Originally Posted by
ivangobike
Loud and clear on the effects of corrosion.
While i cant say for sure as to the cracks between the thread, I will say that the steel is definitely stretched prior to their removal. The weird physics of this must suggest these newer cups are a very cheap, malleable metal, and I always find it weird how the thread lines seems to just separate rather than break off from the stretched steel. Kinda like how certain polycarbons will stretch into long, strong strings after having force applied to them for decades at room temperature.
Like I said above, the problem is due to contaminants in the steel that are in compatible with the steel. The result is a significantly softer material that isn’t as strong as uncontaminated steel. Because of the softness of the metal, it isn’t as brittle as steel so it’s more malleable and stretches rather than breaks as steel would.
The Steel on that bottom right example is worn down super thin on what was the lower half of the cup. The amount of stretch in the steel is relative to the amount of wear inside the cup. And it's been consistent like that for all of them I've seen, especially with the drive side cups as they aren't adjustable.
The wear inside the cup is due to the same softness. The softness of the metal means it wears faster and also elongates. Enough elongation and the cup cracks.
Aluminum is known to loose strength after repeated stress cycles. Once the steel cup wears thin, the aluminum below starts taking the load to a smaller and smaller area. Eventually, the metal is so thin and the aluminum is so fatigued that it starts crushing down, pushing the face of the bearing cup outwards. This particular bearing cup had about a 2-4mm gap between the lower part of it's external face to the lip of the bottom bracket. Also, notice how the thread of this particular cup only starts after a few mm in? That's another cause, only ~2 rounds of thread are holding back from this force, so they start pushing out the lower part of the cup face.
The extreme mileage and downward force from pedaling are what we're looking at being the main effects for these cups.
Again, I have only seen this kind of damage to newer bottom brackets on cheap department store bikes. I’ve used these kinds of cups extensively on bicycle through the 80s and 90s. Many of them were very high mileage bicycles and I’ve never seen any of those bottom brackets crack or pull the threads out. I’ve also remove lots and lots and lots of bottom brackets from bikes of that era and never seen the kind of damage you are detailing. Those bottom brackets were made of much better metal…even the ones on bikes that were of fairly low levels of quality. It’s not been a problem except on bikes from the 2010s and later.