An interesting break

[GilesM]

I'll add a few more pics later, but to answer yellow saddles question, I'm about 10 and 1/2 stone, so not a heavy or powerful rider.

The crankset is probably the oldest thing on the bike, the rings have been replaced several times, the non drive side crank will go in the bin as well, not only has it seen lots of use and abuse, it has also had 10 Scottish winters to put up with, I think I got my money's worth.
I think I only managed to stay upright because of the position and quite wide bars on my hardtail, and I wasn't really pushing too hard, I'm sure if it had been on a road bike, putting in some real effort, I would have had an unpleasant testicle and stem contact before hitting the tarmac.

Thinking about switching my much newer Race Face Atlas cranks from my full suss to my hardtail, using the triple rings currently on the hard tail, they are less than a year old, and that will allow for a shiny new Hope crankset with the double and bash on the full susser. It's only cash.

 

[GilesM]

Probably? I know you are a canny Scot and all that, but a bit of blu-tack and some duct tape is not going to see that back in service.

I'm sure I could have once fixed it with a Toe strap, but in the world of clipless pedals, I've lost all of my toe strap get me home skills.

 

[ColinJ]

Stephen Roche had similar problems in the 1987 Giro, when he felt an oddness in the handling, whilst descending to increase his lead over Visenti, & found at the stage-finish that a blade had cracked/was loose in the crown

Blimey - you would think that once the fork had started to go, that it would just snap. Lucky for Roche that it didn't!

 

[Ajax Bay]

an unpleasant testicle and stem contact before hitting the tarmac

First of all, Giles, very glad that neither 'contact' occurred, and thank you for sharing the failure and images.

If, as you say, that crank's ten years old, assumingg you've done an average of 1000 miles a year on it, that represents 10 million revolutions. Considering that it gets loaded and unloaded once per revolution, that's also10 million loading cycles. That's a lot for an aluminium part (usually the fatigue life for aluminium is assumed to be 10 million cycles, though actual life will depend on what sort of loads it actually sees)! . . .The other crank should be considered suspect. As should any handlebars or stems that are as old and have seen similar use! I'd replace those as well if they're that age

I'd like to explore this.
First: the calculation derived from the assumption: 1000 miles a year for 10 years = 10 million turns of the crank. I don't know if work has been done on average development (ie how far a bike travels for each crank revolution) - for road and MTB - but let's say, for a MTB it's 4m (ie circumference (2.2m) x average gear ratio (1.8 - 40chainwheel/22t sprocket)). Taking McW's 1000 miles a year for 10 years, = 16,000km. So that's 4 million turns of the crank over 10 years (not McW's 10 million).
Second: McW: "any handlebars or stems [should be considered suspect] that . . . have seen similar use! I'd replace those as well if they're that age"
McW: "the fatigue life for aluminium is assumed to be 10 million cycles, though actual life will depend on what sort of loads it actually sees"
Fine. This reflects the fact that Aluminium does not have a distinct fatique limit and will eventually fail even from small stress amplitudes: 10 million is conventionally 'chosen' to represent the fatigue life of the material.
A road bike, if enthusiastically ridden through the year (like mine is), might do 10,000km (I have 559km to go: should just make it). On a road bike the average gear development is more likely in the region of 6m (ie circumference (2.2m) x average gear ratio (2.7 - 50chainwheel/18t sprocket)) so ~1.8 million turns of the crank for 10,000km. Should I be considering prophylactically replacing my bar (think the stem is far more robust/deliberately over-engineered) after its sixth year, as @McWobble recommends? Is creaking a sign of fatique life end approaching (I don't think so) (but no amount of copper slip 'tween stem and bar or retightening seems to reduce this when the bar is under load (ie climbing))?
Noting that the frame and forks are steel (thanks @User ).

 

[Ajax Bay]

Perhaps the question you might prefer to consider is which to replace first, if you think concern about fatique life of the components/frame mentioned is the driving factor.

 

[Tim Hall]

Interesting stuff, and apologies for the slight thread hijack. Does the thickness of the tubing used for handlebars (or any other bike product) have a bearing on the fatigue life? The bars on my Mercian were originally fitted to a Claud Butler I bought in 1980. The frame itself failed about 18 years ago (down tube to head tube joint went Spang!) and the bars are the only original part left. I've been using them fairly constantly over that time, give or take variance such as buying other bikes.

 

[smutchin]

I think I only managed to stay upright because of the position and quite wide bars on my hardtail, and I wasn't really pushing too hard, I'm sure if it had been on a road bike, putting in some real effort, I would have had an unpleasant testicle and stem contact before hitting the tarmac.

Glad to hear you're unhurt. I once snapped a crank on my Brompton when pushing away from the lights and ended up in a heap on the ground – it's lucky the driver behind me wasn't too heavy-footed on his accelerator!

On investigating the broken crank, I noted that like yours it was partly dirty, partly clean, so clearly had already suffered a crack that I hadn't noticed.

 

[GilesM]

To help with some thoughts about how many revolutions this crank completed in it's life:
Average weekly mileage when I'm home approx: 75miles per week
Average number of weeks I'm home each year, approx: 25 weeks
Average annual mileage for this crankset: 70 x 25 = 1750
Age of the crankset: 10 years
Total mileage for this crankset: 17,500 miles
Average gear I use on this bike for most of those miles: 44 X 18 = 64" gear (26" wheels)
Distance covered per rev on 64" gear: 201"
Revs per mile: (1760 x 36)/201 = 315
Total Revs in the cranksets' life time: = 315 x 17500 = 5,512,500

 

[Shaun]

-1 (only because I know no one was injured in the making of that calculation).

 

[Flying Dodo]

I've been on a night ride with Steve "teethgrinder" Abraham, when his handlebars snapped, near the start of a climb. Fairly certain it was a very, very old bike.

I think this sort of incident is one of those things you're vaguely aware could happen, but you shut the thought away and carry on riding.

 

[ColinJ]

You might be over thinking this.

I think so too ... :laugh;

Having said that ... I am about to reuse an old set of drop handlebars on a bike that I am currently building. They have been transplanted a couple of times before over a 17 year period. I think I will give them a very close inspection before using them!

 

[GilesM]

You might be over thinking this.

Not me, just providing a bit of information for a few on here who might want to get involved in some over thinking. I'm satisfied that it was just a used and abused crank that finally failed.

 

[Deleted member 26715]

He overtightened it

 

[Ajax Bay]

I'm trying to deduce what 10 million crank revolutions might mean (in mileage terms) for different bikes (eg road, MTB) and when it might be prudent to change items before they reach their possible 'fatique life'. Or at least subject said vulnerable parts to additional inspection (eg middle 2" of handlebar). "You can call it 'measles' if you like." I call it 'chat'. I thank @GilesM for the extra data (personal to the use he's given the bike), provided in the spirit of exploration.

 

[GilesM]

Off out to my garage to start taking the broken bits off, it's a wee bit chilly, hence a fresh cup of tea will be needed.