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My crank keeps falling off!

samfailedsamfailed Posts: 60
edited September 2019 in MTB workshop & tech
The non-drive side crank on my mountain bike keeps falling off whilst riding! This happened a few months ago and I couldn't work out why but refitted it and tightened and it seemed absolutely fine.

I tightened the Shimano crank bolt and torqued the two bolts equally.

I've ridden it several times since with no issues until yesterday when it came off six miles into a ride without warning.

I've examined all parts and have seen three issues that might be the issue but I'm really not sure.

1. The inner quarter of the inside of the crank appears to be missing the spline. Not sure whether this is supposed to be that way or it's somehow been worn away?

2. There's a rubber/plastic washer ring thing (?) that was on the inner part of the crank where it meets the bike. This has disintergrated and I have now removed it. Will this have made any difference?

3. The thread of the Shimano plastic crank bolt seems very worn.

Could any of these be the problem?
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  • steve_sordysteve_sordy Posts: 2,124
    I no longer have a Shimano crank to go look at, so this is from memory.

    At some point the crank has become loose and the splines on the crank and the spindle may have worn away at each other. I say "may" because there is a short section that is not splined (see exploded view below, zoom in if you have to). It sounds as though you still have enough left to work with though.
    In the meantime it is important that it doesn't come loose again because every time it comes loose could be the last time and then you will be walking back. :shock:

    1. No it's probably not meant to be like that, see above.
    2. No it won't have caused the problem, but it's there for a reason - probably to stop contact between metal parts. Item 6 "Ring" on the exploded view below.
    3. I'm assuming that this is the bit that goes into the end of the spindle. It's quite a wide but short component and probably has a weird shaped hole where the hexagon hole would be if it was an Allen bolt. This is the crank arm fixing bolt. There is a special tool, very cheap that is used, but I used to use the butt end of a screwdriver as it fitted quite well. It does not have to be super tight (use some thread lock, see below). It is supposed to be the last resort to stop the crank from coming off. But as it's plastic, it's not very strong and will not withstand much axial pressure. You can buy these very cheaply and you can get them in alloy if you prefer. It is essential.

    Another essential is the tightness of the clamp bolts. There is a recommended torque for these (dunno what, Google it), but they have to be tight, not hero tight, so don't knacker your alloy crank by over-tightening the steel bolts. One thing I can 100% recommend is to use some "Loctite thread lock" on the threads. This is not superglue, so it will release, but it is strong enough to resist loosening of the threads. You should also use it on the tiny screws that bolt your new discs to the wheel hub. There should also be an odd-shaped metal washer that both clamp bolts pass through. Item 5, "Plate" on the exploded view (EV) below: This probably isn't your exact crank, but you'll get the idea.

    https://si.shimano.com/pdfs/ev/EV-FC-M640-3433.pdf

    All the Shimano Technical Documents for all their cranksets are here below, all you have to is to find what you need! Remove the search item "crankset" to get to ALL the tech docs.

    https://si.shimano.com/#/en/search/Keyw ... e=Crankset

    Your problem happened to me once. It was my fault as I had run out of thread lock, then never got around to redoing the bolts when the new thread lock arrived. It destroyed the retaining bolt and did some minor damage to the splines in the crank. Once the spindle is unrestrained, it can move towards the front rings and cause all sorts of shifting problems, especially if you have 2x or 3x. :shock:

  • There is a special tool, very cheap that is used, but I used to use the butt end of a screwdriver as it fitted quite well. It does not have to be super tight (use some thread lock, see below). It is supposed to be the last resort to stop the crank from coming off. But as it's plastic, it's not very strong and will not withstand much axial pressure. You can buy these very cheaply and you can get them in alloy if you prefer. It is essential.

    Another essential is the tightness of the clamp bolts. There is a recommended torque for these (dunno what, Google it), but they have to be tight, not hero tight, so don't knacker your alloy crank by over-tightening the steel bolts.

    Hmm... I have the special tool for the crank bolt and used it last time this happened. I also tightened the bolts to the recommended torque which is specified on the crank...

    I feel like something else must be going on...
  • Has the bottom bracket been removed at some point? If the spacers which fit between the frame and the cups are wrong it can cause the plastic crank bolt to not engage with the crank spindle as the drive side isn't able to push in far enough so enough spindle comes out the other side.

    I’m speaking from my own experience of exactly the same problem a few years ago.

    I found the plastic centre bolt to be pretty poor, as you say the threads don’t last. I bought an aluminium replacement which takes an Allen key to fit on eBay. Much better than the plastic original, the threads can take a little extra torque without stripping.
  • AlejandrosdogAlejandrosdog Posts: 1,975
    Has the bottom bracket been removed at some point? If the spacers which fit between the frame and the cups are wrong it can cause the plastic crank bolt to not engage with the crank spindle as the drive side isn't able to push in far enough so enough spindle comes out the other side.

    I’m speaking from my own experience of exactly the same problem a few years ago.

    I found the plastic centre bolt to be pretty poor, as you say the threads don’t last. I bought an aluminium replacement which takes an Allen key to fit on eBay. Much better than the plastic original, the threads can take a little extra torque without stripping.

    Stripping threads??? It only needs to be finger tight, hence the size and shape of the tool.
  • steve_sordysteve_sordy Posts: 2,124
    Try the thread lock. What have you got to lose?
  • AlejandrosdogAlejandrosdog Posts: 1,975
    samfailed wrote:

    There is a special tool, very cheap that is used, but I used to use the butt end of a screwdriver as it fitted quite well. It does not have to be super tight (use some thread lock, see below). It is supposed to be the last resort to stop the crank from coming off. But as it's plastic, it's not very strong and will not withstand much axial pressure. You can buy these very cheaply and you can get them in alloy if you prefer. It is essential.

    Another essential is the tightness of the clamp bolts. There is a recommended torque for these (dunno what, Google it), but they have to be tight, not hero tight, so don't knacker your alloy crank by over-tightening the steel bolts.

    Hmm... I have the special tool for the crank bolt and used it last time this happened. I also tightened the bolts to the recommended torque which is specified on the crank...

    I feel like something else must be going on...

    just an idea, incase you've got something the wrong way round......its sometimes worth having a cup of tea laying everything out and starting slowly again without assumptions.

    the order of events is (once everything is clean) ( i also apply a little grease to the threads, and the mating surface. in particular the bolts in the crank arm are susceptible to corrosion and a right pain in the behind to get out if the soft metal head gets rounded out)
    1 ensure the the drive side crankarm is seated fully into the bottom bracket
    2 fit the crank to the spindle ensuring the safety tab locates in the small hole for it on the spindle. if you've lost the safety tab crack on anyway it should work fine without.
    3 screw in the black plastic cap. important! this should be turned only finger tight using the tool. use the correct one for your crank version.
    4 fasten the two pinch bolts in the arm crank arm steadily to between 12 and 14nm. I always use 12nm so that they can be undone again without damaging the head of the bolt. you will need a 5mm hex key on your torque wrench

    it really is that simple.
  • Has the bottom bracket been removed at some point? If the spacers which fit between the frame and the cups are wrong it can cause the plastic crank bolt to not engage with the crank spindle as the drive side isn't able to push in far enough so enough spindle comes out the other side.

    I’m speaking from my own experience of exactly the same problem a few years ago.

    I found the plastic centre bolt to be pretty poor, as you say the threads don’t last. I bought an aluminium replacement which takes an Allen key to fit on eBay. Much better than the plastic original, the threads can take a little extra torque without stripping.

    Stripping threads??? It only needs to be finger tight, hence the size and shape of the tool.

    Yes it only needs to be finger tight, but after experiencing my cranks falling off numerous times (which I did get to the bottom of), I decided I wanted a more robust centre bolt which I could fasten a little more than finger tight for a bit of piece of mind.

    Also the OP has said that his threads are knackered, so a replacement is probably a good idea :wink:
  • AlejandrosdogAlejandrosdog Posts: 1,975
    this thread is a good example of why LBS survive
  • The RookieThe Rookie Posts: 27,748
    The black cap is there SOLELY to preload the bearings (like the top cap on your forks), once the pinch bolts are done up you can remove it and ride without if you want. Doing it up tighter will just screw your BB earlier, if that's what you want then crack on.
  • JBAJBA Posts: 2,706
    samfailed wrote:
    2. There's a rubber/plastic washer ring thing (?) that was on the inner part of the crank where it meets the bike. This has disintergrated and I have now removed it. Will this have made any difference?

    Probably because you overtightened the preload cap and crushed it against the bb.

    As mentioned above the cap is only intended to preload the bb bearings and needs to only be finger tight.

    There is a small section of the NDS crank arm inner that does not have any splines, but not a quarter of it.

    Can you post a few photos of the crank arm and bb splines?
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Bird Zero Mk1 ¦ Giant Revolt Advanced 2 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • AlejandrosdogAlejandrosdog Posts: 1,975
    The Rookie wrote:
    The black cap is there SOLELY to preload the bearings (like the top cap on your forks), once the pinch bolts are done up you can remove it and ride without if you want. Doing it up tighter will just screw your BB earlier, if that's what you want then crack on.
    This :)
  • Just for the record, I’ve never screwed my BB early. It usually lasts for 3000 miles or so. Nothing wrong with going a little beyond finger tight with a better bolt.

    At no point have I suggested hanging off the Allen key. Common sense should decide on how much torque is too much.
  • keef66keef66 Posts: 13,123
    In my experience with Shimano road chainsets, finger tight with the plastic cap is plenty to remove any lateral play. Once the pinch bolts are then tightened up the plastic cap is going nowhere.

    If the OP's crank keeps falling off there's something wrong with the pinch bolts and / or the crank / spindle interface. It's nothing to do with the plastic bolt/cap thing.
  • AlejandrosdogAlejandrosdog Posts: 1,975
    Muppetry
  • larkimlarkim Posts: 2,301
    Once cranks are loose, it's not difficult for the loosening motion to cause damage to the splines of the crank or the BB. Given how easily I stripped a crank's pedal thread on a unicycle when it was unwittingly being ridden back to front, a small wobble can do some quick damage. I'd reckon the crank / BB interface is compromised in some way which leaves the crank feeling snug and tight at first, but once riding pressure is applied it soon moves.

    And as everyone else has said, that preload bolt has nothing to do with it!
    2015 Canyon Nerve AL 6.0 (son #1's)
    2011 Specialized Hardrock Sport Disc (son #3s)
    2013 Decathlon Triban 3 (red) (mine)
    2019 Hoy Bonaly 26" Disc (son #2s)
    2018 Voodoo Bizango (mine)
    2018 Voodoo Maji (wife's)
  • keef66keef66 Posts: 13,123
    Good point. How do you tell which way round a unicycle goes?
  • billycoolbillycool Posts: 833
    I had a crank arm come loose and it never stayed tight after that. The threads and/or splines were knackered.

    Only solution was a full crank replacement.
    "Ride, crash, replace"
  • larkimlarkim Posts: 2,301
    keef66 wrote:
    Good point. How do you tell which way round a unicycle goes?
    Providing you don't spin the seat around, it's obvious I suppose, but if you take the seat out then you have to work it out - I think the most obvious way was that the cranks were marked with stickers on the rear with L and R, but eldest son (the unicyclist) never thought to look so just put the pedals on, put the seat in (newly delivered) and rode it. Cranks destroyed in about 24 hours!
    2015 Canyon Nerve AL 6.0 (son #1's)
    2011 Specialized Hardrock Sport Disc (son #3s)
    2013 Decathlon Triban 3 (red) (mine)
    2019 Hoy Bonaly 26" Disc (son #2s)
    2018 Voodoo Bizango (mine)
    2018 Voodoo Maji (wife's)
  • I'm stumped then. The bike is less than a year old, has never had the bottom bracket removed, the splines look absolutely perfect and has never been a unicycle...

    It's worth noting that this isn't something that happens immediately and took about three 10 mile off-road rides to come loose again. It's not at all noticeable until the very last minute when my foot comes away with the crank attached!

    I've done as Alejandrosdog suggested and just gone through it all step by step with a new plastic cap. It feels absolutely fine but only time will tell...
  • JBAJBA Posts: 2,706
    Make sure you tighten the pinch bolts alternately to ensure they are evenly torqued.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Bird Zero Mk1 ¦ Giant Revolt Advanced 2 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • samfailed wrote:
    I'm stumped then. The bike is less than a year old, has never had the bottom bracket removed, the splines look absolutely perfect and has never been a unicycle...

    It's worth noting that this isn't something that happens immediately and took about three 10 mile off-road rides to come loose again. It's not at all noticeable until the very last minute when my foot comes away with the crank attached!

    I've done as Alejandrosdog suggested and just gone through it all step by step with a new plastic cap. It feels absolutely fine but only time will tell...

    It sounds exactly like what was happening to me when my bottom bracket spacers were fitted incorrectly.

    If the problem keeps persisting then maybe it’s a possibility yours have been fitted incorrectly at the factory (it can happen) since it’s a young bike and you’ve never removed the BB before.

    If it persists I’d take it to a bike shop and have them give it a once over.
  • 10 miles of trails last night and it was perfectly fine. Checked it out when I got home and there's zero play in the cranks!? Checked the bolts and they're fully tightened.
  • AlejandrosdogAlejandrosdog Posts: 1,975
    samfailed wrote:
    10 miles of trails last night and it was perfectly fine. Checked it out when I got home and there's zero play in the cranks!? Checked the bolts and they're fully tightened.

    Yayyyy sometimes going back to the beginning with a cup of tea is just the job.

    :) glad it seems to have worked.
  • cyclecliniccycleclinic Posts: 6,865
    edited August 2019
    What kind of crank is this, hollowtech or octolink.

    If hollowtech the nds arm is pushed on then the plastic preloaded adjuster screwed in till the crank arm is fully on. Then the pinch bolts are done up to 14Nm. In that order. If the crank keeps falling off the spines maybe damaged. Look for shiny blunt areas on the spines.

    If octolink there are two types of octolink bb. Both mutually compatible with each other. Given the age of the bike and no new bb this can't be a cause.
    http://www.thecycleclinic.co.uk -wheel building and other stuff.
  • AlejandrosdogAlejandrosdog Posts: 1,975
    What kind of crank is this, hollowtech or octolink.

    If hollowtech the ends are is pushed on then the plastic preloaded adjuster screwed in till the crank arm is fully on. Then the pinch bolts are done up to 14Nm. In that order. If the crank keeps falling off the spines maybe damaged. Look for shiny blunt areas on the spines.

    If octolink there are two types of octolink bb. Both mutually compatible with each other. Given the age of the bike and no new bb this can be a cause.

    Pissed again?
  • The RookieThe Rookie Posts: 27,748
    What kind of crank is this, hollowtech or octolink.
    Hollowtech is a way of making the crank, (you can get Hollowtech cransk for Octolink BB's), I think you mean hollowtech2 which when the driveside crank has a permanent axle fitment. the OP clearly has HT2 from the description of a plastic preload bolt and pinch bolts, neither of which can be found on Octolink. Lay off the dope!
  • mark~pmark~p Posts: 52
    Assuming HollowTech then the most likely problem is that the splined hole in the crank has worn slightly before the first time it fell off. This means that there will be a small amount of movement, no matter how tight the bolts as the hole is now either bell shaped or tapered in both directions. The splines do not go right to the inside face of the crank so that is normal. Adding excessive preload will just wear the bearings out quicker as others have stated. Unfortunately due to the forces involved when pedalling, the crank will had had it.
    The older Octalink can be similarly affected if it is allowed to get loose. The old square taper is much more forgiving a you can, within reason just tighten it a bit more.

    Your best bet is to either replace the damaged crank or just replace the complete set. Shimano cranks come with threaded bottom brackets with spacers to fit 68mm or 73mm wide BB.
    Having the proper tool is well worth the money.
  • cyclecliniccycleclinic Posts: 6,865
    Initial read it was not clear it was hollowtech2. Also predictive text on a new phone is causing typing issues. The point being if the crank is falling off and the installation is correct the spines are damaged and the cranks need replacing to ensure they stay on.

    There is s problem on this forum. Some are too ready to have a dig. I cant even type nds without the phone trying to type ends.

    I am also quite dyslexic and find it it hard to error check so thanks for the insults. Much appreciated.
    http://www.thecycleclinic.co.uk -wheel building and other stuff.
  • The RookieThe Rookie Posts: 27,748
    None of that affected the fact that the description of the issue meant it could only be HT2 though does it, the phone didn't predicatively ask if it was Octolink!
  • AlejandrosdogAlejandrosdog Posts: 1,975
    The Rookie wrote:
    None of that affected the fact that the description of the issue meant it could only be HT2 though does it, the phone didn't predicatively ask if it was Octolink!

    Maybe he’s already taught the phone all he knows......

    Any way the last message from the op was that the crank is now fine, its been fitted and ridden and the op is out enjoying himself.
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