Sprockets what is the best ratio for a 2 sprocket bike

Discussion in 'Transmission / Drivetrain' started by bcoltsmith1, Aug 26, 2009.

  1. bcoltsmith1

    bcoltsmith1 New Member

    what is the best ratio for a bike that has a sprocket on the back tire and a sprocket on the drive shaft? thanks for your help
     

  2. sparky

    sparky Active Member

    Depends on things like RPM and tire diameter.

    I'm not 100% positive, but I think I have 10 teeth on the drive shaft and 44 teeth on my 20" wheel. I think. Very nice acceleration and top speed for me.
     
  3. bcoltsmith1

    bcoltsmith1 New Member

    thanks my bike has 20" tires, and the rpm of the old partner chainsaw motor 50cc is i think 9000-10000, i heard 6:1 ratio is a good ratio to go by is this true because i want to be able to climb decent sized hills but still have a good speed
     
  4. TREEWK

    TREEWK Member

    As A General Rule Most Of These Small Engines Need Around 18:1 Ratio. If You Used The Golden Eagle Setup You Could Go Without Reduction. The China Kits Have A 4.2:1 Gearing Built Into Them. Hope This Helps. Ron
     
  5. sparky

    sparky Active Member

    I'm not 100% positive on the RPM, but my 33cc engine prolly gets 5500-6000 rpm. It's got a gearbox with a 5:1 ratio. Then I think the driveshaft has 10 teeth, while the wheel has a 44t sprocket on a 20" wheel... for an additional 4.4:1 reduction, making it an overall 22:1 reduction I think.

    I'm 160 lbs, and I accelerate very quickly. Top speed is always 25mph, tho when it warms up... 28 becomes easy, and 30 becomes possible, tho it largely depends on the wind. If I were heavier, I'd prolly keep the gears the same but just pedal assist to accelerate.
     
  6. bcoltsmith1

    bcoltsmith1 New Member

    what do you mean by the gearbox ratio? and to TREEWK im trying to do this build without a bike kit
     
  7. srdavo

    srdavo Active Member

    I agree with the 18:1 reduction.
     
  8. srdavo

    srdavo Active Member

  9. sparky

    sparky Active Member

    Ya gotta have a gearbox... something the clutch grips onto to spin the driveshaft sprocket. That gearbox will have its own reduction.

    So if you got the gearbox I have (5:1) and then wanted 18:1 overall, you'd divide 18/5 = 3.6:1 reduction between driveshaft sprocket and wheel sprocket.... so 10t to 36t would be more appropriate.

    Somewhere in the equation, you've got to account for the tire diameter... but I can't really do math problems right now.
     
  10. TREEWK

    TREEWK Member

    Bcolt. Yes, I Understand That. One Thought, To Keep Cost As Low As Possible. Weld Another Rim Without A Tire To Your Rim (maybe Pop Rivet) And Use As The Driven Belt Pulley. Then Weld A Steel Pulley To The Saw Clutch Drum. Have The Center Of The Motor Pulley Enlarged For Installing And Removal. You Can Not Do This With All Saws. Many Different Clutch Setups. Most Good Saw Motors Will Run A Long Time. A Good Saw Is Worth More Than The Price Of A China Girl Kit. Been In The Tree Service For Many Yrs. Have About 30 Saws. Tell Me What You Have And Will Look At Mine. Or Post Pic Of Clutch Side Of Yours. Ron
     
  11. srdavo

    srdavo Active Member

    A jackshaft is also an alternative to a gearbox.
     
  12. bcoltsmith1

    bcoltsmith1 New Member

    yes thats a good way, the only thing is i dont want to risk breaking my spokes. I have an old partner 500 50cc chainsaw engine its yellow:) any help with finding out more about this motor would be greatly appreciated
     
  13. bcoltsmith1

    bcoltsmith1 New Member

    treewk
    wouldn't i go through alot of belts doing it like that?
     
  14. TREEWK

    TREEWK Member

    bcoltsmith, I SENT YOU A PM. RON
     
  15. bcoltsmith1

    bcoltsmith1 New Member

    i received it
     
  16. TREEWK

    TREEWK Member

    The Secondary Sprockets That You Pm To Me Are 2:1 Ratio (30 To 60). Your Primarey (10 To 60) Is 6:1 As You Stated. You Multiply One Ratio By The Other . 6:1 X 2:1 = 12:1 Final Ratio. I Think Secondary (10 To 30 ) 3:1 X 6:1 Would Give You The Normal 18:1 Ratio. Use Whatever Tooth Counts You Can Find, Try To Be Close To 18:1.
     
  17. bcoltsmith1

    bcoltsmith1 New Member

    lol i see what you mean, i found a gear ratio calculator somewhere on the forum, so i have been playing with the numbers that would be easiest for me. i want the most speed i can get from the motor. and from 18:1 ratio at 9000 (give or take a 1000) and 20" tires i will get 29.7 miles per hour. so i was thinking of using davids from davidsfarm ratio 14:1 (for my weight 220lbs) and the closest i can get with his ratio is 13.64:1 and that sprockets would be easier for me to get. im still uncertain cuz david has ALOT of experience with engines, but so does alot of people on this forum.
    i know im being a little difficult but i want to do it right the first time
     
  18. TREEWK

    TREEWK Member

    Yeah, I Was Not Taking Into Account Your 20 Wheels. So It Looks Like You Are Real Close. Sorry For Steering You A Bit Wrong. I Am So Used To 26" Wheels. Good Luck With The Build. Keep Us Posted . Ron
     
  19. bcoltsmith1

    bcoltsmith1 New Member

    oh no thats fine, i should have mentioned the tire size earlier. and would the center of a bicycle work as a jackshaft?
     
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