Bicycle Mechanics - Why has Shimano changed?

This is very trivial, but I hope someone finds it interesting. :)

The Shimano bottom brackets: UN70, UN71, etc, had an aluminium cup on the drive side,
but I just bought a UN73 with a steel drive side cup?!?!?!?!?

(WOW....fascinating :D)

Why is it so?

They may have experienced excessive wear and stress deformation with the aluminum, or simply an epidemic of stripped threads. (Personally, I would FAR rather strip the cup threads than the shell threads, so I don't see the problem here.) Also, the steel cups are probably slightly cheaper to manufacture, and steel-versus-aluminum could aid their product line differentiation strategy. By the way, I have noticed that some Campag. cartridge BBs use aluminum cups, and others use steel.

Thanks. Pretty boring question, wasn't it? :)

I'll be using it in one of my old steel frames, but I would be hesitant to ram it into one of my alu frames.

It's (almost) interesting to note that, despite of changing to the steel cup, my 113mm UN73 (263g) still weighs about 50g less than some of the UN52 (311g) I have...WOW :D

Thanks. Pretty boring question, wasn't it? :)

I'll be using it in one of my old steel frames, but I would be hesitant to ram it into one of my alu frames.

It's (almost) interesting to note that, despite of changing to the steel cup, my 113mm UN73 (263g) still weighs about 50g less than some of the UN52 (311g) I have...WOW :D
Why not put it in an aluminum frame? The un-73 has a hollow spindle,the un-52 is solid. No mystrey about the weight.

This is very trivial, but I hope someone finds it interesting. :)

The Shimano bottom brackets: UN70, UN71, etc, had an aluminium cup on the drive side,
but I just bought a UN73 with a steel drive side cup?!?!?!?!?

(WOW....fascinating :D)

Why is it so?

Actually...pretty interesting question. As you may or may not know, if you don't have a layer of grease or anti-seize between dissimilar metals, they can fuse together (electrolytic reaction). Most folks who've worked in bike shops have seen a bike that required either the seatpost or stem to be drilled out because the aluminum part had fused to the steel as a result of moisture entering between the two. Being that the bottom bracket is in closer proximity to moisture and that it is reasonably easy to strip out the splined fitting, Shimano might have changed materials after they heard enough reports from shops of seized cups..

Shimano might have changed materials after they heard enough reports from shops of seized cups.. Then shouldn't they make Ti, steel and Aluminum cupped versions?