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Old 12-08-2015, 08:17 PM   #1
1tonne   1tonne is online now
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How to get maximum Joule Creep

-What is Joule Creep?
Joule creep is the effect in which heavier weight bbs exit the barrel at a higher energy level than a lighter bb. This normally occurs because the piston is traveling at different speeds as the bb exits the barrel.

-Piston weight and Speed
Different weight pistons have different effects on Joule Creep. Normally the heavier the piston the more the joule creep but this is up until a point as sometimes a piston can become too heavy and this will lessen the effect (depending on spring strength).
Now the way a light weight piston normally works is, as it is released, it nearly instantly gets up most of its speed and then continues through the cylinder. It may gain a little speed after its initial burst as it travels through the cylinder but most of the speed is somewhere near the start of the movement in the cylinder. In this instant a light bb will move instantly and a heavy bb will also move nearly straight away as there is a quick pressure spike that pushes it. So both bb’s eventually exit the barrel with the piston traveling at full speed. This lessens the effect of joule creep because both the heavy weight bb and the light weight bb are being pushed with equal amounts of power from the piston as the bb’s exit the barrel.
To make joule creep work the piston needs to be traveling at a slower speed for the lighter 0.2gm bb and faster for the heavier bb. To do this we add weight to the piston.
By adding more weight to the piston you will find that the piston will take longer to get up to the maximum speed. So as it is released the piston starts to move forward slowly and the light weight 0.2gm bb will also start to move as it does not take much pressure to push it. By the time the light 0.2gm bb reaches the end of the inner barrel the piston is only part of the way through the cylinder and it is still gaining velocity. The light bb exits the barrel before the piston can get up to its maximum potential. Now look at the difference with the heavy weight bb. The heavy weight bb takes more pressure to move and so once the trigger is pulled the piston moves forward slowly and the heavy bb stays still for a moment as the pressure builds up behind it. Once there is enough pressure behind the bb, the piston is already part of the way down the cylinder and gaining momentum. Now if the length of the inner barrel is correct then the bb should exit it just as the piston slams at the end. So the piston was able to get up to maximum speed just before the bb exited.
So the lighter bb was only able to get up to partial potential as the piston was not at its maximum speed when the bb left the barrel. The heavy bb on the other hand was able to stay still for a little while to let the piston get up its momentum and then the heavy bb started to move. By the time the heavy bb leaves the barrel, the piston is up to its maximum speed.
Now if you continue to add more weight to the piston you will find that it has a negative effect on joule creep. This is because the as the piston is released it starts to move forward slowly and because it is so heavy it only gains a little more speed as it travels through to the end of the cylinder. So when the 0.2gm bb leaves the barrel the piston is only going only a little slower than what it would be with the heavy weight bb. To counter act this, you need a more powerful spring.

-Cylinder Length
If you have a longer cylinder, the piston has more time to travel. This will give the piston more time to speed up. So, if you put weight on your piston, then it will initially travel slowly but it will have a longer period to speed up to its maximum. So this will increase the difference in energy levels between the light and the heavy bb.
It has been said that the more the volume in the cylinder, the more joule creep. I do not believe that a larger volume increases the difference between the energy levels of the lighter bb and the heavier bb. Often, to create more volume, most people will just put a shorter barrel in their gun to create a larger cylinder to barrel ratio. In a lot of stock rifles, this will give a better Cylinder to Barrel to BB Weight ratio for the heavier bb as most inner barrels are too long for heavy weight bb. So it is not the fact that the larger volume creates more joule creep, it is just that you have created a better ratio for the heavy bb. The key to joule creep is in the length of the cylinder matched with the correct weight piston and the correct length inner barrel.
Bolt action sniper rifles often have a longer cylinder than most other guns and they can create a lot of joule creep. In these rifles the volume, although larger, is not the contributing factor to Joule Creep. It is the longer length of the cylinder that creates the extra joule creep as the piston has to travel further.

100% Air Seal
In this, the words “air seal” covers, o-ring, cylinder head, seal between the cylinder head and air nozzle, seal between air nozzle and bucking, bucking rear burst and last of all the barrel bore size.
To get best results for joule creep you need a perfect “air seal”. This is because if your rifle has a leak this is wasted energy that could have been used to push the bb. If the system leaks when the piston is first moving the lighter bb will not move instantly. This will mean that by the time the bb does start to move, the piston would already be part of the way down the cylinder and it would have already gained some speed. So the piston speed will be similar to the speed of the heavy weight bb when it exits as it has had more time to accelerate and this lessens the effect of joule creep.

It is also good to have as tighter bore in your barrel as possible. This means that there is less air escaping around the lighter 0.2gm bb which means the bb should start to move sooner as the pressure will build behind it quicker. The sooner the light bb moves, the less time the piston has to speed up.
With a bigger bore there is a lot more waste air that sneaks around a heavy bb and so the heavy bb does not get up to its full potential where with a tighter bore, it will quickly produce pressure behind the bb and force it out with less air wasted. So with a bigger bore the light 0.2gm piston speed is faster and also the energy output for the heavy bb will be less. What you want is a low piston speed for the lighter bb and high energy output for the heavy bb.

-Cylinder to Barrel to BB weight Ratio
Barrel lengths are very important for joule creep. You need to tune your barrel to the right length. If your barrel is too long or too short, the effect of joule creep can lessen dramatically. For example if your barrel is too long your piston will have plenty of time to speed up and the light 0.2gm bb will exit the barrel with the piston traveling at a similar speed to what the piston would be traveling with the heavy bb, thus lessening joule creep.
Now if you make the barrel too short you can lessen the effect of joule creep too. This is because the heavy bb exits the barrel before the piston has reached the end of the cylinder. So the piston did not get up to full speed before bb exited.
What you have to do is match the right length barrel with the heavy weight bb that you are using. The lighter the bb that you use in game, the longer the barrel needs to be and the heavier the bb that you use in game the shorter the barrel you need. This is because a heavy bb will take more air volume to push it out of the barrel and if the barrel is too long, the piston will hit the cylinder head while the bb is still in the barrel and depending on your setup, this can cause a suction action that lessens the speed of the heavy bb. So, the heavier the bb the shorter the barrel should be but if a barrel is too short then the bb will exit before the piston has reached the end of the cylinder.
The advantage with shorter barrels is that when chronoed with a 0.2gm bb, the bb and piston do not have much time to speed up before the bb exits the barrel, while the heavy bb will get to its full potential. So, the heavier the bb, the shorter the barrel, the larger the difference between the energy of the light bb and the heavy bb but make sure you do not go too short as you want to use the full cylinder volume when using the heavy bb.
Also, one last thing about cylinder to barrel to bb ratio is that you are making full use of your cylinder and so therefore there is no excess air expelled from the barrel as the bb exits. This should increase accuracy.

-Spring Release Rate and Spring length
A lot of my experimenting may differ from other people. This is because different springs can release their energy differently to the springs I have use.
If the spring releases most of it's energy by the time it has reached 3/4's of the way down the cylinder, then the last 1/4 may not benefit the heavy weight bb much more. So in this instance, you may be better off using a shorter barrel than normal. This would mean you would get high energy from the heavy weight bb but the 0.2gm will not be able to accelerate up to full potential.
Spring length also makes a difference with the release rate
If a spring has been cut to be the same length as the cylinder, the spring will apply not much force when the piston has reached the end of its travel because the spring will be fully extended. This will mean that the piston will travel fast at the start of its movement but then only gain very little acceleration after the initial burst.
When making a JC rifle, you want the piston to travel slow as the start of its movement and fast at the end.

My own results:
A&K SR25 (Semi Auto)
Piston Weight: 100gm
Spring: M190 cut down
Inner Barrel Length: 205mm x 6.01mm
0.2gm bb’s = 420fps = 1.63 joules (Hopup Off)
0.45gm bb’s =392fps =3.2 joules (Hopup On)
Joule Gains = 1.57 joules
Note: This setup will put massive strain on your gearbox casing and internals.

So with this setup and a lot of tuning, I was able to nearly double the amount of energy output from my rifle.

For those of you that do not like airsoft science or technology like joule creep, you will be happy to know that I have not used the rifle with this setup in a game.

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Last edited by 1tonne; 11-16-2016 at 04:01 PM.
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Old 12-08-2015, 08:48 PM   #2
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Sweet guide! Now all we need is the right barrel to bb weight for a v2 and v2.5 gearbox and we are set. I bet you used a chrono to calculate the amount of joules you got per shot?
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Old 12-08-2015, 08:52 PM   #3
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How did you go about adding weight to your piston? I've used steel washers behind the piston head in order to add weight and correct AOE which I think put me nearish 100g. I know the Lonex red and pom piston heads are about 20g each and it felt like I doubled the weight with the washers. So I'm probably in the 70-90g range. Did you happen to play around with barrel length and spring weights in order to tune it? I'd be curious to hear what you're experience is with that if you have any. Also, I've included my cylinder setup in my results as it'd also be good to know.

My results:
JG G36C (it's not a DMR, but the only gun I've tried to tune for joule creep)
Piston weight: Lonex red, Lonex POM piston head, washers (unknown exact amount)
Cylinder type: Non-bore up full cylinder (AOE corrected with BOTH sorbo AND washers)
Spring: Prometheus M135
Inner barrel: Prometheus EG 247mm
FPS with .2g: 430fps 1.87j
FPS with .4g: 350fps 2.26j

.39j gain.

@TacticalBeard, you can use this to calculate it.

Last edited by CrusherW9; 12-08-2015 at 08:57 PM.
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Old 12-08-2015, 09:03 PM   #4
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Yeah thanks Crusher for the link. My question is about what barrel for what bb weight depending on the type of gearbox you have. This looks so simple but it must be so complicated to tune lol. Just getting the right piston weight, making sure no air escapes (on an aeg, can't pu your finger at the end of barrel right?).
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Old 12-08-2015, 09:29 PM   #5
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Gearbox type doesn't matter. There isn't a solid rule, at least that I've seen, in terms of how exactly you should have your gun tuned to joule creep. At this point, we (meaning the airsoft community) generally know more cylinder volume, heavier piston, shorter barrel. Even if you tune your piston weight in order to get the most creep with .4's for example, changing barrel length or cylinder volume after that will change your results. So it's a time consuming process.
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Old 12-08-2015, 09:46 PM   #6
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20g for a piston head seems way off. An aluminum head with all the hardware only weighs about 13g.

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Old 12-08-2015, 09:50 PM   #7
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Crusher, there is a difference between the two as they do not have the same cylinder volume...
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Old 12-08-2015, 10:12 PM   #8
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Not necessarily. All of the cylinders are interchangeable (excluding the SR25 and probably some other weird ones). The AUG and MP5K both use a V3 but the cylinder volume will be different between them because of the port difference.

@Corriander, yes, you're right. I thought I remember seeing about 20g at Clandestine but didn't bother to check. I just did and he says it's 8.8g.
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Old 12-08-2015, 10:12 PM   #9
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The way that I worked out how to get so much JC was firstly to get the correct cylinder to barrel ratio.
So I put in a 500mm barrel to start off with. The I would fire the rifle with the heavy bb and the hopup set. Then I worked out the joules and recorded them. I would then cut the barrel 10mm and rechrono the gun. Then work out the joules. The result with the highest energy rating was the best barrel for the bb weight. So for the 0.45gm the 205mm x 6.01mm was the best barrel. (NOTE: I also tested 0.3gm, 0.36gm, 0.4gm, 0.43gm and 0.45gm before I cut the barrel)
Once I had figured out the correct barrel length I then added weight to the piston by moulding lead into a cylinder shape that fitted into the piston.
After experimenting with piston weight I then redid the first stage again with different barrel lengths to see if piston weight effected how long the barrel should be. The results were the same as the first time I did the experiment and so the original results still stood.
Next I experimented with different strength springs. (This took forever)
All this experimenting took me about 6 months and probably a couple of hundred hours. I also spent a large amount on parts as you can stuff things easily.
I have probably had the gearbox apart a couple of hundred times.
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Old 12-08-2015, 10:24 PM   #10
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Interesting to see that the optimal barrel length didn't change after weighing the piston. How much of a difference did the piston weight and spring rate change the creep amount? Also, what's the point of the cut down M190? Was that what you started with and slowly cut coils to re-chrono? Or is it being cut down meant to decrease the acceleration time in order to get the .2 out of the barrel asap?
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Old 12-08-2015, 10:48 PM   #11
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Adding weight to the piston can increase the JC a lot. The more the weight the more JC but you do need to have a spring powerful enough to push the heavy weight.
I had to cut the spring down because I made a lead weight to fit in the piston. The weight was that long that when the piston was full compressed, the weight was only 1mm way from touching the spring guide. Since the weight was so long, this also meant that a normal spring did not have enough room in the piston to compress as a spring has too many coils.
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Old 12-08-2015, 10:59 PM   #12
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Just curious, but would you be willing to publish the data (barrel length, cylinder ratio, piston weight, etc.) for the other BB weights used?

I feel it would be extremely useful to build a gun that chronos well under the joule limit using 0.20/0.25g, but then joule creeps to the joule limit (but not over) on your desired BB weight.
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Old 12-09-2015, 02:56 AM   #13
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Prefer not to at this moment. I would like to keep some info for myself.
I actually finished doing these experiments about 6 months ago and it has take me this long to let people know this info.
So maybe in another 6 months I might put up the info.
One hint to the barrel sizes is that you need to keep the inner barrel under 250mm. So 250mm at most.
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Old 12-09-2015, 03:13 AM   #14
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Very interesting results 1tonne, I already assumed piston weight (and a few other factors) had an influence on joule creep (and efficiency), but I didn't think it'd be this much! I'll do some experimenting with it sometime

Something worth noting is that all joule creep builds are inherently inefficient builds. Both a high cylinder:barrel ratio and a heavy piston make not only .2g bb's inefficient, but the heavier bb's as well. The shorter the barrel and the heavier the piston, the more joule creep, but also the more inefficiency...

Cool stuff 1tonne, thanks for sharing :)
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Old 12-09-2015, 12:41 PM   #15
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Quote:
Originally Posted by Reliku View Post
Something worth noting is that all joule creep builds are inherently inefficient builds. Both a high cylinder:barrel ratio and a heavy piston make not only .2g bb's inefficient, but the heavier bb's as well. The shorter the barrel and the heavier the piston, the more joule creep, but also the more inefficiency...
That is actually a false belief. Most people thought this because they put in a very short barrel and then the joule creep phenomenon occurred.
In actual fact, the shorter barrel with heavy bb weights are not wasting much cylinder volume at all. If any. In my experiment I worked out what barrel length gave the most energy with the heavy bb weight. So this meant I had the correct cylinder to barrel to bb weight ratio. So no energy or cylinder volume was wasted.
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