Question on why Shimano 600 crank cracked?



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Billx

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I own an 88 Trek 1500 with 170mm Ultegra crank arms. This morning after my 98 lb. little brother
stepped on the arm without straddling the seat I noticed a crack that extended across the entire arm
from inside to outside at 180 degrees from the arm extension. My question is what is the likelihood
this crack was caused by little brother's action? I don't recall ever seeing a crack beforehand but
the bike has close to 50,000 miles on it. Has anyone ever heard of the Ultegra crank arm fatiguing
from normal use?
 
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Paul Southworth

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In article <[email protected]>, BillX <[email protected]> wrote:
>I own an 88 Trek 1500 with 170mm Ultegra crank arms. This morning after my 98 lb. little brother
>stepped on the arm without straddling the seat I noticed a crack that extended across the entire
>arm from inside to outside at 180 degrees from the arm extension. My question is what is the
>likelihood this crack was caused by little brother's action?

If it did crack at that moment, then he did you a big favor and may have saved you a trip to the
hospital. You probably put that much weight (more) on it when you stomp up a hill standing up. I
know I put my full load on each foot or damn near as I can get. If I owned it I would probably
retire the crank set since it has delivered a message to you, as clear as you could ever hope for.
Of course it may last another lifetime but I guess I prefer to pay more money to not be afraid of
the equipment, perhaps that is irrational.

--Paul
 
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Chris B

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On Sat, 8 Feb 2003 16:32:05 -0500, "BillX" <[email protected]> wrote:

>I own an 88 Trek 1500 with 170mm Ultegra crank arms. This morning after my 98 lb. little brother
>stepped on the arm without straddling the seat I noticed a crack that extended across the entire
>arm from inside to outside at 180 degrees from the arm extension. My question is what is the
>likelihood this crack was caused by little brother's action? I don't recall ever seeing a crack
>beforehand but the bike has close to 50,000 miles on it. Has anyone ever heard of the Ultegra crank
>arm fatiguing from normal use?
>

http://pardo.net/pardo/bike/pic/fail/FAIL-001.html

Although the cranks pictured are older, any crank will fail, given enough stress and enough time.
The crack is almost certainly a result of fatigue. A failure at 50,000 miles is not too surprising,
especially if you are big or you stand while pedalling or both.

It would be a good idea to replace both cranks now, as the other crank has seen similar use.

He may have saved you from a trip to the dentist, or worse.

Buy him something nice.
 
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