A client came to me with a peculiar problem: His garage door would only close about 75% before the safety cables would become loose.
This is not a particularly common problem, but when he told me what it was I knew what was wrong. The client was a do it yourselfer, and had started the job as a spring replacement. The job seems to have gone well, except for the safety cable slackening. And he told me that he had done the job twice and increased the number of turns on the springs (to increase tautness). He felt that there was something binging on the cable drums or springs,
I knew from the way he explained his situation that he had bought the wrong size garage door springs. In fact the springs were too strong for the job. This raises the question, why would the cable get slack if the spring was too strong? The answer is that the spring bracket twisted out of shape, and caught on the safety cable drum causing it to lock up.
In a case like this you either need to measure the springs in an unwound state, or you should just call a professional.