You must use springs that are matched to the weight of the door. You cannot compensate for the wrong size spring by adjusting the number of winding turns. If you do not know a proper spring size, then you or your spring supplier must calculate a proper size (see below) based on an accurate weight (within 5 pounds) of the door. So you must then in turn have an accurate weight.
Regardless of the material, you will likely need to perform some regular maintenance tasks. A garage door professional can examine common problem areas to ensure all are working as they should. Additionally, it is necessary to examine the integrity of the door itself, since your garage can be especially vulnerable to security issues if your door is not structurally sound.
Now 13 pounds of force must be respected when backed by many hundreds of foot-pounds of stored energy, waiting to be released. Holding this torque is equivalent to stalling a 3 horsepower DC motor. But holding and turning these handles does not require extraordinary human strength. Note that this maximum tangential force depends only on the weight of the door, and the radius of the drums, and is divided by the number of springs (some designs have only one longer spring, as mine did originally, instead of two shorter ones). Higher or lower lift distances imply more or less turns to wind the spring (and thus a different spring geometry), but not more force on each turn.
Speed of a thrown winding bar:: The springs, being in balance with the door, effectively are able to launch a typical 150 lb door at 10.6 mph speed. An 18-inch long by 1/2-inch diameter steel winding bar happens to weigh about 1 pound. Since momentum is conserved, this 150:1 ratio in weight of the door to the winding bar means the fully-wound springs could potentially throw a winding bar at 10.6 mph * 150 = 1590 mph = 2332 ft/sec, assuming the energy were perfectly coupled and transferred. If the energy transfer were only 1/3 efficient, this would still be the 800 ft/sec speed of a typical pistol bullet. Except it is a foot-and-a-half metal spear, not a bullet.
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## Spring index lower limit: DASMA standard 102-2004 provides a lower limit of 6 for the spring index, which is symbol C in our formulas above (section 9.3). Other interesting items in this standard include an upper limit for how much the door sags when raised (no more than 1/120 of its width, section 9.2.1), that each of the two bottom brackets should sustain twice the door assembly's weight (section 9.2.2) for a total safety factor of about 4, and that the lift cables should provide a safety factor of 5.

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You might genuinely need some extra parts when you thought you simply needed a broken spring replaced, and a good serviceman will perform a simple inspection to identify such parts. Nor is it unreasonable for a business to charge separately for a service call versus repair work actually performed. But the best protection for you as a buyer, being somewhat at the mercy of whomever you decide to bring on site, is to understand what is being done, and ask intelligently for a clear explanation or demonstration of why extra parts are required.