Crank loosening causes

[Tim Hall]

Following on from the discussion in the stuck BB thread, what does the panel think are the causes of crank bolts loosening and consequent crank loosening?

Some background: We fitted a new square taper chainset to The Boy's bike a week or two back (mid range Trek hybrid, Altura (I think) chainset, slightest smear of grease on the tapers, done up to five white knuckles. The Boy is 22, 6'2" inches tall and stomps his pedals). After a few days commuting (a couple of miles each way) he reported that the left hand crank had loosened. I tightened it up again, using a long allen key. Next day it was loose (or at least the bolt was loose). He now carries a long allen key with him to tighten it.

First obvious thought was that the square hole in the aluminium crank has been damaged by riding with it loose. Is this likely?
Second though is that the bolt is unscrewing but the crank is remaining tight. Is this likely/possible? If so is Loctite going to be a sensible solution?
Third thought is that the crank ismoving further up the taper. Is this likely? How can I measure if it is happening?

 

[si_c]

I've had this problem myself, am 6'5" and stomp my pedals. I've had numerous problems of the crank bolt loosening and found that the only solution was to tighten that bastard like I just didn't care. I also found that using an allen key just didn't give me the leverage needed to tighten properly, and using a proper tool worked much better.

 

[Yellow Saddle]

There is a discussion of exactly this topic over at the Stuck Cranks fight right now. Have you had a look at that?

 

[Tim Hall]

There is a discussion of exactly this topic over at the Stuck Cranks fight right now. Have you had a look at that?

Yes. That was the thread I meant to reference in my OP, but I put stuck BB instead. Oops. And I guess my thread title should be Crank loosening causes, rather than pedal loosening>

MOD NOTE:
Title changed to 'Crank loosening causes'
We aim to please.

 

[Yellow Saddle]

So, to answer whether damage could be made on the square eye from riding with a loose crank. The answer is a definite yes. Whether that one is damaged is a matter for inspection and experimentation. If he rode it loose for miles and miles, it is ruined.

You say, the next day the crank was loose again or at least the bolt was loose. That's pretty common on square taper ridden by strong and heavy people. The bolt didn't come loose, the crank moved further up the taper, leaving the bolt behind, so to speak. Obviously once the bolt has nothing to push against, it is technically loose and will shake loose further and fall out, if there is not a dust cap in there.

There are two suggestions I can make. Firstly, lubricate the taper before installing the crank and secondly, apply a little Loctite to the bolt, just so that it doesn't rattle or loosen further. However, the Loctite is just cosmetic, if you know what I mean.

To most people it would seem obvious that an object squashed onto a taper (specifically square one) would want to loosen itself if the bolt comes undone. However the vision of metal wanting to move downhill is not reality. By forcing the soft aluminium crank onto the taper, the softer of the two metals squirms and deforms. This deformation (call it stretch if you like) happens in two planes. The first is radial - the hole is dilated by the incoming taper. Secondly, the contact surface of the aluminum deforms like the bristles of a brush when the brush is wiped on sandpaper. The bristles are always a bit behind the handle of the brush. PIcture the brush, under force (both downwards and forwards) on a sheet of course sandpaper. Notice how the bristles slant to the opposite direction of the forward-pushing force? Now, lift the front tip of the brush without releasing the rear end. Notice how the bristles in front shoot forward and assume a perfectly vertical position again? Now push down in front and life the rear. The rear bristles are now also vertical. Transfer this analogy to a square taper crank. As soon as you apply a large force to the crank, the trailing end of the taper loses pressure and the metal corrects. Stand on the pedals in the quarter to three position and you can see how the leading edge of the taper loses pressure and the metal underneath corrects.

If the taper is lubricated, less of the aluminium "stays behind" when the taper is forced on.

Does that help?

 

[Smurfy]

slightest smear of grease on the tapers

Third thought is that the crank ismoving further up the taper. Is this likely? How can I measure if it is happening?

With respect to grease on square tapers:

The risk is that the hole in the relatively soft aluminium crank gets larger and larger, and the stresses around the hole get higher and higher. This happens because the expansion force of the tapered spindle on the crank is much more powerful when the effects of friction are reduced. At some point the crank may be so far onto the spindle that the bolt bottoms out on the spindle and cannot properly secure the crank, or the crank may start cracking at the corners of the square hole. I'm not saying it will happen every time, but many years ago I used to lube the taper for the same reason you suggest, and eventually I had a cracked crank to show for it.

Edit: It's the crank that suffers rather than the spindle, because the spindle is usually steel, and the crank is almost invariably aluminium.

 

[mjr]

First obvious thought was that the square hole in the aluminium crank has been damaged by riding with it loose. Is this likely?

Yes. Never ride a clunking crank further than you have to. I've made this mistake.

Second though is that the bolt is unscrewing but the crank is remaining tight. Is this likely/possible? If so is Loctite going to be a sensible solution?

I think yes, a wobbling crank can work it loose (but see next point). Loctite for that sounds a bit daft to me.

Third thought is that the crank ismoving further up the taper. Is this likely? How can I measure if it is happening?

This happens but if it's that, the crank should stop wobbling and eventually you no longer need to tighten the bolt.

It sounds to me like the left crank may be toast. Some LBSes sell them on their own and some LBSes sometimes let you take old left cranks (from worn-out chainsets) from the scrap bin.

BUT given that, you might as well have a go at tightening it once more.

It's possible that the BB axle is toast but that's quite rare (it's harder than the crank) unless something got in the way or the threads are damaged.

 

[si_c]

The only other thing to check is that you are using the right BB with the right crankset. If you're using an ISO crank with a JIS taper or vice versa, this may be causing a problem. Most of the time it should be ok but see http://www.sheldonbrown.com/bbtaper.html for caveats on that.

 

[Kajjal]

If you are stronger and heavier than most riders this can happen, especially if you ride really hard. But once tightened it should not be happening frequently.

 

[Globalti]

Oh yes, European and Japanese tapers... a minefield.

 

[Username]

Following on from the discussion in the stuck BB thread, what does the panel think are the causes of crank bolts loosening and consequent crank loosening?

Some background: We fitted a new square taper chainset to The Boy's bike a week or two back (mid range Trek hybrid, Altura (I think) chainset, slightest smear of grease on the tapers, done up to five white knuckles. The Boy is 22, 6'2" inches tall and stomps his pedals). After a few days commuting (a couple of miles each way) he reported that the left hand crank had loosened. I tightened it up again, using a long allen key. Next day it was loose (or at least the bolt was loose). He now carries a long allen key with him to tighten it.

First obvious thought was that the square hole in the aluminium crank has been damaged by riding with it loose. Is this likely?
Second though is that the bolt is unscrewing but the crank is remaining tight. Is this likely/possible? If so is Loctite going to be a sensible solution?
Third thought is that the crank ismoving further up the taper. Is this likely? How can I measure if it is happening?

There is only one way no matter what crank your pedals turn. Tighten crank arms bit by bit while tapping them in to stop them loosening. Single tightening actions are no good no matter what bike you ride. Tighten slightly then give them a good whack on both sides while they are still not fully tightened. Tighten them a bit more then give them another good whack on both sides eventually you cant tighten them anymore and they will sit right. Most people are scared to mark new bikes they would rather ride around with loose cranks. Take out your tightening tool before you fully tighten them up and give them a few good whacks to help them sit. Put your tightening tool back in and tighten a bit more keep doing this until they are tight and set right.

 

[Globalti]

Hmmm. I never though of hitting them. I wonder what the technical guru @Yellow Saddle thinks of that idea?

 

[Milkfloat]

Jnr. Milkfloat had a pedal fall off yesterday whilst on the tagalong, I heard a clatter, felt a big wobble, then a few seconds later I was informed that he had nowhere to put his foot. I put it partially back on finger tight, but it only lasted another couple of miles. It is back on now, as tight as I can get it. Something to keep an eye on.

 

[Yellow Saddle]

Hmmm. I never though of hitting them. I wonder what the technical guru @Yellow Saddle thinks of that idea?

It is a poor substitute for greasing the tapers. He doesn't say where they should be whacked but if you understand the mechanism by which a crank taper square releases stress you will also realize that simply standing and bouncing on the cranks, first goofy, then hotfoot, will make them seat higher upon the taper without having to hammer them. The type of hammering required - axial - will not be bearing friendly.

I have described the mechanism above.

Are we still talking crank here or has pedal loosening, a completely different phenomena, been put into the mix?

 

[Ajax Bay]

I guess my thread title should be Crank loosening causes, rather than pedal loosening

Are we still talking crank here or has pedal loosening, a completely different phenomena,

Title edit @Tim Hall ? Is junior @Milkfloat pedaling backwards?