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  1. #1
    RC poster
    Join Date
    Sep 2019
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    2

    Question Need help figuring out where my extra speed is coming from

    Hey everyone! I'm kind of beating my head against a wall here trying to determine where my extra speed is coming from in my rustler 4x4.

    Using digihud pro on my phone (added weight slowing it down), I am clocking in at just over 64mph. I'm completely fine with this top end speed, but I'm not sure how I'm achieving it with the upgrades I've done. I know I'm missing something silly, but I just can't figure it out.

    I currently have swapped the following parts into my car that would impact the speed:
    Max10 SCT ESC Combo Brushless - 4000kV
    18T pinion
    3S lipo

    I run with a flysky nb4 with the endpoints of the throttle set to 100%
    My esc is programmed with endpoints at 100%
    All other parts I've replaced would not impact the speed other than the added weight

    Stock differential for the rustler 4x4 front and rear is 12/47
    Stock spur gear is 54t


    My math for top speed at max voltage is below:
    maxVoltage = 4.2*3(s)
    DiffRatio = 3.92 (stock rustler 47/12 diffs)
    MotorRatio = spur(54) / pinion(18)
    totalRatio = DiffRatio * MotorRatio
    60minutes * ( (KV*maxVoltage)/totalRatio ) / ( feetPERmile / ( ( tireDiam / 304.8 )* pi ) )

    Putting in my values, my final equation is:
    60*(((4000*(3*4.2))/((3.92*(54/18))) / (5280 / ((71.12/304.8)*3.1416)))

    This brings me to 36mph at top voltage, 26mph at bottom theoretical speeds excluding amp pull, wind resistance and tire ballooning.


    What am I missing that is bring me up to 64mph?

  2. #2
    RC Competitor
    Join Date
    Oct 2018
    Posts
    60
    Tire diameter looks awfully low. 71ish mm? Is that what you are running?

    Sent from my SM-G970U using Tapatalk

  3. #3
    RC Qualifier
    Join Date
    Dec 2017
    Location
    North Texas
    Posts
    626
    127mm diameter tire will fix that. (around 55mph)
    71mm is the width

    After accounting for some tire ballooning (since the tires are not belted) you can estimate a diameter of 140mm at 60mph speeds.

  4. #4
    RC poster
    Join Date
    Sep 2019
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    2
    Perfect! thanks so much for the replies, that's exactly what I was missing. For some reason, I was thinking 2.8inch diameter on the tires.... must be width. They are the stock rustler 4x4 talon tires, so 117mm diameter, that puts me at 59mph without ballooning. Assuming an increase of ~10mm with balloon as they are not belted, as you mentioned.... that would be put me at 64MPH theoretical.

    Exactly what I was missing. Appreciate it!

  5. #5
    RC Champion olds97_lss's Avatar
    Join Date
    Dec 2003
    Location
    Dekalb, IL
    Posts
    2,407
    Quote Originally Posted by mfriedenfeld View Post
    Perfect! thanks so much for the replies, that's exactly what I was missing. For some reason, I was thinking 2.8inch diameter on the tires.... must be width. They are the stock rustler 4x4 talon tires, so 117mm diameter, that puts me at 59mph without ballooning. Assuming an increase of ~10mm with balloon as they are not belted, as you mentioned.... that would be put me at 64MPH theoretical.

    Exactly what I was missing. Appreciate it!
    2.8 inch is the rim size.
    Expert rigging at it's best!

  6. #6
    RC Qualifier
    Join Date
    Nov 2018
    Location
    Spokane, WA
    Posts
    345

    Need help figuring out where my extra speed is coming from

    My xmaxx only does 55!

  7. #7
    RC Turnbuckle Jr.
    Join Date
    Oct 2016
    Posts
    4,239
    Quote Originally Posted by Overpowerxmaxx View Post
    My xmaxx only does 55!
    Slow Poke


    Somehow builds are never done....

  8. #8
    RC Qualifier
    Join Date
    Nov 2018
    Location
    Spokane, WA
    Posts
    345
    That was with stock tires pancaked on a gravel cover tarmac stretch


    Sent from my iPhone using Tapatalk

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