ISMI vs Wilson Spring Length

[BangSwitch]

I am trying to get a soft shooting setup from STI DVC 3-Gun with 147gr factory ammo and a 9# recoil spring.

 

I had a 9# Wilson Combat spring (shown on the left) in my inventory.  I picked up two additional ISMI chrome silicon springs (link) for my spares box.  

I was surprised when I put the two springs side-by-side.  The ISMI spring (pictured on the right) is significantly shorter than the Wilson Combat.

 

Why would two springs of the same weight be so different lengthwise?  What accounts for the difference and should I be concerned?

 

[PaladinPrecision]

They are made by different people, just like race cars, you can engineer several different products different ways and come up with comparable results at the end of the race. 

If they varied from one mfg, then I would worry about it. But there will always be variances in the specs that different shops specify when they make something. I would just make sure you use Chrome Silicon springs and go from there. Keep in mind that one brand 9 lb might be 9.4 lbs and another could be 8.5 

[BangSwitch]

3 minutes ago, PaladinPrecision said:

They are made by different people, just like race cars, you can engineer several different products different ways and come up with comparable results at the end of the race. 

...great analogy; especially since I used to own a race car!  

 

If the springs were just a little different in length, I guess I wouldn't be concerned.   I was surprised to see what looks like maybe 15-20% difference.  

I need to fabricate (redneck engineer) a spring testing rig to work with my trigger pull gauge.  It would be nice to know for CERTAIN what weight a spring actually is when I go to put it in service.  

[PaladinPrecision]

I would test them dynamically and see if you can tell the difference ( in this case slow motion analysis, but I doubt there would be a sugnificant difference between 0.5 lb on video)... Or possibly use the longer one as a half weight between 9 and 10 lbs. 

If you were working on a spring rate gauge, I think the best benefit would be the ability to quantify when a spring needs to be retired. But sounds like a lot of work for a 9 dollar spring lol. I would probably work on the low hanging fruit. like dry fire or practice lol 

I would rather test which one loses rate to determine which brand to use. I do like your tag method. I might have to borrow that.

Edited November 19, 2017 by PaladinPrecision

[Guy Neill]

Are they the same wire diameter?  The worry is that a spring will reach solid height (no gaps) when fully compressed (or trying to be compressed) at the full recoil length.

 

In the gun we want the spring to compress to full slide retraction without reaching solid height.  At full slide retraction I think the spring will be at about 1.8 inches.  You might measure the spring wire diameter and multiply by the number of coils to get some approximation of the solid height, which should be less than the 1.8 inches.

 

It may also be that the Wilson is pre-set - meaning it was compressed during manufacture to take a set prior to use.  New springs, once compressed, will take a set and not be quite as long as before being compressed.

 

Guy

[chuckstur]

As used in 1911 parlance, the advertised poundage is not a rate but a force developed at a particular length.  I don't know about other designs.

[lefty o]

with a spring, wire dia, number of coils, length,heat treat all affect its operation. sadly, but  #9 spring isnt just a #9 spring.