Seat Tube Rust-Through and Crack

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Burley tandem, both seat tubes failed. Front tube cracked about 50% around and badly thinned by rust; rear tube cracked about 40% around, presumed thinned by rust.

Service history unknown.

Both seat posts were rusted in place, a fairly common problem unless the posts are removed and re-greased annually.

The construction style is typically durable: 1.4 mm wall thickness seamless Crome-Moly tubing. Also, both cracks are above the weld, rather than at the edge of the weld itself, indicating it is not a heat-induced failure. Finally, the cracks appear to have started on the front and rear of the tubes, whereas typical failures start on the sides of the tubes, where loads are highest.

Neither seat tube had a "vent" hole at the bottom, and both seat tubes failed in unusual configurations. A likely cause of failure was water collected and led to rust-through on the front and weakening the led to a crack on the rear. Note that with a vent hole water can drain in to the bottom bracket, and air can circulate with the outside through frame construction vent holes, typically at the rear dropouts.

One take-away from this failure is that small details can have a significant effect on durability. In this case, it appears failing to drill two small holes was a singificant contribution to the failure.

The frame was repaired by forming new new material to fit around the old failed tubes, extending well beyond the damage area, then brazing it in place.