Constant rate springs are linear devices: the force a spring responds with is directly proportional to the amount (distance) it is compressed from its free length. The distance and force are related by the spring constant which is a property of a given spring.

Since recoil spring manufacturers don't supply conventional spring data, it is useful to know how to calculate a spring constant. The obvious way is to put a weight on the spring and measure the spring's deflection, and there are kits to do this (or you can make your own).

Theoretically the formula for light weight springs (thin wire compared to outside diameter) is k = (G*d^4)/(64 R^3*n). G is the modulus of rigidity; its value is around 11.6*10^6 pounds per square inch for steel spring wire. It is different for other materials, such as stainless steel. d is the diameter of the spring wire. R is the radius from the axis of the spring to the center of the spring wire; numerically, that would be half the quantity (outside diameter of the spring minus the diameter of the spring wire). n is the number of active coils (...don't count coils which are touching each other at the end of the spring).

In the case of any coil recoil spring of the 1911, there are three dimensions which are of prime importance other than the spring constant. They are the free length, the installed length, and the fully-compressed length. The difference between installed length and free length can be called X1 and the distance between fully compressed length and free length can be called X2. When the action is closed, the force tending to keep it closed will be the recoil spring constant times X1. When the slide is drawn fully back, the force trying to return it will be the spring constant times X2.

The length of the spring does not affect the spring constant. Making a spring with the same number of coils, wire diameter, and outside diameter but with a longer free lenght (or permanently stretching one) only changes X1; this causes the force at the installed length to be higher, but it does not change the spring constant.

If you cut a couple of active coils off any spring, you will lighten the maximum force since X1 and X2 will be smaller but you will also make the force curve peakier because the spring constant will be greater.

Work is force times distance or: integral F(s)ds. If you plot the function of force versus distance the area under the curve will be this integral. Since the spring is a linear device, the area under the curve is a triangle and the area of this triangle is one half the ordinate times the abscissa. Since k.X is the force of a spring, the potential energy of a spring (the work absorbed by it) is 1/2 k X^2.

The energy absorbed by a recoil spring when the slide moves from fully closed to fully open is = 1/2 k ((X2^2)- (X1^2)) .

This is equivalent to subtracting the area of the triangle associated with the installed compression of the spring from the area of the triangle associated with full compression of the spring.

Only a small percentage (about 20%) of the kinetic energy of the load can actually be absorbed by the recoil spring, a part goes to work the hammer and compress the main spring, some is lost to friction, and by far the larger part of this kinetic energy goes to impact the frame (again a small percentage when the barrel's lower lugs hit the frame in recoil, and a the larger part at the end of the rearward travel).

At the moment the gun fires (slide and barrel recoiling about only .10") actually very little energy is absorbed by the recoil spring.

I know this is a long and unsolicited explanation and all of you know I'm just a humble aficionado, so please be gentle with your comments