Air Gun Home :: View topic - calling rsterne! calling rsterne! need your analysis brain

Flyby · Member

Hi Bob,
can you look at these shot strings and tell me what makes one spikey versus the other one? Is there drag in the newer tune? I replaced a spacer behind the hammer spring (went from 0.073" to 0.087"). Also, I increased the start fill 100 psi from 2650 to 2750 psi.I checked the o.d fit of the latter to be sure it was OK, and it's a smaller o.d than the former. I like the power gain, as surprising as it was, but the Es and Sd values are pretty large compared to the earlier tune, though accuracy with the pellet seems fine (indoor testing) so far. I did not think the S-16 could be so sensitive to the changes I made.
older tune:

new tune:

retest of the new tune:

thanks!
Flyby out (aka Darryl in the alternate Universe)

rsterne · Moderator

Haven't a clue why that happened, but generally when I see spikes in the velocity string it's either inconsistent pellets or a dragging hammer.... My guess would be the latter.... Maybe a piece of grit?....

Why would you get such a large increase in velocity (180 fps) with only a 0.014" change in hammer spring preload?.... Sounds VERY strange....

Bob

Flyby · Member

The spacers I make for tuning the S16 are all hand-worked (drilled, lapped, sanded, etc). Apparently I flubbed on this one and left a bit of a lobe. That is what caused the drag, and probably also increased the dwell time for the air valve staying open (which explains some of the high FPS). What I have now is a smoother, shot string, made lumpy by the random FPS gaps in the shot string caused by unrecorded chronograph errors (my fault). Still while that first test of the new spacer averaged 39.9 FPE across 60 shots, the new test averages 38.5 FPE across only 58 shots. Again unrecorded errors disrupt the smoothness of the shot string bow. So the modification is valid in the sense that the gain is due to the increase of 0.014" and 100 PSI added to the staert-fill pressure over the old tune in the first spread sheet. I'm amazed at the yield, that's for sure. Notice the the Es and Sd values of this new test are lower than those of that first test of the thicker spacer (2nd spreadsheet in my first post). Only two of the shot strings of the new test are below or near my target goal of: Es = <20 FPS per 10-shot string. I'll take it! Stranger than fiction, ain't it?!
Very Happy

Oh, I added the new spreadsheet to the first post for easier comparison
Flyby out (aka Darryl in the alternate Universe)
EDIT: On second thought it wasn't the spacer at all. More than likely it was the hammer spring binding a bit. When putting everthing back together I recall now that I looked at the spring, and though I did not see any wear marks on it, I did notice it was dry, so I lubed it in case it might be binding under compression. That may have been the cure more than working on the spacer (which did not seem to be a drag source, now that I think about it again).

rsterne · Moderator

Is the extra 0.014" putting the hammer spring virtually into coil bind?.... I can't imagine a hammer spring stiff enough to change 180 fps with that small an adjustment in preload.... How do you cock the gun, pull on a cocking lever with both hands?.... I just can't wrap my brain around that stiff a spring....

Bob

Flyby · Member

I still need to find a replacement hammer spring because the one in the rifle is the original minus a couple of coils cut off. I did that very early on when I first got the rifle used off the Yellow. The first spreadsheet showed the rifle was initially making 32 FPE, then FPS dropped off with nearly each following shot. There was no flat shot string, no bow shot string; Just a "ski slope". Cutting the OEM spring reduced power, and I was on a journey to try to find a way to restore some of the power, plus tune the rifle to a degree of consistency, as shown in the first spread sheet pf my post. I tried softer springs, then softer springs with spacers before actually trying to put a spacer behind the cut OEM spring (Chambers never seems to have any in stock, and so far I haven't had any luck sourcing one anywhere else either). I have spread sheets from where I've tried spacers of many different thicknesses, and start-fill pressures. I don't want to think about the money I've spent on pellets to do all the testing I've done, and I've primarily used the JSB jumbo Heavies. So I feel very fortunate to have used relatively few pellets in coming to this current result.

The rifle cocks easily enough in that it's not at all difficult to rack the bolt back. It takes more force to cock my Daystate Harrier (.177). Besides, with the Logun S-16 the cocking actually occurs when you shove the bolt forward; pulling it back advances the magazine for the next pellet. All very smooth and easy. Not the least fatiguing.

I believe it's the unusual air metering system that has to do with the power this thing is making. All the innards are stainless steel too. To directly answer your question about the spring, I don't think it's in a bind, but maybe it's under a bit of compression. But even if it is, that little bit of lube seems to have done the trick. Just look at the difference in the smoothness of the latest shot string compared to the prior result. I've been thinking of trying some of that tar used on the springs in springer air rifles. Same purpose, I think. Yes, that 0.014" made that difference in power (along with the extra 100 PSI added to the start-fill pressure. I think the hammer spring is a very high quality piece of steel. It's only 2.3" long with the coils being less than 0.050 in thickness (low 40s iirc).
Some images of my first tune, and of my S-16 disassembled (with a few props). BTW, the hammer spring in the photo is a softer spring of the same length as the uncut original spring. Note the white spacer on the spring guide. It's quite thick compared to the one in the rifle now, and that combo did not make near the power of my first best tune (28 FPE). Still, I agree that picking up over 10 FPE on average, with so little done to get there, is a real mind-blower. I'd rather be lucky than good. Wink