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Old 02-25-2020, 08:46 AM   #1
Young Gun
 
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Negative joule creep on DMR

I have a problem when i start to apply hop up for a flat trajectory, my fps decreases, no matter what bb weight I use. So for instance I have 475 fps on 0.2g bb's which equals to 2.09J no hop applied. Whenever i use more and more hop my fps starts to decrease with small amounts until it hits a certain point where i can only achieve 0.4 to 0.5 joules. On heavier bb's where i need to apply more hop, my joules are quite low somewhat in the range of 1.4J to 1.8J (which depends on the weight and hop applied for a flat trajectory) instead of the 2.09J I achieve with no hop applied. I tested it with different lenghts of barrels, different sorts of buckings and nubs and even on a Joule Creep build. On the Joule Creep build i achieved higher Joules with no hop applied on the heavier bb but when I applied some hop, the fps started to drop again which still was achieving JC but quite less.

The set ups I was using:
M150 spring
Barrels from the ranges of 229mm to 620mm all tight bore barrels
I also used a Maple leaf hybrid bucking with a stock TM Gold Match barrel which caused the joule creep.
The buckings and nub swhere the Maple leaf bucking 60 and concave nub
some flat hopped prommy purple and g&g green with concave and flat nubs
The hop up chamber i used was the retro arms ICS model with an R-hop arm installed
For the gearbox I am using a closed cylinder witha stock cylinder and piston head and stock piston, the nozzle is a SHS one. Overall the system has good air seal and no feeding issues everything is working fine besides the joules when using hop.


Is there someone who can sort this out?

I was thinking of getting an R-hop installed to solve the problem because it needs less hop for the same amount of backspin on the bb due to the bigger contact surface it has. But I don't think it will solve the problem when using really heavy bb's like .40g+ which I do think will also need the amount of hop where it starts to decrease in Joules.
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Old 02-25-2020, 12:31 PM   #2
1tonne   1tonne is online now
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This is quite normal depending on the build. Normally the longer the barrel the less joule creep. So use a short barrel. Anywhere from 190-230mm should be fine.
Your piston may be extra light too. The heavier the piston, the more JC occurs. Also you need the air seal to be 100%. Unported cylinder is good. R-hop or Maple style buckings are good.
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Old 02-25-2020, 04:02 PM   #3
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I was planning to do the R-hop on my pdi raven barrel which is 229mm long which is an ideal length as you recommend. I think i'm also going to buy a maple leaf crazy jet of 180mm and compare this setup with the hybrid bucking against the rhop on the pdi barrel. How can I make my piston heavier? I know how to do it on a bolt action but on an aeg it's a different stoy where you have less room to work with. I was planning to use aluminium/steel parts from SHS because of the strength buth I don't think they are a lot heavier. How heavy do you recommend the piston to be for my setup? I know my stock one is around 30g with piston head.

Last edited by Juanasty; 02-25-2020 at 04:14 PM.
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Old 02-25-2020, 04:11 PM   #4
SHEED   SHEED is offline
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Quote:
Originally Posted by Juanasty View Post
I think i'm also going to buy a maple leaf crazy jet of 180mm and compare this setup with the hybrid bucking against the rhop on the pdi barrel.

I'm not sure crazy jet is the way to go, I believe they are ported so you loss air volume. Not that I've used one. If you're aiming for 0.40gs you're going to want the perfect amount of air.

Barrel length and cylinder length, piston weight is where I'd be looking. If you're massively over volume for a 0.2 you 'should' JC on a .40g
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Old 02-25-2020, 04:24 PM   #5
Young Gun
 
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I am using a full cylinder so the air volume should be enough for a crazy jet of 180mm, also the porting is on the end of the barrel so you will keep your pressure almost all the way through your barrel which schould not really influence it for use of heavier bb's although i hope so.
How can I easily add weight to my piston? I don't think it's that easy to do.
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Old 02-25-2020, 08:43 PM   #6
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Quote:
Originally Posted by SHEED View Post
I'm not sure crazy jet is the way to go, I believe they are ported so you loss air volume. Not that I've used one. If you're aiming for 0.40gs you're going to want the perfect amount of air.

Barrel length and cylinder length, piston weight is where I'd be looking. If you're massively over volume for a 0.2 you 'should' JC on a .40g

You can use a Maple 180mm. You could say it is a 170mm because port is about 10mm or so from the end. 170mm would be the shortest I would go. Though that will give good joule creep it will just be hard to get high fps/energy as the bb will not have long to accelerate.


Also, the tighter the bore diameter the more JC
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Old 02-26-2020, 05:14 AM   #7
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My goal was to have a fps between 400 and 450 with .20g bb's. I know i can reach that with my pdi barrel without putting a heavier spring. Though I still haven't figured it out to put extra weight to my piston easily. If the maple leaf barrel shoots too low even with a m170 which is the maximum I want to go then i am going to put in a weaker spring so I can easily switch between an assault class (360fps) and a DMR class(450fps).
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Old 02-26-2020, 12:50 PM   #8
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Adding weight to a piston is hard. Your best bet it to purchase an aluminium piston and maybe an aluminium piston head. A good weight to aim for is about 25-30gm. You can go more or less depending on your setup. I have even made an aeg piston up to 100gm.
You can make weights out of lead since it is easy to melt to go inside the piston. This will obviously put the spring under a little more tension but I have dine this many times. You just need to be careful if you make big weights that there is still enough room for the spring to compress and that the weight does not touch the spring guide when the piston is fully back.
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Old 02-26-2020, 01:14 PM   #9
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I have thought of 2 easy solutions to put on some extra weight on my piston. The first one is using a prometheus next gen piston head which is heavier due to the weight it already has on the back. The second solution is using a longer screw that fits on the pistonhead but isn't too long so it doesn't touch the spring guide and wrap some solder or lead cable around it with some tape to secure it. But first i'm going to see what the results are with the aluminium parts. If i'm not satisfied with the results i'm going to do some testing with putting extra weight on the piston.
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Old 02-26-2020, 01:48 PM   #10
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Your screw idea is similar to some of the weights I have made.
My version is to get a long screw that does not touch the spring guide and then cut a small piece of inner barrel to fit over it. Then melt lead into it. This has worked well and it strong. Your version can probably be heavier since you could wrap more lead tape around it and it should still handle the impact. Mine has a brass inner barrel that does not weigh as much as lead (works well though).
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Old 02-26-2020, 02:19 PM   #11
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Which size of screw will work for this? The lentgh doesn't matter since i can just make it shorter when it needs to be but the bore and thread size do matter for some snug fitting without breaking anything.
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Old 02-26-2020, 02:39 PM   #12
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I just use a longer version of the normal screw. Not sure on the thread size as I just got it out of an assorted screw box.
You will need to make sure the screw does not come apart. The advantage of using an inner barrel meant the screw would tighten up to the barrel. Where in your version, it will tighten up to the solder which can easily move.
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Old 02-26-2020, 03:56 PM   #13
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A quick quistion off topic. With a gun that has a good amount of joule creep with .4g bbs will it also have a good amount of joule creep with something lighter like .3g bb's? Off course it will be much less jc than a .4g bb buth will it still be a significant difference between a .2g bb?
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Old 02-26-2020, 05:54 PM   #14
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Quote:
Originally Posted by Juanasty View Post
A quick quistion off topic. With a gun that has a good amount of joule creep with .4g bbs will it also have a good amount of joule creep with something lighter like .3g bb's? Off course it will be much less jc than a .4g bb buth will it still be a significant difference between a .2g bb?

Most of the time it will still have JC but less than what a heavy bb will have
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Old 02-27-2020, 11:10 AM   #15
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Thank you for all the info given! I'll be busy for the next couple weeks with some testing, i'll post an update soon when I have some results.
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