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Noise from rear cassette when bike is pushed backwards with BBSHD, is this normal?

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    Noise from rear cassette when bike is pushed backwards with BBSHD, is this normal?

    Hi I'm pretty new to mid drive motors and I have a question that I'm sure is easy to answer for those who have been around these motors a while longer. I am building a 7 speed Fat Bike with the BBSHD and I have started the install. I have noticed that when I push the bike backwards it makes my rear cassette click and generally make noise. My chain line although not perfect doesn't seem to be the cause, is this just happening because of the resistance the motor has when rotated backwards? When going forward things are normal. Any thoughts, Thanks

    #2
    very common, personally wouldn't worry. forward noise needs to be addressed.

    Comment


      #3
      I think there is a clutch in there that you might be hearing. Take a look at the internals. Mine does the same.

      Comment


        #4
        Thanks for asking the question, I noticed the same but didn't question it because my drive works so well. It could be something internal such as the clutch, definitely would need to be answered by someone who has had these drives open and understands the nuances. I noticed pushing it slower is rougher than pushing it fast.

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          #5
          Thanks for the responses, I haven't had a chance to get my bike out for a good ride yet but if all goes well after test riding, it sounds like I can just assume the noise is normal for this drive. I to noticed that pushing it slower seems to be rougher than pushing it fast. Thanks again!

          Comment


            #6
            Originally posted by Madman Ebikes View Post
            very common, personally wouldn't worry. forward noise needs to be addressed.
            Just joined the forum, I've just completed my first bbshd build.
            As my motor has the same clonking noise when pushed backwards, as we're now in 2018 I wondered has anybody discovered why this actually occurs?

            Comment


              #7
              Pushing the bike backwards engages the overrunning clutch between the crank and the motor so you are turning the motor rotor (the part with the magnets in these cases). When you turn a permanent magnet DC motor slowly you can feel each time the magnet poles move past the coils - the magnets induce current in the coils when they are moved past them and then the coils generate their own magnetic fields that alternately push and pull the magnets. If you turn it quickly it smooths out from the motor inertia.

              Comment


                #8
                Originally posted by AZguy View Post
                Pushing the bike backwards engages the overrunning clutch between the crank and the motor so you are turning the motor rotor (the part with the magnets in these cases). When you turn a permanent magnet DC motor slowly you can feel each time the magnet poles move past the coils - the magnets induce current in the coils when they are moved past them and then the coils generate their own magnetic fields that alternately push and pull the magnets. If you turn it quickly it smooths out from the motor inertia.
                First of all, I haven't taken one of these apart but read up a bit on them and have seen pictures. From what I read, the BBSHD has a 1:21.9 gear reduction. There is no way you are spinning that motor backwards. There has to be a clutch that allows the wheel to turn backwards when the motor is not engaged and That's what I believe is what we are hearing. The clutch disengages but I'm guessing that there are some type of movable teeth that are still slightly touching the catch grooves and the reason for the clicking and slight resistance when it clicks.

                Keep in mind that I am not disagreeing with you. I am most likely misreading what you typed.

                Comment


                • AZguy
                  AZguy commented
                  Editing a comment
                  I've had them apart so maybe not explaining things or understanding your points.



                  This is what I *know* from hand's on:


                  There are two overrunning clutches in the BBSHD

                  One is a roller type and is in between from the primary reduction from the motor to the secondary reduction to the chain ring axle

                  The other is a pawl clutch between the crank and the and the chain ring shaft

                  The roller clutch is silent when disengaged - e.g. motor not turning and the chain ring shaft is turning

                  The pawl clutch clicks when disengaged - e.g. pedals stationary and the chain ring shaft is turning

                  Rolling the bike backwards engages both clutches so both are silent

                  If you turn the crank backwards while everything else is static you'll hear the pawl clutch click

                  The motor spins backward fairly easily

                  When turning the wheel backwards since the reduction is totally in your favor - i.e. if it's 1:21.9 then every turn of the chain ring shaft you only get 1/21.9 turns of the motor so this works to your advantage

                  The only drag the motor creates turning is due to the electromagnetic effect I described which is significant at low speeds and clunks from the magnets passing across the coils inducing current and magnetic fields in the coils. This is "exaggerated" by the fact the motor is turning *much* slower than the chain ring shaft due to the two reduction gear sets

                #9
                I serendipitously just ran into my notes from the teardown:


                There are eight magnetic poles

                There are twelve coils

                The primary reduction is 9:32

                The secondary reduction is 11:68

                Overall reduction is 9 · 11 : 32 · 68 = 99:2176 ≈ 1:21.98

                Comment


                  #10
                  Many thanks for your replies, really grateful for the detailed explanations, which even as new to this game, I can understand and put my mind at rest.
                  Once again many thanks.

                  Comment


                  • AZguy
                    AZguy commented
                    Editing a comment
                    No worries! We're all about learning from each other...

                  #11
                  AZguy do you recommend replacing the grease of the BBSHD or is the stock adequate? You seem to be well informed on this subject.

                  Comment


                  • AZguy
                    AZguy commented
                    Editing a comment
                    I tore my down at ~2000mi. My greatest concern was that I've seen a lot of chinese machinery that comes horribly greased. The nastiest cheap stuff applied inappropriately, etc.

                    I was very pleasantly surprised to see reasonably high quality greases used reasonably liberally. I'm curious and inclined where a teardown was a "might as well, it's very easy to do" kind of thing. Next time I could do the same thing in likely under an hour. I was also pleased with the engineering. It's a far cry from some german machinery but it was well done and near the top of what I see come out of china. Like everything out of china it maddeningly has zero manufacturer's or service documentation. Can some one say exploded view?

                    It's easy enough to do but unless I thought there was a good reason I wouldn't bother to do the whole teardown. I'm knocking on 3500mi on it now and somewhere around 4000mi I'll likely do the stupid simple task of popping the cover off the secondary reduction and freshening the grease there. Any hint of issues doing that I'd do the rest of the teardown. At this point after the prior practice I could likely do the secondary refresh in 20-30min... Not even a one beer task =]

                  #12
                  AZguy one additional thing about rolling a bbshd backwards
                  your also engaging the primary reduction gear the nylon one which will increase resistance as it engages

                  that's a quick way to test if you've peanut buttered it

                  Comment


                  • AZguy
                    AZguy commented
                    Editing a comment
                    That's what I attempted to explain with "One is a roller type and is in between from the primary reduction from the motor to the secondary reduction to the chain ring axle"

                    The primary reduction is the motor shaft and the nylon gear ahead of the roller clutch that's in the nylon gear. The secondary reduction is the combination of the large steel gear around the pawl clutch around the crank and the geared shaft driven by the roller clutch in the nylon gear....

                    Whew what a mouth full =]
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