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Thread: Gear Ratio/RPM Debate

  1. #1
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    Gear Ratio/RPM Debate

    So I know gear ratio/RPM/MPH threads are all over, but wanted to see if someone could chime in and validate or correct my theory....

    Running on an approx. 1/8 mile banked dirt oval. Currently geared 12/61 and topping out at 5,890RPM's. I'm wanting to get closer to 6,200 RPM's so I was doing the math to determine how many sprocket teeth to add. Here's the math:

    Current gear ratio=5.08333 (12/61)
    RPM=5,890

    5,890/5.08333=1,158.69RPM per 1.0 gear ratio

    6,200RPM/1,158.69=5.3508

    5.3508x12=64.21, or a 64T sprocket.

    I was told that an +/- 1 tooth on the rear equals +/- 200RPM's (talking 35 chain setup). My math gives closer to 97RPM's +/- 1 tooth change. What am I missing, or not factoring in? Mainly just curious and not out to prove a point or to be right. I work with numbers so it's hard to deny the math! Thanks for any input.

  2. #2
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    Might be partially small math rounding errors. 64/12 is 5.3333 not 5.3508

    Also the 200/rpm per tooth rule might be more true with bigger drivers (16-18) and 100 might be rule of thumb for small drivers.

    Don't have the time to crunch numbers right now.

    I have found +-200 per tooth with 15-16 driver size. Might be 150 from the mechanical gear and the other 50 rpm from going faster?

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    Thanks for the feedback. Also you're correct on the 64/12 ratio. When using the exact numbers it came out to 5.3508 or 64.21 which you have to round down to an even number for the sprocket. I know power delivery isn't consistent from the engine throughout the RPM range and affects the numbers as well...

  4. #4
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    Either way you figure it it's still 3 teeth.
    Mathematically it should work.
    In real situation may not.
    Adding teeth will not always raise RPM.

  5. #5
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    Quote Originally Posted by JT1000 View Post
    So I know gear ratio/RPM/MPH threads are all over, but wanted to see if someone could chime in and validate or correct my theory....

    Running on an approx. 1/8 mile banked dirt oval. Currently geared 12/61 and topping out at 5,890RPM's. I'm wanting to get closer to 6,200 RPM's so I was doing the math to determine how many sprocket teeth to add. Here's the math:

    Current gear ratio=5.08333 (12/61)
    RPM=5,890

    5,890/5.08333=1,158.69RPM per 1.0 gear ratio

    6,200RPM/1,158.69=5.3508

    5.3508x12=64.21, or a 64T sprocket.

    I was told that an +/- 1 tooth on the rear equals +/- 200RPM's (talking 35 chain setup). My math gives closer to 97RPM's +/- 1 tooth change. What am I missing, or not factoring in? Mainly just curious and not out to prove a point or to be right. I work with numbers so it's hard to deny the math! Thanks for any input.
    What Motor ? Does it have a Plate in it ?

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    Quote Originally Posted by racing promotor View Post
    What Motor ? Does it have a Plate in it ?
    Non-hemi Predator 212cc, no plate, DynoCams CL-2 cam, Billet flywheel (3.5lbs), Winnie pipe with RLV 90 muffler, rejetted with aftermarket air filter and adaptor. Governor/sensor removed

  7. #7
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    The amount of rpm adjustment you get with the gear change has something to do with where you are in the power band. The +/- 200 rpm rule is old probably early 90s when stock flatheads were turning low 5000 rpm. When you'd get the rpms to the torque peak you'd get a big jump in horsepower and if you got past that it would fall off. So when you got it right it was about 200 rpms.

    As we advanced the engine technology into the 6 even 7,000 rpm ranges the torque became much flatter and the engine rpms didn't move around as much. Now you can expect about 125rpms/tooth.

  8. #8
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    Quote Originally Posted by JT1000 View Post
    Non-hemi Predator 212cc, no plate, DynoCams CL-2 cam, Billet flywheel (3.5lbs), Winnie pipe with RLV 90 muffler, rejetted with aftermarket air filter and adaptor. Governor/sensor removed
    If indeed it's a true 1/8 th mile, an especially if it's a momentum track like my 1/8 th mile, I would be willing to bet a burger your gear bound now with a 12 driver, my track is a small 1/8 th mile momentum and that gearing to turn only 6200 would be like a 15-62 on a blue plate clone just as an example. Sr Clones big pipe use 15/63 ish and turn 6700.

  9. #9
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    What a lot of guys miss also with RPM gain is they look at only gear. IF your off on tires then make a tire change and hit them right you will gain that 200 RPM, so if you also make a gearing change now you over turn it. If your racing a track and you know the gearing well when your fast and you find yourself chasing RPM don't only look a gear use it as a sign your off on tires.

  10. #10
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    Quote Originally Posted by racing promotor View Post
    What a lot of guys miss also with RPM gain is they look at only gear. IF your off on tires then make a tire change and hit them right you will gain that 200 RPM, so if you also make a gearing change now you over turn it. If your racing a track and you know the gearing well when your fast and you find yourself chasing RPM don't only look a gear use it as a sign your off on tires.
    Good point. Happened to us last race!

  11. #11
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    Quote Originally Posted by racing promotor View Post
    What a lot of guys miss also with RPM gain is they look at only gear. IF your off on tires then make a tire change and hit them right you will gain that 200 RPM, so if you also make a gearing change now you over turn it. If your racing a track and you know the gearing well when your fast and you find yourself chasing RPM don't only look a gear use it as a sign your off on tires.
    That's why it's important to watch the climb of engine temp through out the run.
    It can indicate a handling issue

  12. #12
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    Quote Originally Posted by XXX#40 View Post
    That's why it's important to watch the climb of engine temp through out the run.
    It can indicate a handling issue
    sorry if this is a stupid question but can you explain that?
    thanks in advance
    p

  13. #13
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    If the engine temp goes up the kart is tight. That's holding your rpms down. If you free the kart up you'll get more rpms. If you put gear on to get your rpms and then free the kart up you'll turn way more than you should.

    Always make sure your kart is turning before you start changing gears.

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