Short answer. A single to double spring conversion replaces one torsion spring with a pair sized to share the same door. Clopay ships its double car doors with one or two springs, and Wayne Dalton limits its single spring TorqueMaster to doors of 110 lb and under. With two springs, one break leaves half the lift, and each spring works less hard. It needs a second spring, a tube that fits both, and a tech to size them. U&D prices a two spring job at $350 to $700.
What the makers ship
Clopay's torsion spring supplement shows the split. Single car doors, 8 to 9 feet wide, get one spring. Double car doors at 16 feet wide get one or two. On the EZ-SET torsion system Clopay offers single car with one spring, double car with one spring and double car with two springs. So a one spring double door is normal from the factory. It is just not what we would choose.
Wayne Dalton draws a line by weight. Its TorqueMaster single spring system is for doors of 110 lb and below and always sits on the right. Doors of 110 lb and up get two.
Why a single to double spring conversion helps
Three things change with a single to double spring conversion. First, when one spring breaks, the other still carries roughly half the door. It is still a service call, but the door is less likely to slam or trap a car. Second, each spring in a pair lifts half the load, so its stress per turn drops and its cycle life goes up for the same wire. Third, the load on the tube is shared from both sides of the center plate.
Our spring calculator shows this in numbers. Put a door weight in with one spring. Then try two and watch the cycle life move. The page explains why most doors want two springs in more detail.
What changes on the door
The tube has to be long enough for both springs, and the center bearing plate has to carry two stationary cones. On Clopay doors that means longer hex bolts at the plate. Their supplement calls for 1 inch bolts on a single spring and 1 5/8 inch on a double spring at the center bearing plate. One spring has a red winding cone and the other a black one, because they wind in opposite directions.
On a TorqueMaster, Wayne Dalton's instructions run the other way too. A double to single change pulls the left spring and its gears, with a cone left as a spacer. Going from single to double means adding the left spring and drive gear.
Sizing the pair
Two springs are not two copies of the old one. The pair is sized together against the door weight and height. Clopay notes that EZ-SET springs may differ in size depending on door weight, and Wayne Dalton says a TorqueMaster pair can differ in length from each other by design. A tech measures the door, not the old spring. Our measurement guide and wire size page explain the numbers.
Price
From our spring replacement cost page, a single spring replacement runs $220 to $380. A matched pair on a two spring door runs $350 to $700, depending on spring size. A conversion lands in the pair range plus any parts the tube or center plate needs, and we quote it on site before any work starts.
Our opinion is plain. If your one spring double door has just broken a spring, that is the cheapest moment to go to two. You are paying for the visit and the unwinding already.
Book it
Winding a pair is trained tech work with proper bars. Call (778) 873-7282 or see broken spring repair. Parts are on torsion springs. Cones have their own page, winding cones.