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Old 07-02-11, 11:10 PM   #1
Telly
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Cranks and Bottom Bracket

Hello!

A quick question from a newbie...

A week ago I had my left crank and pedal come loose while I was riding. After trying to tighten it a few times, I took my (inexpensive) bike to my LBS, and had the bottom-bracket and crank changed because of wear (I've owned the bike for about a month, and being a overweight rider, have been exceptionally careful on how I ride it).

After a few days of not being able to ride, I took my bike out yesterday evening and once again, the crank came loose. By pure luck, this happened close to a bike shop (not my regular LBS), and the guy tightened the crank saying that it would probably last for a few days, and I would probably need to change the crank again. I actually rode around 30km yesterday after the shop, and so far all seems okay.

My question is: what could be the cause of two cranks (and the BB) failing within a week? Both repair men were sure that it wasn't my weight, and acknowledged it to sub-standard quality; even though the second crank was aluminium and much better quality than the original steel unit?

Last edited by Telly; 07-02-11 at 11:23 PM.
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Old 07-03-11, 12:29 PM   #2
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The quality of the parts do mean something . better made mean better build and will last longer. I think one of the problems you are having is the bolts or nuts holding the cranks onto the BB axle keep coming lose and in turn the crankarms are losing from the axle as you are riding and are getting destroy . One way around this is to use a treadlock on the bolts or nuts . use the blue one, that way the cranks and BB can still be service when needed.
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Old 07-05-11, 08:17 AM   #3
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I did use a tread-lock (loctite if I'm not mistaken). I tried tightening the crank down with another set of tools which fit the crank diameter a bit better, and so far after riding it for 30-40 miles it seems to be okay, although I am a bit comprehensive on starting out on a long ride since I don't want to end up pushing it in 100 degree heat!

Can you suggest a good crank make which I can order over the net? My riding style is non-aggressive, and I use my bike for commuting over paved surfaces only.

Thanks!
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Old 07-05-11, 08:24 AM   #4
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you may want to find a better mechanic. seriously anyone telling you you'll have to replace "their" crank in a couple days is a poor excuse
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Old 07-05-11, 01:01 PM   #5
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you may want to find a better mechanic. seriously anyone telling you you'll have to replace "their" crank in a couple days is a poor excuse
+1
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Old 07-05-11, 03:29 PM   #6
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If you are using a tread lock ,then there nothing to worry about . it will hold ,no matter how far you ride . The only time the tread lock will give is when you break loose the bolt for service .any good name will work like shimano ,or FSA ,just to name a couple , here where price will matter , for a good crank set you will pay around $ 200 .00 for.
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Old 07-05-11, 07:48 PM   #7
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Sure a good crank will cost you $, but even a box store bike shouldn't simply fall apart like this. I advise finding a new mechanic. Also, if the taper on your cranks is bad, just having the bolt secured in may not stop it from coming off again one day. You're probably fine at this point, but keep an eye on it in case it starts to get loose in the rotational direction.
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Old 07-05-11, 08:14 PM   #8
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Thread lock, not tread lock.
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Old 07-05-11, 08:27 PM   #9
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Keep it tourqed down. Check and tighten before it gets loose.
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Old 07-05-11, 09:41 PM   #10
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Keep it tourqed down. Check and tighten before it gets loose.
+1. I've never needed any thread locking compound on a bicycle component. However, you do need to make sure the attaching bolt is torqued properly. Without running to the garage, I am guessing the crank bolt on a square taper or ISIS/Octalink crank is about 30 ft-lb (~40N-m). It is quite a bit of install torque, and I use automotive tools and torque wrench. That said, I've never had a crank bolt (or any properly torqued bolt) come loose.
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