Can someone check my RPM/Speed numbers

Discussion in 'Performance Mods' started by jauguston, May 16, 2010.

  1. jauguston

    jauguston New Member

    I am no good with the calculator posted because I don't know some of the ratios.

    I have a new (1/2 gallon of 32-1 synthetic oil mix used) Silver Grubee Sky Hawk 66/80 in a Schwinn Mesa. I made a speed run on flat pavement a few minutes ago with my Tiny-Tach and my GPS.

    The tires are 26X1.95 and the gearing is stock the way it came from Bikeberry.com. 44t wheel sprocket.

    The tach read 7500 rpm at 30 mph. Is that close to what you guys are seeing? I am getting a SBP shifter kit tomorrow. It being a small world I find out that Jim from SBP lives less than a mile from me.

    Jim
     
    Last edited: May 16, 2010

  2. wildemere

    wildemere Member

    30 is 7000 with that stock setup.

    Something needs adjustment.

    The ratios ratios are, Primary 4.1:1 and Final 4.4 :1

    6000 is a safer redline for engine happyness and life.
     
  3. jauguston

    jauguston New Member

    I think my Garmin GPS is trustworthy. I think my Tiny-Tach is trustworthy. What would need adjustment? What COULD be "adjusted"? The tires are knobby mountain bike tread.

    I sure agree the RPM is too high for engine life. Thus the shifter kit.

    Jim
     
  4. wildemere

    wildemere Member

    I have double checked it for you.

    My money is on the Tiny Tach's calibration

    Get it checked out. Its easy with a Signal Generater.
     

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  5. jauguston

    jauguston New Member

    You are using what-10t on the motor gear 41t on the output/clutch gear? I have never counted them. Then 10t on the output sprocket and 44t on the wheel sprocket-right? I have never counted the teeth on the engine sprocket.

    The number that I don't understand is 26" wheel. What does the wheel size have to do with the overall ratio? I would think the tire OD would be the relevant number. If it was then my numbers would be even more off as the RPM would be less at 30 MPH. I'm puzzled.

    Jim
     
  6. wildemere

    wildemere Member

    I'll try to help.

    You are using what-10t on the motor gear 41t on the output/clutch gear? I have never counted them. Then 10t on the output sprocket and 44t on the wheel sprocket-right? I have never counted the teeth on the engine sprocket.

    The primary numbers are 20t drive & 82t driven, 4.1:1 The HT has a 10t output sprocket.

    The number that I don't understand is 26" wheel. What does the wheel size have to do with the overall ratio.

    Nothing

    I would think the tire OD would be the relevant number. If it was then my numbers would be even more off as the RPM would be less at 30 MPH. I'm puzzled.

    Your 1.95 x 26 tyre is ~26" tall the rim is ~23"

    The area of bicycle tyre sizing is one of the most confusing things relating to bikes.

    From Sheldon Brown:-

    Bicycle tires come in a bewildering variety of sizes. To make matters worse, in the early days of cycling, every country that manufactured bicycles developed its own system of marking the sizes. These different national sizing schemes created a situation in which the same size tire would be known by different numbers in different countries. Even worse, different-sized tires that were not interchangeable with one another were often marked with the same numbers!

    Don't feel bad, even experts are confused!

    The link if you are interested:-

    http://sheldonbrown.com/tire-sizing.html
     
    Last edited: May 16, 2010
  7. jauguston

    jauguston New Member

    Clear as mud:goofy:

    I now understand. What a wacky way to measure tires.

    Thanks for the information.

    Jim
     
  8. loquin

    loquin Active Member

    If you want to check your tire diameter, you should do so 'loaded' - that is, while sitting on the bike.

    Make a mark on a level surface, and place your bike drive wheel's fill valve directly over the mark. While sitting on the bike, move it forward until the fill valve is closest to the surface, and make a mark on the surface directly 'under' the valve.

    Now, measure the distance between the two marks (it should be close to 82 inches for a 26 inch tire.)

    Take your actual distance, and divide by pi (3.14159) to get the actual loaded diameter. Use that diameter in your calculations. It would be pretty easy to have an inch difference between assumed diameter and loaded diameter, if the tire pressure is a little low, and that would make the above calculations 'off' my more than a mile per hour.
     
  9. jauguston

    jauguston New Member

    All of this is academic now as I just finished installing a SBP shifter kit and now have seven speeds. Thanks for all the input.

    Jim
     
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