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The screw to my crank fell off!

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The screw to my crank fell off!

Old 04-02-08, 10:47 PM
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mosquito
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The screw to my crank fell off!

While I was biking today, somewhere along the screw to my left crank fell off. I didn't even notice until my crank got wobbly and almost had an accident. Thank goodness nothing happened to me and that no threads got stripped, I was able to replace it.

Why does this happen and how can I prevent this from happening in the future?
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Old 04-02-08, 11:00 PM
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Sorry but is it a cheap bike?

The 2 walmartesque bikes,(one for me and the wife), I bought when I started commuting both had the cranks start wobbling then fall off. I tried the old piece of thread and hairspray cheapo version of Locktight to try and keep the crank on but it never worked. In the end I paid more for a new bike and I've never had a problem 5 bikes later.

I sure the mechanics on here will tell you something useful though

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Old 04-02-08, 11:07 PM
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No, it's a Crosscheck with sugino cranks. I never used loctite, is that necessary? I was told to do it once but never got around to it and just forgot about it.
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Old 04-02-08, 11:09 PM
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General vibration can loosen screws. The bolt on the left crank is rumored to be more likely to fall out than the right one because of the rotation.

In general, to prevent losing bolts or screws when you ride, make a habit of checking the tightness of all the fasteners on your bike on a regular basis.

That said, I don't regularly check my crank fixing bolts. The bolts on a Shimano square taper bottom bracket should be tightened to about 32 foot-pounds, and is the one thing on my bike that I actually drag out a torque wrench for. My experience is that once it's been properly tightened (by a bike shop if you don't have a torque wrench) and retightened after a couple of hundred miles, it won't fall out again, although it should be rechecked once a year for good measure when you tune up the bike.

EDIT: I don't use Loctite, but a drop of Loctite 242 doesn't strike me as a bad idea.
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Old 04-02-08, 11:10 PM
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Originally Posted by mosquito View Post
No, it's a Crosscheck with sugino cranks. I never used loctite, is that necessary? I was told to do it once but never got around to it and just forgot about it.

Blimey I'm buying myself a Cross check in a couple of weeks! I will be checking this thread regularly!

Come on mechanic guys!!!
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Old 04-02-08, 11:16 PM
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Don't panic, this isn't a "Cross Check Fault" - they use the same BB's and Cranks as many other bikes. If a crank bolt is not tightened enough, it can vibrate out. You shouldn't have to use loctite, but it couldn't hurt. Regardless, it's a very unusual thing to happen.
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Old 04-02-08, 11:16 PM
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Originally Posted by Jonahhobbes View Post
Sorry but is it a cheap bike?
LOL... A few months ago I had two of the five Torx screws fall out of my Campy Record chainrings. And at $20 to replace each one, 'cheap' has nothing to do with it! Riding in cold weather shrinks metals, vibration loosens things up, fatigue/wear and tear, etc. are all causes. Improper installation and using too little or too much torque can too.

One should get in the habit of giving their bike a quick visual look-see every time they ride and a more thorough inspection on a regular basis. And regarding my previous paragraph, I should have taken my own advice. I could have at least spotted it before the second one fell out.
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Old 04-02-08, 11:24 PM
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Originally Posted by dobovedo View Post
LOL... A few months ago I had two of the five Torx screws fall out of my Campy Record chainrings. And at $20 to replace each one, 'cheap' has nothing to do with it! Riding in cold weather shrinks metals, vibration loosens things up, fatigue/wear and tear, etc. are all causes. Improper installation and using too little or too much torque can too.

One should get in the habit of giving their bike a quick visual look-see every time they ride and a more thorough inspection on a regular basis. And regarding my previous paragraph, I should have taken my own advice. I could have at least spotted it before the second one fell out.

Weird I have never had any problems with any cranks on my MTBs I would'nt even know they existed apart from the few times I've replaced peddles. Must be a road bike thing
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Old 04-02-08, 11:33 PM
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My LBS is a long time friend who specializes in Specialized bikes. He works on all bikes and he said the first thing he does before working on any bike is to tighten that screw up. It is common with all bikes and is just a part that needs regular attention. He gives lifetime service with the bikes he sales so I take mine in once a month just for adjustments. He said if I do not take it in that often then check that screw everytime you think about it. Mine has never been loose, but it does happen. Loctite will probably help but I would suggest checking it every once in a while anyway.
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Old 04-02-08, 11:47 PM
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I don't think it had anything to do with my initial tightening of the screw, I turn it pretty hard. That said, I do believe from what dobovedo said that it's from the cold weather and the vibration. I've been biking all winter and the weather is getting a bit warm. But will check the screws anyway.
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Old 04-03-08, 12:01 AM
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I've never had a problem with this on any bike I've had. Put some grease on the crank bolt and tighten that **** down hard.
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Old 04-03-08, 03:55 AM
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Jobst Brandt's advice. Notably, says 'don't retighten'.

http://www.sheldonbrown.com/brandt/i...ng-cranks.html
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Old 04-03-08, 06:48 AM
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In general when cranks are installed, they should be torqued to spec, then ridden a short distance like 1 mile, then retorqued, and they should not fall out nor require loctite or anything of the sort.

If you rode on it with it wobbly, the crank is almost certainly ruined and you'll never be able to get it right again. Honestly I'd probably try anyway because I'm a cheap *******; at least I'd try to make it last until I got a replacement from somewhere cheap.

Last time I wrecked a crank it was from stripping threads out from a pedal that hadn't been removed in over a year (in road salt etc). I had to pay $60 at the LBS for a new crank & chainrings; it was either that or no bike for a few days. I bought a replacement "just in case" next time I saw one in the clearance bin at Nashbar, for $5.
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Old 04-03-08, 07:21 AM
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Originally Posted by aley View Post
General vibration can loosen screws. The bolt on the left crank is rumored to be more likely to fall out than the right one because of the rotation.

In general, to prevent losing bolts or screws when you ride, make a habit of checking the tightness of all the fasteners on your bike on a regular basis.

That said, I don't regularly check my crank fixing bolts. The bolts on a Shimano square taper bottom bracket should be tightened to about 32 foot-pounds, and is the one thing on my bike that I actually drag out a torque wrench for. My experience is that once it's been properly tightened (by a bike shop if you don't have a torque wrench) and retightened after a couple of hundred miles, it won't fall out again, although it should be rechecked once a year for good measure when you tune up the bike.

EDIT: I don't use Loctite, but a drop of Loctite 242 doesn't strike me as a bad idea.
I'd say 32 foot-lb is a little high for a square taper (22 to 26 ft-lb would be better) but, yes, using a torque wrench on crank bolts is never a bad idea. I've never had a crank bolt come loose and I always tighten them to a specific torque.

The bad news, mosquito, is that you crank is probably toast. If it got wobbly on the spindle, you've probably damaged the tapers in the crank. It probably won't hold. The good news is that you get to buy new stuff for your bike. Ya!...sorta
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Old 04-03-08, 07:27 AM
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Find a torque wrench and the crank manual (probably online) and torque it to spec. You may want to keep a close eye on that crank now that you have ridden it loose. That may have damaged the area where the arm meets the BB spindle. The only time I needed to use locktite on crank arms and bolts was when I had FSA carbon cranks, but that's because there a flaw in the design.
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Old 04-03-08, 09:17 AM
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I believe Locktite on the crank bolt is a bad idea for the following reason. The way the crank/spindle interface works is this: the [relatively] soft aluminum crank taper, when first tightened onto the hardened spindle taper only touches in a few places, despite appearances.

After pedaling forces are applied, the small areas of aluminum/steel contact are deformed slightly and now the crank is loose. Not noticeably loose, but loose just the same.

At this point the crank bolt should be retightened. Now the areas of contact are increased so are more resistive to being further deformed.

But deform they do. So the retightening of the crank bolt will need to be done several times. Higher quality parts may reduce this sequence some . . . but never eliminate it.

Eventually the areas of contact between the crank and spindle are large enough to resist any further deformation and the crank bolt will not need to be tightened further . . . and the crank will remain tight.

If you use Locktite on the bolt threads when the above process is started . . . you've fixed the bolt in its current position but this doesn't prevent the natural 'working' of the crank on the spindle. Your crank will eventually be loose and the Locktite bond will make adjustment more difficult.

One other detail I've never seen mentioned is that since nothing is ever machined perfectly . . . if cranks are removed they should be marked in some way so that the crank goes back on the spindle with the 'flats' of the crank on the original 'flats' of the spindle. If they are installed, for example a quarter turn off from the original orientation, the spindle/crank "re-seating" will have to be repeated due to minor differences in the machined tapers.
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Old 04-03-08, 11:41 AM
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I use anti-seize compound on almost all aluminum threads. It is a grease that has micro-sized aluminum particles that help fill in the tiny irregularities on the thread surface, improving contact area and reducing the chance of the thread "seizing."

To me, threadlockers (Loctite produces both a threadlock and anti-seize compond, among other products) seems to be overkill.
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Old 04-03-08, 11:44 AM
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Originally Posted by cyccommute View Post
I'd say 32 foot-lb is a little high for a square taper (22 to 26 ft-lb would be better) but, yes, using a torque wrench on crank bolts is never a bad idea. I've never had a crank bolt come loose and I always tighten them to a specific torque.

The bad news, mosquito, is that you crank is probably toast. If it got wobbly on the spindle, you've probably damaged the tapers in the crank. It probably won't hold. The good news is that you get to buy new stuff for your bike. Ya!...sorta

I agree, the crank is likely dead.
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