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Old 11-12-04, 07:25 AM   #1
Archeomason
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Crank Arm Fell off...

I quick question...

I just bought a bike on Ebay and was out testing it out in my parking lot. While I was cruising, the left crank arm fell off. Is this a bad sign or is it something that can easily be fixed? If it can, is it something that a relative novice can do (I can do basic maintenance etc...). It's a 1982-ish Raleigh Super Record with the standard tapered square spindle...

Anyway, thanks...
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Old 11-12-04, 07:36 AM   #2
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it doesnt sound terminal to me- why did the crank arm fall off? was something missing or loose i.e. a bolt that should have been holding it on?

worse case scenario -you have to buy a new bottom bracket and/or crank arms

you could probably fix it yourself without too much trouble
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Old 11-12-04, 07:59 AM   #3
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Doesn't sound good to me.

Left crank arms falling off is a semi-common problem. The core cause is inadequate torque on the bolt that holds the arm onto the bottom bracket. Now you'd think that if you just put it back on and tighten it up snugly it'd be OK and occassionally that works. With a used bike, however, I'm betting that you have a hosed crankarm. When the bolt that holds the crankarm is loose, it allows the crankarm to wallow around the square spindle and tries to turn that square hole into a round one. The upshot is that the crankarm will never stay tight after that.

The good news is that left crankarms are available singly as repair parts and are usually not too expensive. Any average or better LBS will be able to get one for you, especially if you aren't too picky about having an exact replacement.
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Old 11-12-04, 08:12 AM   #4
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For cotterless cranks....

The bolt does not actually hold the crank arm to the spindle. It is there for the purpose of installing the crank. The crank is press fit onto the spindle using the bolt, after the crank is installed the bolt serves no real purpose and sometimes comes loose.

If the bold does come loose (while crank is still tight on spindle), it is a bad idea to re-torque it as hard as you did when installing the crank, because that will put even more stress on the crank.

If you have ridden the bike with a loose crank, you most likely have hosed the crank, and need to replace it.
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Old 11-12-04, 08:56 AM   #5
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Originally Posted by H23
For cotterless cranks....

The bolt does not actually hold the crank arm to the spindle. It is there for the purpose of installing the crank. The crank is press fit onto the spindle using the bolt, after the crank is installed the bolt serves no real purpose and sometimes comes loose.

If the bold does come loose (while crank is still tight on spindle), it is a bad idea to re-torque it as hard as you did when installing the crank, because that will put even more stress on the crank.
This is all just BS!.
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Old 11-12-04, 12:53 PM   #6
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Quote:
Originally Posted by Retro Grouch
With a used bike, however, I'm betting that you have a hosed crankarm. When the bolt that holds the crankarm is loose, it allows the crankarm to wallow around the square spindle and tries to turn that square hole into a round one. The upshot is that the crankarm will never stay tight after that.

The good news is that left crankarms are available singly as repair parts and are usually not too expensive. Any average or better LBS will be able to get one for you, especially if you aren't too picky about having an exact replacement.
Yeah, what he said. Find a good bike shop, preferably one with a pile of old bikes in the basement. When this happened to me, the dude went downstairs & swapped the crank for me. Wouldn't let me pay him either. Of course it doesn't match the right side, but who cares? The bike is 12 years old & the new arm didn't cost me.
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Old 11-12-04, 01:25 PM   #7
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Originally Posted by sydney
This is all just BS!.
Sydney is absolutely correct. See "Pedal into crank" spec on this page:

http://www.parktool.com/repair_help/torque.shtml
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Old 11-12-04, 03:57 PM   #8
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Sydney is absolutely correct. See "Pedal into crank" spec on this page:

http://www.parktool.com/repair_help/torque.shtml
Not really. Please consult "Subject: 8f.11 Installing Cranks" in the rec.bicycles faq

http://www.faqs.org/faqs/bicycles-faq/part4/.

Jobst gives a nice explanation of how taper interfaces work and why floppy cranks are a bad thing.
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Old 11-12-04, 04:32 PM   #9
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Originally Posted by LarryD
Not really. Please consult "Subject: 8f.11 Installing Cranks" in the rec.bicycles faq

http://www.faqs.org/faqs/bicycles-faq/part4/.

Jobst gives a nice explanation of how taper interfaces work and why floppy cranks are a bad thing.
Yeah, a flloppy crank is a bad thng. They get floppy quicker if not torqued properly ,and even quicker if the bolt ain't there as the OP suggested.Try rining one without a crank bolt. Once the interface gets buggered brom being ridden loose,it is material for the recycle bin.
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Old 11-12-04, 04:35 PM   #10
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Originally Posted by LarryD
Not really. Please consult "Subject: 8f.11 Installing Cranks" in the rec.bicycles faq

http://www.faqs.org/faqs/bicycles-faq/part4/.

Jobst gives a nice explanation of how taper interfaces work and why floppy cranks are a bad thing.
My reply was to support the argument that the crankbolt DOES hold the crank on (Brandt goes so far as to make the point that the dust cap keeps the crank bolt from falling out, and thereby keeping the crank from coming off the axel), and, it takes a substantial torque to do it properly (thus, my link to the torque spec page). Brandt states that to do the job correctly you need to grease the taper AND torque properly. I am in complete agreement with both of those concepts. Of course floppy cranks are bad. But, if I get on my bike and feel a loose crank after a few revs and proceed to stop and fix the problem, then the crank may be ok. If I re-torque the crank bolt and it holds up, then fine. If not, then, yes, it's hosed.

Thanks for that reference, BTW. Very nice.
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Old 11-12-04, 05:04 PM   #11
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Quote:
Originally Posted by cascade168
My reply was to support the argument that the crankbolt DOES hold the crank on (Brandt goes so far as to make the point that the dust cap keeps the crank bolt from falling out, and thereby keeping the crank from coming off the axel), and, it takes a substantial torque to do it properly (thus, my link to the torque spec page). Brandt states that to do the job correctly you need to grease the taper AND torque properly. I am in complete agreement with both of those concepts. Of course floppy cranks are bad. But, if I get on my bike and feel a loose crank after a few revs and proceed to stop and fix the problem, then the crank may be ok. If I re-torque the crank bolt and it holds up, then fine. If not, then, yes, it's hosed.

Thanks for that reference, BTW. Very nice.
The dust cap may keep the bolt from falling out,but it doesn't necessarily keep the bolt from coming loose.Many current cranks use an 8mm allen bolt with no dust cap. I don't use dust caps, cuz they are a PITA.I've also never had a properly installed and torqued crank arm come loose either.
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Old 11-12-04, 06:15 PM   #12
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Originally Posted by sydney
The dust cap may keep the bolt from falling out,but it doesn't necessarily keep the bolt from coming loose.Many current cranks use an 8mm allen bolt with no dust cap. I don't use dust caps, cuz they are a PITA.I've also never had a properly installed and torqued crank arm come loose either.
sydney is absolutely correct.
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Old 11-12-04, 06:22 PM   #13
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I will defer to Jobst Brandt as the ultimate authority on these matters.

I have observed loosening of crank bolts even on properly installed cranks. If the bolt does come loose, it is not advisable to re-torque it with the same torque used to install the crank-- thats what Jobst Brandt says anyways.
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Old 11-12-04, 09:28 PM   #14
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Quote:
Originally Posted by H23
I will defer to Jobst Brandt as the ultimate authority on these matters.

I have observed loosening of crank bolts even on properly installed cranks. If the bolt does come loose, it is not advisable to re-torque it with the same torque used to install the crank-- thats what Jobst Brandt says anyways.
LOL...well, do as you will. Enough very knowledgeable folks disagree with him on enough topics to hardly consider him the ultimate authority on alot of subjects.
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