Spring Loaded

Spring Loaded works out the rate of a coil compression spring from three
measurements you can take with a vernier caliper.

Enter the material diameter, the number of active coils and the inside
diameter, and it gives you the spring rate straight away — in N/mm, kg/mm
and lb/in at the same time, so you don't have to convert anything by hand.

WHAT YOU GET

• Spring rate in N/mm, kg/mm and lb/in
• Mean and outside diameter
• Solid height and total coil count
• Spring index (C)
• Available travel and the load at that travel
• Shear stress at load, with the Wahl correction applied
• Eight selectable materials — spring steel / music wire, chrome silicon
(SAE 9254), chrome vanadium (SAE 6150), stainless steel 302/304, stainless
steel 316, phosphor bronze, titanium 6Al-4V and Inconel X-750

Two optional fields sharpen the answer. Enter the free height and you get the
exact coil-bind travel instead of an estimate. Enter the travel you need and
it tells you the free height required to deliver it.

IT TELLS YOU WHEN THE SPRING WON'T WORK

Most calculators hand you a number and stay quiet. Spring Loaded flags the
things that actually bite:

• A spring index outside the 4–12 band that springs are practical to wind in
• A working stress past what the chosen material will take
• A free height long enough that the spring will buckle rather than compress
• A required travel the spring simply cannot deliver before it binds

CALIBRATE IT TO YOUR OWN SPRINGS

The textbook formula assumes an ideal spring. Real ones come out a few percent
different because of end treatment, presetting, wire tolerance and the actual
shear modulus of the batch.

Rather than guess at that, measure it. Tap Calibrate, enter a spring you
already know the rate of — one with the rate stamped on it, or one you have
put on a scale — and the app solves the formula backwards for that spring's
real shear modulus. The difference becomes a correction applied to every
calculation after it, so the numbers match the springs actually in your hands.

The correction is saved on the device and survives a restart. A CALIBRATED
badge appears on the result and the correction is shown as a percentage, so a
tuned reading is never mistaken for a nominal one. Remove it at any time.
A correction beyond ±10% is flagged, and beyond ±40% treated as a probable
input error — usually the wrong rate unit or a miscounted active coil.

You can calibrate the travel estimate the same way, against the measured
travel of that reference spring.

THE MATHS

k = G·d⁴ / (8·D³·n)

where D is the mean coil diameter (inside diameter plus material diameter),
n is the active coil count and G is the shear modulus of the material. Stress
uses the Wahl correction factor to account for the higher stress on the inside
of the coil.

WHO IT IS FOR

Motorcycle and car suspension work, valve springs, clutch and brake springs,
machine building, small engine repair, model engineering, restoration, and
anyone who has a box of unmarked springs and needs to know what is in it.

NO ACCOUNT, NO INTERNET, NO ADS

Spring Loaded is a calculator, not a service. It works fully offline — try it
in aeroplane mode. There is no sign-in, no advertising, no analytics and no
tracking of any kind. Nothing you type leaves your phone. The only thing
saved on the device is your calibration, and only if you create one.

A NOTE ON ACCURACY

The rates reported are theoretical, derived from the dimensions you enter.
Real springs vary. Calibrate against a known spring to absorb that difference,
and verify any safety-critical spring against a physical test or the
manufacturer's specification.