Garage Door Cable Repair & Replacement

What the cables actually do

Each cable runs from an anchor on the bottom bracket of the door, up the side of the opening, to a grooved drum mounted on the torsion shaft. As the door closes, the cable winds onto its drum groove by groove; as it opens, it unwinds.

Because both cables share the load, they must wind at identical rates. That is what the grooves on the drums are for, and it is why drum position on the shaft is a measured setting rather than a rough alignment.

Fraying, and where it always starts

Cables fray at the two points where they bend most sharply: at the bottom bracket where the cable turns around its anchor, and at the drum where it enters the first groove. That is where to look, and a frayed cable is visible long before it fails — individual strands standing proud of the lay.

Once strands have started to break, the cable's remaining capacity drops quickly, because the load that those strands were carrying transfers to the rest. It is a part with a very short warning period and a very low replacement cost.

Cables that jump the drum

A cable does not have to break to cause a problem. If it comes out of its groove and rides over the top of the winding — usually because the door was closed on an obstruction and the cable went slack — that side of the door will then travel a different distance from the other.

The result is a door that lifts skew, binds in the track, and often gets worse each cycle as the cable works further out of position. Re-seating it means releasing the spring tension safely, re-winding the drum and setting both sides to the same tension.

The most common trigger is something left under the door: a bin, a bicycle, a bag. The door closes onto it, the cables go slack, and one of them jumps.

Rust and the bottom bracket

Cables corrode from the bottom up, for the same reason doors do — that end lives at floor level where water sits. A cable that looks fine at eye level can be significantly rusted in its last 300mm.

The bottom bracket deserves the same attention. It is the anchor point for the cable and it carries the full load of that side of the door. A bracket that has rusted, cracked or worked loose is as dangerous as the cable itself, and it is exactly what people grab when they try to free a stuck door by hand.

When a cable failure is really a spring failure

Cables and springs fail as a system. When a spring loses tension, the door becomes heavier, the cables carry more load than they were designed for and they fatigue faster. When a spring snaps outright, the sudden shock frequently takes a cable with it.

So a cable that has broken on a door with original springs is a signal to check the spring set, not just to fit a new cable. Replacing the cable in isolation leaves the underlying overload in place, and the new cable will fail the same way.

Why forcing a skew door makes it worse

The instinct with a crooked door is to lift the low side by hand or to run the motor and hope it straightens out. Both make things considerably worse.

A door out of square puts its rollers into the track at an angle. Under power, that bends the track, gouges the channel and can pull rollers out of their sockets — turning a straightforward cable replacement into cable, track and roller work together, and sometimes a damaged section as well.

Both, without exception. The two cables were fitted together, have carried the same load for the same number of cycles and are the same age. Fitting one new cable against one aged cable puts the door out of balance from the first movement, because the new cable does not stretch at the same rate as the old one.

The cost difference is small — cable is inexpensive and the labour is in releasing and re-tensioning the system, which has to happen either way.

The larger decision is whether the springs go at the same time. If the spring set is at or past its rated cycle life, doing both at once means one release of tension, one call-out and one labour charge instead of two within a few months.

Cables are cheap, and almost every cable failure we attend was visible for weeks beforehand.

• Look at the last 300mm of both cables occasionally — that is where fraying and rust start
• Never close the door onto an obstruction; if it happens, have the cable seating checked
• Keep water off the floor at the door line, which is what corrodes the cable ends and brackets
• Deal with a fatiguing spring promptly, because a weak spring overloads both cables
• Have the cables inspected at every annual garage door service
• Stop using the door the moment it sits skew, rather than one more cycle to get the car out

That last point is the expensive one. The difference between calling at the moment the door goes crooked and calling after three more cycles is usually the difference between a cable job and a cable, track and roller job.