Heading home from work today I heard a soft thump, thump noise that kept rhythm with the rotating tires. I stopped and did what turned out to be a thorough search for the cause and found this.
My back rim had broken at the side opposite from the valve. The raised edge was hitting the back brake causing the noise. I undid the back brake and headed on home.
After dinner I hauled the Steed to my LBS and got a good replacement wheel. I learned that the break is where the rim is welded together. I can see the weld on the inner side of the wheel midway between two of the spokes. I have used my back brakes more than the front and that is what eventually wore the rim down to where it failed. The front wheel is fine. No need to replace it.
Before all this, on the commute home I happened to meet Jim Phillips as he was leaving his work. This is always a delight. I showed him my new woe and he responded with proper sympathy. I admire the good physical shape he is in and I did my best to keep up with him. This time I would have good reason for a red face when I got home.