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    Crank arm bolt coming loose

    The crank arm bolt that came with my BBHSD keeps coming loose on my bike. I just tightened it down last week and today my crank arm was loose again. On the trail I managed to find someone with an 8mm Allen wrench to tighten it down again but it was already loose again by the time I was almost home. I’m thinking I may need a replacement. Is this a part I can easily find in any bike shop or do I need to order something special since it’s for the bafang crank arm?

    #2
    The bolts are pretty common and surprisingly standardized. But, it may be that the bolt isn't turning and loosening, but instead the crank arm's taper cutout may be expanding and the crank is creeping up the spindle taper - they're known to be a rather "soft" alloy.

    You might try tightening the bolt, then drawing alignment marks on the crank and bolt with a permanent marker, to see if the bolt is loosening.
    Last edited by ncmired; 09-06-2023, 05:54 PM.

    Comment


    • AZguy
      AZguy commented
      Editing a comment
      Yes - I see this as more likely a crank arm problem

      Regardless if the bolt doesn't already have blue loctite, give it a whirl, but this sounds more likely a crank that's not fitting properly on the spindle

    #3
    I would try a new crank arm. They don't cost that much. Once it came loose it may be wallowed out and never fit again.

    Comment


      #4
      I'm with the others thinking that coming loose twice means there is a good chance the arm itself may be damaged. Usually the shaft doesn't get damaged from the arm being loose. The left arm isn't that unusual but it may be safer to get one specifically listed for a BBS. The right arm is fairy unique to the BBS's. Places that sell the BBS kits often sell the arms. IF you get em properly tightened the stock ones seem to hold up fine. Lekkie and a few others sell higher quality arms, they aint cheap but they are nice.

      These do need to get fairly tight. Its hard to describe to a non mechanical person without a torque wrench. Part of what makes it hard to judge is its a wedge fit so once the bolt gets snug you are not done. You need to continue a bit to actually pull the arm onto the wedge of the shaft. If you have a typical L key allen set or a typical bike multi tool sort of thing you could throw in your pack you didn't get it anywhere near tight enough. The 'pro' style tool is L shaped and almost as long as the crank arm. That gives you an idea how tight it has to be. It is possible to get it too tight too so if you buy new arms especially fancy ones like Lekkie maybe worth a trip to your local bike shop and ask them to torque them for you just so you know its right.

      Note that a properly installed arm also requires a special tool to remove them again. Its a common standard tool that everyone that maintains their own BBS should just own. Park CWP-7 is the good quality DIY version. Park HT-8 is the 8mm tool for the bolts.

      Comment


      • Retrorockit
        Retrorockit commented
        Editing a comment
        I'm running Shimano Steps E5000 cranks on mine. 165,170,175mm options. They used to be common and discounted. But I haven't looked in a while.
        I like 175 on a pedal bike, but they didn't do anything for me on the BBSHD.
        I looked.
        Includes Left and Right Crank Arms. Crank Arm Length: 165mm,170mm,175mm. Includes: Left and Right Crank Arms. BB Type: Square. Model: FC-E5000.
        Last edited by Retrorockit; 09-07-2023, 06:41 AM.

      • Retrorockit
        Retrorockit commented
        Editing a comment
        Here are all sizes of FC E5000
        Pit Crew Cycles is a family-owned e-comm shop that was founded by Mark. I started biking as a way to strengthen my back after suffering from several lower back injuries. After falling in love with the sport, I quickly decided to turn my passion into a business. At Pit Crew Cycles, we offer a wide range of components and cycling accessories for all levels of riders. We believe that cycling is the perfect way to explore the world around us and to stay active and healthy. That's why we dedicate our weekends to getting out and enjoying all that the great outdoors has to offer. Whether we are hitting the trails, exploring new roads, or just enjoying a leisurely ride through the park, we always make time to get out and enjoy the beauty of the natural world. Whether you are a seasoned pro or just starting out, we are here to support you on your cycling journey. Thank you for considering Pit Crew Cycles for all of your cycling needs.

      #5
      To see if the crank arm is "creeping" up the spindle:
      1. first, tighten both arms down tightly via the bolt, using an M8 hex socket/ratchet wrench, extended handle key, or similar. As 73Eldo wrote, these bolts need to be (and stay) tight
      2. then remove both bolts, leaving the arms in place
      3. now compare the distance between the end of the spindle and where the crank arm bolt bottoms out in the pocket on both arms - easiest done with a digital or dial caliper. If on one side the end of the spindle is darn near flush with the bottom of the pocket, that arm is toast
      Along with Retrorockits recommendation is the Shimano STEPS FC-E5010 eBike Crank Arm Set. Some unicycle crank arms will work as well.
      Last edited by ncmired; 09-12-2023, 11:03 AM.

      Comment


      • AZguy
        AZguy commented
        Editing a comment
        Lol - "dial caliper"? I still have a vernier (pronounced very-near ;-} ) but like my old dials have long been obsoleted... For a probe (or test) indicator I do still prefer the dial far more than digital...unless hooking to a computer of course...

        Digital calipers can be had for $10-20!!! Get four or five and throw out those old dials =]

      • ncmired
        ncmired commented
        Editing a comment
        LOL, fixed it ... Now behave, you!
        Last edited by ncmired; 09-07-2023, 08:49 AM.

      • Retrorockit
        Retrorockit commented
        Editing a comment
        Don't throw out the dial caliper. I have 3 battery ones. The only one that always works is the dial.
        I just tossed the Bafang cranks right away on general principals (actually they ended up on a TSDZ2, which is just about the same thing).
        Last edited by Retrorockit; 09-07-2023, 08:32 AM.

      #6
      Y’all were right. Took it to the LBS and they gave me a new crank arm. Hopefully that holds up but if not I may upgrade to the Lekkie.

      Comment


        #7
        This is old but most of the crank arm bolts come with a nylon patch applied, some do not. if not Blue locktite is required.

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