The Universe Laughs

So here’s a little irony. On Wednesday which was, I note:

(1) The day before Bike To Work Day in California…

(2) The day after I replied to a commenter that the extra investment in getting a folding bike like mine was well worth it (over getting a cheaper model) since -among other reasons- cheaper models would be more likely to fall apart while mine will last a lifetime…

I set off at 1:10pm on my bike to head (via the bus) to my office at USC, photocopy the final I’d spent the morning refining, and arrive at class at 2:00pm to set the final. Everything went fine, as usual, and as I did one last turn after waiting at the lights to head into the home stretch to catch the bus…

…my bike suddenly lost power. The chain had not slipped or broken though… An examination revealed that the crank arm/lever had completely broken away from the wheel holding the teeth and the chain. To be fair, I put an awful lot of regular stress on it (see below), but isn’t that ironic?

I got off the bike and wheeled it briskly to the bus stop I was headed for, managed to catch my bus, and with a bit of running in the bits that I would have biked, made it to the final only eight minutes late. Happily, I’d scheduled the exam’s length to be 10 minutes shorter than the alloted two hours, so everything worked perfectly.

But how is that for irony? Of all the hundreds of days I’ve been biking to work, on Bike To Work Day, I would not be biking in…It turned out that I happened to have a replacement crank assembly at home and so took the bus and subway home Wednesday, and brought the device in by car the next day. (Sigh. Car because I also had to go to a meeting (I will try to tell you about) in Beverly Hills and then, later, a movie marathon (I will try to tell you about) in Santa Monica). I’d planned to (as usual) bike to work on Bike To Work Day and then bike home and pick up the car for those errands, but since I’d been deprived of that pleasure, I drove and did a sort of loop that day instead of a hairpin.

So the fix? Easy-peasy it turns out. But you need the right tools. Nobody seemed to have a 14mm socket wrench at work, and so I had to wait until today to get one, picked up on a slow, super-hot walk this morning. On my way to Santa Monica last night I stopped off at a bike store to get a “crank-puller”. Yes, really. (We need some of those in physics). Its job is to evenly lift off the crank assembly from the hub. (Type CCP-2 for those of you who might have found this later to guide your own repair.) I brought in my 15mm wrench for the pedal swap I’d have to do. Not a complicated job. Here’s everything to start out:

bike crank fix

Next stop, after popping off the little plastic dust cover on the hub revealing the 14mm hex nut, use the socket wrench to take off the nut. Then, take the outer assembly off the crank-puller and screw by hand it into the recessed threaded hole until it can’t go any further. (Make sure it is not cross-threaded, as this is crucial to make sure that the next step lifts the crank off evenly.) Then you can place the rest of the crank-puller, handle attached, into the interior of the device just screwed in, and turn it all the way in.

bike crank fix   bike crank fix   bike crank fix  

bike crank fix  bike crank fix  bike crank fix  

Eventually, it will begin to tighten, but continue, since further turns will then lift the whole crank nicely off the pin on which it sits. Since the crank arm actually broke off the chain wheel, you can see that it is just the arm that comes off here:

bike crank fix

Well, anyway, one can lift everything off, flick away any stray bits of fatigues metal, and pop on the replacement crank, hooking the chain on first, of course…

bike crank fix  bike crank fix  

… and the hex bolt screwed back in and tightened firmly (but not over-tightened)… the little dust cap replaced… and the pedal swapped from the old crank arm to the new one…

bike crank fix  bike crank fix  bike crank fix  

Job done. All of about 5 or ten minutes, depending upon whether you stop to take pictures every few seconds or not. Here it is:

bike crank fix

You’ll notice that this is a larger lead chain drive attached to the crank than the previous one. This means the bike is faster now (yay!), but my hill-climbing ability has been compromised a bit (boo!). This was in fact the original one, but I preferred a better power ratio on my total of six gears over speed on the flat. I will probably get a crank like the one I had to restore this. Better for my knees, and I don’t need the extra speed so much (only sets you up for more traffic accidents anyway).

I still stand by my earlier statement about it being worthwhile to buy into the quality of the machine, overall, and there’s no other that folds as fast or as compactly while still feeling like a complete full bike on the ride. This was simply the failure of the most stressed part of the bike after four years of faithful service (it takes my entire weight and turns it into the torque that schleps me up some pretty steep hills), which took all of ten minutes to change out. bike crank fix(I’ll pretend not to notice that on this wonderful piece of British-made and designed engineering…the broken part was made in France (Click on right for larger view)…Ahem…) To be fair, I think that over the last few months I’d been noticing a slight strange feeling from that area of the bike, but did not diagnose it and it slipped my mind. I don’t know whether this was a crack developing or whether somehow the nuts had been working loose and if this hastened the failure. I’ll send a note to my friends at the bike shop to ask if this was supposed to fail quite so soon though… seems a bit too soon.

A bit of googling shows that this is, not surprisingly, quite a well-known place where metal under stress tends to come up wanting. There are even learned papers written about the matter, and how to test cranks for metal fatigue and so forth, such as “The failure of bike cranks, International Standards, tests and interpretations, by J. E. Morgan & D. J. Wagg”, in the Journal called Sports Engineering. And here’s a site where you can look at pictures of broken cranks, and spindles. Here’s another at the Open University.

Ah, Physics everywhere.

-cvj

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