Dave wrote:
I've just insulated my Cardale up-and-over garage doors and now need to
increase the spring force to overcome the extra weight. I can't see a
safe way to increase the tension in the central clock-type spring.
What's the secret?

You can fiddle it a very little by cutting the spring, annealing the
end, and reattaching.
Other options are to add external springs, which may well be awkward.