The opener is not lifting the door
Take the counterbalance away and a 218 lb sectional door needs 218 lb to hold it. Put the springs back and the same door needs whatever the springs did not account for, which on a door left alone is usually a handful of pounds either way. That leftover is the imbalance, and it is the only load the opener sees.
This is why the label figures on openers look implausible next to door weights, and why substituting the door weight for the imbalance produces answers that are wrong by more than an order of magnitude. At the defaults the opener is pushing 20 lb — 12 lb of imbalance and 8 lb of drag — against a door twenty times heavier than that.
Drag works both ways and imbalance does not
Drag always opposes motion. Going up it adds to the imbalance, so the opener pushes 20 lb. Coming down it subtracts, so with a door 12 lb heavy the net is 4 lb still helping the door down. Push the drag past the imbalance — a door with stiff rollers and a sticky seal, or one sprung close to balanced — and the sign changes: the opener has to push the door down as well as up.
That is the honest explanation for a door that closes reluctantly and reverses for no visible reason, and it is a measurement rather than a guess. Two readings with a scale, one holding the door and one starting it moving, give both numbers.
Two openers, one job
A trolley unit pushes the door along a rail: 20 lb over 84 in of travel. A wall-mount unit turns the shaft instead: 40 lb-in over 6.68 turns. Multiply each out and both come to 1,680 lb-in of work, or 140 ft-lb, because the drum only changes the shape of the job.
What differs is everything around it. The trolley needs 114 in of rail behind the header — 84 in of travel plus 30 in of head and carriage — and that rail has to live in the backroom. The wall-mount unit needs no rail at all, which is the whole reason it exists, and buys that with a gearbox that has to hold torque at the shaft rather than tension in a chain.
The drum trade, seen from the motor
Going from 4 in drums to 5 in raises the torque by 25 per cent and drops the turns by 20 per cent, and the product of the two does not move. The same trade appears on the spring side of the shaft with the sign reversed. Nothing about a drum is free; it is a lever, and a lever converts.
What is not on this page
No force settings, no duty rating, no capacity, no statement about whether an opener suits a door. Those come from the manufacturer documentation for the unit, which is also where the safety reversal arrangements live. What is here is mechanics — force, torque, distance, work and time — and it stops there deliberately.
It also stops short of the counterbalance itself. Everything above assumes the springs are doing whatever they are doing and simply measures what is left. A wound torsion spring holds enough energy to kill and a cable under load will do the same, so the measurement is a scale on a door, and the counterbalance is somebody else.
Questions people ask
How much force does a garage door opener need?
The imbalance plus the drag, not the door weight. At the defaults that is 12 lb the springs did not take care of plus 8 lb of track and seal drag, so 20 lb going up on a door that weighs 218. Coming down the drag subtracts instead of adding, which leaves 4 lb. Both numbers come off a scale on your own door rather than out of a table.
What is the difference between a trolley opener and a wall mount one, in numbers?
The same work in a different shape. A trolley pushes 20 lb along 84 in of travel; a wall mount unit applies 40 lb-in to the shaft over 6.68 turns. Both are 1,680 lb-in, or 140 ft-lb. The real difference is the rail: the trolley needs 114 in of it behind the header and the wall mount unit needs none, which is why shallow garages end up with wall mount units.
How long is the rail on a garage door opener?
The travel plus whatever the head, carriage and door arm add beyond it — 84 plus 30 is 114 in at the defaults, or 9.5 ft measured back from the header. All of it lives in the backroom, so it competes with the horizontal track and with anything stored above the bay. The figure to put in the second field is the one in the opener instructions.
Why does my door close harder than it opens?
Because drag opposes whichever way the door is going. Going up, drag and imbalance add; coming down, they fight. A door 12 lb heavy with 8 lb of drag still has 4 lb helping it down, but raise the drag past the imbalance and the opener has to push the door closed. That is a measurable thing rather than a mystery: hold the door with a scale, then start it moving with the scale, and the difference is the drag.
Does this tell me what size opener to buy?
No. It computes force, torque, work and time from figures you supply, and states no rating, no duty and no capacity for any unit. Opener selection, force settings and the safety reversal arrangements are all in the manufacturer documentation for the specific opener and door. The counterbalance behind all of it is not a DIY subject at all.