theoretical top speed?

Discussion in 'Motorized Bicycle General Discussion' started by jazz2561, May 14, 2015.

  1. jazz2561

    jazz2561 New Member

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    I have a bike with a jackshaft. The set up is like this: 2.5" pulley on the engine, 4" input pulley on the jackshaft, 12 tooth clutch output, and a 60 tooth sprocket on the wheel. Does anyone know what the theoretical top speed would be?
     
  2. dtv5403

    dtv5403 New Member

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    Need to know wheel size and engine rpm first.
     
    #2 dtv5403, May 14, 2015
    Last edited: May 14, 2015
  3. Legwon

    Legwon New Member

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    There WAS a jackshaft calculator around.
    Try Google if someone here doesn't know.
     
  4. Gbrebes

    Gbrebes Member

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    Poking around on the Internet, I got an 8:1 ratio for your gearing set up. With 26 inch tires and a top rpm of 6,000, I got roughly 58mph. But I'm not too smart with these things, so don't quote me on that.

    Gilbert
     
  5. Gbrebes

    Gbrebes Member

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    With the same set up at 7,000 rpm, I get 67mph.
     
  6. Davezilla

    Davezilla New Member

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    2.5/4= 1.6:1
    12/60= 5
    1.6x5= 8

    Ok the math is spot on... but 8:1 gearing is going to require a lot of torque to be able to accelerate to reach those speeds, not to mention wind resistance which could prevent it from reaching the speed you're looking for...
    What are you using for an engine? 4 stroke or 2 stroke? and is it set up as a single speed all the way or does said engine have an internal transmission like a small motorcycle?

    My guess is you'll need at least 7 or 8 horsepower to reach these speeds, not to mention it will need a lot of low end torque to be able to accelerate well enough for those speeds to be achievable within a reasonable distance...

    It is a good ratio for a small 7 to 12hp diesel engine tho if you got the frame space and the frame is strong enough... if you think about it, small diesel engines produce a LOT of torque for their horsepower and are about the same size as a comparable gas engine, but heavier. Typically a 7hp diesel will feel about the same as a 12 hp gas engine due to the high torque output. the only drawback is that they only like to rev to about 3500 to 4000 rpm but can be ungoverned to get maybe another 1000 rpm out of them, but with 8:1 final gearing it would still be plenty fast...
     
  7. jazz2561

    jazz2561 New Member

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    thanks for the replies. I am using a 5 hp briggs (more like 6 with the carb and header upgrade.) I am running 26" wheels.
     
  8. Gbrebes

    Gbrebes Member

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    Hey Jazz,

    Since you asked for theoretical, I did not get into nuts and bolts. But if we're talking real world, I would recommend a different gear ratio. An 8:1 will give you a faster possible top speed, but it's going to be difficult and slow from a standing start. Most people I talk to recommend a ratio between 10:1 and 12:1.

    I am currently running a Briggs 5hp that is modded to 8hp. I have recently gone with a "bully" type clutch, also known as friction clutch. I started with a 10:1 ratio and I got it up to the 50mph range, but it really took off slow from a stop light, with lots of pedaling and waiting to get in the 20-30nph range. I bought a newer clutch recently and I am switching to a 11:1 ratio.

    My tank has sprung a leak, so I will be repairing it this weekend before I can test the new ratio, but I will let you know how it goes.

    I know you are interested in a modern board track racer. It is absolutely true that they ran super low ratios, but if you see the vintage footage, they could not actually start from a standing stop. They got towed by another motorcycle, popped the clutch, started the engine, and then creeped up to operating speed. The other crazy thing about those guys was no brakes. Going into a turn at 100mph speeds, they would hit the kill switch to turn off the motor and use engine braking. Coming out of the turn, they would let off the kill switch and give it the gas. Crazy times, guys died on those tracks:

    Gilbert
     
  9. jazz2561

    jazz2561 New Member

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    So, with the 8:1 ratio, how fast are we talking? Besides, I have a pedal crank, and adding an electric motor drive system too...
     
    #9 jazz2561, May 15, 2015
    Last edited: May 15, 2015
  10. Gbrebes

    Gbrebes Member

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    It depends on top rpm. I tried to answer that in posts #4 & #5. If you can get the motor up to 6,000rpm, then you would get 58mph, but real world I would expect 5 or more mph less with wind resistance. If you could get the Briggs to spin at 7,000, you could achieve 67mph, but I would minus out at least 5-10mph for wind resistance. Also to get your Briggs up to 7,000rpm would likely take a performance cam, billet rod, and stiffer valve springs.

    Good luck,

    Gilbert
     
  11. dtv5403

    dtv5403 New Member

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    Not to mention a billet flywheel
     
  12. jazz2561

    jazz2561 New Member

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    from the looks of it, I can spin about 4,000 rpm. I am not sure if the engine still has the governor or not.
     
  13. Gbrebes

    Gbrebes Member

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    With a top rpm of 4,000, I get a top speed of 39mph. If the governor is still operational, you need to disable it.
     
  14. jazz2561

    jazz2561 New Member

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    ok, I can get that done.
     

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