Volume, short stroking and what is really needed

[SSPKali]

Hi all,

Now GBBR weather is here I am fettling my franken TM&C M733.

I was running 18:1 gears (2 from pick up / 2 from release) with Warhead 35k and 7.4v or 9.9v. Was snappy but a bit noisy. M110 spring giving 1.0J on 0.28g

Noise was probably due to the pad on the cylinder head being pounded into submission as it was the OG TM one and probably old enough to vote! Plus the nozzle was lose and wobbly contributing to midcap syndrome.

New SHS cylinder head on its way, POM piston head and some SHS 13:1 gears to fit. The cylinder is 3/4 ported and probably has a ~36mm air stroke. This is probably ok for the 300mm barrel and 0.28g BBs but might be slightly under according to some?

By dry fitting it with an old sacrificial gear and cycling it looks like -4 on pick up -1 on release is what is needed to stop over travel on the piston….this seems rather extreme!

I was going to start at -2 pick up as gears will be cycling faster and I don’t want tappet timing issues! 
 

What are peoples thoughts? Any experience with this?View attachment 126617

 

[Pseudotectonic]

You actually don't need short stroking, the "over travel" actually increases your FPS because it accelerates the piston before it goes into the compression zone as opposed to accelerates from within the compression zone, so the overall compression happens faster i.e. there is actually gain in air pressure from the "pre-travel"

Also try putting on the tappet delayer chip on the sector gear to see if it improves feeding, if there is any feeding issues

 

[Leo Greer]

"Over travel" is a good thing, not a bad thing. I have no clue why people try to avoid it. That extra space for your piston to accelerate increases your FPS efficiency by a significant amount (by increasing air pressure), allowing you to use a weaker spring and have an all-round better time.

Heck, if you're not having PME issues there's no reason to short-stroke either.

 

[EDcase]

Yes, the whole point of having a ported cylinder is to allow the piston to accelerate before beginning the compression phase.

(Short stroking may be more useful with pistols)

I may have heard that cats like short stroking... ?

 

[SSPKali]

Ended up short stroking to a) drop power and b) speed up cycle time.

Seems my M110 spring was actually a M115 so 3 from pick up / 1 from release to get down to 285fps on a 0.28g which seems best performance from testing. 0.25g were a bit light, 0.32g needed lots of hop and dropped a load of speed.

Level four pre-cocking on the Perun AB++ makes for very snappy response and full auto is hovering around 19-20RPS for the once a year I use it.

Still being released beyond the port so getting some acceleration before making an air seal. Not sure something as light as a polymer piston is going to have much momentum to make a noticeable difference but this is airsoft so ??

 

[Sewdhull]

From a physics point of view, even if the piston is not compressing air its still receiving the energy from the spring which it will then transfer to a smaller amount of air depending on the porting of course raising the air pressure.

The speed of compression is higher which creates higher pressures which is more force on the bb even if the energy in the spring and therefore the air is the same.

If you don't need the air volume you don't need a full cylinder but that's not a reason to short stroke. If you only compress the spring by 60% of its usual level that's 60% of its normal energy and if you don't need the energy, then why not just use a weaker spring?

Physics is physics even if it's airsoft.

I reduced my spring rate to get my fps down, which raised the rps of course. 20rps shouldn't cause any issues in normal guns.

 

[SSPKali]

Ok, but if you don’t need the air volume why waste time and battery energy pulling the piston all the way back for every shot? 

 

[Sewdhull]

Its not wasted energy, the same air is being compressed but faster and because the bb moves as soon as the pressure is enough to move it, a faster build up of pressure means the pressure is higher earlier as the bb moves along the barrel and why you need a stronger spring to compensate for the loss of spring travel if you compress it less, to maintain the fps. 

If you choose to have a shorter travel for the piston you must put the similar energy in to get similar energy energy out, but since you are increasing the load you will need more current and so the electrical losses are higher. The mechanical stresses are higher also. 

If you are compressing more air than the gun can use it is wasting energy, which is why cylinders are ported to match the barrel length (volume). Since air leaks out from various places a heavier BB will need more air because the time the leaks are happening are longer as the BB needs more time to exit the barrel. 

So far as time is concerned, if you get 20rps short stroked on a stronger spring or 20rps on a normal stroke and weaker spring, there no loss of cycle time or wasted time compressing the spring.

If you put a certain amount of energy into a spring over time and then the same energy in a shorter time, there is no time wasted if the energy gets used.

People will short stroke to avoid PME, where the spring doesn't push the cylinder back into position before the gear comes around for the next cycle, but this is something that happens past 25rps and would initially be compensated with a stronger spring to get the piston back sooner perhaps in combination with short stroking to keep the fps under control, or various mixtures of the 2. 

It's worth bearing in mind that you don't get something for nothing, if you have more RPS you will use more energy than lower RPS in the same time however you do it. Electrically  you want the least current to minimise the losses and mechanically optimise the energy use needed to get your outcome. Compressing a stronger spring is less efficient than a weaker one in the same time for example, but efficiency is not always the goal.

If you want to have a really high ROF you'll need to sacrifice electrical efficiency and mechanical efficiency to get closer to your goal. Motors will work hard to compress the spring, the gears will have more friction because the pressures are higher on the teeth, and all the insides will experience higher forces making wear higher, damage more likely and timing of the bits that care about timing more important and less forgiving.

 

[Leo Greer]

I should add, the amount of “cycle time reduction” you get from SSing is functionally ineffective, since you could just use a weaker spring and achieve the same faster cycle.

The only time I think it’s useful to SS (period) is to use a very stiff spring at high RPS while keeping the FPS down. This allows for low FPS and high RPS with no PME. And by high RPS, I mean 35-40 RPS SSGs.

 

[SSPKali]

Spare 13:1 gears and M105 spring ordered as I like empirically testing such things, also interested to see if it is quieter with longer stroke / softer spring. Happy with performance now but it does give a bit of a SMACK at end of the stroke (even with POM head and padded cylinder end!) ?

 

[Sewdhull]

It's useful to know how your own setup performs if you like to fiddle. I'm the same

It's difficult to remove that smacking, some used an airbrake but it's very difficult because of the way the guns work.

If the cylinder maintained pressure maybe you could do it, but it might rebounding anyway so I dunno

 

[Leo Greer]

Silicone cylinder head buffer mod, if you have enough space. Remove the previous buffer, sand the area rough, use fishtank silicone to create a buffer, and then cut it to the correct size. In my experience this mod performs much better than sorbo or other typical buffer materials. Additionally, silicone instantly rebounds to its original volume, so your cylinder volume is the same each shot (vs sorbo where your FPS can get wonky due to the material not being elastic enough).

 

[SSPKali]

Silicone cylinder head buffer mod, if you have enough space. Remove the previous buffer, sand the area rough, use fishtank silicone to create a buffer, and then cut it to the correct size. In my experience this mod performs much better than sorbo or other typical buffer materials. Additionally, silicone instantly rebounds to its original volume, so your cylinder volume is the same each shot (vs sorbo where your FPS can get wonky due to the material not being elastic enough).

Never cone across that before, although I have seen the silicone pads that places sell to replace the std rubber pad on the cylinder head. 
 

How durable is the silicone? I have a spare SHS cylinder head so can give it a go.

 

[Sewdhull]

Rtv silicone varies in hardness. You'd want a recommendation for some. 

 

[Leo Greer]

Never cone across that before, although I have seen the silicone pads that places sell to replace the std rubber pad on the cylinder head. 
 

How durable is the silicone? I have a spare SHS cylinder head so can give it a go.

Rtv silicone varies in hardness. You'd want a recommendation for some. 

I and others have had good consistent success with fishtank silicone or others silicone meant to bind glass. As for exact brand, I use Locktite brand (I believe) but I haven't done much experimenting since what I have works. If you'd like some more in-depth info, there's a thread on the subject by the creator on the AirsoftSniperForum under Long Range AEGs.

 

[SSPKali]

I have a few different shore rating spacers on their way to me and I’ll give them a go. I read up on the silicone mod and it sounds a bit fiddly but can be tuned to suit the build and customised.

Just rebuilt the gear box with a full tooth sector and M105 spring - might be running a little quieter and an extra beep of pre-cocking it is still very snappy.
 

Still need to check FPS but that is a bit of a PITA and I need to take it apart again for the buffer pad ?

 

[SSPKali]

So the M105 spring (which should give about 105m/s) was giving 118m/s! Seems the air seal and shimming is really good.
 

M95 ordered from AK2M4 and fitted with the 40shore piston bumper on the cylinder head and full tooth sector gear.

Now out putting 278fps on 0.28g with a little hop and seems a little quieter. 6 beeps of precocking and it is really snappy and can give 21rps although I have it on 3rd burst to conserve ammo.

Turningy 1000mAh 7.4v fits in the hand guard really well and I have heat shrinked the cables to the barrel to keep them out the way and make battery changes less of a ball ache.