Airgun World - - More Tuning Options -

The stiff­ness of a spring is a mea­sure (called the spring ‘rate’) of how much force is needed to com­press it per unit of dis­tance, such as pounds force per inch, and so it is equally a mea­sure of the force the spring can ex­ert on the pis­ton as it ex­pands dur­ing the com­pres­sion stroke.

The stiff­ness of a spring varies by its wire di­am­e­ter raised to the power of four, so wire di­am­e­ter is the dom­i­nant fac­tor in de­ter­min­ing stiff­ness, and the wire di­am­e­ter of a spring can on its own tell you quite a lot about what the spring will prob­a­bly do to the shot cy­cle of your ri­fle.

2.9mm di­am­e­ter wire will pro­duce a spring at the soft end of the range, which will need a fair amount of preload, to give a gen­tle re­coil and re­duced surge. For a given muz­zle en­ergy, the soft spring should gen­er­ate lower peak cylin­der pres­sure and tem­per­a­ture.

3mm to 3.1mm di­am­e­ter wire is prob­a­bly the near­est to a ‘stan­dard’ air­gun spring likely to be used by man­u­fac­tur­ers for UK muz­zle en­ergy lev­els. It will need less preload than 2.9mm wire, will give a slightly quicker feel to the shot cy­cle, but with more surge travel.

3.2mm and above di­am­e­ter wire will pro­duce a stiff spring which, at UK muz­zle en­ergy lev­els, will need min­i­mal preload. The shot cy­cle will feel de­cid­edly ‘quick’, and there will be greater surge travel. Due to the min­i­mal preload force, the pis­ton will not loi­ter in the vicin­ity of the cylin­der end, so that for a given muz­zle en­ergy, the stiff spring will have to gen­er­ate higher pres­sure and tem­per­a­ture.

An­other fac­tor that af­fects spring stiff­ness is the di­am­e­ter of the coil, and stiff­ness varies in­versely as the mean di­am­e­ter to the power of three. Most air­gun main­springs are in the re­gion of 19-21mm (most are 20 point some­thing mm), so the lim­ited range of di­am­e­ters means that it has noth­ing like the effect on stiff­ness of the wire di­am­e­ter.

The third fac­tor that de­ter­mines spring stiff­ness is the length of the wire, taken as the num­ber of coils, and the longer the wire, the greater the num­ber of coils, the less stiff the spring.

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