Feeling a bit cranky today...

[Spinners]

And a bit lucky as well too.

I set off for work on my 10-mile commute and I was literally 30 metres from the house (that's the lucky part) when my left crank came right off and was left dangling from my SPD shoe

From the first pedal stroke things didn't feel right and I thought I might have a loose cleat but, before I could home in on the problem, off came the crank. Anyway, back home for a quick change of bike and the ride to work got me thinking.

After decades of problem free 'square taper' chainsets (Shimano MTB and Campag road) I've been using Shimano external BB's without issue (and this Tiagra 4600 chainset has been on at least three bikes now) so what went wrong here?

I don't have a torque meter but consider that I have a good 'feel' for tightening things up but on this occasion, clearly I didn't tighten it up enough. I have two different lengths of 5mm allen keys and always use the longer one for tightening the bolts on the left crank. After work today, after checking for damage, I put the crank on and gave it some extra welly but nice and evenly on the two bolts.

BTW I always use grease on these bolts so should I have used threadlock?

[reohn2]

I tend do a quick check of all nut and bolts about once ever couple of months or when I'm cleaning the chain

[philvantwo]

Maybe you got some grease on the bottom of the bolt head, that could have made it work loose?
There's a label on the crank that tells you how tight it's got to be, get a torque wrench!!
Or maybe shimano put these stickers on for fun!

[slowster]

From the Shimano manual:

The two left crank arm mounting bolts should be tightened in stages rather than fully tightened at once. Use a torque wrench to check that the final tightening torques are within the range of 12 - 14 N·m. Furthermore, after riding approximately 100km (60 miles), use a torque wrench to re-check the tightening torques. It is also important to periodically check the tightening torques. If the tightening torques are too weak or if the mounting bolts are not tightened alternately in stages, the left crank arm may come off and the bicycle may fall over, and serious injury may occur as a result.

The only Shimano Hollowtech 2 type chainset I have fitted had factory applied grease on the left hand crank bolts.

[Polisman]

I've always fitted Shimano cranks completely bone dry. Never had an issue

[Samuel D]

[…] I put the crank on and gave it some extra welly but nice and evenly on the two bolts.

These two bolts: was at least one of them lying on the ground after the crank fell off?

There is a “stopper plate” with a “plate pin” that, when fitted right, prevents the crank falling off even if the bolts get loose. I wonder if you had that the right way around.

That’s the Shimano terminology from this installation instructions PDF:

https://si.shimano.com/pdfs/si/SI-1MG0A-001-ENG.pdf

A torque wrench is pretty useful for these bolts that have a narrow window between too-loose and damage from excessive torque.

[PDQ Mobile]

I've always fitted Shimano cranks completely bone dry. Never had an issue

Until you came to get them off?

I am just the opposite. I oil or grease everything- (except braking sufaces!)
Some places I use thread locker too though.

[Brucey]

I've seen quite a few of these cranks come loose and I think there are usually various stages of the failure.

When the crank is initially tightened the bolt torque has to

a) overcome friction in the splined interface i.e. to allow the joint to settle
b) provide a clamping force onto the spindle
c) deform the hole in the crank so that it clamps down on the spindle
d) overcome friction under the bolt heads and in the bolt threads

I think what happens is that the joint settles more in initial service and as the instructions say, at this point you need to retorque the bolts. If there has been much movement then it is an excellent idea to clean the splined interface before reassembling and retorqueing.

However if the joint has moved too much then the fit of the parts goes well off; more and more of the bolt torque is used (wasted) on c) and d). Eventually there isn't anywhere near enough spent on a) and b) and the crank falls off. Worn cranks increasingly cause the bolt force to be used in deforming the crank enough to clamp down at all, and when the crank is deformed thusly, the bolt holes simply are not straight any more; you quite often see that the bolts are binding on the heads (because they are cocked in the bore they are sitting isn't) and the shanks of the bolts are chewing into the sides of the holes a bit lower down. All a bit of a mess! Worse yet the bolts start to bend as they are tightened and they bend as many times as you turn the bolt, so they can and do fail.

With a slightly worn crank you might be better off fitting a stud and using a special nut that fits in the counterbores, e,g, a modified version of the kind of M6 nut that is used on V-brake blocks. This would mitigate the bending and make for lower friction when the crank is tightened. But the fit doesn't have to be very bad before the clamping force simply won't be enough; at that point it is game over, really, and new parts are required.

cheers

[Spinners]

[…] I put the crank on and gave it some extra welly but nice and evenly on the two bolts.

These two bolts: was at least one of them lying on the ground after the crank fell off?

There is a “stopper plate” with a “plate pin” that, when fitted right, prevents the crank falling off even if the bolts get loose. I wonder if you had that the right way around.

That’s the Shimano terminology from this installation instructions PDF:

https://si.shimano.com/pdfs/si/SI-1MG0A-001-ENG.pdf

A torque wrench is pretty useful for these bolts that have a narrow window between too-loose and damage from excessive torque.

Both bolts were still on the crank but I can't recall seeing the stopper plate and plate pin. Are these parts plastic? I'll check out the installation instructions next time I'm on my PC. Cheers.

[Samuel D]

Both bolts were still on the crank but I can't recall seeing the stopper plate and plate pin. Are these parts plastic?

The plate is plastic with a steel pin.

Brucey’s point (a), above, is why I grease the spindle splines before fitting the left crank. Shimano recommends this in cases where the joint creaks. Their words from the PDF for the FC-4600 above:

“If a squeaking noise is heard coming from the bottom bracket axle and the left crank arm connector, apply grease to the connector and then tighten it to the specified torque.”

Also, I don’t think Shimano would supply bottom brackets with this much grease if the splines weren’t supposed to be greased. It’s impossible to push the spindle through that without greasing the splines. And once greased, they’re not easily degreased. A wipe of a rag won’t do it.

I mention all this because creaking is evidence of movement, the first stage of your crank falling off.

Having suffered a couple of bouts of creaking from this interface (with the FC-5600), I now pay more attention to these words from the Shimano PDF:

“The two left crank arm mounting bolts should be tightened alternately in stages rather than each bolt being fully tightened all at once. Use a torque wrench to check that the final tightening torques are within the range of 12 - 14 N·m. Furthermore, after riding approximately 100 km (60 miles), use a torque wrench to re-check the tightening torques. It is also important to periodically check the tightening torques. If the tightening torques are too weak or if the mounting bolts are not tightened alternately in stages, the left crank arm may come off and the bicycle may fall over, and serious injury may occur as a result.”

Brucey has previously explained some nuances of reliably achieving this torque, although Shimano neglects to mention this and I’m sure many mechanics don’t know. Essentially the bolts should be set with a significant final swing of the wrench. That means backing them off a bit to check the torque values after the first 100 km and “periodically”.

It’s a lot of careful work compared to square-taper, isn’t it. Not that they were immune to problems.

[Polisman]

Square tapers, I loved them! Only ever had one issue with a left crank, after forgetting the sage advice to tap them onto the spindle with a good few whacks of a rubber mallet before locking off the bolt. Doh!

[Brucey]

re grease on the splines/bolts.

I think that as Samuel says shimano intend there to be grease present on initial assembly. If the bolts are greased the first time (and/or have paste threadlock on the shanks, which acts as a lubricant during assembly i.e. before it goes off) then they need to be similarly lubricated whenever they are retorqued too, so IMHO lubed bolts is the way to go. Much of the friction is below the head of the bolt and these surfaces tend to pick up on one another; cleaning / regreasing there isn't a bad idea. [NB I don't think these bolts commonly back out and allow the joint to loosen; if this were a worry I'd suggest threadlock on the bolts and/or marking the heads to see if they move or not.]

However (c.f. greasing of square tapers) once the joint has settled it isn't so clear that grease on the splines themselves is of comparable benefit. One thing is for sure; the mix of grease and wear debris that is usually found whenever there have been even tiny movements of the crank is nothing but harmful.

FWIW for this type of joint I don't recommend the use of any kind of anti-seize on the splines, so not copper ease, not graphite grease, not MoS2 grease. The use of such grease will encourage fretting movement and furthermore it isn't so likely to be pushed out of the joint on initial assembly. The latter point means that, having been assembled, the joint can appear to 'settle' (i.e. slacken) even without being ridden on. So I would expect shimano's usual grease to be suitable for this job (even if it is no good in hubs or anything else that moves and sees the weather/road salt).

cheers

[Spinners]

Some good advice there guys. I've not much to add except that I'd done 322km on this fresh build (old groupset onto a new aluminium frame) and was enjoying squeak free cycling... but must admit that the indexing was getting a bit noisy in certain gears on my last couple of rides which I now realise was the drive side crank moving outboard slightly.

One of my other bikes has Tiagra 4700 on it (shop-built so I can only assume by a pro-mechanic) and taking my long handle 5mm allen key to the two bolts showed that they were very tight.

So I think I'll ask Santa for a Torque Wrench and also take away a lesson learned about checking these things after about 100km following a build or refit.

[Samuel D]

If Santa needs a pointer, the torque wrench kit that Wiggle is selling for just 25 quid is surprisingly good. The bits are hard and the wrench is convenient to use – a lot more convenient than the discontinued Park Tool beam-type torque wrenches that I now have instead.

[Spinners]

If Santa needs a pointer, the torque wrench kit that Wiggle is selling for just 25 quid is surprisingly good. The bits are hard and the wrench is convenient to use – a lot more convenient than the discontinued Park Tool beam-type torque wrenches that I now have instead.

Thanks mate. Chief Elf has ordered one ; )