Need Help Finding a Sprocket That Fits a 20" Wheel

Discussion in 'Hubs, Gearboxes, Sprockets and Chains' started by pulsar2121, Sep 1, 2016.

  1. pulsar2121

    pulsar2121 Member

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    This post is not about gearing issues, but rather fitment issues. I'm currently working on a BMX build and I'm having trouble finding a sprocket that fits the spoke pattern of the 20" wheels.

    I'm trying to get the standard 9-hole sandwhich mount sprocket to fit and it seems like no matter how I rotate the rubber ring, one of the holes aligns with a spoke intersection.

    Does anyone know what kind of adapter and sprocket set up would fit this wheel? FYI, the hub is fairly large since it has a coaster brake.
     
  2. Kioshk

    Kioshk Active Member

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    In my own experience, I've gotten away with less bolts than that. 6 with a rag-joint, 4 when I jerry-rigged a sprocket; routed to fit over a disc-brake interface. You should be just fine with 8 bolts.
     
  3. mogollonmonster

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    Geeze, here I go again... Grab your (or your buddies) trusty, not rusty, calipers and measure the hub body, in between the spoke flanges. 1.5 is a really common coaster brake hub diameter. Manic Mechanic sells that size and reducer bushings. Pirate Cycles sells a number of sizes, but I cannot speak of their quality. The MM unit is very well made, and fits my hub like a glove.

    This style sprocket adapter allows for you to properly align the drive sprockets, which is pretty critical for a number of reasons.

    Kiosk is right though, one missing bolt out of nine won't hurt much, at least not any more than the crappy ragjoint itself. The rag joint is one of the weakest links in the ChinaGirl kit, losing points only to the Scrapaloy hardware.
     
  4. pulsar2121

    pulsar2121 Member

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    I like the idea of the hub mounted sprocket much better than having an out-of-round spoke mounted sprocket. I bought a digital caliper at Harbor Freight and measured up the hub on the rear wheel.

    It's 1.70 inches in diameter. I did some searching on bikeberry.com and ebay and all I'm able to find are adapters for hubs that are 1.5 inches or smaller. Do you know here I could find an adapter that fits my hub?
     
  5. mogollonmonster

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    Gasbike and bikeberry are NOT your friends. Check with pirate cycles, if not them try www.mmbikeparts.com I'm pretty sure George has them in that size. Higher quality parts and workmanship.
     
  6. MotorBicycleRacing

    MotorBicycleRacing Active Member

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    How many spokes on your wheel?

    9 bolt rag joints are made for 36 spoke wheels.
     
  7. curtisfox

    curtisfox Well-Known Member

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    Yes, and hub adapter won't fit through wheels with more spokes, either i don't think.......Curt
     
  8. MotorBicycleRacing

    MotorBicycleRacing Active Member

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    hub adapters fit on 48 spoke wheels because 3 divided into 48 = 16.

    But I don't know of any made for 1.7" diameter.
     
  9. curtisfox

    curtisfox Well-Known Member

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    Good to know! Have not used any yet. I sorta make eveerything i need...........Curt
     
  10. Wild Bill

    Wild Bill New Member

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    The problem with using a rag joint or hub adapter on a 20" wheel is that you cant gear it as high as you need to get a decent top end speed if you are running a china girl engine. I laced up a disc brake hub and there are smaller sprockets available to bolt to the disc hub. I make up my own and I have run as small as a 20t but as long as you have an engine that turns higher rpms than stock anywhere from a 25t to 28t work pretty good. My Sons race bike hits 46-47mph on a 28t. There are disc mount sprockets on ebay that go down to around 26t. A 30t or bigger on a stock engine will be pretty slow.

    My race bike runs 20" wheels and uses a hub adapter with a 34t sprocket but that engine is a KTM and turns a lot more rpm than a china girl so I can use the adapter with bigger sprocket like 34T and still hit around 47 mph which is what I need tor the track we normally race on.
     

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