Pedal / Crank Thread Failure
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raleighnut
Legendary Member
- Location
- One of the 'Elite'
Well that's fubarred. 
Drago
Legendary Member
- Location
- Suburban Poshshire
That'll sell on Retrobikes as 'NOS'.
- Location
- Glasgow
Yellow Saddle
Guru
- Location
- Loch side.
What happened? One of three things: it wasn't torqued properly and worked loose or, it cracked and then came loose or, it was torqued too much, which caused the deformation around the edge of the hole, which lead to a stress riser and gave rise to the crack. Although that is a very nice photo (it is nice to see someone who has a camera with a focus function) it doesn't show the inside of the hole at the "cracked" area, so I can't say if that is a crack or not.
That's a left crank, right?
You are a powerful rider, right?
Powerful/heavy riders expose a particular weakness in crank/pedal attachment. A pedal attaches via the wrong mechanism. It should have had a lug nut fitting such as a wheel nut with a taper. The flat collar on the pedal eats into the crank and leads to this type of problem. It won't happen with a conical section, but unfortunately the industry either doesn't understand that or, the problem is not prevalent enough to bother with it and fight against legacy compatibility issues.
The thread can easily be fixed with an insert, but the collar damage does not make a repair worthwhile.
That's a left crank, right?
You are a powerful rider, right?
Powerful/heavy riders expose a particular weakness in crank/pedal attachment. A pedal attaches via the wrong mechanism. It should have had a lug nut fitting such as a wheel nut with a taper. The flat collar on the pedal eats into the crank and leads to this type of problem. It won't happen with a conical section, but unfortunately the industry either doesn't understand that or, the problem is not prevalent enough to bother with it and fight against legacy compatibility issues.
The thread can easily be fixed with an insert, but the collar damage does not make a repair worthwhile.
Last edited:
Yellow Saddle
Guru
- Location
- Loch side.
[QUOTE 5428697, member: 9609"]I didn't notice what might be a crack until I viewed the photo, i will investigate that further this morning, it may just be a flaw in the casting ?
I do tend to under tighten pedals in some sort of daft belief that they just tighten themselves up. I'm thinking this has come a little loose and once that happens failure of some sort wil just be a matter of time.[/QUOTE]
Cranks are forged, there's very little chance of a flaw in them at exactly the same spot as a crack from a burr would originate.
I do tend to under tighten pedals in some sort of daft belief that they just tighten themselves up. I'm thinking this has come a little loose and once that happens failure of some sort wil just be a matter of time.[/QUOTE]
Cranks are forged, there's very little chance of a flaw in them at exactly the same spot as a crack from a burr would originate.
raleighnut
Legendary Member
- Location
- One of the 'Elite'
[QUOTE 5428697, member: 9609"]I didn't notice what might be a crack until I viewed the photo, i will investigate that further this morning, it may just be a flaw in the casting ?
I do tend to under tighten pedals in some sort of daft belief that they just tighten themselves up. I'm thinking this has come a little loose and once that happens failure of some sort wil just be a matter of time.[/QUOTE]
I don't tighten pedals massively, just give em a good final tweak when fitting and have never had one go like that in normal use. I have had a pedal strip it's threads like that but I'd had a hefty 'off' that kind of bent the pedal a bit (well it felt weird) and that failed about a month later.
I do tend to under tighten pedals in some sort of daft belief that they just tighten themselves up. I'm thinking this has come a little loose and once that happens failure of some sort wil just be a matter of time.[/QUOTE]
I don't tighten pedals massively, just give em a good final tweak when fitting and have never had one go like that in normal use. I have had a pedal strip it's threads like that but I'd had a hefty 'off' that kind of bent the pedal a bit (well it felt weird) and that failed about a month later.
Yellow Saddle
Guru
- Location
- Loch side.
[QUOTE 5429475, member: 9609"]Bizarrely that must have just been a dirty mark or something on the earlier picture 
I cleaned it up before this image
View attachment 436628
So what are we going for here as the cause ?
pedal either not tightened up correctly (may be I just done it finger tight and got distracted before spanner) or pedal worked loose
and then over the next 2000 mile the sideways play increased, millionths of a mm at first until the final hilly ride of last night when failure took place.[/QUOTE]
It wasn't tightened enough, so the shoulder of the hard steel pedal threaded part could dig into the crank as the pedal moved diagonally inside the crank with each pedal stroke. Eventually the shoulder worked enough crank material sideways (flowery bits) to increase the movement between male and female thread enough to fret the softer aluminium away.
You can feel what I mean if you loosely insert a pedal into a good crank and finger-tighten it. You'll see that the pedal can move laterally.
I cleaned it up before this image
View attachment 436628
So what are we going for here as the cause ?
pedal either not tightened up correctly (may be I just done it finger tight and got distracted before spanner) or pedal worked loose
and then over the next 2000 mile the sideways play increased, millionths of a mm at first until the final hilly ride of last night when failure took place.[/QUOTE]
It wasn't tightened enough, so the shoulder of the hard steel pedal threaded part could dig into the crank as the pedal moved diagonally inside the crank with each pedal stroke. Eventually the shoulder worked enough crank material sideways (flowery bits) to increase the movement between male and female thread enough to fret the softer aluminium away.
You can feel what I mean if you loosely insert a pedal into a good crank and finger-tighten it. You'll see that the pedal can move laterally.
Nigelnightmare
Über Member
Unless your left leg is shorter, use cranks of the same length.
Otherwise you might start getting problems with your neck/shoulders/back/hips/knees etc.
Otherwise you might start getting problems with your neck/shoulders/back/hips/knees etc.
