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Square taper crank damage

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Square taper crank damage

Old 11-16-11, 01:20 AM
  #1  
nicoth
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Square taper crank damage

Hello. How do you tell if the holes in the crank arm have been deformed? I'm interested for two reasons: my bike makes squeaking/creaking noise while pedaling, and it feels mushier than it used too (not as stiff). I've tried removing, cleaning, and reinstalling the crankset. Can you visually tell if the holes in the arms are damaged? THanks.
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Old 11-16-11, 03:21 AM
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Probably being aluminium, it's more likely they're cracked than deformed. Have you whipped them off and inspected them?
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Old 11-16-11, 04:56 AM
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Originally Posted by nicoth View Post
Hello. How do you tell if the holes in the crank arm have been deformed? I'm interested for two reasons: my bike makes squeaking/creaking noise while pedaling, and it feels mushier than it used too (not as stiff). I've tried removing, cleaning, and reinstalling the crankset. Can you visually tell if the holes in the arms are damaged? THanks.
Have you inspected the BB for wear?
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Old 11-16-11, 01:18 PM
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If they are deformed the cranks will move on the spindle and loosen the bolts. Once the bolts are loose the crank can move freely on the spindle which further deforms the hole. Then you have trouble riding for more than a few minutes before the crank is loose again.
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Old 11-16-11, 01:38 PM
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I used to have a problem with squeaking and I thought it was my bb. I checked it out and it turned out that the square taper on my crank arm was pretty worn. I replaced the cranks and the squeaking stopped. So you might want to check the taper on your cranks too.
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Old 11-16-11, 07:51 PM
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Originally Posted by mikezs View Post
Probably being aluminium, it's more likely they're cracked than deformed. Have you whipped them off and inspected them?
Sadly, reality does not agree with mikezs today. Aluminum square taper cranks often get rounded out and loose like d_D says. It is almost always the left crank. But it usually only happens when the cranks are not properly installed. I have seen it happen occaisionally on older bikes, too, though.

THe characteristic 'creak' of a loose crank arm can generally be differentiated from other creaks with the following test:

1. With all your weight on the saddle, push really hard down on the left pedal when it is in front
2. Take your foot off the left pedal
3. Use the right pedal to turn the cranks so that the cranks are level (at 3:00 and 9:00) and so the left pedal is at the back, keeping your weight on the saddle and left foot off the left pedal
4. Gently place your foot back on the left pedal
5. Stand up suddenly and listen and feel.
If your square taper it getting worn or damaged and is squeaking, this procedure will give you the best chance of reproducing the squeak. You will be able to hear it and possibly feel it in your foot.
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Old 11-16-11, 10:00 PM
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If the holes you refer to are the ones that attach to the BB spindle, you can take the spindle out (or leave it in the BB) and paint the square ends of the spindle with Dye-Chem and push the cranks onto the spindle ends, then remove the cranks and inspect the Dye-Chem for uneven rub-off. This will show even minute mating surface irregularities. Generally, however, spindle holes worn enough to cause problems will be apparent just by looking at and feeling the cranks when pressed onto the axle.
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Old 11-16-11, 10:24 PM
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Rounded square taper cranks are almost always the result of insufficient torque on the fixing bolts. If you don't tighten them to spec they WILL loosen and destroy your crankset. This is the one part on a bicycle I would argue a torque wrench is absolutely necessary unless you've done the job before or have a good feel for 40ft*lbs.
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Old 11-17-11, 01:22 AM
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I had a bike thdonath did that- I had broken seat stays at the seat tube braisings Oh!- DO NOt lube Your "flats" ror nyou WILL overtighten them deforming your cranks and possibly void any warranty
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Old 11-17-11, 09:56 PM
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Originally Posted by neurocop View Post
If the holes you refer to are the ones that attach to the BB spindle, you can take the spindle out (or leave it in the BB) and paint the square ends of the spindle with Dye-Chem and push the cranks onto the spindle ends, then remove the cranks and inspect the Dye-Chem for uneven rub-off. This will show even minute mating surface irregularities. Generally, however, spindle holes worn enough to cause problems will be apparent just by looking at and feeling the cranks when pressed onto the axle.
Good point. Often the damage can be seen. If the 'flats' inside the crank are not perfectly flat - if there is a characteristic dent on each of the flats caused by the corners of the spindle when it is slightly misaligned - then the crank is damaged and will likely never stay on the spindle even if they are properly torqued on. The Dye Chem is a great idea, too.
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Old 11-18-11, 02:36 AM
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Originally Posted by LarDasse74 View Post
Sadly, reality does not agree with mikezs today.
Just say I'm wrong and get it over with

Having only fixed bikes i own or of close friends/family I haven't seen too many crank issues, but I have seen a crank that snaps at the eye more than once, hence my assumption.
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Old 11-18-11, 03:00 AM
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Originally Posted by mikezs View Post
..I have seen a crank that snaps at the eye more than once, hence my assumption.
There's a nice write-up about split cranks by Jobst Brandt somewhere.

But basically, if you were to try to torque a crank until it split in one go, you'd have your work cut out for you. What happens is that if you were to torque a crank to spec and wait a week, then parts would have "settled", or "bedded in" and you'd be able to torque it again.
This sequence can then be repeated until either the crank has split, or made contact with the BB.
A typical chain of events is a rider noticing a loose crank, torques the snot out of it, ride it a day, detects another wobble, torques it again in a renewed attempt at solving the problem, until drastic failure.
Or you have the overly keen guys who keep on going over their bikes, refreshing the torque wherever they can, eventually splitting their cranks through misguided maintenance.
Do note that the loss of torque between initial assembly and use isn't a problem, and normal for this design. If initial torque is OK, with no previous damage to the parts, then square taper is actually one of the more reliable designs available.

In theory this should mean that even if you're doing things right, there should be a limit to how many times a square taper crank can be removed and reinstalled, but it's not something I've ever encountered.

Last edited by dabac; 11-18-11 at 03:11 AM.
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Old 11-18-11, 07:58 AM
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Originally Posted by mikezs View Post
Just say I'm wrong and get it over with

Having only fixed bikes i own or of close friends/family I haven't seen too many crank issues, but I have seen a crank that snaps at the eye more than once, hence my assumption.
It does happen, but 'rounded out' cranks are, in my experience, much more common.
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