CZ P-10C - Striker Spring and HBI Trigger Rates

One of my pet peeves is how gun companies rate springs.  Springs are typically rated in force/distance.  Gun companies typically label springs with just pounds.  A Governent 1911's recoil spring weight is the pounds exerted at 1.625" of compression.  Commander? 1.125"  Officers ACP? 0.700".  So when I look at a Glock 5.5# striker spring, what is that actually measuring? 5.5 pounds per inch?  5.5 pounds when cocked in the gun?  A 5.5 pound predicted trigger pull weight?

Who cares right?  Springs are cheap.  Just buy a few and measure the trigger pulls.  But trigger pull doesn't tell the whole story about how a trigger feels.  Take-up and reset are also effected by different spring rates and lengths, as is the energy delivered to the primer.  I'm also a huge nerd and like to geek out on little details.  So if your eyes are glazing over, scroll down to the results below.

I figured that the only way to predict the effects the striker springs in my P-10C, would be to find their spring rate (pounds/inch) and calculate the spring pressure when the striker is in various positions in the slide.  I picked up a simple spring rater from Brownells and setup a super scientific spring lab.

The Spring "Lab"

Adjusting for 1" of Compression

I used a camera tripod to hang the scale and assembled each spring into the rater.  I pulled the red spring tube down to the table, measured the distance, then adjusted the legs.  I repeated this until the tube consistently hovered 1" above the table surface.  Then I could take a weight reading.  Pulling down on the red tube would compress the spring until it hit the table.  This gave me a rough spring rate in lbs/inch.

Then I measured the length of each springs at rest, and the length of the spring when installed on the striker.  I got 1.371".  Then I had to figure out the distance the striker compresses the spring when it catches on the trigger bar, essentially the half-cocked position.  I removed the striker spring, then measured the distance from the strikers rear face to the exit of the slide at full forward and half-cock.  Subtracting one from the other gave me an approximate distance of 0.316".  With the trigger held to the rear, I could measure the fully cocked striker at 0.437"  Unfortunately, I didn't have enough hands to get photos of measuring the striker positions.

Striker Spring Length Installed

With these measurements I could calculate spring pressure at the forward, half-cock, and break positions.  Here's the raw spreadsheet:

Here it is broken down to % reduction in spring pressure at three striker positions.

FWD is when the striker is fully forward in the slide and free of the trigger.  This is the last bit of "shove" the spring can give the striker before its momentum carries it into the primer.

Half is when the striker is caught on the trigger (trigger forward) at the start of pre-travel.

Break is when the striker is at maximum compression as it's released from the trigger.

Below are the trigger weights for each spring with HBI Theta Trigger, measured from the hook at the bottom of the trigger.

While it's obvious my measurements aren't 100% accurate, the spring weight data does correlate with the observed trigger pull weight.  However, the returns seem to diminish as decreasing striker weight does not scale linearly with weight at the trigger.  The HBI 3.5# reduces striker load at the break by 9% and results in a 9% lighter trigger.  The Wolff 5.0# reduces striker load by 32.6% at the break, but only 19.9% at the trigger. Interestingly enough, the lighter 5# spring has more spring pressure at the full forward position.  

Obviously there are a ton of other variables that effect trigger pull weight.  I'm not about to try to calculate the friction generated by each spring due to the slightly different diameters and number of coils.  No two springs are alike, and neither are guns.  But in my sample size of one, I'm going with the Wolff 5# Glock spring.

***Update***

I figured I should also calculate the amount of energy the spring can deliver to the striker.  I calculated the potential energy in Joules of each spring at the fully cocked position, just before the striker goes forward.  The results were interesting.

Due to the amount of preload, the lightest springs don't necessarily impart less energy.  With its longer starting length, the 3.5# HBI trigger imparts 7% more force on the striker than the stock one even though it reduced trigger pull by 9%.  Again, my measurements aren't perfect and I did not take into account the inertia of each spring or friction.