Spring sizing (wire, diameter, length)
Spring sizing is the specification that matches a torsion spring to a door: wire diameter, inside diameter, overall length and wind direction, chosen against the door’s measured weight and drum geometry. The same door width can take dozens of different specs — which is why springs are measured, never guessed.
How the sizing math works, in owner terms
A torsion spring is an energy budget written in steel: at the door’s closed position it must hold enough wound torque to counter the full door weight through the drums’ leverage, and it must deliver that torque smoothly across the turns the door’s travel consumes. The wire diameter and coil geometry set how much torque each turn stores; the length sets how many turns fit within safe stress; the drum radius (from the cable-drum entry) sets how much torque the weight demands. Solve those against the measured pounds and the four numbers fall out — a genuine calculation, standardized in the trade’s rate books and calculators.
The same math explains the sizing decisions owners actually meet. Cycle upgrades work by re-solving the equation with longer, heavier-wire springs that flex less per operation — same lift, more steel, doubled calendar. Single-to-paired conversions split one spring’s duty across two smaller ones sharing the shaft. And the cardinal sin — reusing an old spec after the door changed (new insulation, added struts, replaced panels) — fails precisely because the equation’s left side moved. The spec sheet from the last service belongs with the house papers; the next visit prices faster and truer with it in hand.
The four numbers, decoded
| Number | What it sets | Typical residential range |
|---|---|---|
| Wire diameter | strength + fatigue life | ~0.192" to 0.273" and up by door weight |
| Inside diameter | hardware family fit | 1¾" and 2" dominate homes |
| Overall length | torque per turn, total safe turns | sized to the weight equation |
| Wind direction | which way the coil tightens | left + right mirrored on pairs |
| (Derived) cycle rating | fatigue life at working torque | 10k standard; 15–25k+ upgrades |
Using the spec like an owner
Ask for the numbers on the invoice
A spring line reading “torsion spring, pair” tells the next technician nothing. Wire, ID, length, wind and rating on paper is a five-second ask that upgrades every future visit.
Colour codes are the shorthand
The paint dabs on cones and coils encode wire size and wind direction by industry convention — the trade’s way of reading a spec at a glance. Useful cross-check; never the substitute for measurement.
Weight changes re-open the spec
Insulation kits, strut additions, panel swaps to a different construction: each moves the door’s pounds, and each is a reason the balance test follows the work. The spring spec serves the door as it IS.
Wrong-spec symptoms mimic other faults
Floor-heavy doors get blamed on openers; slamming doors on travel limits. The balance test arbitrates before parts money moves — the recurring moral of this entire glossary.
Spring sizing questions
Why can’t springs be ordered by door size like blinds?
Because weight, not width, is the variable that matters — and width predicts weight badly. A 16-foot uninsulated builder single and a 16-foot polyurethane sandwich double share a dimension and nothing else: their weights can differ by well over half, and the spring spec follows the pounds. Construction, insulation tier, added struts, even accumulated paint on an old wood door all move the number. The measured weight (scale under the released door, or arithmetic from the components) is where every legitimate spec starts.
What does each of the four numbers actually control?
Wire diameter sets the spring’s strength and, jointly with length, its fatigue life; inside diameter must match the shaft and cone hardware family (1¾-inch and 2-inch are the residential staples); length tunes the torque delivered per turn and how many total turns the spring can safely hold; and wind direction — left or right, mirrored on paired systems — makes the spring tighten rather than loosen as the door closes. Change any one and the balance math changes with it.
What happens when a spring is close-but-wrong?
The door works, badly, for a while. Slightly under-sprung: heavy at the floor, an opener carrying real weight toward its force ceiling, gears wearing on the spring’s behalf. Slightly over-sprung: a door that wants to fly open, slams at the top, and beats its top fixtures. Both states also spend cycle life faster — the spring flexes outside its intended torque band every operation. The balance test catches all of it, which is why it is the handback proof and the annual check both.