Have had a steel tandem frame break at 50,000 miles and again at 56,000 miles. Sold that tandem with 64,000 miles on the odo.
While most folks will not put that many miles on a tandem it shows that material fatigue can happen.
Have put 56,000 and 57,000 miles on 2 other steel tandems.
Have also broken an experimental steel fork at 13,000 miles.
Our mantra is that after 50,000-some miles perhaps it is not a bad idea to design another tandem for ourselves.
Been riding full carbon (Zona) tandem for the last 10 years and still extremely happy with our choice.
Buy the best you can afford . . . watcha gonna do, wait 'til you get older?
Pedal on TWOgether!
Rudy and Kay/zonatandem