EamonPVD

Hey all,

Noticed a small crack or scrape on the inside of my crank. Crank is a shimano rx100. Can anyone tell from the pictures how serious this is?

prathmann

Can't see any pictures, but I'd avoid riding on any crank that has a visible crack. Lots of reports of rather nasty crashes resulting from cranks that failed unexpectedly and suddenly. And it's likely to happen just when you're pushing really hard on a climb and are least able to recover your balance.

Add - after managing to see your images it's a rather unusual location for a crack. I'd try polishing the area and see if it disappears.

79pmooney

I'd be concerned. Mark the bottom of that crack exactly with tape or a marker. Go for your next ride. Look at it when you get home. Any growth?

I would fully expect that crack to continue down to the hole for the BB axle, at which point I would expect the hole to open enough that your next pedal stroke will pull the crank off (probably leading to a crash).

The crank still has a lot of material in that area so that should buy you time but if you see crack growth, that is all you have; time before the inevitable. Watch that crack carefully. Any growth and start looking for a replacement.

I've never seen a crack there on a crank. I have seen three cranks fail close up; one mine, two belonging to riders immediately in front of me. I got lucky and broke it going slow and coming out of the saddle. The other two crashed.

Ben

fietsbob

Replace, to be safe...

FBinNY 

Yes, it's dangerous to ride, if you consider the consequences of breaking a crank arm while you're putting your full weight on it dangerous.

Aluminum is prone to notch failures, and cracks like this grow until they reach critical and the part fails. Of course that failure will happen at maximum load. So, maybe you can continue to ride it around town avoiding hard starts and hill climbing, but you want to deal with it before it deals with you.

dsbrantjr

+1 on replacing it as soon as possible and staying in the saddle in the meantime. The crack will only grow and consider what is likely to occur when it lets go if you are inclined to adopt a wait-and-see attitude about it. Even if you manage not to crash you will likely impact the frame with a delicate area of your body, to put it, well, delicately.

CliffordK

That might be a reasonable approach. If it was a crack, it would be propagating up from the bottom bracket spindle (taper?). There is what looks like a second "crack" coming up through the middle of the spider just to right of the first crack. But, those could both be artifacts.

I assume nothing is visible form the front side of the crankset, and the crankset is solid, not hollow.

So, if you wish to be certain, pull the crank, then progressively sand down from say 220 grit, 320 grit, 400 grit, 600 grit, and maybe 1000 grit sandpaper, and see if the cracks disappear. Then just watch the area.

If the marks can't be polished out, I agree with the others, best just replace. Better to error on the safe side.

79pmooney

I haven't seen or heard of that yet. But your foot will come down on the road hard with pedal and what's left of the crank attached (assuming you are cleated in). If you have speed on, that could work out not so well. But your manhood probably will stay intact. Every instance I have seen has happened as the rider came out of the saddle.

Ben

dksix

That's something too likely to be very bad if it lets go, wouldn't be worth the risk for me.

DiabloScott

Yeah dangerous.

I've broken several cranks, only one time did I not get hurt... fortunately never really bad.

That is an unusual place for a crack, but you never know. One crank (Campy!) had a little rock or something inside the arm - totally invisible until the crank broke and then you could see exactly why - the crack was in an odd place because that's where the rock was, and I didn't notice until too late.

70sSanO

Just go to eBay and get a Shimano 105 crank. If your RX100 is an A550 (7 speed) get a 1055 if it is A551 get a 1056 (8 speed) and use existing BB. Might even be able to get a right arm only. If it were me, there is no way I'd risk breaking the crank arm under load. Dr visit will cost you more.

John

BikeLite

Yes, if you value your life.

Andrew R Stewart 

I would suggest that the crack/future failure is not along a path that is most likely to see a end of arm/pedal separation from what is still attached to the spindle. The crack looks to be radial to the spindle, not tangential in it's orientation.


But if this was an airplane or mass transportation device and it did fail there could be massive claims. Being that it's the OP's bike and if he only rides alone the risk is mostly limited to him.


The prudent thing is to replace as soon as it's possible. Andy

headasunder

If you have taken a vow of chastity there is no need to replace it otherwise as per all the others suggestions replace it asap.

sweeks

Add me to the list of those ^^ who recommend replacement ASAP.

It's an odd place for a crank to crack. It *could* be a forging mark of some kind. A machine shop could do a fluorescent dye test to see if it's actually a crack.
Steve

70sSanO

If you have ever ridden a mountain bike with platform pedals and had your foot slip off, you would understand. Thankfully it never happened out of the saddle.

John

Marcus_Ti

Odd place for a sq-taper crank?

Remember you're talking a clamp interface with precisely 1.5-butt-loads of compressive-force on it (That crank bolt should be torqued to 300-400 in-lbs). Look closely at the bowl of that interface in the pic, there are lots of finer crack-looking flaws in the metal. It also is an older model, and has been mounted (torqued to spec) and pulled off a few times probably.


Not that it ultimately matters....that crank should be a wall ornament post haste.

grizzly59

If it's a crack, throw the crank arm away. Are you sure it's a crack? Not "flash" from the mould? I would get some 400 grit or so black auto body sandpaper and sand that smooth. Did the crack disappear? I can't tell from the pic. If you have a friend who's a welder or something like that you might have a dye penetration test done. If I couldn't prove it's not a crack I would toss the crank arm.

KLiNCK

Good catch!
That's what preventative maintenance is all about!
Replace crank ASAP.

FBinNY 

Most definitely NOT flash from a mold (mould). There's no parting line there, to cause flash.

If you look closely, you can see how it seems that the crank arm is trying to twist away from the disc part of the chainring spider. If it also lines up with the corner of a square spindle, then that too, might be causative.

HOWEVER, While the risk of failure is pretty high, the consequences may not be as dire as some would have you believe. Over my cycling career, I've broken a number of cranks and pedal spindles (at least 5 cranks, maybe more), and none led to any injury of any kind, or even a crash, though once or twice it led to loss of control, and swerving before I cold recover. Had I been in heavy traffic, it could have been worse.

The rest of the time, it was no worse than stamping my foot very hard into the road.

sweeks

Aluminum does pretty well under compressive loading. Its tensile strength is less, and when it is subjected to alternating tensile and compressive loading it is susceptible to cyclic fatigue. It's unlikely that repeated compression from installation would cause any problems; there is tensile stress when the crank extractor is used, but if it could cause cracks, they would be be in the threaded area on the outboard side of the crank, not the inboard side, and they would not likely be radially oriented.

That said, the crack-like structure seen in the images is in a part of the crank that is likely subjected to intermittent tensile stress, so I'd like to retract my remark about this being an odd place for a crack. I'd be interested to know whether the crack is seen on the outboard side of the crank. Also, the dye test would be useful to rule out a surface imperfection that isn't a safety risk. Probably cheaper and easier to just replace the crank though.
Steve