Spring-Power paradox: Cutting the spring to increase power?

[bennedose]

Earlier this year I had given my SDB 50 for respringing and the dealer proudly announced that he had put a powerful Cherokee spring in and demonstrarted how the cocking effort was ridiculously high. That was all very well but I found that the power that the rifle generated, as shown by muzzle velocity had gone down to useless levels, associated with a loud report with every shot and por accuracy. The only thing that was increased was the cocking effort/.

Many months ago I had reported on this forum that out of frustration I cut 2 inches off the spring and was amazed to see a phenomenal jump in performance. Muzzle velocity went up from 90-110 mps to 150 mps and accuracy became a real darling. However I did not have a chance to check outy this strange phenomenon until the last few days.

That Cherokee spring broke about 10 days ago and I bought a new local spring as replacement because I had more pressing issues on my mind (the rifle was auto-firing). After I set the auto-fire right I had a chance to play with the spring.

I tested the rifle with three different spring/spring configurations.

I put in a brand new 12 inch 40 spiral "Marshal shot" spring. The cocking effort was high but worse, the rifle was barely generating 75 meters per second (250 fps). I could alnmot shoot a target at 20 meters and then go have a cup of tean and return to hear the target being hit.

I also had a 11.5 inch local spring from which I cut off 1.5 inches (removed 4-5 coils) This was again giving me velocities of 100-110 mps (350 fps) - but accuracy was reasonable.

I decided to go radical and cut the spring down to about 9 inches by trimming another 3 coils off, leaving 31 coils (out of the original 40)

Once again my SDB showed a remarkable jump in performance. I started registering 140 to 145 meters per second (450 fps) and accuracy appears to be excellent. The performance is almost back to what it was before the Cherokee spring broke - and it may well get better as the spring gets set.

This is a really weird thing. One gets used to the idea that a powerful spring will make a Bofors out of a pea-shooter, but that is obviously wrong. There is a "sweet spot" in spring power/length that squeezes the best performance out of a springer. If your rifle needs a huge cocking effort, makes loud reports, and keeps you unhappy, try cutting your spring size. High effort and loud noises are for constipated people, not air rifles.

[brihacharan]

Bennedose wrote........
I decided to go radical and cut the spring down to about 9 cms by trimming another 3 coils off, leaving 31 coils (out of the original 40)
Once again my SDB showed a remarkable jump in performance. I started registering 140 to 145 meters per second (450 fps) and accuracy appears to be excellent.

> This certainly proves the point...
> The length / no. of coils in a spring have to match the optimum stroke length to obtain maximum 'swept volume' for the AR to shoot smooth & accurate.
> When the spring is too long (with excess no. of coils) it gets compressed to an extent during its installation - thereby reducing its compression strength - hence drop in velocity...
> While with reduced no. of coils the gap between the coils are not affected - which gives the spring its optimum compression strength & helps deliver a swept volume in accordance with the design of the AR.
> Given is a link which shows how this takes place....www.arld1.com/rifledynamicssmaller.html?
> The attached diagram shows the formula for working out the correct length of the spring when compressed...


> Its evident from the above that the 'optimum no. of coils' of a spring should be "30"...
Briha

[bennedose]

Many thanks for the useful info Briha

[Basu]

It is suggested that if one needs change the spring then it is always better to use new leather seal.
The old seal ,once taken out looses its set position and so it can not retain the compression.
The new seal will take position as per the pressure of the spring and so providing maximum power output after 100-200 shot.

Basu

[bennedose]

It is suggested that if one needs change the spring then it is always better to use new leather seal.
The old seal ,once taken out looses its set position and so it can not retain the compression.
The new seal will take position as per the pressure of the spring and so providing maximum power output after 100-200 shot.

Basu

I am sure you are right. But I still have 2 more springs to play with. When the experimentation is done I will change the seal. Part of the pleasure is that I have found the weirdest looking workbench at home which is custom made (by pure chance) to use as a spring compressor using my car.

[harshvardhan]

Stronger spring reduces velocity for piston bounce
While trying add 20 grms weight by adding washers
Inside the piston.hope it will give a boost
My 30 coiled cherokke 27 provide me 550 fps in my
Sdb 45 with awaysome accuracy

[bennedose]

Stronger spring reduces velocity for piston bounce
While trying add 20 grms weight by adding washers
Inside the piston.hope it will give a boost
My 30 coiled cherokke 27 provide me 550 fps in my
Sdb 45 with awaysome accuracy

Harshvardhan - I didn't exactly understand what you have suggested. Can you expand in some more detail?

[harshvardhan]

If the piston is lighter then with strong spring its bounces back.result is less velocity and poor accuracy.to overcome this you can add some wahers inside the piston.I suggest add weight approximately 20 grams.I have overcomed similar problem earlier and resolved by adding weight to the piston.hope it will solve your problem too

[brihacharan]

If the piston is lighter then with strong spring its bounces back.result is less velocity and poor accuracy.to overcome this you can add some washers inside the piston.I suggest add weight approximately 20 grams.I have overcome similar problem earlier and resolved by adding weight to the piston.hope it will solve your problem too

Hi Harshvardhan,
> What Bennedose is wanting to know (If I am right) is....
1. Where have you inserted the washers in the piston?
2. If you have inserted them inside the piston then the spring will jut out a bit longer & may cause cocking problem...
3. It would be best if you could make a drawing (even a sketch would do)and post it...
Briha

[harshvardhan]

Thanks brihaji.I will try to do it
In sdbs still there is about 2 cm area available
I have drawn a rough sketch...

u can utilize about half inch area inside the piston..it will not create any cocking problem

Thanks,

[brihacharan]

Hi Harsh,
Thanks for your prompt response
Your diagram very clearly explains what you have achieved....
Perhaps "bennedose" will attempt it and tell us his 'success story' soon....
Briha

[harshvardhan]

Thanks brihaji.
With our shared knowledges we are making
Indian ars heavenly

[Basu]

This is a very strange paradox.
Only reason could be the bouncing of piston, that affects smooth acceleration of pellet due fall in pressure .
A heavier piston carries more MV so bouncing reduces, thus allows pellet to accelerate.
My SDB 45 delivers 600 fps with 11.6 grns AimcoRH with 38 coil stock spring and piston.
To my knowledge IHP 35 piston weigh 290 grms and SDB 45 should be 270 grms.
May be piston weight makes the difference.

Basu

[bennedose]

Thank you all for your interesting inputs - will try changing the weight/inertia of piston.

[brihacharan]

Hi Guys,
> What an interesting topic for discussion
> Here are a few random thoughts on the subject....

Piston weight does affect performance but it is a tricky balancing act. The spring produces kinetic energy in the piston. The kinetic energy is used to compress the air. The piston should use up all of its kinetic energy just as it comes to rest. At which time the pellet has moved far enough down the barrel so that the air pressure remaining against the piston is just enough to balance out the spring preload.

Then you have no piston slam and no piston bounce. That is the ideal. But it is rarely perfect. Excessive piston bounce means you are not getting full power. Excessive piston slam can break things and hurt accuracy.

Hence as thumb rule the solution is:
• Excessive bounce - Increase the weight.
• Excessive slam - Decrease the weight.

If you are increasing the spring energy, it is possible that you will need to increase the piston weight in order to achieve maximum power. In my considered opinion performance wise pellet weight may also have a co-relation with the optimum piston weight.

In retrospect the design of an air rifle is totally based on the basic principles of mechanical engineering & physics. Now some food for thought…. Even air guns designed & manufactured abroad undergo “TUNING” after purchase by which:

1. Top Hats / Washers are added to increase the piston weight
2. Springs are cut to reduce spring wobble / twang
3. Piston weight is reduced by drilling holes in it
4. DELRIN buttons are added to reduce friction
5. Reduction in the size of the piston rod to enhance swept volume

So what do all these add up to – perhaps to arrive at the optimum energy deliverance that result in smoothness, accuracy & a pleasurable shooting experience!!!

Probably this is one reason why “Air Guns” are an “enigma” that both captures & captivates the aficionados’ imagination & enterprise to achieve near perfection in its performance!!!!
Briha