Tuning Fx Crown: Looking for the "lost Knee"

[Doc_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).

 

[Doc_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!

 

[Doc_pafumo]

Following the previous conversation, I maxed out the hammer spring on the FX Crown until there was no more slack at position 23 of the power wheel. The regulator is set at 93 bar, but I still can't find the plateau. As I increase the power wheel, the velocity keeps rising. In fact, I’m now noticing more inconsistency in the velocities I had chosen as ideal for the 18 grain JSB pellets, with greater spread and reduced accuracy. Too much hammer spring? Should I increase the regulator pressure?

 

[SilentSquirrel]

Oh poop!
I may just learn something here
FOLLOWING

just my 2 curious
Edward

 

[Trucker3573]

This has been a bit confusing of a read for me. When I do it with any gun I don’t use the power wheel whatsoever. All that is for is determining the “range” of your tune when it is finished. How far up or down you want to move from the knee. Just pick a number on it and dial too it. Now take the internal adjuster screw and turn it way down. Side note, this is assuming you want to shoot 18 gr pellets out of the 700 barrel. I don’t think you will need more reg. 95 should be close. Now that that internal adjuster is backed way down you start shooting over a chrono. Note speeds and start slowly increasing hammer tension with the screw, leave power wheel alone. You are looking for the spot where increasing the screw or hammer tension , causes no increase in velocity. Then you have found the knee. The velocity you achieve will tell you if the reg pressure is correct. If you want to use a variety of pellets I would suggest do this tuning method on the heaviest projectile you plan to use. Then dialing down with the bigger power wheel when shooting lighter ammo.

 

[NAProf]

Following the previous conversation, I maxed out the hammer spring on the FX Crown until there was no more slack at position 23 of the power wheel. The regulator is set at 93 bar, but I still can't find the plateau. As I increase the power wheel, the velocity keeps rising. In fact, I’m now noticing more inconsistency in the velocities I had chosen as ideal for the 18 grain JSB pellets, with greater spread and reduced accuracy. Too much hammer spring? Should I increase the regulator pressure?

Hi Doc,
Yep, if you have maxed out spring compression you only have two choices. A heavier hammer weight (you'll have to back off on the internal hammer spring adjuster because of the larger hammer head just to get the rifle to cock) or increase the reg pressure. It is way simplier to adjust reg pressure. If you watch the tuning video from Sub12Airgunners (given above) you will find that he sets the hammer spring to a mid-range value and the Power Wheel to something mid range also. Then he plays with the reg pressure to find where the velocity stops increasing.

I can't recommend that video enough. Watch it a few times and try to follow what he is doing. The Crown is a very versatile and accurate rifle. Keep at it to get what you want!

Cheers,
Greg

 

[nervoustrig]

A (mostly) complete list of things that make it easier to reach the velocity plateau, in roughly ascending order of effort/invasiveness/expense:

• increase hammer spring tension (preload)
• reduce regulator pressure
• install stronger hammer spring
• increase hammer weight
• change poppet to a harder plastic like PEEK or PEI
• increase hammer stroke
• reduce valve throat diameter (i.e. poppet sealing diameter)
• change the valve from a conventional knock-open type to a balanced valve or pilot valve

These measures can be done alone or in combination. Not all will apply to every gun on the market, and each has its own set of pros & cons to weigh that could probably fill a small book but hopefully it will be useful to look over the available options.

 

[nervoustrig]

As I increase the power wheel, the velocity keeps rising. In fact, I’m now noticing more inconsistency in the velocities I had chosen as ideal for the 18 grain JSB pellets, with greater spread and reduced accuracy. Too much hammer spring? Should I increase the regulator pressure?

The opposite of each...it would need either more hammer spring or a lower regulator setpoint (or both).

 

[Doc_pafumo]

Honestly, I'm a bit lost. What I do get is that I should mainly use the IHS screw to adjust hammer spring preload, and not rely too much on the power wheel. The goal is to shoot JSB 18 grain pellets at around 870–890 fps, with the lowest spread possible, so I’m aiming for a plateau speed around 915–920 fps.

What’s confusing is that some tuners as Meathead Marksman go top-down — they run high reg pressures, like 150 bar or more, and then back off the hammer spring from max until they hit a stable speed.

But in the Sub12Airgunner video, he does it the other way around: starts with a low reg pressure, then slowly increases it in 5-bar steps while also gradually increasing the hammer spring preload until he hits his target velocity.

So yeah, still figuring things out. I’ll let you know once I dial it in. Right now I think it makes more sense to start low and work up — drop the reg pressure, back off the hammer spring a bit, and increase step by step while testing.

 

[Trucker3573]

Honestly, I'm a bit lost. What I do get is that I should mainly use the IHS screw to adjust hammer spring preload, and not rely too much on the power wheel. The goal is to shoot JSB 18 grain pellets at around 870–890 fps, with the lowest spread possible, so I’m aiming for a plateau speed around 915–920 fps.

What’s confusing is that some tuners as Meathead Marksman go top-down — they run high reg pressures, like 150 bar or more, and then back off the hammer spring from max until they hit a stable speed.

But in the Sub12Airgunner video, he does it the other way around: starts with a low reg pressure, then slowly increases it in 5-bar steps while also gradually increasing the hammer spring preload until he hits his target velocity.

So yeah, still figuring things out. I’ll let you know once I dial it in. Right now I think it makes more sense to start low and work up — drop the reg pressure, back off the hammer spring a bit, and increase step by step while testing.

I would definitely start lower not high. If you start with the HS maxed that will mesh with a crown with a reg pressure somewhere around 150 bar. For what you are trying to do we know that is WAY too much. No point in over smashing the valve with a maxed hammer or starting where you know right off you will have to lower it. I’m not versed in crowns but I do realize they will need a bit more reg to accomplish the same thing an impact or maverick will. With your long barrel I think you might be very surprised what a 100 bar reg will do with 18 gr pellets. I would start down there with the internal screw backed way off and work it upward slowly. Just remember to take several shots at each point not just one.

 

[Doc_pafumo]

Alright, it’s working!!! I backed off the IHS until there was a good amount of free play at position 23. With the regulator set to 60 bar, I'm getting a plateau velocity of 830–835 fps. Tomorrow I’ll start increasing the reg pressure in 5-bar steps...

 

[Trucker3573]

Alright, it’s working!!! I backed off the IHS until there was a good amount of free play at position 23. With the regulator set to 60 bar, I'm getting a plateau velocity of 830–835 fps. Tomorrow I’ll start increasing the reg pressure in 5-bar steps...

Awesome to hear. That approach should work excellent. Please update this as I’m curious where you end up for reg pressure

 

[Doc_pafumo]

I'm still fine-tuning, and several doubts have come up.

The regulator pressure is set at approximately 72 bar, and with that, the plateau velocity sits right in the 874–879 fps range, which holds steady after several 1/4 turns loosening the hammer spring adjustment screw. Eventually, if I keep loosening the screw, the FPS begin to drop gradually: 869, 865, 862 fps… and this is where my questions arise:

1. Is that FPS drop the "knee" of the velocity curve, or is it just a normal downward slope within the plateau, before actually reaching the knee?


2. If that drop isn’t the knee yet (because it's too gradual), what kind of FPS drop should I expect to clearly identify I'm hitting the knee? Should it be a sharper drop—like 20, 30, or even 40 fps?


3. If my ideal shooting velocity is in the 875–880 fps range, should I aim for a plateau velocity that matches that exact ideal (while keeping the hammer spring as light as possible), or should I aim for a plateau that’s higher than the ideal velocity—and why? How much higher: 10, 15, or 20 fps above 875–880?


4. Is it advisable to fine-tune while the rifle is connected to the SCUBA tank to keep bottle pressure constant, or does that not really matter?


5. Lastly, what extreme spread (ES) value should be considered acceptable to consider the tuning process complete?



Thanks for your help.

 

[EPG]

I'm still fine-tuning, and several doubts have come up.

The regulator pressure is set at approximately 72 bar, and with that, the plateau velocity sits right in the 874–879 fps range, which holds steady after several 1/4 turns loosening the hammer spring adjustment screw. Eventually, if I keep loosening the screw, the FPS begin to drop gradually: 869, 865, 862 fps… and this is where my questions arise:

1. Is that FPS drop the "knee" of the velocity curve, or is it just a normal downward slope within the plateau, before actually reaching the knee?


2. If that drop isn’t the knee yet (because it's too gradual), what kind of FPS drop should I expect to clearly identify I'm hitting the knee? Should it be a sharper drop—like 20, 30, or even 40 fps?


3. If my ideal shooting velocity is in the 875–880 fps range, should I aim for a plateau velocity that matches that exact ideal (while keeping the hammer spring as light as possible), or should I aim for a plateau that’s higher than the ideal velocity—and why? How much higher: 10, 15, or 20 fps above 875–880?


4. Is it advisable to fine-tune while the rifle is connected to the SCUBA tank to keep bottle pressure constant, or does that not really matter?


5. Lastly, what extreme spread (ES) value should be considered acceptable to consider the tuning process complete?



Thanks for your help.

Hi Doc,

Here are some answers:

#1: You are “at the knee” of the velocity curve when you reach peak velocity.

#2: The knee is indicated by reaching peak velocity, it is not indicated by any “rate of drop” after reaching peak velocity.

#3: The is a lot of information published on this site as well as an excellent HAM (Hard Air Magazine) article that describe an ideal tune for most PCPs is the “96%” tune. If your target velocity is 880 fps, you tune (set reg pressure) so that max velocity is reached at 926fps. Then you dial the hammer spring preload back down to 880fps and you are at the “96%” tune. (880 is 96% of 926)

#4: You have a big 480cc air tank on your Crown. That is sufficient for keeping pressure stable enough while tuning.

#5: Extreme spread is overrated in regards to target accuracy - especially at distances under 50 yards. However, a reasonable spread with your Crown would be 10-12 fps.

Additional comments:
I’ve got two Crowns and have tuned a boatload of PCPs. I prefer to have regulator pressure of at least 90bar - ideally 100bar.

The reason is that your “regulator” (stack of bellville washers) is working really hard if you have 250bar of pressure on one side and 72bar on the other. If your tune requires 72bar, you might consider only filling your Crown to 200bar.

The “96%” tune is a good starting point- it is not some iron clad rule. You might find 98% or 95% of max velocity works best with your rifle. Remember that this tuning process is tuning your rifle to the balance point where it is producing the most power for a set plenum air pressure. It will be more efficient with air use the further you tune “above the knee” - i.e. if you tune to 90% of max velocity - the tradeoff will be a choppier shot string - more shot-to-shot velocity spikes.

Some rifles prefer to be exactly at the knee - JimD who shoots in our 30-Yard Challenge has such a rifle that performs best exactly “at the knee”.

I’d advise that you search AGN and find a link to the HAM magazine article on the 96% tune - and read it twice… then we’ll answer more questions

Edit: I found it for you:

Tuning Regulated PCP Airguns - Hard Air Magazine

The first thing you need to understand when tuning regulated PCP airguns is that for any PCP there is a maximum velocity for a given pellet,

hardairmagazine.com

-Ed