington Supercell

Rubber ball refers to a solid homogeneous elastomer acting as a spring to gradually slow down the gun. A typical rubber ball will compress and store the energy and return nearly 100% of it, accelerating the gun in the opposite direction just like a superball bounce! This is not desirable trait, the pad should absorb some of the energy - i.e. you want some hysteresis. Sorbothane used in KickEez pads has quite a lot of hysteresis by design.
Here's a picture of what Sorbothane material does. What's interesting is the loading curve shows a straight line - i.e. the viscoelastic properties do not help to make the pad softer, rather they return the gun slowly to the resting position after deflection. In my experience, the slope of this loading line is too steep to be comfortable...
Image


Enough of the background and definitions, let's look at some performance curves. I used the same measurement technique as before. Because of limitations in test equipment, I can only repeatably measure the _decompression_ curve. Looking at the image above, it means the curved part as the load is reduced and the pad returns to rest. I cannot measure the compression curve, which should be a straight-ish line for these pads. Anecdotal evidence suggests the pads which exhibit large hysteresis seem to have a non-linear compression curve as well.

All measurements are quasi-static. All data below has been converted from absolute pounds force into psi, which eliminates the variation in pad width. I put the starting flag and stop sign on the left of Y-axis to show the starting and ending pressure (i.e. unloading).


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 Post subject: Re: recoil pad wormhole - a technical perspective
PostPosted: Wed Dec 17, 2014 2:05 pm 
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The more gentle the slope of the curve, the softer the pad.

Explanation of the results:
Pachmayr F325 came off my buddy's 1969 Browning A5 (a terrific gun). I'm pretty sure the pad dates to 1969, the rubber is as hard as a rock and it's labeled as a Browning pad. The F325 pad itself should be fairly soft with it's 30deg column buckling design. I'm sure a new pad would perform similar to the Limbsaver 10542 results. Here is a pic of the F325 which clearly show the open column buckling design.
Image

Moving to the right, next up is the KickEez 401-8-L. This pad really surprised me. It was significantly harder than my buddy's same KickEez on his Browning 725 Sporting. I understand KickEez recently changed the formulation of the Sorbothane material which retained the hysteresis but increased the durometer. It's noticeable. Further, this pad is very dense and heavy, depending on where you like the mass on the gun this is a good or bad thing. Not much to see in the pics or the cross section. It's a solid hunk of specialized visco-elastic rubber. I don't get the hype, I don't think this pad is all that.
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next is the CZ OEM pad. This is basically a 0.6" Pachmayr SC100 pad....
Image

...which is similar in design to the Pachmayr D752B. I tested 2 different D752B's, one I bought of ebay (which I'm pretty sure was NOS, rather hard), and the other from my buddies SKB Weatherby Orion which was really supple. First pics and cross section
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This pad design is neat. It's made from neoprene so really tough and abrasion resistant yet still soft. It has two different air chambers in the middle and solid rubber on either end by the screw holes. As a result, this pad is noticeably and measurably softer in the middle (see top vs. center measurements). There's a co-molded metal plate to provide support to the sidewalls and screws. The thin ribs do little to support, but the middle rib does act as a buckling column. If this pad is installed over an open ended stock it will feel slightly softer than if installed over a flat even surface as the air will be compressed in the two chambers if there's no way for it to escape.

Notice the D752B-L-1 is softer than the D752B-M-1. Same construction, but the air channels are smaller on the M pad. As a result, this pad will feel stiffer than an equivalent sized L pad. Thus, if you want a softer D752B, get the larger pad and grind the sidewalls thinner. Riflemeister taught me this trick.


Next is the Remington SuperCell 19483. Not much to see, it's a solid piece of closed cell foam, and is very light and low density. It is one of the softest pads I tested and imagine is very comfortable. What's interesting is the shape of the curve. It doesn't have the curved unloading profile which is characteristic of high hysteresis materials, it's rather linear, which means the foam is acting more like a pure spring. I had to measure it 3 times just to make sure I didn't screw up, but each time it was repeatable behavior.
Image



Next up is the Limbsaver grind-to-fit 10542. I have a lot of miles with this pad and love it. I can't get a cross-section without destroying it, but by poking around the internals feel similar to the construction of the slip-on Limbsaver 10547.
Now this pad is really interesting. The outer wall is thinner than the D752B, coupled with the lower durometer it makes the interface to the shoulder compliant. Inside there is a series of ribs and columns (literally columns). There is a longitudinal stepped rib and a mid lateral rib. The columns in the top air chamber come right to the stock interface surface. The columns and rib in only the bottom air chamber stop short about 0.225" below the surface. This is really clever as it creates uneven stiffness from top to bottom the net result being a moment which wants to push the muzzle down, thus counteracting muzzle rise! Smart.
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My offer still stands, if anyone wants me to measure performance of some different pads PM me and we'll chat.

sorry for the long post, I hope it dispels some misconceptions and inaccuracies about recoil pad design and will help make you a more informed consumer.


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 Post subject: Re: recoil pad wormhole - a technical perspective
PostPosted: Wed Dec 17, 2014 3:04 pm 
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Great post!

I'm still absorbing it all (pun intended!)

I love the Pachmayr SC100's. I wish Remington offered aftermarket Supercells for Brownings and Berettas. Don't think that's going to happen though. I'm thinking about ordering one for my 1981 mint Remington 700 BDL .30-06. Although I've seen a lot of reviews that the fit is not always great on older models.

So far I like the new Browning Inflex pads on the 725 and new A-5. It will be interesting to see how they hold up over time.


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 Post subject: Re: recoil pad wormhole - a technical perspective
PostPosted: Mon Aug 31, 2020 8:49 am 
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Posts: 18
IheartShotguns wrote:
I thought I'd take up the recoil pad topic once again. I never felt like I did it sufficient justice.


Hey IheartShotguns, have you done any recent research on the Limbsaver nitro pads? They are grind to fit and appear to be 1/2" longer/thicker than their classic pads. According to Limbsaver they use the same technology, and the extra 1/2" should give extra cushioning.

Great work on all this testing-I'm switching from kicked to limbsaver because of what I see in your charts.

Pad in question-
https://www.amazon.com/LimbSaver-Grind- ... B00A2OAU2W


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 Post subject: Re: recoil pad wormhole - a technical perspective
PostPosted: Mon Aug 31, 2020 9:00 am 
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Very interesting, I use the Pachmayr 325 a lot when I put a new one on.


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 Post subject: Re: recoil pad wormhole - a technical perspective
PostPosted: Mon Aug 31, 2020 9:15 am 
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One thing I have learned, from owning a Precision Fit Stock, is that if you have too much travel in the recoil absorbing mechanism, your shoulder is fine, but your pistol grip hand gets beaten up. The tang hits your thumb and the trigger guard starts hitting your middle finger every time you shoot. I stiffened up the recoil absorber and that helped a lot. I also stopped doing the chicken wing hold and that really helped with my right hand problems.

The chicken wing hold. This photo is of me shooting muzzle loading trap with black powder.
Image

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 Post subject: Re: recoil pad wormhole - a technical perspective
PostPosted: Wed Sep 02, 2020 6:13 am 
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A couple more for you to consider
- microcell, by cervalatti (Hastings)
- isis green pad, x pad (UK)
- falcon pad-has an air bladder
-kick eeze, dual action. 2 densities of foam. one is much sqishier




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