Hello,
After my ride today, I noticed that one of my cranks appears to be cracked. The crack runs across the top of the crank and about halfway across the back, widthwise.
I have the Qu-Ax aluminum cranks.
These things are solid aluminum, aren’t they? Is there some kind of finish that could be degrading, or is my crank just going to snap in half one day when I drop off a curb?
Given that those are the Qu-Ax cranks, I would expect that the cracks are structural and that you’re not far from having them fail. You might want to get a set of stronger cranks on order.
In my experience cranks usually snap off when starting or stopping rapidly. I’m with Tom though, it’s time for a new set of the same at the very least.
I’ve never seen cranks fail like that. So I don’t think thats a crack. I might be wrong but I’ve never seen even a picture of a crack right down the middle of an al crank. Aluminium cranks usually twist.
I’ve been fairly silly on quax ISIS cranks on a 20" freestyle and they always held up.
Either way, a crank breaking whilst going down a curb wouldn’t hurt that much so I would just ride them until they fail. I’ve had pedals snap off whilst riding and it’s not really that bad. If you are doing trials and are a full size adult it might be a good idea to upgrade to something tougher before they twist and become useless though!
I have seen Qu-Ax cranks break clean through right at that point.
Not doing trials, but I’m definitely a “full size adult.”
I guess this is my excuse to get a pair of 137/165 moments.
Thanks for the feedback!
I’d love to see a picture if you have one. I’m considering buying a nimbus eclipse and putting qu-ax cranks on like these because they are lighter than the ventures. But if they are just gonna snap… 
People say those QuAx cranks aren’t very strong (softer alloy than others apparently) but the pair or 145s on my 36er seems completely fine after a few years, even after being dropped a few times. So they (or at least my pair) are strong enough for road and light xc riding. I’m not particularly light either (12 st).
The OP’s cranks do look dodgy though - I think I’d replace them if only for peace of mind.
The only cranks I’ve had that happen with were a pair of ProWheels on my old muni (square-taper). They developed very similar surface cracks and I replaced them with another pair the same, but they lasted a few years of rocky muni riding and never actually broke (although they may have done if I carried on using them for much longer).
Rob
But you’re young and bouncy Brendan 
I have run the QuAx lightweight cranks and they worked well until I started getting some slippage at the spline. I run the 170’s and weigh 200#, not cracks or failures.
If you want some strong and light cranks, the Venture II are very nice, I have 165’s and 150’s, nice low Q. The dual hole moments are super heavy, not worth the trouble unless you switch crank length often.
I’m around 200 lbs as well.
I’m looking at the dual hole moments because, on my (increasingly rare 
) offroad excursions, I could often use more power and control, and I would much rather swap pedals than cranks for these days.
Heavier cranks aren’t really a problem for me; I’m not spinning 200rpm anyway, nor hopping onto chest-high obstacles. 
Steel cranks usually twist. Aluminum usually snap. It’s just the way the metal fatigues.
corbin
Why would you need a picture? The crank broke clean through. It was used on a basketball unicycle.
They’re also highly prone to stripping at the pedal threads.
I used them on my racing unicycle (they are very light), but I consider them ideal for special purposes only.
Yeah, had some of those on an earlier pair as well.
I’ve still never seen aluminium cranks snapped there. Pics?
Just google it.
I’ve seen cold laps happen in steel forgings where the material is gathered at one end for a through hole. I’m not very experienced with aluminum. Some of those breaks might be from some legacy problems of the original forging.
Has anyone tried shot peening? I see a lot of two wheeled contraptions (TWCs) have components available that are shot peened aluminum.
It seems like a shot peened aluminum frame might be quite an advancement.
