One Torsion Spring or Two? When (and How) to Convert
Plenty of 16-foot doors leave the factory with a single torsion spring. It works — until it breaks, usually at 6 a.m. with the car inside. Here's the case for converting to two springs, and the math that makes the conversion straightforward.
The math: divide by two
A door needs a fixed amount of torque per turn — its required IPPT. Whether one spring delivers it or two springs split it, the total is the same:
Two springs each need exactly half the IPPT of the single spring they replace.
That's the whole conversion formula. A 100 IPPT single becomes a pair of 50s. The conversion flow in DoorBot's spring calculator does the lookup: enter the existing spring, get matched pairs ranked by cycle life.
Why two is better
- When one breaks, the door doesn't free-fall. The surviving spring carries half the load — the door is heavy but controllable, not a 200 lb guillotine. This is the safety argument, and it's enough by itself.
- Longer life at the same load. Each spring in a pair runs at lower stress for its size, which compounds into more cycles. Compare options in the cycle life calculator.
- Less shaft stress. Balanced torque on both sides of the center bracket instead of all of it on one end.
The honest downsides
Two springs cost more than one, take longer to install, and need a shaft long enough to fit both (almost always fine on residential). On a light single-car door, a quality single is perfectly defensible. On anything 14 feet and up, or any door over ~150 lbs, pairs win.
Doing the conversion right
- Confirm the door weight — don't trust the old spring to have been correct. Estimate or weigh it.
- Size the pair with the convert flow, matching wire, ID, and length between the two springs. Mismatched pairs load unevenly and fail early.
- Mind the winds: one left-wound, one right-wound — red cone right wind, black cone left — on the correct sides of the center bracket.
- Follow the procedure. Winding bars, not screwdrivers; the full sequence is in the torsion spring replacement procedure. Spring winding is the most dangerous task in door work — treat it that way.
Verdict: if you're already replacing a broken single on a double-wide door, the marginal cost of converting to a pair is small and the upside is permanent. Run the numbers and quote it both ways.