Bottom bracket axle/crank failure

[Bhud]

90-ish year old broken axle.

They call this a bottom bracket axle, I think. Don't know why - I'd call it a crank, but whatever. This piece connects the pedals on my (only) pedal bike. It's very old, and I felt like sharing this here because (a) cycling forums, universally, suck, and (b) I'm amazed this has happened.

English road bike from the 1930s. My legs have got so much stronger over the past month or so. Anyway, the bike has flat bars, but is otherwise standard, and I've been absolutely hammering it on a BMX track and gravel paths every other evening. So, what happened was, my legs got stronger than this axle could handle, and it's only got 3 gears, and so when I went up a very steep little hill my legs could handle it but this part couldn't. It sheared internally with torsional force. Fortunately, was able to source another part (it's a precise part made by Bayliss) which is on its way to me.

I'm just sharing this for mechanical interest as well as historic. A Google search suggests that this happens between 25000 and 50000 miles, which is absolutely staggering mileage. Not sure if I believe that, but it's definitely metal fatigue anyway. If you put the two parts together, they don't join together perfectly, because they twisted before they broke. Think it's called "plasticity" or something. But I feel strong putting out torsional Nm like that!

https://i.ibb.co/hB57Pqh/broken-axle.jpg

[stinkwheel]

It's snapped through the cotter pin area so probably not a torsional fracture because that part should be fully supported within the crank. It's probably reveiving a fair bit of shear force or even flex there. I bet the cotter pin doesn't make particularly good surface contact across the entire cutaway.

If you want to prevent that happening, I'd suggest you "bed" the cotter pin by applying blue to the flat, fitting, removing and filing/grinding/sanding down the high spots until it makes maximum surface area contact. A bit like they do with the wedges on power hammer swage blocks.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Bhud]

That's interesting, thanks. I'll file down the pins then before fitting the new crank.

[stinkwheel]

In fairness, you can even see on your photo the limited amount of contact the cotter was making on that spindle. Looks like there was only about 2mm front and back leaving the entire centre unsupported. Compare that to the amount of contact you get on even a (now comparatively obsolete) square taper.

EDIT: The cotter pin was probably slightly bent then refitted. In fairness, I don't think I ever saw many straight ones. I think it used to be pretty common to try a few in until you got a "good" one.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Bhud]

I don't feel like trying out a few of them. Besides, the next one is in America.

I think, what I'll do is just install the new crank when I get it, after flattening the pins and filing them down as much as possible. Next time it breaks, the whole bike's getting binned and I'll get an 80s mountain bike.

[stinkwheel]

[Bhud]

[ThunderGuts]

For something so apparently small and insignificant (although of course, it does an important job), bottom brackets can be a minefield.

Here are some useful resources:

https://www.bikeradar.com/advice/buyers-guides/the-complete-guide-to-bottom-bracket-standards/

Sheldon Brown (the late, great) is a mine of information if you don't already know about it; it's a clunky site but is very useful for the home bicycle mechanic:
https://www.sheldonbrown.com/cribsheet-bottombrackets.html

I'd normally recommend anyone the Shimano UN55 as a workhorse BB but they sadly stopped making them, this is the replacement (might not be the right size/taper etc.. though). I think you'll need to get out the verniers/rule and check all the dims though. If your pedals will fit onto a square taper BB (again, there are a few variations) then you can probably get away with measuring the old axle length and transferring this across, otherwise you might find issues with the chainring not clearing the stays and/or the chainline being out.

Linky: https://www.tredz.co.uk/.Shimano-BB-UN300-Tapered-Bottom-Bracket-British-Thread_232243.htm?sku=764674&utm_source=google&utm_medium=cpc&utm_campaign=Bottom%20Brackets&gclid=Cj0KCQjwz96WBhC8ARIsAATR252cS2aCCMJBx9mvWLQqLLD0cVduZBJqu0SrNk0ImkGZ-j-KPSqKsZAaAocMEALw_wcB

You will need a bottom bracket tool for whatever one you go with (which you may already have?) - the Shimano one is pretty simple and inexpensive I think.

Lastly - massive kudos for riding around on a 90 year old bike! I thought I was in a minority doing 20 miles a day on a 25 year old bike, but you are on another level.
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TG.

[stinkwheel]

Not quite as simple as the above because while you will almost certainly be able to get a sealed bottom bracket with the correct threading quite easily, it will be a taper type and the original cranks for this bike are cotter pin fitting. So it would need different cranks fitting too.. Unless someone is making a sealed BB with cotter pin fittings.

When I said BSA threading, the majority of bicycles with a threaded bottom bracket use BSA thread sizes... Except for the ones that don't (see Sheldon Brown above). You could walk into halfords today and buy a bike with a BSA threaded bottom bracket.

EDIT: Well spank my arse and call me Charlie. If your bottom bracket axle is 136mm wide, you're onto a winner.
https://www.sjscycles.co.uk/bottom-brackets/cottered-sealed-bottom-bracket-136-mm/
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[ThunderGuts]

You see Charlie, you don't have any faith . . .

But yes, good point on the cottered bit - I'd skimmed past that detail.
____________________
TG.