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rsterne · Moderator

One of the most unique ways of tuning the MRod and PRod is the addition of a stack of O-rings to form a buffer between the valve and hammer, called the "bstaley mod" after it's inventor.... It requires a hammer with an adjustable striker extending past the face, such as found in the MRod, PRod, and Challenger.... so the first part of trying this on a Disco is to fit such a hammer.... I chose the one from a Challenger, along with the RVA (rear velocity adjuster aka power adjuster) from a Challenger as well.... You need an adjuster with a hole through the screw so that you can insert a 1/8" allen key to turn the striker to adjust it.... Full CCW provides the maximum hammer stroke before it hits the valve stem, and as you turn the adjuster "in" CW the stroke reduces by 1/28" (0.036") per turn.... Since the end of the stem is fixed (when closed), what actually happens is that the hammer slides back that much, increasing the preload on the hammer spring by the same amount.... However, since the outside face of the hammer is what catches on the sear, the spring length and therefore it's force when cocked stays constant.... These have to some extent a cancelling effect, and the first 3-4 turns adjusting the stroke (in a conventional installation, without the O-rings) seems to have little effect on the velocity.... Once you screw the adjuster out a long ways, drastically reducing the travel, that is no longer the case, and the velocity starts to drop more rapidly.... Here is a photo of the Challenger hammer sitting beside a stock Disco hammer.... Note that I haven't had to cut a notch in the top of the Challenger hammer to clear the 4-48 Disco breech screw because I'm using a 22XX breech with the forward screw location.... In a Disco, the hammer will have to be notched to miss the screw like the Disco hammer is....

Note that the main body of the hammers are the same length, but the striker sticks out 0.080" past the front of the hammer when fully in CCW (as shown).... This means that compared to a Disco hammer, the stroke will be reduced (and the preload increased) by (at least) that amount.... A stock Disco has a hammer travel of 0.58", so when you install a Challenger hammer, that drops to 0.50" (maximum) or less (by 0.036" per turn on the striker).... For lower powered (stock or less) guns that will not be an issue, but it may hamper power tunes, requiring more preload and/or a stiffer hammer spring than would be required if a longer (rather than shorter) stroke was available.... This can be addressed by shortening the valve stem to gain back the lost stroke, and I'll be looking at that much later in this thread as it develops.... There is one #113 O-ring sitting on the front of each hammer.... Note that the Challenger striker, when fully in, comes just flush with the top of a single O-ring (because of the taper on the edge of the hammer).... We will be starting with four of them.... and they are 0.103" thick, so the hammer will make contact with the O-ring stack when the striker (when fully in) is about 0.31" from the back of the valve.... Since a stock Disco valve stem protrudes 0.32" from the back of the valve, that will mean that the valve will only open about 0.010" before the hammer first contacts the O-rings....

At this point, we need a quick review of how unregulated PCPs work.... At the beginning of the shot string, the air pressure is at its highest, and the force holding the valve closed (and closing it once it is open) is at its highest.... Since the hammer strike is (for any given tune) a constant, the valve will get knocked open a tiny amount (the lift), typically about 1/16".... This releases a very small amount of very high pressure air for a very short time (the dwell) to accelerate the pellet.... As the pressure drops, so do the forces resisting the hammer, so the lift increases gradually, and with it the dwell.... The pulse of air gets longer in duration but less in pressure, with the mass of air released remaining relatively constant.... As the pressure continues to drop, the valve opens further and further, and eventually there is not enough pressure to maintain the same mass of air escaping, and the velocity drops, ending the useful shot string, and you have to refill the gun.... At that point, the lift is typically about 1/8", about double what it was when you started.... The velocity in a properly tuned PCP starts out a few percent below the peak, rises, plateaus a bit, and then falls again.... The trick in tuning a PCP is to maintain the velocity within an acceptable Extreme Spread (ES) while getting as many shots as possible.... I usually use 4% as my acceptable ES at 50 yards, narrowing that to only 2% for 100 yards....

I have had a few discussions with bstaley, trying to understand the principles involved, and the basic idea is that as the valve lift increases during the shot string, the hammer, at some point (depending on how you have the stroke adjusted) starts to impact the O-ring buffer.... This reduces the valve lift in the latter part of the shot string.... By adjusting the hammer spring preload and the length of the striker you can govern at what point in the shot string the buffer starts to limit the valve lift, and what the maximum lift at the end of the string is.... Here is a quote from the inventor....

rsterne · Moderator

To continue, I first wondered how much "give" there was in the stack of O-rings.... Did it simply sit there like a rock and stop the hammer completely once it contacted it, or did it indeed act like a "cushion" and gradually slow the hammer by varying degrees depending on the hardness (durometer) of the O-rings, the height of the stack, and how hard the hammer hit it?.... I had to test out bstaley's assertion that the stiffness of the buffer was, in fact, important.... To do that, I used a trick I have done many times before to measure the valve lift in a PCP.... I made up a lightweight indicator that looks like a common nail with the head captured between the hammer spring and the inside of the hammer.... It is made from a piece of 1/8" aluminum rod and a thin 5-40 nut threaded onto one end and peened in place.... Once in place, it moves with the hammer.... It is long enough to protrude through the hole in the center of the RVA adjusting screw, and carries a 1/8" ID O-ring on it.... To use it, you simply cock the gun, slide the O-ring up against the back of the screw and then fire.... The O-ring slides back on the rod, stopping when the valve is at maximum lift.... By measuring the gap between the O-ring and the screw, you have a direct measurement of how far the valve opened.... Once fired, it looks like this....

I have used this device on enough PCPs, at enough pressures, to realize that the range of lift values (at least in .22 through .30 cal) is pretty constant for any USABLE tune.... This is because any tune that produces a proper bell-curve works pretty much the same, regardless of pressure.... At the beginning of a shot string, the lift is typically about 0.050" to .060".... and by the end it runs about 0.100" to 0.120".... The only time the lift seems to go over 1/8" (on the many guns I have tested) is when the pressure is very low (ie you are past the usable portion of the shot string), or you have so much hammer strike that the first shot is the fastest and you have no bell-curve at all (really the same thing, if you think about it).... Therefore, the RANGE of lift values from the beginning to the end of most shot strings runs about 0.050" to 0.060"....

So how much does a bstaley O-ring stack compress when struck by a hammer?.... To determine that, I removed the striker from the Challenger hammer so that it couldn't hit the valve stem.... Now the front outer face of the hammer sat right against the O-ring stack at rest.... When you cocked the gun and fired it, the O-ring gap to the RVA screw showed how much the stack compressed when struck by the hammer.... I must admit, I was quite surprised to find that in my very first test, that was 0.040", as shown in the above photo.... There is one other piece to the puzzle, however.... How much was the stack of O-rings compressed just from the hammer sitting against it with the preload of the hammer spring?.... To measure that, I measured from the end of the indicator rod to the back of the screw, then removed the hammer spring, and measured it again.... The difference was the static compression of the O-ring stack from the hammer spring preload.... I used four different setups to get a feel for what is happening, and the results are below....

I used a stack of four #113 O-rings.... The first 3 tests were with 70 Durometer, and I tested one setup with 90 D.... "Stock Disco" means a Disco hammer spring at stock preload (like with a stock end cap, and yes I allowed for the thickness of the nut on the indicator rod).... "Max. Disco" is the same spring, set at coil bind.... "Max. 0.040" is my heavy hammer spring (made from 0.040" wire) set at coil bind.... The "Total" number is the important one, it represents the total distance the O-ring stack compresses on hammer impact from a relaxed state.... I came to the following conclusions from this (limited) testing....

A 90D O-ring is roughly twice as stiff as a 70D O-ring when acting as a buffer....
The spring rate is not even close to linear, there is much more give as the O-ring first begins to distort from circular than once it has collapsed somewhat....
This progressive spring rate means that the initial compression is fairly easy (look at the static numbers) but the final amount of compression doesn't increase much even with a big increase in hammer energy....

When thinking about how the NUMBER of O-rings in the stack affects the travel and stiffness, I compared them to the way a Belleville spring washer works.... As you add disc springs in series, you gain travel and the (total) spring rate goes down.... I think stacking O-rings works in exactly the same way.... Each O-ring is doing an equal share of the work.... If, for example, you only used two O-rings instead of four, I would expect the deflection of the stack to be half the distance at the same load.... ie the above numbers would be cut in half for a stack of two.... This remains untested at the present time.... There may be a small friction component that could skew the results slightly, of course....

The important "discovery " I made was simply to confirm bstaley's statement that the stiffness of the buffer changes the way it responds to the hammer strike and how much of that strike energy is being absorbed by the buffer.... This opens a big can of worms from an experimental point of view, of course, drastically increasing the number of variables.... However, I think it can be summed up as follows:

THE STIFFNESS OF THE BUFFER IS PROPORTIONAL TO THE HARDNESS OF THE O-RINGS, AND INVERSELY PROPORTIONAL TO THEIR NUMBER (A TALLER STACK IS SOFTER AND HAS MORE TRAVEL).

THE BUFFER DEFINITELY GIVES IN A PROGRESSIVE MANNER, A SIGNIFICANT AMOUNT RELATIVE TO THE TOTAL LIFT, AND THEREFORE HAS THE ABILITY TO MODULATE THE LIFT, NOT JUST LIMIT IT.

I know that bstaley already has a feel for all of this, but I didn't, so I hope I'm not just stating the obvious. In the next post I hope to get into the "nuts and bolts" of what happens when you install a bstaley mod....

Bob

rsterne · Moderator

I finally got a chance to get back to working on this today.... A couple of days ago, I got a chance to fire a few shots with four 70 Durometer O-rings installed as a buffer, and it wasn't quite thick enough.... With the striker in the Challenger hammer fully CCW, it just touched the valve stem at the same time it hit the buffer.... Today I picked up a couple of #113 "Backer Rings", which are a 90 Durometer flat O-ring only 0.055" thick (about 1/2 as thick as a #113 O-ring), that are flat on one side and concave on the other.... They are used to decrease the clearance and improve the sealing when using a 70D O-ring where the tolerances are too big to seal at the expected pressure, and are placed on the low pressure side of the O-ring.... In this case, I placed one against the back of the valve as a spacer (I could have used a metal washer, but this was an item that is readily available at any hydraulic shop, and will be consistent in dimensions, so a good choice).... It slid in just like installing an O-ring, and then I slid four 70D # 113 O-rings down against it.... This made the total thickness of the buffer 0.467", and when the hammer was dropped in, it just went "thud" against the bumper.... I started my testing by setting the striker fully CCW, and with no O-rings in the gun I shot a string to find out what hammer spring preload was needed.... Well, it turned out that with the reduced hammer travel with the Challenger hammer, the string looked the best with the Disco spring at maximum, just shy of coil bind.... All the testing was conducted at that spring setting, and all testing was done with JSB Express 14.3 gr. pellets....

I shot a string with the hammer stroke set to maximum (full CCW), and then repeated, turning the stroke adjuster in one turn at a time, stopping at 4 turns in.... Here are the results, to be used as a baseline....

You will notice that decreasing the stroke causes a loss in velocity, and eventually the first shot at 96% requires less than a 2000 psi fill.... Shot #1 on the graphs was at 2000 psi in all cases, but only shots above 96% of the peak velocity are shown.... The efficiency was relatively constant at between 1.04 and 1.20 FPE/CI, acceptable but not stellar.... with the most efficient being at 3 turns In, peaking at 838 fps and averaging 21.6 FPE for 12 shots using the 65 cc tube (half what a Disco has).... I then pulled the gun apart and installed the buffer, consisting of four 70 Durometer O-rings plus a Backer, and I repeated the tests, starting from 4 turns in where I stopped before.... Here are the results from 4 turns in down to 1 turn in, plotted on the same vertical axis (velocity) as the above for easy comparison....

The first thing to note is that the orange line, at 4 turns in on the striker, showed virtually identical velocity to the curve above with no buffer installed for the first half of the shot string.... At that point, I assume the hammer started to make contact with the buffer, and the velocity dropped like a stone.... In fact, the first shot after the peak (820 fps) was down 50 fps, so that setting was completely useless.... I was pretty disheartened at this point.... However, when I backed the striker out a turn, to 3 turns in, things changed in a hurry.... First of all, note that instead of the velocity increasing as the striker is backed out, it DECREASES because the hammer starts hitting the buffer and the valve has less lift.... Although the velocity was 60 fps slower than the same setting without the buffer, THE SHOT COUNT DOUBLED.... Now I was equally shocked, but in a good way!.... When I calculated the efficiency, it was well up, at 1.39 FPE/CI, and that string started at 2000 psi and continued all the way down to 850 psi before the velocity dropped more than 4% below the peak.... I have NEVER seen that wide a usuable pressure range on an unregulated PCP before....

With the striker turned out another turn, at 2 turns in, the curve was similar but shorter, only 16 shots, ending at 1200 psi.... I would assume that intermediate settings, with varying degrees of buffer interaction with the hammer, would produce intermediate results.... When I backed the striker out another turn, to 1 turn in, the velocity dropped, and the gun was a LOT quieter.... By the shape of the curve, it would appear that there is too much hammer strike (or too low a fill pressure) to get the longest shot count at that velocity, and that may also be true of the previous string at 2 turns in.... All of these strings had efficiencies between 1.26 and 1.41 FPE/CI, which is very good, although the velocities are, admittedly, quite a bit less than the gun shot without the buffer in place.... That is consistent with what bstaley has always said about this system, that it is at its best for low to medium power tunes, not flat out power....

Now you may be wondering why I haven't shown the results for the striker set to maximum travel with the 70D buffer.... Well, the buffer was pretty well engaged fully, and I didn't even know whether the gun would fire.... Anyway, I filled it to 2000 psi, and started shooting.... The first shots were under 400 fps.... 40 shots later, at 1650 psi, the velocity had climbed to 430 fps.... After another 35 shots, it peaked at 447 fps at about 1300 psi.... Add another 30 shots, the pressure was down to 1000 psi, and the velocity had fallen to 96% of it's peak.... That meant I got 65 shots with a 4% ES from 1650 psi down to 1000, at an efficiency of 2.11 FPE/CI.... For those of you that aren't impressed, that is the HIGHEST EFFICIENCY I have personally ever seen in any PCP.... For those of you that like the other units, it is less than 7.8 barCC/FPE.... One other thing.... the gun was so quiet the loudest sound was the pellet striking the backstop....

For any of you (including myself) that doubted whether or not the bstaley O-ring buffer and striker tuning system can deliver results, I can certainly tell you that I no longer have any doubts.... For detuning a gun a bit and extending the shot string, it shows huge promise, IMO.... Whether or not it is possible to get "better" results at higher power I have no idea, although I hope to find out.... Bear in mind, this is only the first time I have played with this idea, I am by no means an expert at tuning with it.... On first blush, it would appear that there may be some very strange (and undesirable) characteristics if the buffer starts to engage too late in the shot string, as evidenced by the "4 turns in" curve with the buffer in place.... This might not be the case with a firmer buffer (ie either shorter or using harder O-rings).... and it might disappear with the striker adjusted just slightly differently.... However, once the buffer is working, it would appear that playing with the striker position and the spring preload could well provide some very efficient shot strings at medium power settings....

The thing I found quite shocking, was how well the bstaley mod performed at low power.... Anyone who is interesting in seriously detuning a gun for a ton of shots at low power should consider trying this system.... The basis of tuning with it is that you set the velocity you want with the striker position, and then fine tune the fill and refill pressures with the hammer spring preload just as you normally would, at least that is my assumption.... My next experiment will be with a stack of four 90 Durometer O-rings, which will provide a much stiffer buffer.... I have absolutely NO idea what to expect....

Bob

rsterne · Moderator

Here is the string for the four 70 Durometer O-rings plus a Backer ring with the stroke set at maximum (full CCW).... With the bstaley method, backing the striker out produces the lowest velocity....

That string was from 1650 psi down to 1000, and represents the portion within 96% of the peak velocity.... I shot 40 shots starting from 2000 psi, where the velocity was just under 400 fps, just to get to the start of this string.... The efficiency was a staggering 2.11 FPE/CI.... Anyone wanting to set up a PCP for the Canadian non-PAL requirments (under 500 fps) should have a serious look at doing the bstaley mod.... If you were careful with the height of the O-ring stack and the length of the hammer spring, you could acheive a huge shot count with no adjustable components (ie using a stock hammer).... This rifle only has a 2260 tube, with 65 cc of volume.... Here is a photo....

This was just thrown together from parts to give me a test bed.... It uses a stock Disco valve, transfer port, and 24" barrel, a Crosman steel 22XX breech, 2260 tube and cut down stock, a PRod gauge port and a Disco fill fitting.... The adjustable hammer is from a Challenger / PRod, and the RVA / power adjuster is also from a Challenger, running a Disco spring.... The performance will mirror a stock Disco, but with only half the air volume and half the shot count.... That saves time and pellets during testing changes....

Bob

rsterne · Moderator

After having a delay due to a burst disc failing and draining my SCUBA tank, and subsequently having to refill it (a long process with a ShoeBox).... I got back to doing more testing today.... I installed a stack of four 90 Durometer O-rings, this time without the 0.055" thick backer ring.... I figured that since the O-rings deflect a lot less than the 70D, I would have sufficient adjustment range between Max. CCW and 4 turns in (the settings I used before) to see what we need to see.... Well, at max. CCW (minimum lift) I got just about what I expected, but the rest of the results leave me totally confused.... Here is what happened....

I used the same colours for all the stroke settings as on the other two graphs.... The vertical scale is the same, but it is shifted down 50 fps in velocity to include all the curves.... The graph is even the same length as the original (no O-ring) graph.... The black "max. stroke" curve starts at nearly maximum velocity, indicating that the hammer spring preload is too high.... The efficiency is 1.23 FPE/CI, in between what I got unbuffered and with the 70D buffer.... I think that if the entire curve was included, it would be a pretty usable string, and the data fits well with what I got using the 70D buffer.... Now for the bad news....

First of all, the other four curves show the OPPOSITE trend for the velocity to what you would expect in a bstaley gun.... As you turn the striker in CW, the valve lift increases, and the velocity is suppose to increase with it.... That was plainly the case with the 70D buffer.... With the 90D O-rings, however, the highest velocity occurred at only 1 turn in, and the velocity DROPPED as I turned the striker in CW, with the lowest velocity (not counting the black line) occurring with the striker in 4 turns.... Three of the curves, on the face of it, look pretty decent in length, 14-15 shots.... The problem is, that the efficiency of all four of these curves was WORSE that the unbuffered gun.... It ran from 0.93 to 1.08 FPE/CI, improving as the velocity dropped.... The velocity with the buffer ran about 30-40 fps lower than the unbuffered gun, and the efficiency was poor as well, so this setup is pretty much a bust, IMO.... The big question is WHY ???

A day or two ago, I had a request to measure the valve lift with a buffer in place.... While I had the stack of four 90D O-rings in the gun, I added the Lift Indicator Rod, set the striker to Max. CCW, readjusted the preload to just shy of cioil bind, and measured the lift at three pressures, 2000, 1500, and 1000 psi.... I then pulled the gun apart, wound the striker in four turns CW, and repeated the measurements.... Here are the results....

A word of caution here.... I was having trouble with the O-ring cocking on the rod (ie not staying perpendicular to it), so it was hard to measure.... I would give a tolerance of about 0.005" plus/minus to the above numbers, but the TREND is pretty clear.... There is VERY little lift occuring when the striker is Max CCW, but it is still about twice as much at 1000 psi as it is at 2000.... and it only takes about 1/64" of lift to produce 640 fps (13 FPE) at 2000 psi.... AMAZING !!! ....

I'll be looking forward to seeing your analysis of these rather confusing results.... So far, it would appear that with a Disco, the 70D O-rings are far superior.... So where do we go from here?....

Bob

rsterne · Moderator

Well, I learned a valuable lesson about the O-ring buffers, today.... If the hammer isn't hitting them, they can shift out of place and cause weird results.... I began to suspect that last night, and bstaley suggested it this morning, but I had already confirmed it.... I think the slight air blast coming out around the stem of the valve pushes them back a bit, or maybe just makes a space between them if the hammer isn't already contacting them as it would be before that blast of air occurs if the buffer was even partially engaged.... In addition, I made a new Lift Indicator Rod, even lighter and with a thinner head than the original.... This one is made from 5/32" OD aluminum hobby tubing, and only weighs 1.4 grams and only requires 1/2 turn adjustment on the preload.... The larger diameter means it doesn't wiggle around in the hole in the RVA adjusting screw, and the larger O-ring tends to stay nice and straight, allowing much more accurate and consistent measurements.... I then redid my previous lift measurements, and took a bunch of new ones, with the following results....

I have confirmed that at 4 turns in, with either O-ring stack, the hammer does NOT contact the buffer (or so lightly as to have no effect), providing the O-rings haven't shifted.... Therefore, those curves on the previous graphs should be ignored.... Anything happening there is a result of the buffer not remaining properly in place.... In addition, the previous chart I posted about the lift numbers with the 90D buffer are incorrect, the numbers above are the accurate ones....

Note that the lift at 1000 psi with no buffer in place changes 0.022" between max. stroke (full CCW) and 4 turns in, or about 0.005" per turn, and that should be linear.... That means that at 3 turns in, with no buffer, the lift at 1000 psi should be about 0.086".... I made another test, with the 90D buffer at 3 turns in, and the lift was 0.084", so the buffer is either not in contact, or it is just barely touching it at 1000 psi.... It would NOT be producing any changes to the shot string, so again any difference between that string and the unbuffered one would be due to the stack moving around.... I also tested the 70D buffer with the backer ring at 3 turns in, and the lift was 0.075" at 1000 psi.... This indicates that the hammer is just making contact at that pressure, but it would not be making contact with the buffer at over 1200 psi.... so the data for that string (unfortunately, the best string I had initially) may be somewhat suspect.... Perhaps that is why I couldn't duplicate it last night.... On the other hand, if the hammer is contacting the buffer at 1000 psi and below (only), that may be the reason that first string extended down to 850 psi....

Now that I have a better indication of what range of adjustments will and will not work, I am going to go back and retest both the 90D and 70D (with backer) stacks at the appropriate settings, but this time with 1/2 turn increments.... I'm hoping for a lot more consistent results....

Bob

rsterne · Moderator

I completed the second set of tests, changing the striker setting by only a half turn at a time instead of a full turn.... I also eliminated the settings where I was sure the buffer was not playing a role.... All strings were started at 2000 psi, and the Disco hammer spring was set just shy of coil bind as before.... and only shots within a 4% ES are graphed.... The four 70 Durometer O-rings had a 90D Backer ring under the stack.... and here are the results....

Basically, once the striker is screwed in 2 turns CW or more from the fully CCW position, it is ineffective in altering the shot string to any significant degree.... These results are different for the 3 turns out curve (blue line) which I found in the first test.... My only explanation is that the buffer in that test was moving out of place slightly, and interfering with the hammer in just the right way to skew the results.... That curve was a lower velocity, longer, and flatter, but since I haven't been able to duplicate it in several tries, all I can say is that it was a tease, unfortunately.... The 2 and 3 turn in shot strings are virtually identical to the unbuffered results....

When we explore the striker adjustment range from maximum CCW (minimum lift) to 2 turns in, however, we have some very interesting things happening.... As the velocity drops, the efficiency increases, and the strings get longer.... At 1.5 turn in, the average velocity is 775 fps (at 1.18 FPE/CI), and a 1 turn in, it drops to 718 fps (at 1.36 FPE/CI).... In both cases, a large portion of the first part of the shot string appears to be missing, an indication that the hammer strike is too great for the 2000 psi fill pressure.... For both of those strings, further experimentation is needed.... At 1/2 turn in, the velocity is only 617 fps, but the gun got 32 shots within 4% ES, and the efficiency was 1.65 FPE/CI, very efficient for a 12 FPE gun.... If you remember from the first tests with the 70D buffer, at maximum CCW, the velocity was about 430 fps and the gun did 65 shots at 2.11 FPE/CI between 1650 psi and 1000.... This soft, tall buffer is excellent for drastically reducing the power of the gun and raising the shot count and efficiency.... If that is what you are after, this is something you should definitely consider....

Now to look at the stiffer buffer, consisting of four 90 Durometer O-rings.... Here are the results....

With this stiffer buffer, and without the extra height of the 0.055" thick backer ring, the buffer was basically ineffective once the adjuster was past 1 turn in.... The top group of curves pretty much mirror those of the unbuffered gun at the same adjustments, although for some reason they seem to peak at a slightly higher velocity, although no higher than the unbuffered gun did at maximum.... The efficiencies are similar as well, so IMO no advantage can be found operating in that adjustment range....

Once again, however, when we explore from maximum CCW (minimum lift) over the first part of the striker adjustment, we see more shots at increased efficiency, but at lower velocity.... The shot string at 1/2 turn in (791 fps average) started out with the first shot the fastest, so it definitely needs to be explored with less hammer spring preload.... With the striker at maximum CCW, the velocity averaged 671 fps, and it looked like it too, could benefit from a bit less hammer energy for the 2000 psi fill.... The efficiency of that 15 shot string was 1.30 FPE/CI, and I have a feeling that both can be increased a bit.... However, when you consider that the 1 turn in string with the 70D buffer was higher velocity AND more efficient, I have my doubts if the 90D buffer will show any advantages over the softer one....

There are a couple of general observations I have to make.... When the striker adjustment first starts to have a serious affect on the velocity, it appears that the shot string shifts to the left, ie it peaks at higher pressure.... In an unbuffered gun, that usually indicates too much hammer strike, and that is likely the same case here.... Only adjusting the preload to a lower value will let us find out if there is more shots and efficiency hiding there.... Once the velocity is reduced further, however, the shot string returns to the original peak pressure, and eventually, as the velocity is lowered enough, then the fill pressure must follow, just as in detuning by hammer energy alone.... While the basic idea of reducing lift directly by decreasing the striker engagement with the valve is simple in concept, and very effective.... it still needs that fine tuning of trying various combinations of hammer spring preload and/or fill (and refill) pressure to find the optimum setting....

When time permits, I plan to concentrate on those parts of the envelope where less hammer strike is needed to optimize the results.... In the meantime, there is certainly lots here to chew on and discuss....

Bob

rsterne · Moderator

I had a chance this evening to do some testing of preload changes on the 90 Durometer stack for the striker position that was 1/2 turn out.... That tune had a shot string that started high and declined, so I wanted to see what would happen if I reduced the hammer spring preload, if the gun would react in a "normal" fashion even with the buffer.... Well, as I expected, it did.... The colours have changed on this graph for clarity, so don't use them to refer to earlier graphs....

All the solid lines are at maximum hammer spring preload, as before.... The black line is with the striker full CCW (0 turns), the solid red line is 1/2 turn in on the striker, and the blue line is 1 turn in on the striker.... The other two red lines show what happens with the preload reduced two and four turns.... As you can see, the peak velocity drops and shifts to a lower pressure, reacting just like a conventional unregulated PCP, as the preload is reduced.... In addition, the efficiency increases, and the shot count increases, again as per usual.... The interesting thing about the efficiency is that it increases from top to bottom on that graph, starting at 1.06 FPE/CI for the blue line, and progressing to 1.30 FPE/CI for the black line.... I have no reason to believe that any "magic" will happen in between, although in between tunes are certainly possible.... As an example, if you wanted to tune for the flattest curve peaking at 800 fps, you would use a striker setting roughly 3/4 of a turn in CW, and a preload likely in the neighbourhood of 2 turns out from coil bind.... and you could expect about 12 shots at an efficiency of about 1.20 FPE/CI.... Using a full size Disco tube, that would be about 24-25 shots within a 4% ES.... The problem is, that is basically no better than you can get with a stock Disco.... From what I have seen so far (and I'm running out of ideas), the 90 Durometer buffer doesn't seem to offer a significant improvement over stock.... I detuned a Disco with just an RVA (preload adjuster) and achieved 33 shots at 18.6 FPE within 4% ES from 1800 psi down to 1000.... That is 614 FPE from 452 CI of air, which works out to 1.36 FPE/CI.... That is more power AND more efficiency than anything I've seen so far with the 90 Durometer buffer.... It is possible, of course, that shortening the valve stem and increasing the hammer stroke might provide better results.... I'm still hoping that the 70 Durometer buffer will provide greater efficiencies as well....

Bob

rsterne · Moderator

I decided that for the Preload testing on the 70 Durometer buffer I would try removing the 0.055" thick Backer Ring.... Here is the test at Maximum CCW on the striker, and maximum preload (just shy of coil bind).... shown on the graph as the black line with square markers....

You can see that my intuition was pretty good about using the Backer Ring to check out the lower end of the velocity range possible using a bstaley buffer.... Without it, and with the striker set fully CCW (minimum lift) the average velocity of the string was 750 fps, about halfway between the 1 turn in and the 1.5 turn in strings.... However, this is perfect to explore reducing the preload, as most of the first part of the shot string is misssing.... The plan is to try a few reduced preload settings with this striker adjustment, and then wind it in 1/2 turn and test again.... Incidently, once again on this graph, the efficiency increases from top to bottom, from 1.07 FPE/CI at 2 turns in to 1.65 FPE/CI at 1/2 turn in, with the "Max. No Backer" string coming in at 1.20 FPE/CI....

Bob

rsterne · Moderator

I completed the preload adjustment trials this afternoon, working on the strings that were "left shifted" ie the first shots were the fastest when the hammer spring preload was at maximum.... Here are the results for the stack of four 90 Durometer O-rings....

The string at 1/2 turn in on the striker (solid red line) was shifted left.... I shot additional strings at 2 turns and 4 turns out on the preload, shown in the dotted and dashed red lines.... The string at 4 turns out was the longest and the most efficient, returning 14 shots averaging 738 fps with an efficiency of 1.25 FPE/CI.... The highest efficiency overall was still the minimum lift string (black line - striker fully CCW), which was 15 shots at 670 fps. and 1.30 FPE/CI.... That is decent efficiency, but not as good as I shot with a stock Disco with the power just dialed down to the same level with an RVA.... On to the stack of four 70 Durometer O-rings (without the backer ring)....

The first thing to note is that at 1 turn in on the striker (the blue line), the shot string is almost identical to the same curve using 90 D Orings (top), and indeed virtually identical to the unbuffered gun at the same setting.... In other words, by that time the buffer is pretty much ineffective.... The set of red lines (1/2 turn in on the striker) are the same as on the first graph, solid is maximum preload, dotted is 2 turns out, and dashed is 4 turns out.... The most efficient of those was the latter, 16 shots at 758 fps with an efficiency of 1.24 FPE/CI.... The black lines are at fully CCW on the striker, using the same line types as above.... Once again, the curve at 4 turns out on the preload was the most efficient, giving 19 shots at 713 fps (16 FPE) and 1.31 FPE/CI.... Once again the efficiencies were beaten by a stock Disco detuned with a simple RVA, but in this case there were more shots in the string within a 4% ES than I have been able to achieve before at these power levels.... The bottom string on that graph would be 38-40 shots using a full size Disco tube....

I can now offer the following conclusions for my testing of the four #113 O-ring bstaley buffers in a stock Disco fitted with a Challenger hammer and RVA....

1. The 90 Durometer O-rings provided a wider range of velocities for the same range of striker adjustment.... eg. 1 turn was ~175 fps compared to ~125 fps....

2. The 70 Durometer O-rings required an additional spacer to get under 700 fps, but when used with a 0.055" thick backer ring could be adjusted down to about 440 fps (6 FPE).... I preferred the 70D....

3. With both buffers, as the buffer first becomes engaged, the peak of the velocity curve shifts to higher pressures, and must be compensated for with reduced hammer spring preload.... This is over a range of about 1 turn for the 90D buffer and about 1.5 turns for the 70D buffer....

4. I was unable to achieve increased efficiency during this first stage of buffer engagement, compared to what could be achieved at similar velocities with a simple RVA.... At the lower power levels, however, longer than usual shot counts were obtained with the 70D buffer....

5. As the buffer becomes fully engaged, the velocity drops rapidly, and the peak of the curve shifts back to the original pressure range.... Eventually, as the velocity is decreased further, the peak shifts to lower pressures, and the fill pressure must be reduced accordingly....

6. During this second portion of the buffer use, with the 70D buffer with backer ring, the efficiency climbed to higher levels than I have acheived in a Disco with a simple RVA, peaking at 2.21 FPE/CI, an extremely high value, at 6 FPE.... These high efficiencies were obtained with very large shot counts as well....

The bstaley buffer, when used in a stock Disco, provides the ability to change the velocity over a wide range, in a very quick and simple manner, without having to adjust the fill pressure.... I didn't find any gains in efficiency unless the power was reduced below 15 FPE using the 70D buffer.... although some lengthening of the shot strings occurred when settings were optimized for any given velocity.... I personally preferred the 70D buffer over the harder 90D O-rings.... although I didn't test the 90D below 670 fps.... When the Disco is detuned to below 600 fps using the 70D buffer, the efficiency skyrockets, and the shot strings soar....

All in all, an interesting experiment.... I learned a lot....

Bob

rsterne · Moderator

Today I got a chance to try a buffer made from three 70 Durometer O-rings instead of four.... That required replacing the flanged striked in the Challenger hammer with one made from a 1/4-28 SHSS with the point ground flat.... The hammer ended up slighty lighter, but with the striker set flush with the end of the hammer, the stroke increased from 0.50" (with the flanged one fully retracted CCW) back to the stock travel of 0.58".... This increased the velocity of the shot strings, making the first shot pretty much the fastest, and moving the energy of the gun up to 24 FPE with the striker extended a turn, which made the buffer ineffective.... I left the hammer spring preload just shy of coil bind and shot a string with the striker flush, extended CW 1/2 turn, CW 1 turn, and also with the striker below flush (CCW) by 1/2 turn.... Those results are show in the graph below as solid lines.... The efficiencies are all below 1.0 FPE/CI, as we would expect with the excessive hammer strike eliminating most of the first half of the shot strings....

The buffer is ineffective with the striker 1 turn CW, and having little effect at 1/2 turn CW, but becomes effective when flush, and the velocity continues to drop as the striker is recessed in the face of the hammer.... I didn't try it further CCW than 1/2 turn, however, as I was interested in velocities not too far below stock.... I then optimized the hammer spring preload for three striker settings, with the buffer just disengaged, starting to engage, and well engaged, and those are the dotted lines.... The colours are the same as the strings taken at maximum preload.... The dotted red line is fairly close to stock Disco power and shot count, although down a bit on efficiency at 1.07 FPE/CI.... The dotted green line is with the striker flush with the end of the Challenger hammer, with the preload set 5 turns out from coil bind.... That turned out to be the optimum setting for a 2000 psi fill, and the string is 17 shots at 18 FPE for an efficiency of 1.29 FPE/CI.... The dotted black line was with the striker 1/2 turn recessed in the hammer face and the preload out 6 turns, and returned 20 shots at 16 FPE with an efficiency of 1.34 FPE/CI.... Once again, these efficiencies, while pretty decent, are no better than what I can get with a simple RVA in a stock Disco.... However, you can still use a 2000 psi fill, and because of that, the strings are longer....

In reality, there is no practical difference between today's testing and what I did last week with the stock Challenger striker and one more O-ring in the buffer.... Either setup can be adjusted for about the same velocities and efficiencies.... although today's strings are 1 shot longer than with the taller buffer.... When you consider that with a full size Disco tube, the shot count will double from the above.... 40 shots within a 4% ES at 16 FPE is a pretty decent string starting from 2000 psi and shooting down to 1150....

Bob

ZipSnipe · Veteran Member

Wow you have been busy Bob, great work as always!!!!

rsterne · Moderator

Yesterday I took my test gun apart again and removed another O-ring from the buffer, so it is now just two #113s of 70 Durometer.... In order to allow the hammer to engage the buffer, I simply wound the setscrew I am using for a striker out 3 turns (0.108") to allow for the missing O-ring.... Note that as you back the striker out CCW below flush, you increase the hammer throw (to where it first contacts the stem) beyond stock but reduce the preload by the same amount.... At 3 turns below flush, the hammer stroke is now 0.69" and the preload is effectively at 3 turns out, even though the spring remains at coil bind when cocked.... After a few shots to determine where the buffer started to engage the hammer face (2 turns below flush) I shot some strings at maxmum preload.... and then at reduced preload on the last string to move the curve back below 2000 psi....

If you look at the top three curves, you get a perfect picture of what happens as the buffer progressively engages the face of the hammer.... The blue line, with the striker recessed 2 turns into the hammer face, shows the buffer not engaged at all.... The red line, at 2.5 turns recessed, show the buffer starting to engage on the 4th shot in the string, and surpressing the velocity in the second half, shortening the string by 2 shots.... The solid green line, at 3 turns below flush, shows the buffer with increasing engagement, and the entire first half of the shot string is missing, it would require filling to over 2000 psi to see it.... or reducing the hammer spring preload to move the curve to the right.... That is what is taking place with the dotted green line, which is still 3 turns below flush on the striker, but has the preload backed out 5 turns to create an entire shot string without filling past 2000 psi....

As it turns out, I now have three shot strings all at around 750 fps with the preload optimized, but with buffers of 2, 3, and 4 O-rings of 70D.... When I look at the data, I find the following:

Four 70D: 16 shots @ 758 fps (18.2 FPE) @ 1.24 FPE/CI (equals 32 shots in full size Disco tube of 135 cc)
Three 70D: 17 shots @ 748 fps (17.8 FPE) @ 1.29 FPE/CI (equals 34 shots)
Two 70D: 15 shots @ 754 fps (18.1 FPE) @ 1.40 FPE/CI (equals 30 shots)

It is dangerous to drawn any firm conclusions from such a statistically small sample, particularly when the average fps and FPE are not identical.... For example, the small decrease in velocity from 758 fps down to 748 fps could account for part of the increase in efficiency from 1.24 to 1.29 FPE/CI, and maybe for the extra shot as well.... However, there seems to be a slight trend towards increased efficiency with the shorter, and therefore stiffer buffer, but maybe at the cost of a lost shot or two.... Statistically speaking, they are pretty much a wash, so you would be hard pressed to make a choice between them.... When you compare the above with what I got by detuning a stock Disco with just an RVA to about the same power, there is again nothing to choose from.... I got 33 shots at 18.6 FPE within 4% ES with an efficiency of 1.36 FPE/CI from 1800 psi down to 1000....

Bob

rsterne · Moderator

Yesterday I got the chance to rip apart my test gun and mod the Disco valve.... I drilled the throat to 0.234" (still the stock 0.156" stem), and bored out the exhaust port with a 5/32" mill on a 20* angle.... By the time I was done, both had an equivalent diameter of 0.174".... I drilled the transfer port to 0.162" (about as big as you can go without danger of the wall kinking), but out of curiousity (and because I only have one .22 cal barrel) I left the barrel port stock at 0.134".... I was therefore quite surprised when the gun made 31 FPE with 14.3 gr. pellets.... I didn't record a shot string at maximum preload because the 2nd shot was the fastest at 988 fps, but I shot strings at 2 through 8 turns out on the RVA and here are the results....

The average power is 28.3, 25.7, 23.6 and 21.0 FPE.... and the efficiency is 1.02, 1.17, 1.19, and 1.12 FPE/CI respectively.... The pressure shown in the above graph is only approximate, just to give you an idea of the pressure range for each string.... the starting pressures are very close.... These tests are only to establish a baseline for the testing using a bstaley buffer consisting of three 70 Durometer #113 O-rings.... I am using a Challenger hammer with a 1/4-28 SHSS with the point ground flat as a striker, a Disco hammer spring, and a Challenger RVA.... The above tests were conducted with the set-screw flush with the front of the hammer, so the stroke was 0.58" which is stock for a Disco....Remember, for a full volume Disco reservoir, the above shot counts would double....

We're really busy in the Motel right now, so I have no idea when I will be able to get to doing the bstaley tests, unfortunately....

Bob

redlock · Member

Bob, you need to organize all of your data and publish a book on airguns. If the father and son team of Messrs Cardew can publish their findings then you can too. This is especially true, I think, because what you research is more interesting and so are your findings.
Red.