Today’s cautionary tale: Don’t ignore BB creaks

Svendo

Veteran
Location
Walsden
I’ve had a bit of a BB creak for the last couple of weeks, out of the saddle. Assuming it was something else like it nearly always is I’d greased the drop outs and it’d nearly gone. I also thought it could be the new cassette settling into the freehub body gouges.
Turned out it was from the BB area as the crank axle gave out on the way to work, first press setting off from lights.
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BurningLegs

Senior Member
Crikey! Hope you didn’t hit the deck too hard?!
 
Location
Loch side.
Very interesting those failures. It is a great example of how a tube fails in torsion.

The fact that the crack started on the left, just beyond the bearing tells us that a crank's spindle only experiences torque from the left leg. Further, the crack started at a spot of rush, which caused a localised weakness that started the crack and it spread spirally to the right until the point where there just wasn't enough meat left to support a heavy start.

There's beauty in failure too.

Edit: Spot of rust, not rush. Shame on no-one for not pointing it out.
 
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C R

Über Member
Location
Worcester
And to think the bike magazines hark on about how stiff these 2 piece designs are !
I would have thought that increased stiffness would make that type of failure more likely.
 
OP
Svendo

Svendo

Veteran
Location
Walsden
Very interesting those failures. It is a great example of how a tube fails in torsion.

The fact that the crack started on the left, just beyond the bearing tells us that a crank's spindle only experiences torque from the left leg. Further, the crack started at a spot of rush, which caused a localised weakness that started the crack and it spread spirally to the right until the point where there just wasn't enough meat left to support a heavy start.

There's beauty in failure too.
Hi YS I was hoping for your opinion. The circumferential crack nearest the left crank goes further round, about 180° altogether. I put the current BB on in January and the axle looked ok then.
 
OP
Svendo

Svendo

Veteran
Location
Walsden
Crikey! Hope you didn’t hit the deck too hard?!
I knew that your legs were strong, but that is just crazy! :eek:

I assume that you were not injured? It could have resulted in a terrible crash.
Felt like a chain breaking, luckily I hadn’t clipped in on the left and my right cleat disengaged so I didn’t go down. Looked down and after a confused split second thought ‘oh, that’ll do it!’ Gathered the bits up and moved to the side. Only injury is a row of grazes from the chainring, and a flat rear tyre. Had to chuck the chain set bits into the backpack, fix the puncture then walk/ scoot to work. Only 15 minutes late. Mostly annoyed I’ve lost mileage!
Just very grateful it’s not when Svendo Jnr #1 or #2 were on the back!
I’ve already ordered a 2nd hand 6700 chain set & a BB. Now at least the chain rings and cranks will match.
 
OP
Svendo

Svendo

Veteran
Location
Walsden
As a matter of interest, do you stand on the pedals a lot?
Yes I do, I’m a terrible honking grinder. (That could mean something quite different in other contexts!)
It’s one side benefit of the child seat, as then I can’t balance out of the saddle. Or at least I need to balance so much I get no benefit.
 
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