One spring or both? Why a replacement spring gets measured first

The spring that let go on your door was built for that door and no other. Not for the model, not for the brand. For the exact weight of those panels and the exact distance they travel.

That is easy to miss in Howard County, where a subdivision can go up with what looks like the same door on every garage, and where the detached garages out toward Russiaville and Sharpsville hold doors nobody has weighed since the day they were hung. Insulation, a steel skin over wood, a glass row up top, a replacement bottom panel. Each one changes what the spring has to hold, which is why the two questions everybody asks on the phone, whether both springs need doing and whether you can get one like the old one, are answered with a tape measure.

If one garage door spring breaks, should both be replaced?

On a door with two torsion springs, yes, replace the pair. The two are specified together, wound to one rating for a single door weight, and they share every opening equally. That makes the intact one the same age and the same rating as the one that let go, which is why it is the next call rather than a spare. Garage door spring repair on a two-spring door is quoted as a pair for that reason.

The rating is what makes that concrete. A standard torsion spring is built for 10,000 cycles, and on a two-spring door both springs wind and unwind on every one of them, so their counts never diverge. The survivor is the same part as the one on the floor, a few weeks behind. A standard torsion pair runs $220-$375 here, quoted as one line because it is one job, and what moves a spring price inside that spread is on the published price list.

The honest exception is a spring that was replaced recently. If one side was done a year or two ago and the older original is the one that just broke, the newer spring still has most of its life, and a single replacement matched to it is defensible. Say so when you call, because it changes what gets ordered.

How is a replacement garage door spring sized?

By four numbers off the spring itself, plus the door it came off. Wire diameter, inside diameter, overall length, and wind direction. Those four specify one spring out of hundreds that look alike on a shelf, and all four get measured in your garage rather than guessed. None of them can be read from a photo.

• Wire diameter. The thickness of the steel the coil is wound from, and the number the rating hangs on. It gets measured across a run of coils squeezed tight together and divided out, because two wire sizes a few thousandths apart look identical lying on a shelf.
• Inside diameter. The bore of the coil, taken across the inside of the spring or read off the stamp in the cone.
• Overall length. The relaxed length of the coil. On a broken spring both halves get slid back together first, and the broken section counts.
• Wind direction. Right wound or left wound, covered below.

Then the door gets its own numbers. Width, height, panel material, insulation or glass, track radius. Those decide the weight and the travel the spring has to match, and they are read off the door in front of you. A wood door that has spent decades soaking up humidity in an unheated garage does not weigh what it weighed new.

Why are there left-wound and right-wound springs?

Because a two-spring shaft is a mirrored pair. One spring is coiled clockwise and the other counterclockwise, so that both wind up as the shaft turns one way and both unwind as the door comes down. The winding cones are usually color-coded, one color per direction, which is how the wind gets confirmed before a spring goes anywhere near the shaft.

A left-wound spring in a right-wound position does not fit in a way you can work around. Wind it the direction the door needs and you are opening the coil instead of tightening it, which puts no lift on the drum and stresses the steel backwards. Turning it end for end solves nothing either, since the stationary cone bolts to the center bracket and the drums are handed as well.

Which system you have decides whether any of this applies at all, since extension springs are counterbalanced on a different principle and carry their own rules. Telling the two systems apart takes a look from the garage floor.

What happens when the wrong spring goes on a door?

The door stops being weightless. A correctly sprung door holds still anywhere in its travel because the spring is giving back very nearly what gravity is taking. Get the size wrong and that balance breaks in one of two directions, and which of the two it is shows up within a day of use.

Undersprung, the door feels heavy at the floor. The last foot of the close lands harder than it should, the opener labors on the way up and may stall partway, and lifting it by hand after pulling the release is a two-arm job. Oversprung, the opposite: the door creeps open on its own, lifts off the bottom seal, and the opener has to drag it back down against the spring.

Either way the opener carries the difference. It was built to steer a balanced door, not to lift an unbalanced one, and the usual response to a door that will not seat or will not rise is to crank the force limits to cover it. That leaves the safety reverse reading a door whose normal effort is already wrong. An undersized spring also spends its cycle rating early, working past the load it was rated for.

Winding a spring to tension is the one job that stays on our side of the line, for the reasons the maintenance guide sets out.

Why do some doors have one spring and others two?

Width and weight. A narrow single-car door can be counterbalanced by one spring sized to carry all of it, and plenty of older detached single-car garages are hung exactly that way. A wide two-car door is heavier and longer, and splitting the load across a mirrored pair keeps the shaft from twisting unevenly along its length.

The pair rule changes shape on a single-spring door. Nothing is ageing alongside the broken part, and the replacement gets sized for the whole door weight rather than half of it. What comes up instead is whether the door should have had two all along, since a single spring that keeps failing early on a heavy door is one doing a pair’s work.

Extension setups are always two, one per side, and they get specified as a pair the same way, each spring sized to half the door rather than all of it.

Does the door get balanced after new springs go on?

Yes, and it is the part of the job the springs exist for. Balance means the door stays where you leave it with the opener disconnected. Stop it about halfway up, take your hands off, and it should hold there. Drifting down means undersprung, creeping up means oversprung, and either way the tension gets adjusted in quarter turns until it sits still.

That check is where the tension actually gets set, which is why springs are wound on site instead of arriving pre-tensioned. Two doors with identical part numbers can still want a different number of turns.

Balance depends on the cables and drums too. If one drum sits a turn off, or one cable has stretched, the door hangs crooked no matter how well the spring is sized, so the cables get a look while the tension is off anyway. What a worn lift cable looks like is a check of its own. Opener force limits get set last, once the door is balanced.

What is worth writing down before the next one breaks?

The spring spec and the door size, written somewhere on the door jamb where the next person can find them. Your invoice should carry the four numbers already. Copy them onto a strip of tape or a label inside the jamb, add the door width and height and whether the setup is one spring or two, and date it.

The label outlasts anyone remembering the numbers. It is still on the jamb when the door changes hands, and it is there for the tenant or the house-sitter who has never looked above the opening. Next time a spring goes, the right one gets loaded the first time instead of after a return trip out to Galveston or Oakford.