Anyone know why this keeps coming undone?

[Flying Dodo]

remind me, for mere mortals, what was wrong with square taper bottom brackets?

There are 2 answers:-
1) With an external BB, you can have bigger bearings, which in theory last longer, along with a larger diameter spindle which you can make lighter and thinner than those on a traditional bottom bracket.
2) It's satisfies the N+1 methology as well as trying to re-invent the wheel, in order to persuade gullible MAMILS that they have to have the shiny new bling things.

Both of these answers are correct.

 

[GrumpyGregry]

They're from the past.

For mere mortals that's usually a good thing!

 

[Acyclo]

remind me, for mere mortals, what was wrong with square taper bottom brackets?

1 The square hole in the crank arm concentrates stresses, making cracks more likely.
2 The taper causes stretching of the arm hole.
3 Pedaling forces create a loosening force.
4 The retaining bolt has no locking mechanism. It relies on friction.
5 Removal can be difficult.

 

[Tim Hall]

6. I've never had a problem with one.

 

[Shut Up Legs]

Ooh! A square taper BB vs. cartridge BB debate!

@gaz: When you tighten the 2 crank bolts on the left crank, ensure you alternate between them as you tighten, to avoid having one much tighter than the other. Also, when you tighten up the lock-nut (aka 'pre-load cap'), only tighten it up just enough to remove almost all the crank wobble. If you over-tighten it, the cranks will press too hard against the BB bearing cartridges, and cause the bearings to wear out too soon.

 

[PpPete]

remind me, for mere mortals, what was wrong with square taper bottom brackets?

Hang on a cotton-pickin' minute here... is this the same GregCollins who took issue with my expression of similar sentiments with regard to quill stems ?

 

[GrumpyGregry]

Hang on a cotton-pickin' minute here... is this the same GregCollins who took issue with my expression of similar sentiments with regard to quill stems ?

It is.

 

[Ed no-more-lemons]

remind me, for mere mortals, what was wrong with square taper bottom brackets?

Alway been pretty darn good, your old square taper.
Not that I've ever owned anything else.

 

[the_mikey]

The 'thing' is to pre-load the bearings before you tighten up the bolts on the crank arm, when it's done in that order, it shouldn't come undone, and even if it does it will be of little consequence apart from making life difficult (assuming it's lost or missing) next time you want to remove and refit your crank arm.

Never put loctite on it!

 

[gaz]

The 'thing' is to pre-load the bearings before you tighten up the bolts on the crank arm, when it's done in that order, it shouldn't come undone, and even if it does it will be of little consequence apart from making life difficult (assuming it's lost or missing) next time you want to remove and refit your crank arm.

Never put loctite on it!

Too late, got loctite on it to keep in place.

 

[Smurfy]

Too late, got loctite on it to keep in place.

I would've tried the PTFE tape idea I posted earlier (and seconded by Ian), purely on the basis that the friction/torque during insertion will be similar to the friction/torque required for removal. With Loctite, once it's set you could have one hell of a job removing it*.

* I know all about this after spending several hours removing two immersion heaters which were locked solid because a plumber had used a liquid sealant rather than PTFE tape on the threads. The only way I could release the heaters from the bosses in the hot water tank was by carefully collapsing them

 

[Cyclopathic]

6. I've never had a problem with one.

7 Neither have I. Until I read all the problems I could have with them I thought that they were perfectly good. I've certainly never had the problem the op seems to have. Besides it's not just friction that holds them on, there is also a large nut. And anyway, isn't everything just held together by varying degrees od friction. So there.