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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 11-01-05, 11:00 AM   #1
cooker
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He's GOOD...

My bottom bracket may need to be overhauled or replaced soon, but for the moment it's ok. I noticed creaking on every pedal stroke, usually as my right foot came over the top, and only if I applied torque. If I pedalled gently, no noise.

I walked into the LBS, and noticed the guy at the service desk wasn't the head mechanic...bummer. I started to explain the problem to the guy, who seemed a little perplexed, and as I did, the head mechanic happened to walk in the front door 30 feet away, heard the tail end of my explanation from across the room, and yelled to his assistant "tighten the left crank!"

Problem solved, at least for now. If it comes loose again, it may need more work.

RGC
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Old 11-01-05, 11:13 AM   #2
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Quote:
Originally Posted by cooker
My bottom bracket may need to be overhauled or replaced soon, but for the moment it's ok. I noticed creaking on every pedal stroke, usually as my right foot came over the top, and only if I applied torque. If I pedalled gently, no noise.

I walked into the LBS, and noticed the guy at the service desk wasn't the head mechanic...bummer. I started to explain the problem to the guy, who seemed a little perplexed, and as I did, the head mechanic happened to walk in the front door 30 feet away, heard the tail end of my explanation from across the room, and yelled to his assistant "tighten the left crank!"

Problem solved, at least for now. If it comes loose again, it may need more work.

RGC
Yeah...If it comes loose again, good chance the arm is hosed.
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Old 11-01-05, 12:01 PM   #3
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Might want to try a drop of blue loctite on that crank bolt as well. Saves the hassle and cost of having to tighten that crank again at a later time... or even worse having to replace that crankarm.
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Old 11-02-05, 04:21 AM   #4
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It's good to see someone praising a shop for once.
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Old 11-02-05, 04:59 AM   #5
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Originally Posted by DannoXYZ
Might want to try a drop of blue loctite on that crank bolt as well. Saves the hassle and cost of having to tighten that crank again at a later time... or even worse having to replace that crankarm.
No.
What was described happens when a square taper crank is ridden for some time with a loose crank bolt. Probably from missing it's first tuneup or even from being missed on assembly. (There's no guarantee when it happened)
It's not noticeable at first but slowly the Bottom Bracket (BB) spindle (being made of hardened steel) begins to hog out the hole in the softer (Aluminum) crank. By the time you notice it it's usually too late.
Tightening the crank arm pushes it a small bit up the taper of the BB spindle which gives you only a small area (Like a thread or two's width of "new" contact on a higher (fatter) part of the spindle. The fact that the rest of the hole has been hogged out means that with in a short time your crank will start flopping again. Tightening it again will have the same reaction. Usually after about the third time tightening it you'll wail on the bolt causing it to shear. (Always fun)
Bottom line: The crank on that side is likely hosed. Time to replace either the arm or the crankset. There's likely NOTHING wrong with the BB itself, and loctite won't help the problem at all.
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Old 11-02-05, 05:02 AM   #6
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Quote:
Originally Posted by Raiyn
to replace either the arm or the crankset. There's likely NOTHING wrong with the BB itself, and loctite won't help the problem at all.
He's right, but JB Weld on the other hand.........

JUST KIDDING, DO NOT TRY THIS
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Old 11-02-05, 10:59 AM   #7
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Quote:
Originally Posted by Raiyn
Bottom line: The crank on that side is likely hosed. Time to replace either the arm or the crankset. There's likely NOTHING wrong with the BB itself, and loctite won't help the problem at all.
Thanks, Raiyn, that was my guess, and it started to click again a few km later. Time for new crank(s). Are you saying the spindle is possibly still intact? (I'll get it checked out, anyway).
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Old 11-02-05, 11:43 AM   #8
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speaking of praise,i have to throw in a plug for wilbur at helen's in santa monica. the last three visits i was able to get work done while i wait, in like 30 minutes tops. cut a steerer tube. replaced headset bearings and star nut. fine tuned campy drivetrain. the last visit was no charge.
keep in mind i didn't buy the bike at the shop, and they are a high volume, busy place. top notch mechanics and customer service.
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Old 11-02-05, 12:26 PM   #9
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Quote:
Originally Posted by Raiyn
No.
What was described happens when a square taper crank is ridden for some time with a loose crank bolt. Probably from missing it's first tuneup or even from being missed on assembly. (There's no guarantee when it happened)
It's not noticeable at first but slowly the Bottom Bracket (BB) spindle (being made of hardened steel) begins to hog out the hole in the softer (Aluminum) crank. By the time you notice it it's usually too late.
Tightening the crank arm pushes it a small bit up the taper of the BB spindle which gives you only a small area (Like a thread or two's width of "new" contact on a higher (fatter) part of the spindle. The fact that the rest of the hole has been hogged out means that with in a short time your crank will start flopping again. Tightening it again will have the same reaction. Usually after about the third time tightening it you'll wail on the bolt causing it to shear. (Always fun)
Bottom line: The crank on that side is likely hosed. Time to replace either the arm or the crankset. There's likely NOTHING wrong with the BB itself, and loctite won't help the problem at all.
Will it hurt? Certainly doesn't go back in time and fix anything because what's done is done. IF however, at the very beginning when they assembled the bike, they had used some loctite on the same bolt, and it wasn't checked and remained loose a 5-10lb*ft of initial torque, the loctite would have prevented the later loosening that occured to damage the crank-arm. It's a bit of a safety net to catch mistakes. Sure if the mistake never occured, with or without loctite would be the same result.

In this case, we still don't know what kind of condition this crankarm is in. Until he takes it off and gets a picture and measure the hole and its squareness, no one has any more of a clue as to what will happen next.

Cooker, the spindle's fine. The worse-case scenario is you've got a damaged crankarm.
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Old 11-02-05, 02:08 PM   #10
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I would take the arm off and clean everything then grease the axle, the arm and the bolt, no loctite. If it goes back together and you can torque it to spec without the bolt going all the way in ( you will know its all the way in when it stops abruptly and you can't turn it even one milimeter more) then you should be ok. If the bolt goes all the way in, new crank time or you may be able to get away with a slightly larger axle, if thats a lot cheaper than a new arm or crankset.

Danno, what probably happened is that it was first put together dry (ungreased), and even though it may have been torqued right, the there was not a good enough interference fit (or press fit) between the arm and the axle. After use, the arm moved around and at that point the bolt no longer was applying tension to the press fit of the arm and the axle. At that point the bolt had not backed out out, and even if there had been locktite on it, the arm would have been loose.
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Old 11-02-05, 02:25 PM   #11
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Spindle will more than likely be alright, and you should just be out a left crankarm. Might (might, not will, might) be a warranty.
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Old 11-02-05, 04:54 PM   #12
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Thanks, everyone...it's an older bike, no warranty, but now I have a good understanding of the likely problem and solution when I go back to the LBS.
R
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Old 11-02-05, 06:02 PM   #13
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Quote:
Originally Posted by San Rensho
Danno, what probably happened is that it was first put together dry (ungreased), and even though it may have been torqued right, the there was not a good enough interference fit (or press fit) between the arm and the axle. After use, the arm moved around and at that point the bolt no longer was applying tension to the press fit of the arm and the axle. At that point the bolt had not backed out out, and even if there had been locktite on it, the arm would have been loose.
Where does the space come from that allows the arm to move? Even a bolt that's finger-tight on the crank will not allow the crankarm to wiggle. The only way to generate some lateral space for the crank-arm to wiggle is to back off the bolt.

At my shop, SOP was to ALWAYS mark the bolt and crank-arm with a line of nail-polish/touch-up paint that crosses both the bolt and the inside-edge of the crank-arm. That way on visual inspection you can tell if the bolt moved or not. And you can can also verify that the bolt had been torqued to spec even with the dust-cap on. If there's no paint, the crankarm gets torqued and marked before it leaves the shop. You'd be surprised how many bikes come back with loose crank bolts that had been torqued to spec. In every single one of those cases, the bolt had twisted loose somehow.
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Old 11-02-05, 08:16 PM   #14
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I guess I was just speculating as to what happened, but the space would come from the crank arm moving further in on the axle. The end of the axle is tapered and the hole on the crank is tapered. The bolt pushes on the crank arm only, wedgding it against the end of the axle. If that wedge, the press fit, is not tight, the arm can start moving around and get loose, sliding further in and now the bolt is no longer pushing against the arm. So in this (possible) situation, the bolt hasn't backed off, the arm has moved inward, away from the bolt. And I guess loctite wouldn't hurt.

Actually, if the bolt had been locktited and marked, as you described, and if the crank arm had come loose and the bolt had not backed out, that would tend to prove my theory that the crank moved further in on the axle.

How many angels can dance on the head of a pin? The number escapes me now.
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Old 11-02-05, 09:37 PM   #15
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Actually, if the bolt had been locktited and marked, as you described, and if the crank arm had come loose and the bolt had not backed out, that would tend to prove my theory that the crank moved further in on the axle.
Except that I have NEVER seen a crank come loose were the bolt didn't back out. The only way a crankarm can go up a spindle is by tightening the bolt. Due to the angle of the taper, pedaling forces on the arm will always push it off the spindle. So... if the arm is pushed up on the taper as far up as it will go... and the bolt is installed finger-tight so that there's no play in the arm. The ONLY way for the crankarm to come loose is if the bolt backs out.

The problem here is that the shop made a mistake. All I'm suggesting is a 2nd-level backup plan in case that mistake is made. In the auto/motorcycle-racing that I'm used to, there's actually four levels of safety. First comes self-locking bolts (wobbly threads), then threadlocker, lockwashers (star or serrated). Finally, safety-wire is the last step in securing a fastener.

Sure 95% of cranks never ever have any problems, I'm not talking about those and neither is the OP. I'm talking about a technique that will ensure 100% security even with mistakes made in torquing the bolt, and without changing the existing methods and procedures of fastening the crankarm, we'll still be stuck with a 95% success rate. Here's some background on bolted joints.
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