Found that annoying little creak today

longun

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Retrobike Rider
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Riding to work on a pace, sun shining, breeze on my face. Couple of heavy pedals to get up that wee hill..... mmmmm.... its creaking down there near the bb

Almost in the workshop,use the new speed bump for a quick happy jump and ........... ping




















Old cranks, fatty riding and a small jump.....
 

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ishaw

Gold Trader
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Crikey. I've got a set where there is a stress crack up the inside of the arm where the pedal goes, but that's some crack. Hope you didn't injure yourself.
 

Frankenorange

Orange 🍊 Fan
Stunning!

Hope you didn't hurt yourself. It looks like the axle taper split the arm. Sort of like a log splitter.

I have seen a few pics of Middleburns that have wee cracks at the taper. Probably the highest point of stress but could also be a compatability issue with particular axles upping the stress in that area.
 

markoc

Orange 🍊 Fan
Ouch! Hope you're OK - that's quite a failure that I'd have thought was a good while in the making.

Overtightened at some point and worked its way out from there?

Found a crack in a Lava Dome frame the same way - speedbump, entry to work, decent sized hop and "ping". Oops.
 

markoc

Orange 🍊 Fan
What's interesting to me (a non-engineer) is that the crack is diametrically across the corners of both sides of the square taper.

If a weakness emerged I'd have thought it would have spread from a single point, ie a hairline crack at one corner of the square taper that then went along the crank - yet this has sheared on both sides of the arm, with no way for the "tear" to progress across the gap to take the BB. Presumably landed with horizontal cranks (as we would) and the force did the rest.

Scary stuff. Might well take mine off tonight and have a proper good check!
 

Mickeyspinn

rBotM Winner
What's interesting to me (a non-engineer) is that the crack is diametrically across the corners of both sides of the square taper.

If a weakness emerged I'd have thought it would have spread from a single point, ie a hairline crack at one corner of the square taper that then went along the crank - yet this has sheared on both sides of the arm, with no way for the "tear" to progress across the gap to take the BB. Presumably landed with horizontal cranks (as we would) and the force did the rest.

Scary stuff. Might well take mine off tonight and have a proper good check!
In its defence it's clearly broken along its grain...
So it was made properly,It's the gauntlet of running old lightweight stuff !
 

greencat

Senior Retro Guru
Wasn't there an issue with some lightweight cranks where they would fracture along similar lines and to solve it they simply rotated the square of the taper by a quarter turn or similar?
 

markoc

Orange 🍊 Fan
In its defence it's clearly broken along its grain...
So it was made properly,It's the gauntlet of running old lightweight stuff !

Didn't realise it would have a grain as machined from billet. Would make sense. Every days a school day :)
 
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