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How to rework rear sprockets for mid drive systems...

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  • How to rework rear sprockets for mid drive systems...

    Here’s a photo show and tell about the wonderful rear cassette modification I’ve learned from other mid drive users.

    Standard 9 speed rear derailleur and the resulting poor chain line connected to a single front ring:
    Click image for larger version  Name:	IMG_20170707_134204.jpg Views:	1 Size:	364.3 KB ID:	39258

    The straightest and best chain line is somewhere in the middle of the cassette gear cluster. By the time you reach granny or greyhound positions the chain is suffering a fairly extreme angle.

    Do we need all those gears/cogs in between the High-Med-Low ranges? Nope, those are needed with human legs looking for every ounce of efficiency they can scrape together. When you tug 1500W of battery power on the chain the need for “narrow” bicycle ranges becomes unnecessary.

    And who wants to spend all their time shifting? These modern and robust BBSHD drive kits can jump 10 teeth at time without any problems or stress. What we really need for them is 3-5 wide gear ranges.

    So in this example I’m going to rework 3 gears to 14T, 24T, 32T or High, Medium, Low gear ratios.

    We start with a basic 8 speed cassette gear cluster. I like cheap steel Sunrace 8 speed gear sets as they most often don’t use rivets to hold the gears in shape:
    Click image for larger version  Name:	s-l500.jpg Views:	1 Size:	74.2 KB ID:	39171

    Even if you do have rivets, they can easily be ground away so that disassembled looks something like this:
    Click image for larger version  Name:	IMG_5099.jpg Views:	1 Size:	216.0 KB ID:	39172

    Don’t worry about keeping them in order because it seems they’ll only assemble correct orientation on the cassette spider:
    Click image for larger version  Name:	IMG_5100.jpg Views:	1 Size:	194.0 KB ID:	39173

  • #2
    Basically make up whatever combo you wish. I’m doing 14T, 24T, 32T on this spider

    Here’s the resulting 3 speed range
    Click image for larger version  Name:	IMG_5102.jpeg Views:	1 Size:	140.9 KB ID:	39175

    The inner-outer gears serve as spacers. I actually changed my mind and moved the 3 speeds out towards the axle one more cog space.

    These tools are very helpful for doing this sort of work:
    Click image for larger version  Name:	IMG_5108.JPG Views:	1 Size:	196.0 KB ID:	39176

    Click image for larger version  Name:	IMG_5109.JPG Views:	1 Size:	230.0 KB ID:	39177

    Click image for larger version  Name:	IMG_5115.JPG Views:	1 Size:	198.4 KB ID:	39178

    Next thing to do is to adjust the “limits” so that we’re only shifting the 3 desired gears:
    Click image for larger version  Name:	IMG_5118.JPG Views:	1 Size:	191.4 KB ID:	39179

    The Low limit needed a longer screw to accommodate the new narrower range.


    • #3
      Resulting 3 speed derailleur chain line looks good now:
      Click image for larger version

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      • #4
        I did this with my cassette. Huge difference, well worth the fairly minimal effort. I kept 5 gears though. I have a tiny amount of play in my freehub, hoping it was like that before I did the mod.


        • #5
          I have a hub motor with the very common Imperial threaded male freewheel mount. I use a common freewheel, which happens to be a 5-speed Sunrace 14-28T freewheel. I don't have room for a 7-speed freewheel, which I what I wanted because the smallest cog has only 12T.

          I see that you did this with a cassette. Would I have any luck trying to do this with a freewheel?


          • commuter ebikes
            commuter ebikes commented
            Editing a comment
            Video on taking apart a freewheel (as opposed to a cassette): If the cogs are threaded onto the housing (3:43 in the video), then it looks like this would work.
            Last edited by commuter ebikes; 07-07-2017, 10:26 AM.

          • ykick
            ykick commented
            Editing a comment
            Hmm? I watched the video but never got anything about how to re-arrange cogs on the freewheel gear cluster? Great info for cleaning and reworking the ratchet pawls and stuff but I'm not seeing a way to move the gears around?

          • commuter ebikes
            commuter ebikes commented
            Editing a comment
            ykick, you are right. When I posted that video, I assumed that one had to execute the work in the video in order to allow the freewheel cogs to be unscrewed. I assumed that the inner shaft had a lip which prevented the cogs from becoming unscrewed.

            Upon looking at my freewheel, however, I see that there is a lock nut securing the cogs in place. In order to rearrange the cogs on a freewheel, which are threaded, one uses a pin spanner to remove the aforementioned lock nut at which point one is free to arrange the cogs in any order desired.

            I still plan to take an 11-13T cog off of a 7-speed freewheel and put it on to my 5-speed freewheel, but I have yet to see if the threads match on the two different cogs that I plan to switch out.

            Thirty lashes with a wet noodle for posting an irrelevant video!

        • #6
          Thanks all for the input and we'll hopefully get some more savvy computer folks to better clean this mess up? But yeah, it's proven a good way to solve some of the annoyances created by the marriage of so much power thorough single chain ring applied to rear gear clusters.

          I've also done it with some 9 speed derailleurs but I prefer older 8 speed for this: cheap, solid and easy to source.


          • paxtana
            paxtana commented
            Editing a comment
            Thanks for this! Did what I could to clean it up, added a top pic for a not-good chainline since that pic was broken on the post and I couldn't recover it.

        • #7
          I'm getting ready to build a mid motored mountain bike with a 9 speed cassette. I am curious as to what affect the increase of gear diameter from one gear to the next has on the shifting performance/efficiency. Thanks


          • ykick
            ykick commented
            Editing a comment
            I find little difference shifting 2T increments or 10T increments.

            Performance/efficiency? How efficient is it to change up/down 2T 4X as opposed to 10T 1X?

            The distinction here is that bicycle gear cluster increments designed "narrow" for leg powered systems. When a motor tugs 750-1500W on a chain, narrow range gear increment's no longer a benefit. And IMO, it adds a lot of wear tear to any derailleur system while also wasting a bunch of time fiddling with gear selection more than is really needed.

        • #8
          That is a good question. I have come to the conclusion that it depends more on the quality of your rear derailleur than on the spacing of the sprocket sizes. All of my bikes have been around $500 and the shifting was not the best with a wide variety of cassettes and freewheels. I just bought a Paratrooper Pro, $1000 and the shifting is flawless.


          • Giant Rider
            Giant Rider commented
            Editing a comment
            I have a good quality Sram derailleur that shifts rock solid when tuned properly, even under moderate leg power so with a little luck it should be fine. I am not going racing here all it needs to do is get a fat old man over the hill and through the woods. LOL

        • #9
          Appreciate the guide... My Sunrace CSM98 9-speed gets here today to begin modifications. Cogs to select from are: 11, 13, 15, 18, 21, 24, 28, 32, 36.

          Drive cogs will be 15, 21, 28, 36 with spacers on the outsides.

          My initial thought was to try and center the chain line on the middle 21 and 28. Does this make sense? also are there advantages to specific spacer cog order/placement other than centering the chain line on the driven cogs?


          • ykick
            ykick commented
            Editing a comment
            I honestly don’t know if you’ll ever need a 32T? I’ve yet to need anything bigger/lower than 28T on 26” wheels running a 42T chain ring. But you are in SF so 32T might be a good idea?

            I do find myself sometimes wishing I had something a little smaller/higher where I run 24T. 21T might be a good idea there? A couple narrow increments in the middle ranges could be useful?

            As far as what order to replace the cogs used as spacers? Obviously, the large ones won’t work too well outboard. But I just play around with everything until it looks good. I get funny looks from bicycle mechanics who notice this odd arrangement….

        • #10
          I do a lot of throttle only and have 29er/700c wheels so I find the granny gear useful for takeoffs after I turn the first revolution with the crank.