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Old 08-01-07, 05:26 PM   #1
Daily Commute
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Damaged Crank, Replace only one arm?

One of my cranks is damaged. It was creaking, and two days in a row, I tightened the bolt after my commute home, and two days in a row, it was loose the next day. Today, I tightened it 2/3 of a turn.

The LBS sold me a crank puller to take a look at it (I prefer to buy the tool instead of the labor, especially since advice comes with MSRP). The mechanic said that if the problem was the crank arm, I could just replace the one arm. Well, I pulled the crank, and it's worn on all four sides of the opening.

I notice that online places only sell crank sets. If the LBS can match come close or match the original crank, is there any problem with buying only one crank arm?

I ride a lightly-loaded Surly Cross Check for commuting every day.
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Old 08-01-07, 05:49 PM   #2
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As long as you find a matching arm, there's nothing inherently wrong with just replacing the one arm. The only problem is, it's usually quite difficult to find a matching arm, or at least at such a price as to be worth it versus replacing the whole crankset.
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Old 08-01-07, 06:17 PM   #3
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As long as you find a matching arm, there's nothing inherently wrong with just replacing the one arm. The only problem is, it's usually quite difficult to find a matching arm, or at least at such a price as to be worth it versus replacing the whole crankset.
When you say "matching arm" are you referring to the length and Q-factor? Other than that, you can use whatever crank-arm you want as long as it fits the type bottom bracket you use.
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Old 08-01-07, 07:11 PM   #4
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When you say "matching arm" are you referring to the length and Q-factor? Other than that, you can use whatever crank-arm you want as long as it fits the type bottom bracket you use.
As long as I had the right size and generally the same weight, would I even notice on a loaded steel commuting bike?
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Old 08-01-07, 07:37 PM   #5
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As long as you find a matching arm, there's nothing inherently wrong with just replacing the one arm. The only problem is, it's usually quite difficult to find a matching arm, or at least at such a price as to be worth it versus replacing the whole crankset.
Replacement left crankarms are a common repair part. For a bread and butter crank you should be able to get one that's a reasonably close visual match for $20.00.
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Old 08-02-07, 03:12 AM   #6
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Replacement left crankarms are a common repair part. For a bread and butter crank you should be able to get one that's a reasonably close visual match for $20.00.
Thanks. I'll stop by the LBS after work for a crank arm.
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Old 08-02-07, 03:39 AM   #7
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The thing is, replacing a crank arm - if it's of a different make, probably won't match. I tried it last month, at least for square taper arms different - makers put the hole at a different angle. So it's impossible to get them parallel to each other. It's hard to explain without pictures.
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Old 08-02-07, 06:45 AM   #8
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The thing is, replacing a crank arm - if it's of a different make, probably won't match. I tried it last month, at least for square taper arms different - makers put the hole at a different angle. So it's impossible to get them parallel to each other. It's hard to explain without pictures.
in theory all cranks are forged and machined so that the pedal spindle is perpendicular to the centerline of the bike. there's inevitably some deviation, but unless you get a bad one that got past quality control, it's probably nothing the average joe can notice.

what you will find is that the q factor on a basic replacement arm may vary greatly from that of the original.
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Old 08-02-07, 10:00 AM   #9
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Unless the crankarm is cracked you can salvage it.
Typically the crank arm bottoms out on the flared portion of the spindle (where it goes from square to round). If this is the case you can rework the crank to remove some material from the crank arm in this area. The two options would be to add a generous chamfer on all four sides of the spindle hole (flat file or dremmel) or to remove material (1/8") from the inside face of the crank (grinder or hacksaw).
The intend is to allow the worn crank arm to slide further up the taper to a point where it is making good contact with the taper.

I have done both methods on seperate bikes and have had no more loose crank issues on either.
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Old 08-02-07, 04:20 PM   #10
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I like DIY bike repairs, but reworking the crank arm seemed like more than I wanted to do. So I bought the $15 arm. The LBS offered to use their pedal wrench to loosen my pedal (so I could use my cone wrenches to complete the job at home), but they couldn't get it loose. They even took the pedal off and put it in a vise.

So I'm buying new pedals. Fortunately, I was looking at getting new ones anyway. The advantage of paying MSRP for the crank arm, crank puller and pedals is that I get exactly what I need and some free labor. Not bad. (Oops, I forgot to have them see if they could get my left pedal off!)
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Old 08-03-07, 07:20 AM   #11
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I think if you're crank arm's damaged, your BB spindle probably is as well. I'd replace it all, especially since BB's are only ~$25. But yes, you can replace just one arm, you just have to find one.
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Old 08-03-07, 09:46 AM   #12
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I think if you're crank arm's damaged, your BB spindle probably is as well. I'd replace it all, especially since BB's are only ~$25. But yes, you can replace just one arm, you just have to find one.
Unless it's a steel arm, which would occur cheaper bikes, the aluminum arm will wear long before the steel crank spindle. I replaced many, many left crank arms in my life and have not seen a problem with them not mating and staying on the used spindle.
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