Extension spring
A long spring mounted parallel to each horizontal track that stretches as the garage door closes, used mainly on older and lighter doors.
- Torsion springs on a shaft above the opening
- Extension springs alongside the horizontal tracks
- Safety cable threaded through the spring — ask for these if yours lack them
Extension springs run alongside the horizontal tracks on both sides of the door and work by stretching. They are simpler and cheaper than torsion springs and are still found on many older single-width doors.
The drawback is the failure mode: an extension spring under tension that snaps can travel across the garage. Safety cables threaded through the centre of each spring contain it if it breaks, and any extension spring without one should have one added — the part costs very little.
Replacing a pair of extension springs costs less than a torsion pair, but extension systems balance the door less evenly and tend to need more frequent adjustment.
If your door has extension springs, the most valuable thing you can ask for is safety cables: a length of cable threaded through the centre of each spring and anchored at both ends. They cost very little, they are often absent on older installations, and they are what stops a snapping spring crossing the garage. Many technicians will fit them as part of a spring replacement if asked, and it is worth asking explicitly rather than assuming.
Extension systems also depend on a pulley at each end of the spring and a cable routed around it, which gives them more moving parts to wear than a torsion setup. If your door is on extension springs and both were fitted at the same time, replace them as a pair for the same reason torsion springs are replaced in pairs — matched springs keep the door level, and one new spring pulling against one tired one racks the door in its tracks. Many installers will quote to convert an extension door to torsion instead; on a door with adequate headroom that is usually money well spent rather than an upsell. Conversion also removes the springs from head height along the tracks, which is the part of an extension system that makes people nervous in the first place. Whichever system you keep, ask the technician to show you the safety cables before they leave. It takes ten seconds, and it is the one component people most often assume is fitted when it is not.