Gear ratios for HF bike -- please help!

Discussion in 'Motorized Bicycle General Discussion' started by cschade, Aug 11, 2014.

  1. cschade

    cschade Member

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    I'm bulking a motorized bike with a 3hp, 79 cc Harbor Freight engine that's on sale for 79.99. Max rpm is 3600 without making internal mods. I'm using a 12 tooth clutch off the engine to a jackshaft in. The jackshaft out is the standard bike gears (the large three sprockets in the front) running to the usual rear freewheel gears. I don't know the exact tooth counts but the bike is a standard Huffy Blades. Gears are average sized. My question is -- what size should my jackshaft in sprocket be? I want to be able to go ~40 mph or so, but still have enough power to climb hills. What size sprocket should I use (tooth count) and how fast can I expect to go on it. I'm a 130 lb rider, and the hills we have are quite steep -- typical New England countryside if you know what I mean.

    Thank you!

    brnot
     
    #1 cschade, Aug 11, 2014
    Last edited: Aug 12, 2014
  2. cschade

    cschade Member

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    Re: Gear ratios for HF bike -- pleased help!

    Bump.........
     
  3. KCvale

    KCvale Well-Known Member

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    Re: Gear ratios for Harbor Freight bike -- please help!

    That won't work, you need way more gear reduction than that and how did you plan to keep you feet from getting beat to dead by the pedals when the motor turned the pedals?

    Check out SickBikeParts 4-stroke jackshaft kit.
    http://www.sickbikeparts.com/catalog/product_info.php?cPath=21&products_id=110

    And that is with a gear reduction transfer case, you are planning to run direct off the engine so you need even more reduction.
     
  4. Dan

    Dan Staff
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    Re: Gear ratios for HF bike -- pleased help!

    Dunno if you have seen this but is a great program; http://jimsitton.net/ratiocalc/ Really comes in handy.

    I am told this is ill-advised but on my HF, I only removed the externals for the governor. I've never had a problem and it maxes out at 5,000 RPM.

    But again, lots of folks think it is not a good idea to leave the gov in. Just wanted to mention the 5K RPM.
     
  5. cschade

    cschade Member

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    Re: Gear ratios for Harbor Freight bike -- please help!

    Well, pedals are not required where I live. HACKSAW!! Thank you for your suggestion, though.
     
    #5 cschade, Aug 12, 2014
    Last edited: Aug 12, 2014
  6. cschade

    cschade Member

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    Re: Gear ratios for HF bike -- pleased help!

    Thanks for the suggestions guys. I downloaded the program you selected, and this is what I came up with. Does this sound reasonable, as far as gearing?
     

    Attached Files:

  7. cschade

    cschade Member

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    Re: Gear ratios for HF bike -- pleased help!

    How come my thread was tagged as "flimsy illegal motorcycle"?? Could someone please explain?
     
  8. The_Aleman

    The_Aleman New Member

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    Re: Gear ratios for HF bike -- pleased help!

    Motorized bicycle with no pedals? Check.

    That's basically what you're building...
     
  9. Dan

    Dan Staff
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    Re: Gear ratios for HF bike -- pleased help!

    Pedals are not required where I live, either.

    " Connecticut law distinguishes the difference between a motorcycle and a motor-
    driven cycle, commonly known as a moped. A motor-driven cycle is defined as any
    motorcycle, motor scooter, or bicycle with an attached motor that has a seat height of
    at least 26 inches and a motor that produces no more than five brake horsepower. You
    must have a valid driver’s license to operate a motor-driven cycle, but do not need a
    motorcycle endorsement."
     
  10. runnermike

    runnermike New Member

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    Re: Gear ratios for HF bike -- pleased help!

    Ive read the first post a few times and still am a bit lost. So, from the clutch you are going to run to a jackshaft, then straight to the crank sprocket, then to the rear sprockets?

    I am not as well educated as some of the members here, but to me, in my opinion, doesn't really sound safe. Maybe I am missing something and not fully understanding...
     
  11. curtisfox

    curtisfox Well-Known Member

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    Re: Gear ratios for HF bike -- pleased help!

    You need to revers the second set of sprockets and even go smaller then 28, you should end up with about a 12:1 gear ratio to start with otherwise the engine will not have enough power to pull it..........................Curt
     
  12. Dan

    Dan Staff
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    Re: Gear ratios for HF bike -- pleased help!

    I agree with Curtis, Cschade. Is a whole lot to take in all at once, I know. But I think going with the ratio you are thinking, you would have to be going around 20 MPH before engaging the engine.

    The HFs have lots of tourqe. Which is a good thing but any thing much less then 12:1, and you just won't have any low end.

    Just a suggestion or what I do with 4 strokes, aim for 35 MPH, (plenty fast on a street bicycle) and 12:1. Ya gotta remember there are lots of opinions and thoughts on this. I know it makes it all that much more confusing. But that is part of the fun and learning process. If I may be so bold as to offer another suggestion, (and do take em all with a grain of salt. Especially mine) But for a first build, ere on the side of caution and go for safe and functional first. The really great part of this crazy is you build it and can alter/experiment at will. As you go on, push the limits.

    Really enjoy your enthusiasm and imagination. Stay excited and build some thing cool! What ever help you need, there are some great folks here who enjoy lending a hand or thoughts.

    I think every one here will back me up on this but the first time she fires up and your tooling down the street, is a great high!

    r.ly.
     
  13. cschade

    cschade Member

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    Re: Gear ratios for HF bike -- pleased help!

    Thank you for the encouragement Dan! This is actually the second motorbike I've built. The first was a WeedEater friction drive. The fastest I ever got on it was 34.4 MPH on a slight downhill. It lasted for about 2 months before it gave out. The first time I rode it, was one of the happiest days in my life.

    What size sprockets would you recommend for clutch, jackshaft in, jackshaft out, and rear wheel? I am open to ANY suggestions you have.

    .cs.
     
  14. Dan

    Dan Staff
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    Wow, your a real DIYer. That really is cool.

    Having 0 experience with using the bike gears, I dug around a bit. This link gives exact ratios; http://motorbicycling.com/showthread.php?t=32814 Which is 16:1 and only 24 MPH but a jumping off point, any way. (His rear sprocket is a 30T. dunno what yours is)

    I searched with the words shift kit to make part shopping and basic concept similar. This is what came up; http://www.google.com/cse?cx=partne...#gsc.tab=0&gsc.q=HF with shift kit&gsc.page=1

    But some one will join in and help, I'm sure.
     
  15. KCvale

    KCvale Well-Known Member

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    Re: Gear ratios for HF bike -- pleased help!

    Put a $300 Q-matic on you HF 79cc and run it direct drive with a 44T like this one I built, it moves along great.

    [​IMG]
     
  16. Dan

    Dan Staff
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    Just playing with the calc. 12T/30T, 10T/40T to 26T/30T works out to 11.54:1 and 33 MPH at 5,000 RPM. (RPM really depends on the weather but that is what mine does when happy and weather permitting. most days just a tad less)

    Am just playing around. I forgot you said you lived in a hilly area. HFs are not fun to pedal. Really wide.
     
  17. cschade

    cschade Member

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    Thanks for those ideas Dan. I'd really not rather have a separate jackshaft running to the pedal cranks, in other words, I want to stick to using the pedal cranks as the jackshaft. I'd also rather not buy a pre made solution (DIYer through and through). So, the bike rear gears need to stay the same. The sprockets I have choice on are the clutch (11 or 12 teeth), jackshaft in, and jackshaft out. Ideas?
     
  18. FFV8

    FFV8 New Member

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    Experience tells me that with no pedals you will need a 14:1 overall ratio with a 79cc engine.

    Pedals are not needed here in NV either. Without pedals, it must launch on the clutch. I have seen various ratios tried within our group, and one guy has to flintstone launch the bike at every stoplight - I think he is at 10:1

    Using a Hilliard Extreme Duty clutch with the 3000 rpm springs installed helps a lot. You can at least get the engine up on it's torque peak at launch.

    Here are some pics of the bikes with various jackshaft arrangements:
    http://motorbicycling.com/showthread.php?t=54897

    All of those 79's are running a PZ19 style carb of one kind or another, and they will all hit 6200 rpm

    .
     
  19. cschade

    cschade Member

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    So to hit a 14:1 ratio, what size sprockets do i need for clutch, jackshaft in, and jackshaft out, assuming a 28 tooth rear sprocket?
     
  20. FFV8

    FFV8 New Member

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    Sprocket choices depend on how much fabrication skill you have, and what soer of equipment you have to use.

    You should do most of your reduction i the first stage. You are planning on using the bottom bracket bearings for a jackshaft, and they are not high speed items. I would really recommend replacing them with sealed ball bearings.

    The 28T final sprocket limits you to a few choices, none of them easy bolt up stuff.

    12/65 > 10/28 just makes it. A 65T #35 chain sprocket would have to be fitted to the jackshaft input, and a 10T sprocket on the output.

    12/72 > 12/28 works too, but the 72T #35 sprocket is over 9" OD with a chain wrapped around it.

    .
     

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