On 5/17/2021 2:53 PM, Ade wrote:

Making bikes light is one thing, making them last is another.

My last allow bike was a Specialised Allez. The frame was light enough,
about 1.5KG (3 lbs in old speak). I weighed it when I stripped it for
return, it had cracked due to fatigue. It was a thing of beauty, much
nicer than the horrid oversized carbon frame they replaced it with.

My previous alloy Trek failed at about the same distance, ~10,000 miles.

I thought the benefit of titanium was that like steel it had a fatigue
limit, meaning it wouldn't inevitably eventually fatigue like aluminium.
I know aluminium bikes can be made so the fatigue failure takes a long
time, but I suspect this adds weight.

Well, as we've mentioned, steel and titanium alloys can and do fail by
fatigue - specifically, when a local stress exceeds the fatigue limit.
The way to prevent that is usually to make things heavier.

The difference with aluminum alloys is that at least in principle, every
aluminum bit subject to fatigue loading will _eventually_ fatigue. But
if it fails after enough billions of fatigue cycles, the original
designer usually doesn't care.

--
- Frank Krygowski