Torsion vs. extension garage-door springs — and the required safety cable

The spring counterbalances the door’s weight; the opener just guides it. Torsion springs mount on a shaft above the header (balanced, longer-lived, contained failure — today’s standard, needs ~10″ headroom). Extension springs run beside the tracks for low headroom — and every one MUST have a safety cable threaded through it (UL 325 / 16 CFR 1211) or a break flies loose.

What this diagram shows

Two garage-door counterbalance systems compared, because the spring — not the opener — lifts the door weight. On the left, a TORSION system: a spring wound on a shaft mounted above the header, winding lift cables onto drums at each end. It gives balanced lift, longer life, and a contained failure mode, which is why it is today’s standard, but it needs about 10 inches of headroom. On the right, an EXTENSION system: springs stretched horizontally alongside the tracks, which fits low-headroom and retrofit jobs. Every extension spring must have a continuous safety cable threaded through it, shown in red, per UL 325 and 16 CFR Part 1211 — without that cable a spring that snaps becomes a projectile across the garage. The safety cable is the single most-missed garage-door safety item.