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Left crank arm fell off, need advice.

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Left crank arm fell off, need advice.

Old 08-09-14, 07:01 AM
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Left crank arm fell off, need advice.


The bike was given to me by the previous flat mate, according to him just needed new inner tubes and tyre , I just fixed those and took the bike for ride that's when I notice a clicking sound, then the left crank arm started to wobble and finally fell off, luckily no accidents.

I don't have the any tools to assemble the arm and tight the screw again so before spending any money in tools and/or parts I would like your advice.

Is the crank arm still useable ? Do I need a screw ? Do I need to replace the bottom bracket ?

I also notice that the front tyre is a little wobbly.

RidgeBack Bullit 7005
Crank set: Shimano Sora AL FC-3303

My expectation was to use the bike as commuter ~14 miles per day

Thanks for your time.
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Last edited by jrafael; 08-09-14 at 07:11 AM. Reason: bike details
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Old 08-09-14, 07:21 AM
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That crank arm has come off several times and is badly damaged. You need to get a new one from a bike shop. Get one the same length and have them install it so it's properly tightened, with a new bolt. The right one should be checked for tightness at the same time.

Last edited by cny-bikeman; 08-09-14 at 12:17 PM.
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Old 08-09-14, 07:29 AM
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The crank arm is junk now, it was loose for some time judging by the damage. You will need a new arm and don't be surprised if the bottom bracket is loose as well. I don't know if you will be able to find just a left arm or not, maybe at a shop that has a junkyard. I've always just bought the whole crankset and replaced it when needed, but maybe you can find just the left arm someplace.

Last edited by mcrow; 08-09-14 at 07:41 AM.
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Old 08-09-14, 10:37 AM
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Certainly you DO need a new and longer bolt to lock the arm on. The one you have is stripped so it's no good anymore. And since there were clearly only a few threads on the end that engaged it's a sign that you want a longer bolt that will allow for grabbing a few more turns. Ideally any bolt should engage for at LEAST 1.5 times the diameter for heavy duty use like we're talking for this sort of job.

I gather that this is a basic beat 'em up bike that you can ride around and if it gets stolen or hacked up it's not the end of the world. Given this I'd get the new longer screw and GREASE the crank arm and spindle and tighten the bugger back on. I'd even go so far as to suggest that you back up the other side with a few boards or a rock and some padding and give the deformed crank arm a few whacks with a wood board for padding and a good size hammer to aid in forming the aluminium back into place to some extent. Then tighten the new locking screw down HARD with a socket and ratchet.

Then go for some short rides around your flat and check the arm frequently for loosening. Don't go far since you may be walking the bike back if things come lose again too quickly. On the other hand if it seats on well enough to get around then you saved yourself the cost of a new arm and moving the pedal over.

I gather this means you would need to buy some tools. But that's OK. They won't go bad and just might prove useful for other uses And if the arm really is dead and should be put out of its misery like the others are saying then at least you tried. You won't need a crank puller tool if this happens because it'll loosen itself off for you.

The spindle on this bike's bottom bracket isn't doing anyone any favours either. I don't recall ever seeing one with such large cutaways on the corners. It looks more octagonal than square with relieved edges. The real damage was due to the arm loosening off the taper. But the small size of the flats didn't help any once things started to go.
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Old 08-09-14, 11:48 AM
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Since you indicate your plan is to commute on this bike, you'd want to know that the parts were not going to have the same problem again.

The parts that you provided photos of are all shot. you should replace all of the parts (crankset with crank arm bolts, and bottom bracket).

Have the parts installed, and use a torque wrench to make sure that the crank arm bolts are correctly tightened to avoid the kind of damage pictured in the photos in the future.
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Old 08-09-14, 11:50 AM
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Buy a new one, arm and bolt.
and a wrench so you check it for tightness regularly

replace whole crankset and BB if you want to ..
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Old 08-09-14, 04:37 PM
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You can probably find just a left side "Square taper" crank arm. Yours is most likely 170mm or 172.5 mm. Look for the size stamped on one of the crankarms. You don't need the same model as long as it's "square taper", and the square is the same orientation relative to the arm. Notice the 2 "points" of the square are in line with the crank arm. Check your local shop to see if they have one in their "misc parts box". Ebay is another option.

I have 1 bike with a 170mm crank on the left and 172.5 on the right. So close that I would never notice.
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Old 08-09-14, 04:45 PM
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Here we go again.

That's a fairly common problem. So common that many, maybe even most, bike shops stock replacement left crankarms. As another poster mentioned, you'd like for the crank arm length to be the same. It'll be stamped on the inside bottom edge of the arm. The orientation of the square hole has to match too so check to be sure it's the same as your old arm.

It takes quite a bit of torque to attach a crank arm properly. 30 ft/lbs is a typical spec. All too many folks, even bike shop mechanics, try to tighten those bolts with a wimpy, little, 6 inch long allen key. You're not likely to get to 30 ft/lbs that way. That's why so many left crank arms come off.
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