crank arm keeps falling off! what am I doing wrong?

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  • crank arm keeps falling off! what am I doing wrong?
  • Premier Icon jonathan
    Subscriber

    Have a good look at the splines. If they have been hammered and they’re worn there then they don’t really tighten properly again

    MoseyMTB
    Member

    Are you pushing the black securing pin down?

    orangeboy
    Member

    I’d guess the splines are worn. You might get away with just a left hand arm
    But can be had very cheaply on-line inc bb and rings I’d prob change the lot or upgrade

    You have tightened the pinch bolts evenly

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    That’s what I thought, splines don’t look great. Must be letting crap in and working loose.

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    I’m goona sound like a tool now! Whats the black securing pin?!?!?

    Is that the plastic shim type thing that goes between the pinch bolts? If so yeah….

    MoseyMTB
    Member

    The black plastic between the pinch bolts . You push it in and it secures in a hole on the spindle.

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    Mosey, I’ll check that thats fitted properly tomorrow. Mine looks knackered to be honest, so maybe thats the problem.

    3 things need to fail for this to happen. Or 1 thing.

    1: the metal 5mm hex bolts arent done up enough. Or too much. Or are stripped. Or: “what bolts”?
    2: the plastic securing pin has failed.
    3: the plastic pretensioner is missing. Or is stripped.

    4: You bought them used. and 1-3 werent followed.

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    I think it’s the securing pin. I assume if it’s supposed to connect with the spindle, I think mine is broken and isn’t reaching! Can you buy replacements?

    Premier Icon rOcKeTdOg
    Subscriber

    Too many spacers on the BB, 68mm needs more than 73mm, too many spacers & there’s not enough spindle for the crank arm

    STATO
    Member

    The plastic shim the the gap in the cranks has a thin metal pin which goes in a hole in the axle, this is so you know the arm is located correctly, it is not strong enough to hold a loose crank on, it just bends.

    The plastic pre-load cap that threads in the end of the axle is just that, for bearing pre-load. Its a bit more substantial and may help hold a loose crank on, but thats not its job, it will break eventually and your loose crank will fall off.

    You problem is your crank is loose. It was probably ridden loose and didnt fall off straight away (due to things pointed out above) so you probably damaged the splines. Next time you re-fitted it the damaged splines didnt fold so bolts had to be tighter, deforming crank more, ergo, it falls off again. Repeat.

    All of this is of no importance tho. your crank is done, throw it away and buy another. If the axel is ok you may be able to save a bit of money just buying a non-drive arm, but full chainsets are so cheap its prob not worth it.

    shedfull
    Member

    You need to torque up the pinch bolts equally, meaning you do one, then the other, then back to the first, then the second again and so on. If you don’t, the first is effectively released by tightening the second, so you end up with one correctly torqued and one under torqued. Uneven torquing means that the crank wobbles on the tighter one as you pedal and knackers the splines.

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    Just fiddled around with the securing pin and I think it wasn’t pushed in far enough or i’d put it back together wrong after cleaning it all. I’ll give it another go before buying new ones!

    Although I can hear SLX calling me, couple more beers and i might pull the trigger……..

    Cheers all, hopefully i’ll manage more than about 2 miles next ride!!

    The securing pin will not hold your cranks on. It’s a last ditch safety measure, not a means to an end. Something else has to fail (the pinch bolts/splines) before you rely on this.

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    Stato – that sounds about right, i’ll give it one more go but if fails again it’s new cranks time! 😀

    Like I say they have been hammered for a couple of years so it’s probably about time for new ones anyway. The thread on the preload thing is knackered as well so I reckon it’s time to get the credit card out!

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    Deore HT2 crankset, non drive side keeps coming loose and falling off. I’ve stripped and cleaned eveything but it keeps happening. Driving me bloody crazy! Am I missing something or doing something fundamentally wrong? I’m sure they’re fitted right. They have been hammered so maybe they are just worn out. Is time for a new set?

    Premier Icon gofasterstripes
    Subscriber

    If you post good photos we can probably guess if it’s worn out.

    The thread on the preload thing is knackered as well so I reckon it’s time to get the credit card out!

    I’ve done ’em up with a D-shaped file, but not with a credit card.

    Premier Icon martinhutch
    Subscriber

    You’ve probably knacked it now, but my fault was not making sure that the drive side was snug against the BB – so not enough spline showing to make a secure fit. Has to be tapped through really firmly.

    Premier Icon jeffl
    Subscriber

    I’ve had exactly the same problem with a new bike and some deore cranks. I can only assume when it was put together the pinch bolts weren’t torqued up correctly. Finally got a low range torque wrench, the cheapo topeak one. Now done up to 12nm and we’ll see how they do.

    I’ve also got some thread lock to use on the pinch bolts if they come loose again. We’ll see how it goes.

    pipnet1
    Member

    I can’t be the only person who simply throws those plastic shim/locator things away? They just get in the way. Never had a problem with out it.

    They also haven’t seen a torque wrench. In fact there is nothing on my bikes, or any previous bikes, that have ever seen a torque wrench.

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