Cracked drums
Should have said this earlier. Steel and cast iron do not fatigue with age. They only begin the fatigue process if the stress applied exceeds about half the yield stress (which is called the endurance limit). If the stress is below that, then it'll last forever. Or for 120 years and counting like the Forth Bridge and many other steel and cast iron Victorian structures. The Tay Bridge fell down because flaws in the castings were filled with silly putty. So don't repair cracks in your front hubs with plastic padding.........
If you want to crack detect them, a pretty good job can be done with "simple household materials". Clean the paint off, immerse them in hot paraffin, take them out, wait until the paraffin has evaporated off the surface, then dust them with talc and blow the excess off. The talc sticks to the cracks because the cracks still hold unevaporated paraffin.
It is safe to conclude that if there are no cracks then the fatigue process (caused by alternating stress above the endurance limit) hasn't started yet.