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stripped crank or BB or something like that.


mix
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hey Guys and Girls.

 

so i have a problem that im sure is pretty common or at least has happened in the past to a few of you, i went out for a ride yesterday and nearing the end of my ride for the second time my left arm on my crank fell off, now the bb is stripped apparently, it is not an octalink it is a square join (i realise at this point im sounding useless and extremely ignorant of the topic, YES im a complete Newbie) anyway so this is the second time it has happened i took it to my LBS and they said they will have a look and see what i need to replace, crankset and bb were mentioned.

 

now i dont want to be ripped off by these guys, so please can you tell me what i should do and what needs to be done also what BB i should get if i do need one and so on and so forth. Obviously i dont want to spend a large amount of dolla.

 

Oh my bike is a Merida matts 400d xc.

 

i would really appreciate help and mockery is only tollerated if i get help :) thanks.

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No worries mix...

 

Can you tell us how the crank fell off? Normally, there's a bolt that fastens the crank itself to the Bottom bracket, and unless that fails (strips or just plain shears off) the crank shouldn't come "off" perse. Another way that it could have failed is that the BB shaft would have sheared off. I find that very hard to imagine, which is why I'm going to rule that bit out.

 

I'm going to assume that the bolt that fastens the crank to the BB spindle (the square bolt-like thing that the cranks atttach to) was either over-tightened and stripped, or that the BB / fastening bolt had some sort of structural fault which enabled the stripping to occur int he first place. Another thing that could have happened was that the bolt WASN'T tightened, which would have enabled the bolt to work itself loose over rough terrain, thus allowing the crank to fall off.

 

2 options now, really. Either you could do it the bush-mechanic way, and get a slightly bigger guage stainless steel bolt to replace the one that is now gone (I'm assuming it's gone because the crank fell off - it wouldn't have fallen off if the bolt hadn't come undone) and use that to fasten the crank to the now-stripped BB spindle. You can source a bolt like this at your local bolt merchant / store (specialized store - not builders whorehouse) You can also then get a tapping tool, to "tap" a new thread into the BB Spindle, so that the new bolt fits properly without having to forcibly insert a new thread as you screw the bolt in.

 

2nd choice - get a new Square taper BB, and some proper BB fastening bolts so that this doesn't happen again. A new Square taper BB shouldn't be more than R 150 (even taking into consideration the inflationary LBS pricing) and the crank-fastening bolts should come with it. Just make sure that the bolts that are supplied have a hard rubber washer, approx 3mm thick, to help the bolt "sit" in place.

 

But if you're not in any way mechanically minded, I'd take it to the LBS and ask them to sort it out for you. They will use the second option (well, I hope they will!) but you'll have a place to take it back to should it not work..

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Thank you. in all honesty im not mechanically minded when it comes to bicycles.

 

is it a complete waste in buying a new deore crankset which comes with a bb?

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The Deore crankset and external BB would be nice upgrade (what is the model of the cranks you have now?) but not entirely necessary.

 

Like Capt Mayhem says - you'll be best off with a new BB for less than R150 and some new bolts. The crank arm coming off should not have damaged the crankset at all, so that should be perfectly reuseable. Check that the bolt didn't just fall out cos it was loose - if it was, all you'll need is a new bolt.

 

If you decide to upgrade, please consider donating your old cranks to Build-a-Bike (excuse the shameless plug!)

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What usually happens here is that the bolt works loose and the crank follows. The issue is that with the crank being aluminium and the bb taper being steel, it tends to damage the taper on the inside of the crankarm. So yes, you're probably in for a new crankarm. It's usually the left one, and if you ask your bike shop nicely they may be able to sell you just that one.

 

Make sure that you check the square taper on your bb as well - it usually survives but can sometimes be rounded off, in which case you'll need a new bb too.

 

Otherwise, use it as an excuse to upgrade and get a Deore external bearing crankset. About a grand, and pretty bombprooof if installed properly.

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Thank you. in all honesty im not mechanically minded when it comes to bicycles.

 

is it a complete waste in buying a new deore crankset which comes with a bb?

 

Shimano SLX, BB included. And remember to have the BB shell faced before the BB is fitted. :thumbup:

ROTOR BB's are also available now, cheaper than Shimano and bearings are available standards.

 

Change now. All worth it.

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As CPTmayhey says, you can fix it.

 

Other option is that, if the thread is gone, you can get the thread drilled out and you can have a coil put in. then it would be as good as new.

 

As droo says, the alu crank arm square taper can also stretch / get worn if you ride with it loose, so have a look at that too.

 

Alt B is that you get a new crankset - nice upgrade :thumbup:

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Any chance of a picture to see what the problem might be? As mentioned it may just be the bolt that needs replacing. Think about using a drop of thread locker as well when you re-install.

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The Deore crankset and external BB would be nice upgrade (what is the model of the cranks you have now?) but not entirely necessary.

 

Like Capt Mayhem says - you'll be best off with a new BB for less than R150 and some new bolts. The crank arm coming off should not have damaged the crankset at all, so that should be perfectly reuseable. Check that the bolt didn't just fall out cos it was loose - if it was, all you'll need is a new bolt.

 

If you decide to upgrade, please consider donating your old cranks to Build-a-Bike (excuse the shameless plug!)

 

no prob on the plug. i will see what i can do i normally pass my things onto my cousin or brother in law.

 

I have the standard shimano crankset that came with my bike.

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What usually happens here is that the bolt works loose and the crank follows. The issue is that with the crank being aluminium and the bb taper being steel, it tends to damage the taper on the inside of the crankarm. So yes, you're probably in for a new crankarm. It's usually the left one, and if you ask your bike shop nicely they may be able to sell you just that one.

 

Make sure that you check the square taper on your bb as well - it usually survives but can sometimes be rounded off, in which case you'll need a new bb too.

 

Otherwise, use it as an excuse to upgrade and get a Deore external bearing crankset. About a grand, and pretty bombprooof if installed properly.

 

that is exactly what has happened, it is the third time it has happened and this time happened after about 8 kms which is pretty upsetting. i did check the bb and it is rounded off, the left arm thread is also prettyw worn, my wife is happy with me getting the uprgade and so i think its a no brainer :) i can get the deore crankset with external bb for 870 installed does that sound good? is this maybe not the external bearing?

installed properly? how would i know?

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Shimano SLX, BB included. And remember to have the BB shell faced before the BB is fitted. :thumbup:

ROTOR BB's are also available now, cheaper than Shimano and bearings are available standards.

 

Change now. All worth it.

BB shell faced before fitted? excuse me??

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Any chance of a picture to see what the problem might be? As mentioned it may just be the bolt that needs replacing. Think about using a drop of thread locker as well when you re-install.

 

sorry already at my LBS.

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BB shell faced before fitted? excuse me??

 

Getting the shell faced means that the LBS workshop dude completely levels the outside face of the BB shell so that the bearings are completely flush with the frame.

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Getting the shell faced means that the LBS workshop dude completely levels the outside face of the BB shell so that the bearings are completely flush with the frame.

 

ok great will ask him to do that. what does this do?

 

man i sound like an ignorant wanker.

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If it looks like this, then it's the external BB model....

 

http://lp1.pinkbike.org/p4pb3224712/p4pb3224712.jpg

 

thats the one. thanks.

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