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.
Your ad had no cheap coupons or promises.. Just business. Service was more than prompt. No attempt to fix something that was not broke or did not try to sell me anything I did not need. He stated the price at the beginning and stuck to it. (I was so pleased, I did not bother to mention a senior discount.) Very, very pleased that I have found someone I can trust and rely on. GaryRichard