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Thread: Help with gear ratios

  1. #1

  2. #2
    10mph = 4.47 m/s
    0.3m Diameter (300mm) * PI = 0.94m circumference
    0.94*4.47=4.75

    Therefore the wheel needs to be turning 4.75 times per second to get 10 mph

    Taking the motor speed at 5500 rpm
    5500/60 = 91.7 revolutions per second

    91.7/4.75 = 19.3

    Therefore you need a gear ratio of about 20:1

  3. #3
    The quick shorthand calc is-

    output rpm*wheel diameter in inches*0.003=speed in mph

    so, rearranging that-

    speed in mph / (wheel diameter in inches * 0.003)

    It's not perfectly accurate, but it's close.

  4. #4
    I just use this because I'm too lazy to do the maths myself

  5. #5

  6. #6
    Yeah, it's the only bit of maths I can do really :P

    You're using pretty big wheels, so a high gear ratio isn't too surprising; I think the more common wheel diameter for a heavy is closer to 100mm, so they're more likely to get away with single stage. But as you say, 5:1 and 4:1 are fairly easy gear ratios to setup.

  7. #7
    Most heavyweights now are geared for 15mph rather than 10; 10's a bit slow for a heavyweight. Plus most heavys have smaller wheels, for example Dystopia has 6" wheels. A good rule is the wheels diameter in inches:1 as a gear ratio when dealing with Mags or Bosch 750s

  8. #8
    Funny how the Dutch/Belgian heavies get by with much larger gearratios.

    20-1 ain't uncommon. Bullfrog and TAN are in that range. 10" to 16" wheels.

    Gravity is around the 15-1 range. 10" cart or solid HDPE wheels.

    On the other hand, those machines use 2 stage gearboxes.

  9. #9
    With regards to gear ratios and top speeds I have found they are disconnected with the real world of fighting robots.

    Saint is not slow by any means and that works out (In theory) at about 9.25mph in fact is is probably closer to 8.25mph.

    Gabriel is a little faster but probably only just into the 10mph in reality. These robots have very little problem with the arena surface so they can in practice reach these speeds.

    Most people drive tin snail flippers. yes the wheels could with no resistance rotate to an expected speed of 15mph none has managed this in practice as they get stuck on the arena floor, experience roll resistance and other factors that slow them down, There are exceptions to this like Storm 2, and now Brutus. these are designed to use kinetic energy to knock and push there opponents into the arena barrier or pit. both are exceptionally expensive machines to build and run.

    so remember in the robot scene as with most things in life

    High speed costs!!! If you want a robot to be cheap and fun keep the speed down, the quality high, and the concept clever

  10. #10

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