Figuring out gear ratios- 1983 z50r with ct70 10" wheels

Discussion in 'General' started by RadRacer203, Sep 10, 2020.

  1. RadRacer203

    RadRacer203 Member

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    So I've been continually upgrading my '83 z50r that started as a bin of parts a few years ago. This time I've got a set of 10" ct70 wheels, I believe from 1979 with a 35 tooth sprocket. I'm just having a little trouble wrapping my head around the ratios and was wondering if anyone had any suggestions. I've got the stock front sprocket (12 tooth) so I have some room to make that smaller and I have room to put a larger one on the rear too. Ideally I want it to be about the stock ratio so a top speed of 25 or 30 with enough grunt to haul me around.
     
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  3. racerx

    racerx Administrator
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    Unless the engine has been tuned to make a lot more power, with an emphasis on torque...i.e. increased displacement...the gearing needs to be the same, in revs per mile, regardless of tire/wheel size. Measure the circumference of both tires, divide the larger number by the smaller number, to get the % change. Next, multiply the existing sprocket combo by that number, which will be 1.xx, to get the new final drive ratio. From there, you either multiply the CS sprocket tooth count, or divide the wheel sprocket tooth count by the target ratio.
     
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  4. RadRacer203

    RadRacer203 Member

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    Awesome, thanks! It's a stock engine so I'll definitely be going for a stock ratio. Hopefully in the future I can get my hands on a cheap ct70 engine and throw that in there but that's a long ways off
     
  5. racerx

    racerx Administrator
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    You only need a CT70 top end: cylinder & piston, head, carb and an angled intake to clear the tank. For that matter, you could opt for a 52mm (88c) bore-up kit for the same money. Your Z50 motor is, essentially, the same lower end used with every 6v/3-speed motor from 49cc up 89cc (Cub 90, which was never sold in the US).
     
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  6. RadRacer203

    RadRacer203 Member

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    I know I can just swap the top end but I really want that 4th gear too if it'll fit in my frame
     
  7. racerx

    racerx Administrator
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    Cases are the same. Thus, a 4-speed tranny will fit the same as it would in a CT70 engine. For that matter, you can pair either transmission with either clutch type. OEM 4-speed transmssions are a bit hard to find. High-end Japanese (Takegawa, Kitaco) are on the expensive side. AHP 4-speed is inexpensive, readily available. Quality isn't Japanese aftermarket, or OEM. It's way better than the super-cheap, Chinese stuff though and has been on the market close to 15 years now. IOW, it should be fine for your project, if you're looking to save some bucks.
     
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  8. RadRacer203

    RadRacer203 Member

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    Huh, I had assumed all ct70's were 4 speeds like my XL70. I never knew that most were 3 speeds. I guess I'll just be looking for an sl70 or xl70 motor then
     
  9. racerx

    racerx Administrator
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    May as well improve your chances just that much more and add CL70 to that list. Those are 4-speed/manual clutch too.
     
  10. RadRacer203

    RadRacer203 Member

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    I'll put that engine on the list too! I found someone selling a couple sl70 motors today but they're about 200 miles away. I'm going to see if my buddy will pick one up for me
     
  11. RadRacer203

    RadRacer203 Member

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    Out of curiosity, when I eventually get one of these 4 speed motors, can I swap the z50 semiauto clutch onto it? I'm not sure I would, but maybe because full manual might be a giveaway if I want to try to sneak in registration as a moped
     
  12. kirrbby

    kirrbby Well-Known Member

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    SL70s are 1down 3up shift pattern. They say a semi auto clutch makes it hard to hit neutral with that shift pattern.
    A CT70H is 4down. Some CL70s are 4down. And some ATC70s are 4up, or 4down 4 speeds, with a semi auto clutch...but they are pullstart only.

    XL70s are all 1down 3up.
     
  13. racerx

    racerx Administrator
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    Absolutely, transmission type is independent of everything else. As for the issue finding neutral with a 1-N-2 shift pattern, consider that Honda used that with some semi-automatics. ST70 Dax models that were sold in the EU, came with 1-N-2-3 shift pattern. They also came fitted with a heel/toe shifter.
     
  14. RadRacer203

    RadRacer203 Member

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    Awesome, thanks for the input. When I eventually get one of these motors I think that's what I'm going to do. I don't know how I forgot about the shift pattern considering I own an XL70 but I don't think it should be too much of an issue for what I'm using this bike for.
     
  15. racerx

    racerx Administrator
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    I don't really buy the shift pattern being an inherent problem with a semi-auto clutch. Can you imagine Honda selling a problematic setup, for decades? A low-dollar transmission may be a large part of the problem, precision machining is critical when it comes to a trans that will reliably land in neutral, when desired. A lot of the Chinese engines went to an all-down shift pattern, the cheapest solution. Mostly, it comes down to proper idle speed & preload adjustment. A number of riders dislike a heel/toe shifter; I'm not among them. A toe-only lever provides less control, imo.
     

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