Medic Zero

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This happened to me as I went to take off from the left turn lane at a busy stop light, early in the second day of an overnight, two day bike ride. Had to catch a bus into the next town, as the town it happened about two miles outside of didn't have a bike shop, at all.

I know my weight is a factor here (280 lbs), but I suspect the pedal extenders also probably put some additional stress on the crank in a way I guess it wasn't really designed for. Nonetheless, I never even considered that such a failure was a possibility prior to this happening to me. I guess I just assumed that a cast piece of aluminum was basically unbreakable in this context.

To be fair, in addition to my weight, I ride for nearly two hours a day, seven days a week over rough roads, and occasionally end up hopping off a curb during my commute, so I suppose that poor crank has been subjected to continuous vibration and occasional severe stress.

eajohnson

Haven't personally seen that but it happens. With a stereo microscope it's usually possible to examine the surfaces and get some idea of how the failure progressed. https://en.wikipedia.org/wiki/Metal_fatigue You might be amused to note, that an example used to illustrated the wikipedia article is an aluminum bicycle crank arm.

Glad you're OK, a sudden failure like that could have ended up very badly for you!

FBinNY 

Aluminum has a definite fatigue life. One of the factors is the amount of deflection. Your weight alone would shorten the fatigue life of cranks if you used the full weight such as in hill climbing. But combine that with pedal extenders which push the pedal outward and the twisting torque is increased. Combine both and this is what can happen.

We probably don't see as much of this as we might because of the simple fact that there aren't that many high mileage heavy riders. Also, regular riders (regardless of weight) tend to buy better equipment and are less likely to use pedal extenders.

As you point out, there are a number of aggravating factors besides your weight, so while the actual event was probably a surprise (I broke a number of cranks and pedals over the years, and know it's always a surprise), the fact that it happened shouldn't be.

When you replace this, consider a better level crank, preferably one made for off road use, but even then there's no guaranty of infinite life.

ThermionicScott 

M0, could you take a close-up picture of the broken ends? If it looks like this, indicating that the crack has been in progress for a while, it might be a good idea to start inspecting for cracks more often:


(Picture from pardo.net)

Medic Zero

Thanks! I appreciate the insight and advice (and your presence here on this site, always helping)!

A bike shop on the trip replaced it with one from their spare parts bin. It's also a fairly low end crank, but it is wider and has a curvature to it, so I'm hoping structurally it'll be stronger. Since the failure (luckily!) was non-drive side, a full upgrade to more expensive cranks wasn't what I was looking for at that moment, but I'll be taking your advice to heart and be seeking something designed for off-road abuse as a replacement crankset. It'll probably be fairly low priority with the other expenses I have stacked up ahead of it, but hopefully I'll come across a deal somewhere and can get one and get it mounted before the next failure!

Medic Zero

Unfortunately, the failure happened in the middle of a two day bike ride away from home. I was tempted to keep part or both halves of the broken crank, but I was already close to capacity in the limited luggage I had aboard the bike this time and am trying to mend my pack-rat ways! If I had a fireplace mantle, it probably would have ended up there, but as it is, it'd probably just be more junk spilling out of my storage space! I think I was also expecting the mechanic at the shop to offer it to me as a souvenir, and kind of forgot about it when I squared my (nominal) tab with him and was happy to be mounted again! FWIW, I recall it appearing similar to the one you posted, and I've been trying to track down several different mystery creaks and noises recently, one of which magically disappeared when the crank snapped! My girlfriend also snapped a photo with her phone, I doubt it shows any more detail, but I'll see if I can't post it here.

As I said, I never expected such a failure to occur, so I never thought to look for cracks mid-crank! I'll add it to my inspection list for sure now that you mention it. I was wondering if this failure would have showed any outward signs beforehand, and was assuming that everything was happening internally until it snapped. Does anyone know if sometimes this is visible as cracks or discoloration prior to failure?

Medic Zero

Unfortunately, it looks as if the photo she snapped after I got it over to the entrance of a nearby business complex to regroup doesn't show any more detail than my photo:



Other than evidence that I am overdue to replace the feet on my kickstand!

ThermionicScott 

Aw, rats! I kinda like seeing pictures of broken parts (there were similar threads on my vintage VW forums), but I am glad no serious harm came to you.

These breaks can happen gradually, and are not as uncommon as one would hope. Very thin cracks can be seen as it happens, but you have to look closely. More broken cranks here: https://pardo.net/bike/pic/fail-001/000.html#Crank

Medic Zero

Very interesting link, thank you! The pictured spindle failures were a surprise! Another component I never expected to have something like that happen to!

Also, they mention another thing that could very well have contributed to the short life of my crank arm: pedal strike. I have yet to mount the 180mm crankset I picked up for my tourer, but running pedal extenders on the 175's I run on my commuter means that once in a blue moon I will have a pedal strike when cornering like a motorcyclist through a fast turn and pedaling. Given my size and the pedal extender putting the force of the impact further out than the crank was designed for (not necessarily the impact, they probably aren't designed for that so much!, more stress in general), I imagine such a strike really transfers some force through the crank. I seem to recall actually having a pedal strike recently as well.

The good news it that I had a number of creaks and mystery noises I was trying to track down, and between my LBS retensioning my rear wheel, me wrapping duct tape around the crack in my metal fender, and the this crank being replaced, my bike is finally back to being quiet!

Medic Zero

RE: the bolded bit, you would've loved the time my bus ran out of oil on the freeway and threw a rod through the case then! I heard it happen and heard shrapnel bouncing all around inside the engine compartment! When I coasted her off the causeway (over rice paddies and a river) and popped the decklid, at first I didn't see anything. When I peeked over the doghouse I finally spotted the end of the rod protruding through the engine and then spotted the shrapnel - pieces of the cast engine block! This must have been about 1992 though, no pictures to share, sorry!

icemonkey

Used to see this type of failure all the time, old nuovo record cranks used to get stress fractures at the base of the spiders, early dogbone style cranks (I think crank bros) used to shed pedal ends with alarming frequency. In fact during the 90s I would see broken cranks weekly on city bikes. Mountain bikes just ate cranks. The new stressed relieved designed are so much better. I am not an expert (...but...) in your case just looking at the little bit you have left can tell you a lot, is it cupped - an indication of a radical failure, a flatter fan like break could be a forging error and a series of c shapes as the pick above show an evolution of cracking.

Whatever you do, invest in quality cranks - imagine that happening at a busy intersection...

Kimmo

Old crank looks like it might've been a C-section - a pretty bad idea IMO under a 280lb rider using pedal extenders.

Most of the external BB cranks I've seen seem to be a fair bit beefier than square-taper cranks. Cheap stuff like Sora may have a nominal C-section, but it's chunky enough to entirely eclipse the cross-section of most 70s cranks.

Medic Zero

I don't have to imagine it, it did! I was at the intersection of a two highways and an arterial, in the left turn lane at a stop light! I was in pole position, with a line of cars behind us, when the light turned green and I stood on the pedal to take off it snapped! My girlfriend was behind me on one of her bikes and said the pedal and attached crank remnant hit the ground with such force that it bounced several times!

Luckily, although I smacked my right inner thigh hard on the top tube, no worse happened. I had to negotiate my way through two lanes of traffic on foot, after picking up my pedal out of the road and then lost about 17 miles out of our ride, walking two into the next town, and then catching a bus to the town beyond that where the nearest bike shop was.

Medic Zero

RE: the bolded bit: that's part of why I'm sharing here and in the Clydesdale's forums. Despite being quite active on Bike Forums for the past four years, my experience is the first I've heard of even a crank failure, hence the topic title here. I'd also never had it happen or heard of it happen during the six years I spent car free and cycling everyday when I was at my healthy weight of 210 pounds. Two of those years were spent as a bicycle courier as well.

I'm not really interested in changing to outboard bearings, surely there are quality cranks in square taper?

miamijim

What's the part number?

Maybe one of these? FC-CT90, FC-M290 and FC-MC12. These numbers are located on the back or inner side of the right crank arm.

Kimmo

Why not? Wouldn't it make sense to employ a stronger system when you're putting so much load on it? The square-taper interface would be pretty close to its limits in your application, IMO.

As I said, the strongest-looking crank arm designs I've seen all seem to be external BB types; I guess because it doesn't make sense to make the crank arm any stronger than the BB interface maybe.

I'm not sure it's quality you're after so much as plain strength; quality is generally correlated to strength/weight. Cheap cranks may well be best if they happen to be over-engineered.

Just get whatever cranks use the most metal, I reckon.

HillRider

Not really, at least not new current models. Square taper is two or more generations out of date so you will have to find a good quality used or NOS crank.

Shimano dropped square tapers when it first went to Octalink in the 9-speed era and then Hollowtech II with it's external bearings for 10-speed. The last Shimano square taper cranks date from 8-speeds. Campy continued with square taper into the 10-speed era but dropped it for their external bearing designs several years ago.

I agree with Kimmo. Why not replace it with a lighter, stiffer and stronger system.

Kimmo

Also, Hollowtech II, and other external BB systems to a slightly lesser extent, are an absolute breeze to work on next to the elbow grease and potential dramas of square taper.

cyccommute 

I have broken a crank in the past in a similar manner. The crank, however, was an aluminum crank with a carbon fiber insert...a Race Face Next, to be precise. That, for those of you who aren't up on your 90's moutain bike parts, was a high end crank but not the best design. It had too little aluminum to be structurally sound.

Breaking it also taught me something (along with other aluminum failures) about aluminum and aluminum parts failure that many people don't understand. Aluminum doesn't shatter or fail suddenly. It complains a lot prior to failure but most people, myself included, fail to pay attention to the warnings. This crank creaked like crazy for quite a while before failing. Yes, when it failed, it did so suddenly(ish) but the warning signs were there before failure.

I don't know what your issue with external bearings is Medic Zero but if you want to stick to square taper and use pedal extenders, you should buy quality parts. Specifically, you should look for a crank that has been cold forged. Most inexpensive cranks have been cast from melted aluminum which isn't nearly as strong as cold forged parts. Sheldon Brown has a good explanation for why.

That said, I would suggest at least looking at the external bearing cranks. They are dead simple to install. I'd suggest looking at a Race Face Atlas which is an all mountain/downhill crank. Very strong and very good. Shimano SLX would also be a good choice. The Shimano is a little easier to install.

icemonkey

Ouch. It happened to a customer (at a London shop I was at decades ago) on Mall during rush hour sprinting off from lights. Nearly killed him, and I don't think he ever had kids.

HillRider

I certainly remember the raft of boutique CNC'ed "billet" aluminum cranks that hit the market in the '90's. Many were made by those with good CNC programing skills but poor metallurgical and mechanical knowledge and there were many reports of breakage.

Matariki

Happened to me earlier this year. Drive side arm broke while going up hill out of the saddle. I hit the ground instantly (seemed like it). I'm 61 and 120 lbs lighter than you so weight and power probably have less to do with aluminum fatigue than mileage does.

fietsbob

Been shown over time, to be one of those; 3 choices Pick 2, situations.. light, or strong, or durable ..

3alarmer 

[h=2]Virtues of the Cottered Crank...[/h]

FBinNY 

For what it's worth, I've broken about 6 cranks in my lifetime. All were steel, and the first crank I didn't break was a Campy Record crank from 1967. Since then I've ridden only with forged aluminum cranks and never had a failure.