Tuning Fx Crown: Looking for the "lost Knee"

[pafumo]

700 mm barrel. 95 bar regulator. Caliber 0.22. JSB 18 grain pellets. Front wheel: MEDIUM and dual trasnfer port in Slugs.
I fired 5 shots in each position of the power wheel (23 to 1) and calculate the arithmetic mean of each group of 5 shots. There is a downward slope but I can't find the famous knee. Be careful, theinflection from 11 to 12 is due to there´s a fault in my rear wheel and it goes directly from position 13 to 11. Any suggestion is appreciated...do I maximize the hammer, do I increase regulator pressure? I summarize by showing the graph.

 

[nervoustrig]

If the power wheel isn’t capable of reaching the velocity plateau, what you want to do is increase the baseline hammer spring tension using the adjuster at the back of the rifle. It’s a grub screw you adjust with a 1.5mm hex key.

Or alternatively, you can try reducing the regulator pressure so the existing range of hammer adjustment will allow it to knock open the valve sufficiently to reach max velocity.

Go with option A if you want a velocity over 900fps, or option B if you want under 900fps.

 

[Raider03]

@nervoustrig, Increasing the hammer wheel only decreases the velocity. Isn't the hammer completely past the peak and decreasing baseline hammer spring tension would raise the velocity at the lower settings ( the peak is off the chart too the left for the 95bar reg setting)?
Thanks.

 

[nervoustrig]

You may be right. I thought the graph was simply plotted in reverse.

If each increase of HST with the power wheel produces a velocity decrease, the hammer may be overdriving the valve stem, causing the hammer to crash against the valve body and rebound. When/if that happens, the valve doesn't stay open as long as it should, releases less air, thus velocity goes down. In which case the approach would be to reduce the baseline hammer spring tension (counter-clockwise with the 1.5mm wrench).

 

[pafumo]

Thx for your help. The graphic is inverted...1=23 in power wheel, 2=22, 3= 21...so, the velocity is down descrecing hammer spring power

 

[NAProf]

Hi Guys,
IMHO Nervoustrig had it right the first time. I would expect the knee in a 700mm barrel would be above 900 fps. I think it will help pafumo to understand a bit more about how the mechanical parts of the Crown action affect projectile velocity and accuracy. I say this because 90 bar is probably about right for a 18.13 gr .22 pellet in a 700mm barrel. The trick is finding the right combination of settings. I also agree that the Medium position on the Transfer Prot Wheel is good for this case and the Slug side of the barrel port is also fine.

There are two things that pafumo is missing. The Internal Hammer Spring adjuster (I call it IHS) and the additional hammer weight head size and weight. Turns out the weight of the additional hammer weight is not as important for pellets as the head size. That's because the additional hammer weights available range from about 10g to 13g (not counting the Tungsten weight at 27g which really only should be used for .30 size projectile) while the head sizes range from 1mm to 8mm. Each click on the Power Wheel (PW) changes the amount of hammer spring compression by about 0.2mm. So all 23 clicks would get you at most 4.6mm of change. The IHS changes hammer spring compression by 0.5mm per revolution. You can adjust that from a minimum of 2mm to a max of 8mm. The IHS and the PW work together so too little of the IHS sticking out of the back of the spring carrier adjuster will not even contact the PW at low settings.

Nervoustrig has it right in taking off the stock and adjusting the IHS. You could likely screw it in 4-6 turns and be pretty close to getting what you want. Take a look at Sub12Airgunners video on the Crown Disassembly to see all the parts (and maybe take the rifle apart yourself). They also have an excellent video on tuning the Crown.
Cheers,
Greg

 

[Raider03]

Hi Guys,
IMHO Nervoustrig had it right the first time. I would expect the knee in a 700mm barrel would be above 900 fps. I think it will help pafumo to understand a bit more about how the mechanical parts of the Crown action affect projectile velocity and accuracy. I say this because 90 bar is probably about right for a 18.13 gr .22 pellet in a 700mm barrel. The trick is finding the right combination of settings. I also agree that the Medium position on the Transfer Prot Wheel is good for this case and the Slug side of the barrel port is also fine.

There are two things that pafumo is missing. The Internal Hammer Spring adjuster (I call it IHS) and the additional hammer weight head size and weight. Turns out the weight of the additional hammer weight is not as important for pellets as the head size. That's because the additional hammer weights available range from about 10g to 13g (not counting the Tungsten weight at 27g which really only should be used for .30 size projectile) while the head sizes range from 1mm to 8mm. Each click on the Power Wheel (PW) changes the amount of hammer spring compression by about 0.2mm. So all 23 clicks would get you at most 4.6mm of change. The IHS changes hammer spring compression by 0.5mm per revolution. You can adjust that from a minimum of 2mm to a max of 8mm. The IHS and the PW work together so too little of the IHS sticking out of the back of the spring carrier adjuster will not even contact the PW at low settings.

Nervoustrig has it right in taking off the stock and adjusting the IHS. You could likely screw it in 4-6 turns and be pretty close to getting what you want. Take a look at Sub12Airgunners video on the Crown Disassembly to see all the parts (and maybe take the rifle apart yourself). They also have an excellent video on tuning the Crown.
Cheers,
Greg

IMHO, the chart turned out to be plotted backwards and it was reasonable to ask for clarification since that wasn't included initially. Not to say you're wrong...

 

[Jay308]

Best YT I’ve seen explaining tuning to the knee, good luck!