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Karate Monkey (2020) + BBSHD Chainring Size

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    Karate Monkey (2020) + BBSHD Chainring Size

    Hey everyone,

    I previously completed a conversion on my old police bike with a 68mm BBSHD kit, paired with a Luna Eclipse 42T chainring. The chainline was properly aligned. However, due to the bike's age, I was kind of at the limit of what I could upgrade on the bike, and the rim brakes just weren't cutting it.

    So I ended up purchasing a completed Karate Monkey to replace it, after researching that the 68mm size "should fit", going off of some posts I've seen here and there. Well, now I've got the bike, and am noticing that when attempting to install the BBSHD + Eclipse chainring, I don't have enough space between the motor and chainstay.

    Realizing this error, I pumped the brakes, and began doing some MORE research, and discovered that I missed the max chain size for the Karate Monkey is:
    32t Boost 1x, 24/36t Boost 2x, 28t 1x, 24/34t 2x

    I was hoping to get some advice on how to proceed - it appears to me that my solutions are either pick up the mighty mini (but I am concerned the motor will still have the chainring offset too much), or to swap the bike out for another frame. I specifically picked the Karate Monkey for supposed compatibility with my existing 68mm BBSHD kit and due to the fact the inside of the frame is coated, and I plan on this being a hunting/mountain/do-everything bike.

    This is the eclipse hitting the chainstay

    This shows the crank clearance and a better perspective of the motor

    And this is the amount of thread I have on the other side of the BBSHD

    tl;dr would the mighty-mini work for me on a 2020 Karate Monkey frame? Would there be any cassette considerations if so?

    Thank you so much for your time!

    Updating for future people who come by - after asking Reddit, it seems that you need roughly 66mm from the center of the bottom bracket to the chainstay to ensure a flush fit of the BBSHD. Unfortunately with the Karate Monkey 2020, even if I was able to cram my eclipse onto there I would still have insane chain alignment problems. I think this is going to be an exchange, and chalked up to a failure on my part to do adequate research!


      Hi Horvaticus, Just wondering if you solved you issues?o know. I wwas just about to buy a Katate Monkey as such a perfect do it all 29er and a 68-73mm BBSHD motor and battery conversion kit. The Karate Monkey is sweet frame. Steel, rack mounts, good HA etc BUT i understand your issues! I was thinking perhaps a 100mmm BBSHD wuld work or did you solved with spacers and mayeb a the 42T and bringing the larest sprockets away from the midline? Ir did you just chnage bikes. Finding a a good modern 29er with rack mounts is really hard! I started to think a lot about why cotic/stanton/surly etc and other modern 29ers have larger chain stays and lote of other things ans found a mounting parameters diagram AND the templetes on the Bafang USA website. Issue is i dont have the Frame/complete to check it all.

      Last edited by spinningmagnets; 05-29-2021, 05:51 PM.


        The wider unit doesn't help with the chainline which is usually the issue. Even if you don't need any spacers the gearbox takes up a lot of space which is why they designed them with a 19mm offset ring that gets you back to a stock chain line for a 3x bike which was still pretty common when these were designed. Problems come when the frame was designed for 2x and 1x which also tend to be a lot smaller than the 3x's were. The 1x designs kick out sooner and often further for fatter tires so there is no room for the gear case. You may also not have room for a 40t or bigger ring which is what it takes to get that 19mm offset so you are stuck with minimal offset rings which are only 9mm offset. So you start talking some spacers plus the gear case and loosing 10mm off offset chain ring options you could be 10 or more MM further out than the stock chain line was which could loose you 3 or 4 gears.

        Its really hard to tell from the photos how fast that flat part of the chainstay kicks out. From what I can see it looks like there may be decent room but none of the photos I have fond are from a high enough angle to really see the curve. I did see a couple photos of a BBSHD on a KM but could not find any details about the build and the photos were from the wrong side to see what chain ring was used.


          @Eldo Thnaks for you response. I totally understand 1*'s kicking out further for larger tyres., much more so than 3* or even 2* and every frame design is different.. geneneral a lot of modern HT's do seem to flare the chainsays out sooner for clearance these days. I have looked at so soooo many frames and conversion online and you're right; often photos are not from directly above (plan view) and therefore do not show chainline so well.

          What is very interesting is, Matt from EMPowered cycles replied to my question on this matter just today and said "Converted many Surly KM and the 42T Lekkie is the best ring choice by far so i'm I'm absolutely bamoozed. I've aksed him what year/model he's done.


            Another thing to note is the BBS02 has a slightly smaller and thinner gear case which can help on some frames but I think has less options when it comes to chainrings.

            If you are willing to do some grinding on the gear case of your new motor you can often gain some space. Usually where the gear case its is on an angle of the gear case and a curve on the frame so if you grind off 1mm that could mean 2mm less of spacers needed on the BB. I did that on one of by builds that has a flat bar chainstay like what the KM seems to have and that helped get things tucked in nice. It was easy for me to do mentally because it was my 3rd build and I picked up the unit used and it was well used at that so my grinding wasn't the first scratches. I can see how it may be mentally hard to do on a new unit. On the 02 its basically just a cover so you could likely grind the whole thing away and other than keeping dirt out not have any issues. On the HD's the cover supports a bearing but it does that over the whole circle so I don't think a thin spot on one side would really hurt.

            If you really like the KM and can actually get one I would say go for it. Photos and Surly specs look OK and the one set of photos I found with a BBSHD would seem to indicate that there is a way to make it work. That note about the 42t sounds encouraging too especially because it was not followed up with info like having to re stack the cassette or make something custom.


              Here is what I did to fit a BBS02B on a bike with similar interference issues,
              This is what I started with. There is 3mm of spacers between the bb and motor housing. And the chain line is horrible.
              Click image for larger version

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              This is the area I filed away on the back of the reduction gear housing.
              Click image for larger version

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              This is the final instal. There are no spacers between the bb and motor housing, the chainline is 3mm better and the modification wasn't detrimental to the structural integrity of the case.
              Click image for larger version

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              in the above picture you can see the approximately .25mm clearance between the housing and chainstay.
              Click image for larger version

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              Of course this is a do it at your own risk modification. I have had zero problems from it. The big thing is to not go all the way through, that would allow dirt and water to get in easier. I planned for that eventuality and had a plan in place to seal any breakthrough.
              I determined for myself that the part of the BBS02B case that I was thinning out wasn't structural and wouldn't cause catastrophic failure.
              And I determined that gaining 3mm better chainline was worth the effort and tiny risk.
              Your milage may vary....


                Dshue Cheers for the post. I can see what you've done and why you state its possibly not recomended BUT its possibly fine. Your pic is good and clear. The BBS02 also has 'marginally thinner' gear housing. The frame design of the KM will be slightly different to yours so it may be ok or not. It would be good to actually see a 2020 KM with the BBSHD fitted including the chianline. I can't actually find any photots of this frame and a fitted BBSHD. Darn those wide chainstays. I would just take this risk but dont have time to buy a bike to see if it will work and if not sell it and then by another bike. COVID has also resulted in the 2nd hand market being inflated. There are other Surly models which can hadle larger front chainwheels which i can see would themselves be fine but they dont have that nice sweepspot geo of the KM. The search contunues...On another note the 'Sun race' is full of BBSHD's - incredible little motor and a few Surly's too - the Troll or the Bridge club, but they don't take suspension forks. Surly designed them not too and rather for bigger tyres - makes a lot of sense for touring; especially epic tours BUT not good for any technical singletrack. Such a wierd problem this. I raced downhill, cross country then road bikes and Ironman, plus some super long distance events and even have a custom geo Ti road bike i got at cost to race ultras (which i can't
                ride now and is living in garage on the 1% chance i ever recover). Never had to think about chainstay width being an issue until now, especially not for an electric motor. New problems/new oportunites ommmmmm


                  Has anyone successfully fitted a BBSHD on a 2017+ Karate Monkey??? I found the one linked below but I believe that was an older model KM and I think the frame changed around 2017. Thinking about buying this setup but don't want to buy the bike and order the motor to find out it's just not going to work. Anyone? Let's revive this thread! Anyone done this?

                  Last edited by Slugger; 03-31-2022, 08:45 PM.