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Crank arm gone done fell off

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I hit a pot hole hard at the weekend, at the bottom of a hill, doing 55km/hr and my Ultegra crank arm came loose The plastic tension bolt had sheared off and one of the 5mm bolts was completely loose, the other was tight. I tightened it with an allen key, not super tight as I wasn't sure if I needed the tension bolt or not and it came loose again. Took it to my LBS, they had a look at the crank arm, said it looked fine and I got a new tension bolt. They ended up screwing it in for me. I took it for a quick ride round the block and it seemed OK. But I have an Ironman in 3 weeks.... I don't really want the crank arm coming off during the race. That will definitely be game over...
 
I'm puzzled as to why it came off. Years of BMXing and MTBing, lots of punishment on the bikes and I've never had a crank do that. Any ideas why this could have happened? I installed the crank a few months ago, it was off my mates bike and near new. I'm reasonably OK with bike maintenance stuff, but perhaps I ****ed this up? Not sure how though. LBS mechanic suggested I had not slid the crank arm on hard enough so it was almost flush with bottom bracket. I thought this would be impossible with the tension bolt? The only other thing is I didn't tighten the 5mm bolts enough, but that would be a bit retarded, although I can sometimes go full retard.
 
 
 
30801_shimano_ultegra_fc_6800_crank_arm_fixing_bolt.jpg
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12 minutes ago, KieranR said:

Go back to racing BMx and MTB. 

Don't get to wear lycra.

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1 hour ago, zed said:

Years of BMXing and MTBing, lots of punishment on the bikes and I've never had a crank do that. 

Bring back the cotter-pin. :)

I loved the chunks out of the ankle.

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4 minutes ago, Ex-Hasbeen said:

Bring back the cotter-pin. :)

I loved the chunks out of the ankle.

haha that brings back memories and bear-trap pedals on the shins... 

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Put loctite on it if you’re worried. Had the same thing happen at Nepean last year halfway thru the ride, day over. Replaced the bolt, 1 drop of loctite on it, no problems since despite checking it regularly

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I had something similar happen at a race at Redcliffe.

Genuinely JRA. 
Bike shop mechanic had seen that type of thing a bit, but was never sure when people told him the alleged circumstances.  
Once they fixed it, never had an issue again. 

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This actually happened to my little brother on his roadie about two years ago, after discussing things with me and his LBS he decided it was better to retire from cycling altogether and take up ultra running (walking).

maybe this is an option for you also? 

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So that "tensioning" bolt is just to preload the bearings and tighten the NDS crank arm against them.

The pinch bolts and retaining clip do all the hard clamping work.

 

My guess is the LBS was correct, You may not have loaded the NDS arm far enough over the crank spindle meaning the retaining clip didn't engage the little keeper slot in conjunction with no enough righty tighty.

 

 

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2 hours ago, zed said:
I hit a pot hole hard at the weekend, at the bottom of a hill, doing 55km/hr and my Ultegra crank arm came loose The plastic tension bolt had sheared off and one of the 5mm bolts was completely loose, the other was tight. I tightened it with an allen key, not super tight as I wasn't sure if I needed the tension bolt or not and it came loose again. Took it to my LBS, they had a look at the crank arm, said it looked fine and I got a new tension bolt. They ended up screwing it in for me. I took it for a quick ride round the block and it seemed OK. But I have an Ironman in 3 weeks.... I don't really want the crank arm coming off during the race. That will definitely be game over...
 
I'm puzzled as to why it came off. Years of BMXing and MTBing, lots of punishment on the bikes and I've never had a crank do that. Any ideas why this could have happened? I installed the crank a few months ago, it was off my mates bike and near new. I'm reasonably OK with bike maintenance stuff, but perhaps I ****ed this up? Not sure how though. LBS mechanic suggested I had not slid the crank arm on hard enough so it was almost flush with bottom bracket. I thought this would be impossible with the tension bolt? The only other thing is I didn't tighten the 5mm bolts enough, but that would be a bit retarded, although I can sometimes go full retard.
 
 
 
30801_shimano_ultegra_fc_6800_crank_arm_fixing_bolt.jpg

Definitely get it fully checked out. 

I did MMM in 1992 and after a crank issue and tightened by bike shop in came off 9km into a 180k bike ride. Lots of rolling and waiting and some pedalling 1 legged I managed to finish. But by this stage my ITB was so shot I hobbled the marathon. 

Finished 83 I think...... 8th last.

My mates said they would have pulled out and through my bike through the LBS front window the next day.

Moral: if I’m doubt replace pedals etc if you think it may reoccur. Back then I naive and money conscious- big mistake

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58 minutes ago, pieman said:

There's a little clip in crank and off that's engaged the other stuff shouldn't matter...

 

 

Screenshot_20191111-124420_Google.jpg

What's that circular thing?  I found something very similar looking to that on the floor of the storeroom where we keep our bikes...

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40 minutes ago, SomeTri said:

What's that circular thing?  I found something very similar looking to that on the floor of the storeroom where we keep our bikes...

Pretty sure thats just an inboard seal?

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38 minutes ago, SomeTri said:

made of metal?

Yep, fairly light gauge though. More than likely aluminium.

 

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Just now, Jim Shortz said:

Ouch 

I remember that 

My girlfriend waxed my legs and was WTF at my shins... 

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