Old girl started sagging a little driver side rear. Has anyone had springs re-arched? Any specs? 1928 Coach

I had the springs on my 32 re-arched over 20 years ago and they are still OK. Success will depend on the condition of the springs and the expertise of the shop doing the work. The abuse they will get today will be no where near what it was when the car was new and the roads were poor..

Successfully re-arching springs takes skill and patience. From a materials science perspective you have to deflect the spring past the elastic limit but not past the yield strength. The difference in deflection at those 2 limits is pretty small. If you deflect the spring far enough to exceed the yield strength you will alter the defection properties of the material enough that the spring will probably sag after a little bit of use.

I have seen videos of people doing this with a special support fixture they put in a press. They will deflect a short segment of the spring just enough to change the set of the arch slightly, release that deflection, and move to the next segment on the spring. They keep working back and forth the length of the spring until they have the uniform arch they want.

The other trick is making each successive leaf conform to the arch pf the prior leaf.