Developing a bstaley Disco

[rsterne]

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....

The main thing to understand is that as the pressure in the tank decreases, more and more of the hammer's energy transfers to the buffer and less to opening the poppet further and for a longer duration. So the stiffness of the buffer will definitely have an effect on the resultant shot string....

Screwing the striker in CW (when viewed from the rear of the gun) will increase the valve lift before the hammer contacts the buffer.... causing it to become effective later in the shot string.... If you screw the striker in far enough, the hammer will no longer hit the buffer at all.... and the gun will operate "normally", albeit with much reduced hammer stroke.... This is the first principle of the bstaley tune....

AS YOU SCREW IN THE STRIKER CLOCKWISE, IT INCREASES THE VALVE LIFT, INCREASING THE POWER, AND THE BUFFER BECOMES EFFECTIVE LATER IN THE SHOT STRING IF AT ALL.

Start digesting that while I work on my next post....

Bob

[rsterne]

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

[AirGunEric]

Having had no prior readings on the "bstaley mod"- my question is; what is the point of such a modification in addition to an RVA and an adjustable hammer pre-load? It strikes me that by creating this buffer, the obvious result would be less hammer strike at lower reservoir pressures, less duration and then less propellant consumption and a weaker and weaker shot as the air pressure diminishes- i.e. shortening the usable shot string than would be the case if no such 'buffer' were in place.

[rsterne]

The idea is that in some guns (maybe not all), the self-regulation of the valve (ie increase in lift) as the air pressure reduces is too great.... I'm sure you have heard PCPs get louder as the shot string progresses?.... That is because the efficiency is less in the latter part of the string.... With this mod the idea is to "knock the top off" the bell curve, flattening it, and then retaining some of that air for later shots, which use progressively less volume of air, but at a slightly higher pressure, thereby producing more shots overall.... I've NEVER tried it, this experiment is a first for me.... however, the success that this method of tuning has produced in the MROds and PRods is very good....

It also makes the tuning process easier overall, as once the ideal hammer spring preload has been found for the fill pressure, backing out the striker reduces the lift and hence reduces the power.... Again, I've never tried it, but the claims are that strings detuned in this way are significantly flatter, and hence longer, than those achieved by strangling up the transfer port.... The efficiency is supposedly higher.... and that's what I want to explore.... There is defintely something going on here that needs invesigation....

Bob

[rsterne]

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]

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

[AirGunEric]

Not being funny- but as the end result was a "detuning" of the gun to gain shot count- did you do the same tests using the RVA and hammer adjustment screw to achieve the same velocities? I'm not suggesting anything in particular, but if you didn't de-tune the gun to, for example, 430fps with the already existing parts before adding the washers and check the shot count- what we have is a nice graph with no actual comparison to a "before" state.

[rsterne]

This specific gun, no, but I have a fair amount of experience with Discos and what they can and cannot do by simply detuning them with an RVA.... As an example, I detuned a dead stock Disco with an RVA to 18.6 FPE and got 33 shots within 4% ES.... The fill pressure was 1800 psi and the gun had to be refilled at 1000 psi.... That works out to 614 FPE total from 452 CI of air, an efficiency of 1.36 FPE/CI.... Compare that to the "blue" (3 turns in) string with the O-ring buffer, doubling the shot count to allow for the 135 cc Disco reservoir instead of the 65 cc available with the 2260 tube.... The efficiency was similar, at 1.39 FPE/CI, but the shot count would be 50 shots, because the string started at 2000 psi and extended all the way down to 850.... a much wider pressure range than I have ever observed in any unregulated PCP.... Simply put, the gun with the bstaley mod gave 50% more shots within the same 4% ES....

Looking at the string of 65 shots averaging 438 fps, I don't have a direct comparison to an unregulated Disco.... simply because it isn't practical to detune one that far with just a hammer strike adustment, in fact I don't think you could do it even with a clipped 2240 spring.... I recently tested this very same gun in stock Disco trim (same 2260 tube), with a 0.070" transfer port, and I got 23 shots at 626 fps within 4% ES, starting at 2000 psi and ending at 1050.... but the efficiency was only 1.10 FPE/CI.... I will eventually be testing it with a Challenger transfer port (0.047") and expect to be close to 500 fps, and will report on that when the testing is complete.... I did build one non-PAL 2260, using a stock Disco valve, and in order to get the velocity under 500 fps I had to use a regulator set to under 500 psi.... The efficiency was only about 1.0 - 1.1 FPE/CI.... The efficiency of the "black" tune with the bstaley buffer was 2.11 FPE/CI, and I have NEVER had any gun break the 2.0 FPE/CI mark before.... THAT really made me sit up and take notice....

I have no horse in this race.... In fact, I had serious doubts about the bstaley mod, and wondered if part of the success was the detuning that was taking place.... The claims were that the shot strings were flatter and longer for the same amount of air used, ie more efficienct.... So far, my results seem to be bearing that out.... However, it would appear that to obtain these results, you have to give up about 20% in FPE compared to what the gun will shoot without the buffer.... For many, that will not be of any use.... I tend to max. out my experiments to see how much power they will deliver, and then detune them a bit for use.... However, the detuning I do is seldom even 10%, let alone 20%.... At this point, I can't tell you if I will find a place in my guns for a bstaley mod or not....

There are two things I REALLY like about this tuning method, however....

1. By using a hammer with an adjustable striker in this manner, you can tune the gun over a wide velocity range without having to drastically change the fill and refill pressures.... If you try and detune just with hammer strike (and I don't care whether you do it with springs, travel, or hammer weight), by the time you cut the power of a PCP in half, you will have to drop the pressure range you use.... The bstaley tune allows the velocity to be adjusted over a wide range without a corresponding drop in pressures....

2. This ability to reduce velocity while maintaining a reasonable fill pressure (or maybe the same fill pressure), can be taken to extremes, something I haven't seen in conventionally tuned PCPs.... Try and retune one to non-PAL velocities (using an RVA) and the fill and refill pressures drop wayyyyyyyyyyyy down.... That causes the shot count to drop, because you aren't getting all of the benefit of the lower power.... I actually got over 110 shots from 65 cc, and the velocity was still 420 fps when I quit.... and 65 of those shots were between 430 and 447 fps.... That's only a 10 psi drop per shot on a tube half the volume of a Disco.... Something is going on here that needs further investigation....

Bob

[AirGunEric]

Let me ask once other question- interference- ? Do the o-rings have the ability to twist/shift in the tube and thereby block the hammer from contacting the valve stem completely, or are they being fastened together in some way to prevent this, or... ?

[rsterne]

The O-rings are in a simple stack, the OD is a snug fit in the tube (ie no play).... They are not fastened together in any way, they just sit in a stack up against the back of the valve.... The total valve lift we are dealing with without any buffer is about 0.050"-0.060" at the beginning of the string to 0.100"-0.120" at the end.... When I measured how far the O-ring stack could flex on hammer impact (in this gun), I got about 0.060" for a 4 high stack of 70 Durometer.... and that is with FULL hammer impact, ie after the O-ring stack flexes that far, there is no energy left to open the valve.... In other words, each O-ring distorts from a 0.103" circular cross section to flatten by about 0.015" or so, although the top one may in fact distort more and the one touching the valve less.... I haven't seen any indication that the O-rings are "getting out of place", I would think that would turn up as large jumps in velocity.... I would think that if one got wildly out of place, it would remain so, and turn up on disassembly.... Nor have I heard/read any reports of that happening, although I would agree that is is theoretically possible.... When you go to remove them, you have to hook them and pull them out of the tube....

Bob