The left crank arm just fell off

When commuting to work this morning, I noticed that my left pedal was wobbling. I stopped to check, and suddenly had the complete crank arm in my hand

I have a 15km commute to work (into the nearest town), most of it in rural areas. Luckily, I had reached the urban area, so I dropped in at the tire shop where I buy both my motorbike and car tires. I borrowed an Unbrako key and remounted the crank arm. I guess I lost max 5 minutes on the incident.

However, I must say i'm quite surprized that this could happen. The bike is my Haibike with Bosch CX smart system, and it has done approx 550km since i got in in September. Quite a new bike in other words.

When I get home, I will take the crank arm off and inspect it. Is there anything I should do / check before I torque it down to the torque value stated on the screw's head? Should there be any grease on the splines? Anything else? I guess it could be wise to check the other crank arm as well.

I found my left crank loose the other day too. I do remove mine regularly for maintenance so it may be my fault but it was a surprise! You could try blue threadlock, enough to bind the thread but not so much that you won't be able to undo it again if needed.

Put a little grease on the bolts.

Tighten them to the specified torque.

Then after fitting put some nail polish on them as tell tales so you can see if they're loosening.

irie said:

Put a little grease on the bolts.

Tighten them to the specified torque.

Then after fitting put some nail polish on them as tell tales so you can see if they're loosening.

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I did what you suggested. Used some copper paste on the threads, torqued to 50 Nm, marked it with Tipp-Ex, as I had no nail polish handy. The Tipp-ex bottle sits by my lathe, I sometimes use it to mark out when working in the lathe.

So then I just have to cast my eye on the markings from time to time. Lets hope the Tipp-Ex doesn't wear off. If it does, I gues I'll have to invest in some nail polish (or ask my wife if she has a spare bottle).

milwaukee do a nice 'paint pen' designed for this job, doesn't come off in rain etc... yellow or white. I use yellow on the DH bike bolts.

irie said:

Put a little grease on the bolts.

Tighten them to the specified torque.

Then after fitting put some nail polish on them as tell tales so you can see if they're loosening.

View attachment 135729

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This is correct, grease to the bolt and then correct torque. It's almost impossible to get ISIS crank to fall off if this is done right.

Anders said:

I did what you suggested. Used some copper paste on the threads, torqued to 50 Nm, marked it with Tipp-Ex, as I had no nail polish handy. The Tipp-ex bottle sits by my lathe, I sometimes use it to mark out when working in the lathe.

So then I just have to cast my eye on the markings from time to time. Lets hope the Tipp-Ex doesn't wear off. If it does, I gues I'll have to invest in some nail polish (or ask my wife if she has a spare bottle).

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My problem with selecting the nail polish was which of my wife's colours to choose. Was a bit tricky tbh

Wtf, Mrs Weeksy has hers in colour order. She has 3 of these!!!!

Weeksy said:

Wtf, Mrs Weeksy has hers in colour order. She has 3 of these!!!!

View attachment 135975

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If those ^^^ are in "colour order" then you and/or your Mrs needs to go to Specsavers

Lol that was just a representation from internet, not her actual boxes

Don’t you guys occasionally check all the bolts on your bikes? I probably go over my bikes every 2-3 rides and make sure particular bolts are snug. It takes less than 5 minutes. I check my axles, seat clamp, brakes, derailleur, suspension pivots, and sometimes the stem. All you need is 3 hex wrenches….4,5, & 6mm.

Desert_Turtle said:

Don’t you guys occasionally check all the bolts on your bikes? I probably go over my bikes every 2-3 rides and make sure particular bolts are snug. It takes less than 5 minutes. I check my axles, seat clamp, brakes, derailleur, suspension pivots, and sometimes the stem. All you need is 3 hex wrenches….4,5, & 6mm.

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If you're not using a torque wrench, you're doing it wrong...

Desert_Turtle said:

Don’t you guys occasionally check all the bolts on your bikes? I probably go over my bikes every 2-3 rides and make sure particular bolts are snug. It takes less than 5 minutes. I check my axles, seat clamp, brakes, derailleur, suspension pivots, and sometimes the stem. All you need is 3 hex wrenches….4,5, & 6mm.

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No, I must admit that I don't.
I assumed a brand new bike would have been assembled correctly, meaning the bolts had been torqued to spec.

On the other hand, I have always done my own wrenching on my motorbikes and have never experienced things falling off.
As this is my first e-bike, I just assumed I was safe until I started replacing parts.
But I obviously was wrong, I now understand that I have to check all bolts that I haven't yet touched myself.

Anders said:

No, I must admit that I don't.
I assumed a brand new bike would have been assembled correctly, meaning the bolts had been torqued to spec.

On the other hand, I have always done my own wrenching on my motorbikes and have never experienced things falling off.
As this is my first e-bike, I just assumed I was safe until I started replacing parts.
But I obviously was wrong, I now understand that I have to check all bolts that I haven't yet touched myself.

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Yep, wrong assumption.

Shit comes loose particularly on new bikes and rotating components.

Every new weeks do a bolt check. It only takes 5 mins to throw and Allen key into a hole and see if its unduly loose.

Anders said:

No, I must admit that I don't.
I assumed a brand new bike would have been assembled correctly, meaning the bolts had been torqued to spec.

On the other hand, I have always done my own wrenching on my motorbikes and have never experienced things falling off.
As this is my first e-bike, I just assumed I was safe until I started replacing parts.
But I obviously was wrong, I now understand that I have to check all bolts that I haven't yet touched myself.

Click to expand...

Yep, wrong assumption.

Shit comes loose particularly on new bikes and rotating components.

Every new weeks do a bolt check. It only takes 5 mins to throw and Allen key into a hole and see if its unduly loose.

Paint pens are only a couple of quid and won't come off in the rain/washing the bike.

I put a single dot at 12 o'clock on all suspension and linkage bolts and a witness mark on the crank arm bolts.
I've had a few bolts come loose and this makes it super easy and quick to check if any are working loose.

RickBullotta said:

If you're not using a torque wrench, you're doing it wrong...

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RickBullotta said:

If you're not using a torque wrench, you're doing it wrong...

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I definitely torque certain things to spec. I have several different tools that I use….i have park t-handles with certain torque settings and more than one actual torque wrenches. I love my tools.

Desert_Turtle said:

I definitely torque certain things to spec. I have several different tools that I use….i have park t-handles with certain torque settings and more than one actual torque wrenches. I love my tools.

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I torque everything to spec.

The reason is that engineers know better than me when specifying torque settings.

When I've purchased crank bolts they've always come with blue thread locker on them and still torque them up regularly never had problems

Hattori-Hanzo said:

Paint pens are only a couple of quid and won't come off in the rain/washing the bike.

I put a single dot at 12 o'clock on all suspension and linkage bolts and a witness mark on the crank arm bolts.
I've had a few bolts come loose and this makes it super easy and quick to check if any are working loose.

Click to expand...

I'd have dots all over my bike if I used paint pen every time I did something up.....

Plummet said:

I'd have dots all over my bike if I used paint pen every time I did something up.....

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Indeed, but they are only the size of a pin head so unless you're really looking you won't notice them.
Not like in the motorcycle industry where they put a great big line across all bolts.

They are good as a quick visual check when out on the trails though

Desert_Turtle said:

Don’t you guys occasionally check all the bolts on your bikes? I probably go over my bikes every 2-3 rides and make sure particular bolts are snug. It takes less than 5 minutes. I check my axles, seat clamp, brakes, derailleur, suspension pivots, and sometimes the stem. All you need is 3 hex wrenches….4,5, & 6mm.

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Every few rides in trail bikes.

DH bike gets bolt checked at lunch on race/practice days and a check in evening if back to back days.