whats the best gear ratio for 35cc 4 strokes

Discussion in '4-Stroke Engines' started by ducatiluke, May 29, 2008.

  1. ducatiluke

    ducatiluke New Member


    Engine lets say honda gx35 or subaru robin eho35
    or even the gx31

    Ratio's Ive seen ratio's range from 16:1 to 22:1 on 26 " rims

    terrain say flat slight inclines

    should friction drive run lower ratios compared to chain or belt drive

    trial by error ends up costing a lot of time,money and effort , has anyone done the hard work?

    any response will be very appreciated

  2. loquin

    loquin Active Member

    'ratios' aren't a consideration with friction drives. Only two variable factors come into play - the engine RPM, and the roller diameter. Nothing else.

    With a friction drive, the speed is calculated as follows:
    v = pi * d * RPM * 60 / (12 * 5280)
    • pi = 3.1416 (pi is the ratio of a circle's circumference to its diameter)
    • v = velocity, in MPH
    • d = roller diameter, in inches
    • RPM is maximum engine RPM (revolutions per minute)
    • the 60, 12, and 5280 are conversion constants, used in converting units of minutes and inches to hours and miles.
    The above formula can be simplified by combining constants (and a slight rounding) to:
    v = d * RPM / 336.1
    Now, for a general calculation, usable with both English and metric units,
    v = d * RPM / k
    • v = velocity, in MPH
    • d = roller diameter, in inches
    • RPM is maximum engine RPM (revolutions per minute)
    • k = 336.1 for english (roller diameter in inches, speed in MPH) units
    • k = 5305 for metric (roller diameter in mm, speed in KPH) units

    Note: Wheel diameter doesn't enter into the picture for friction drive. This is because a friction drive just uses the wheel as a transfer roller. To transfer the speed of a point on the surface of the roller to the ground.

    However, with chain or belt drive systems, the wheel diameter DOES come into play, along with the system gear ratios between engine and rear hub. This is because these types of systems apply power at the HUB of the wheel, and not at the outer surface of the tire. The total ratios from the motor to the hub, determine how fast the wheel will turn, but NOT how fast the bike will go. You must convert the rotation speed of the wheel to the amount of distance that a point on the circumference of the wheel will move during a given time to calculate how fast the bike will move.

    The speed is calculated as follows (jumping straight to the simplified formula) :
    v = RPM * D / (R * k)
    • v = velocity
    • RPM = Engine speed, revolutions per minute
    • D = Wheel Diameter, including the tire
    • R = Overall reduction ratio, engine to wheel
    • k is a constant, equal to:
      • 336.1 for English units (Wheel diameter in inches, speed in MPH
      • 5305 for metric units (wheel diameter in mm, speed in KPH
    Last edited: May 30, 2008
  3. loquin

    loquin Active Member

    Now, the other factors that you need to consider are the weight of the rider, the desired top speed, and the acceleration desired. All other things being equal, a heavier rider will take longer to get up to speed than a lighter rider. Decreasing the top speed increases the acceleration. Likewise, increasing the top speed will reduce the acceleration. Acceleration is also directly related to the bike's hill-climbing ability. The greater the acceleration the motor can provide, the greater the slope that the engine can overcome on its own. (However, this is not a linear relationship; it is a trigonometric relationship, proportional to the inverse-tangent of the slope percentage)

    A roller diameter of 1.25 inches will give you a speed of about 26 MPH at 7000 RPM. A 1 inch roller results in a speed of 20.8 MPH at the same engine RPM.

    It's all a compromise. The faster you can go, the slower you will get up to speed, and the less slope you can take unassisted. The slower the top end, the better the take-off and the steeper the slope.

    Considering all the above, let's use me as an example. :smile: I'm a big guy (ex-defensive tackle, over 300 pounds) who wanted decent acceleration, but didn't need a top speed a lot over 20 MPH, as Arizona's laws can REALLY penalize you if you go over 20. I'm happy with a friction drive with a 1.25 inch roller.
    Last edited: May 30, 2008
  4. loquin

    loquin Active Member

    Finally, all the above being said, here's an excel spreadsheet that you can use for your calculations. (If you don't have excel, you can download/install OpenOffice free of charge. Recommended!)

    Attached Files:

    Last edited: May 30, 2008
  5. ducatiluke

    ducatiluke New Member

    Fantastic exactly what I was looking for !!!


    I do realize wheel diameter in friction drive setups do not affect ratio
    But I compared friction drive and chain drive effective ratio and found that friction drive all seem to run lower overall ratio I assume its because they are less efficient

    pictures is a friend's home made friction drive engine is rack mount , and centralized

    its also pedal start with a centrifical clutch

    Attached Files:

    Last edited: May 31, 2008