[Closed] my crank fell off today....why?

Lack of maintenance 😆

My point being that there’s not an epidemic of these failures therefore there must be something faulty about yours. I have outlined the most likely fault 😳

lack of the preload cap or the little pin won't cause the arm to fall off.

Loose pinch bolts and/or damaged splines is the cause.

How can you tell pinch bolts were still torqued up correctly if the arm's no longer ont he axle?

Throw that stupid plastic one away and get an alloy one from Ebay, they are under a fiver. Basically it fell off because it's old and worn. People say you don't need the torque plastic bit to keep it on but in my experience you do if the cranks a few years old. As I say get an alloy one from ebay, they come in all colours and are usually just a couple of quid. Filter results to UK only as there are loads in china.

cinch bolts needed allen key to open them. cheched splines, all seem intact and fine.
crank went on fine when I installed it, the pin sat in the hole on the axle fine.

proper odd.

and it was all new in august.

[i]cinch bolts needed allen key to open them[/i]

aren't they threadlocked when new?

Given the time-in-service is relatively short I’d also like to add ‘incorrect fitting’ to the list of reasons for the failure 8)

Did you notice any play in the cranks beforehand?

The plastic cap doesn't really do anything except preload the bearing, the little plastic tab which locates into the axle hole isn't under any load so it's a bit bizarre that the non-driveside crank bolts are still tight.

I'm grasping at straws really, but have you had cause to swap out the crank bolts at all?

[edit]

I note that from the OP's update that the entire crank is only a couple of months old and that everything would be in good order.

Like I said, bizarre.

Mine did the same, once it happened the first time, it would happen again, then again.... It was the only crankset i had that did it and i'd owned a dozen HTII cranksets by then. I'm not convinced if i ever got to the bottom of it. It may have simply been the preload nut jobbie as stated though.

maybe you just hadn't done them up tight enough then.
It's a bit disconcerting when it happens int it. Did you lower the saddle and scoot bone shaker style to your destination? That'd be a sight to see.

Throw that stupid plastic one away and get an alloy one from Ebay, they are under a fiver. Basically it fell off because it's old and worn. People say you don't need the torque plastic bit to keep it on but in my experience you do if the cranks a few years old. As I say get an alloy one from ebay, they come in all colours and are usually just a couple of quid. Filter results to UK only as there are loads in china.

This - had exactly the same thing on my 105s. I slightly bolloxed them doing a roadside repair with minimal tools and a rock, but it hasn't worked loose since replacing the end cap and refitting.

The baby jesus hates you.......or aliens.

The plastic cap doesn't really do anything except preload the bearing, the little plastic tab which locates into the axle hole isn't under any load so it's a bit bizarre that the non-driveside crank bolts are still tight.

This was my understanding, but it seems to have a role - my pinch bolts were correctly torqued on install but the plastic plug had come out - the cranks seem to work their way down the spline without it.

Given the time-in-service is relatively short I’d also like to add ‘incorrect fitting’ to the list of reasons for the failure

yeah right, cos I have never built a bike up before.

maybe you just hadn't done them up tight enough then.
It's a bit disconcerting when it happens int it. Did you lower the saddle and scoot bone shaker style to your destination? That'd be a sight to see.

I sat sulking until my lad collected me.

refitted it,and had a spin around and it seems fine.

alloy preload thing will be ordederd

For info, if you get some of the Perdos type thick width tyre levers, they actually fit perfectly inside the standard nut thing for tightening it up trailside

weeksy - Member

For info, if you get some of the Perdos type thick width tyre levers, they actually fit perfectly inside the standard nut thing for tightening it up trailside

While not as easy to use, from the depths of my mind I seem to remember that a 20p will also fit into the pre-load nut & can be used as asn emergency tightener.....
Could be a load of rubbish though (I need to remember to try this later).

For info, if you get some of the Perdos type thick width tyre levers, they actually fit perfectly inside the standard nut thing for tightening it up trailside

You can usually find a small rock that'll fit too, I've had this a number of times in the past. I can only put it down to me fitting them wrong somehow, it put me off shimano cranks for ages but my current set are fine. In my experience if it's happened once it's going to keep on happening but maybe a metal preload thing will help.

+1 for Pedros tyre levers and 20p coins.

I have had an XT crank loosen itself, but this was entirely down to my own ineptitude when fitting with PF46 axle spacers - not I hasten to add, that I'm suggesting that the OP has fitted cranks incorrectly!

Might be worth making a warranty claim.

yeah right, cos I have never built a bike up before.

He didn't say that. You have a product which is successfully fitted to literally millions of bikes, 99.9999% of which have no problems. Yours fell off after 3 months of ownership. There's no inherent issue with the product, neither generally nor in this specific case. Ergo the obvious cause is that it was fitted wrong.

I had one come loose, it didn't actually fall off, it rotated slightly on the splines, which is probably more damaging. Just one of those things, it obviously wasn't tight. Be glad it's done no damage, shrug your shoulders and move on.

alloy preload thing ordered. didn't realise alloy ones were available.

cheers all.

cinch bolts needed allen key to open them.

Doesn't mean they were done up correctly, just that they were tight in their threads.

If you're relying on the bearing pre-load cap to keep your crank arms on, there's a bigger problem with the cranks.

One of the reasons that it is plastic is that it makes it harder to overload the bearings. Also why you should only tighten it up using your fingers, not a tool with any leverage. It should be playing absolutely no part in keeping the crank on once the pinch bolts are done up.

Never had any problem just jamming a normal tyre lever in on the rare occasions it's necessary to do up one of those end caps without the tool.

I would err on the side of installation issue, but have seen a friend's cranks exhibit similar behavior. Alloy nut seemed to slow it down but not stop it. Eventually that fell off in the bushes and we ended up using zip ties through the centre of the axle and around each arm to maintain preload to get home.

I have also seen someone happily riding around for some time without the end cap at all because he hadn't got round to ordering one yet.

warning,

its still possible to fit inccorectly even with the blingy alloy nut (that does nothing the plastic one didnt do ;))

I've been riding round for 8 or 9 years without an end cap on one of my bikes.

You can pretty much get the right end load by just pushing the two halves of the crank together....... then getting someone else to nip the first cinch bolt up.

did you grease the splines before fitting?

Aren't the splines pre-greased? I'm pretty sure that both of my XT cranksets were.

Also check spacers.
If there is too many on one side there could not be enough support on left hand arm.
One lad I know did this.
Arm dropped off in the middle of nowhere!
He brought no tools!
Lucky i was there!
Max

IME if it's fallen off once the soft alloy splines in the pedal are now damaged (which is not particularly visible) and will keep coming loose unless you over torque the pinch bolts, and even then it might still come loose.

Also check spacers.
If there is too many on one side there could not be enough support on left hand arm.

Only if there are too many full stop, doesn't matter where they are. If you have a 73mm shell and have >1 spacer then that could be a contributory factor.

new HT2 cranks often need a re-pinch a while after installing. they take a bit of bedding in - maybe too much grease or something.

Is it an MTB crank on a [url= http://singletrackworld.com/forum/topic/mtb-crankset-to-replace-a-road-cranksetok ]road BB? [/url]

Only if there are too many full stop, doesn't matter where they are. If you have a 73mm shell and have >1 spacer then that could be a contributory factor.

there was only 1 spacer, on the drive side. now removed this and refitted the whole lot. it seems ok, we will see.

Is it an MTB crank on a road BB?

yes it is. deore crankset onto a tiagra bb.

It's hollowtech 2 isn't it?
So you have a right hand tiagra crank and a left hand deore?
Why not just use a pair of tiagras?

One of the reasons that it is plastic is that it makes it harder to overload the bearings. Also why you should only tighten it up using your fingers, not a tool with any leverage. It should be playing absolutely no part in keeping the crank on once the pinch bolts are done up.

Saint cranks come with an alloy pre load cap.

It's hollowtech 2 isn't it?
So you have a right hand tiagra crank and a left hand deore?
Why not just use a pair of tiagras?

yes.

no I have a deore crankset on a tiagra bb.

Ah, clear now, you mean the bearings. Which might be a problem, i think they are thicker. by a mm or so. So thicker bearings and a spacer, hmmm. Might just not have had enough engagement.

space is removed. bb on my bike is thicker than a bb in my spares box. just measured it.
spacers removed. hopefully now sorted.

This happened to a friend, once it had loosened it would never stay on again, no visible damage but the damn thing was goosed for some reason.

It only fell off after he lost the plastic cap, whether that was a symptom or a cause I don’t know. I always assumed they play no part in holding the crank on but after seeing it with my own eyes I’m no longer convinced.

JAG - Member

Given the time-in-service is relatively short I’d also like to add ‘incorrect fitting’ to the list of reasons for the failure

ton - Member

yeah right, cos I have never built a bike up before.

😀

IME if it's fallen off once the soft alloy splines in the pedal are now damaged (which is not particularly visible)

This. Due to incorrect fitting on my part. The end cap does not have a role in holding the bugger on, but if the crank/axle join fails, it's the last thing to go before your crank is bouncing along the trail.

Are we sure MTB cranks work ok on road bbs? Cos there certainly weren't many replies to that original thread. Meaning, to me, that nobody does this!
Maybe it's just spacers, but maybe not enough (or too much?) spline is available for the left arm to attach to. I'd measure to be sure.

it has done 6 weeks commuting since fitted. hopefully now the spacers are removed it will be fine. fingers crossed.

There is also that little hook in the crank arm which is meant to help prevent the arm falling off if things get loose. It shouldn't close fully if the crank isn't seated correctly, as far as I am aware.

HT2 cranks are only compatible with their own bearings ie road cranks ->road bb’s, mtb cranks -> mtb bb’s. There is 1mm difference between the cup sizes that cannot be correctly adjusted for by the 1.5mm bb spacers.
Ditch the tiagra bearings and fit deore.