Airsoft Labs by CMEE

Guide

Spring

The basics

The simplest thing is to pick a preset matching the label on the packaging (M100, M120, SP110…) in the Spring → preset field. The presets are calibrated to the nominal energy: M100 ≈ 1.00 J, M120 ≈ 1.44 J, SP110 ≈ 1.51 J and so on.

The simulator supports linear and progressive springs. Linear ones (marked M) have all their coils evenly spaced, whereas on progressive ones (marked SP) the coils are unevenly spaced.

If you’re sure your spring doesn’t match the presets (it’s more likely that the replica you measured it in isn’t optimally set up) – you can adjust the Stiffness parameter until the simulation output gives you the energy you want.

Measuring a spring's free length with a tape measure

Measuring the spring guide bearing

Measuring the spring guide bearing thickness with calipers

Measure the thickness of the ball bearing (or metal / plastic spacer) on the spring guide with calipers, in millimetres, measured from the plane in the gearbox from which its length is also measured (to the front edge of the cylinder). Enter that thickness into Spring → spring guide bearing.

For guides without a bearing you normally enter 0 – but watch how the guide seats: some sit a bit forward of that plane even without a bearing, so you have to include that offset in the value.

For those who count coils on the spring

A linear and a progressive spring of equal power (1.8 J)
Top linear, bottom progressive spring. Both 1.8 J.

What the spring markings mean: the M mark is for linear springs. On those, compressing the spring by two inches takes twice the force needed to compress it by just one inch. The force grows linearly with compression – hence the name. The SP mark denotes progressive springs. On those the force doesn’t grow linearly with compression, but increases progressively. That makes it easier for the motor to get going and reduces wear on the mechanism over time. The main advantage of progressive over linear springs is that, thanks to their uneven coils, they damp firing vibrations faster and therefore ring less.

The number on M springs tells you the speed (in m/s) a 0.20 g BB would reach in a properly dimensioned replica: M120 means 120 m/s. Non-linear SP springs tie the same number to a 0.25 g BB – which automatically means 25 % more energy than an M spring with the same number.

Incidentally, 25 % more energy means only 11.8 % more speed, because energy grows with the square of speed and in linear proportion to mass.

Don’t blindly trust the number

One manufacturer’s springs can differ quite a bit from another’s springs with the same marking. The problem is that the measurement method isn’t precisely defined. Yes, it measures the BB’s energy at the muzzle – but there’s no convention for how the replica should be set up or which BB mass to use.

The Lab presets hit the nominal value on an ideally set-up replica, so if your chronograph disagrees with the simulation – before you doubt the model, doubt the spring’s declared rating, or the accuracy of the data you fed the simulator. Cheaper chronographs can also show all sorts of things.

I’ve noticed that since around 2023 quality control has slipped at many – even brand-name – makers, with two “identical” springs varying quite a bit in power. So it’s best to keep your own reference replica in which you chronograph a spring’s actual power.

Advanced: custom spring

For non-standard springs you can bypass the preset and enter everything manually:

  • stiffness k (N/mm) – how many newtons it takes to compress the spring by 1 mm. You can measure it with a weight of known mass: put the weight on the spring and watch how far it compresses – but be sure to start from the fully relaxed position. E.g. a 1 kg weight (≈ 9.8 N) compressing the spring by 10 mm means k ≈ 0.98 N/mm.
  • free length – tape measure, spring unloaded
  • spring mass – a scale reading to 0.1 g will do

Modelling a non-linear spring

For non-linear springs, besides stiffness, free length and mass, you also have to enter the wire thickness Wire ⌀. You add coil groups by clicking the Add group button and then entering the coil count (need not be a whole number) and the pitch in the group. There are usually two alternating density groups. The terminator – the compressed and partly ground-down coil at each end of the spring – isn’t counted.

For an example, look at how the SP springs are modelled in the presets.

Bearings

On the spring guide, and/or in the piston, usually sit bearings – most often an axial (thrust) one with its ball race, or metal / nylon spacers.

If the spring is too strong, you can dial its power down by removing one of those bearings – or just a part of one, e.g. the ball race – which reduces the thickness. But careful: if you pull a bearing out of the piston, the complete piston with its head must be re-weighed and the new mass entered into piston + head mass.

The thicknesses go into spring guide bearing (with the spring) and piston head bearing (in the piston settings block).

Cutting the spring

Another way to bring energy down: cut off a coil or two, which lowers the spring’s preload. The catch is that a shortened spring becomes stiffer per millimetre (fewer active coils doing the same job), but the drop in preload dominates.

The process is fairly counterintuitive, so if you shorten it “by eye” you’ll very likely get it wrong. That’s why a spring is usually cut a little at a time, checking on the chronograph.

The simulator can make this much easier: under Advanced there’s a spring cutoff setting – enter how many millimetres you’d cut off and it works out the resulting output power for you.

Tip: a stronger spring is not a free win. The simulations show that efficiency drops with power – a weaker spring converts over 60 % of its work into BB energy, while the strongest ones fall below 50 %. More violent compression means more heat and more losses. So: the weakest spring that reaches your target energy is also the most efficient one – don’t “over-spring and compensate”.