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  1. #1
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    Calculating Speed

    I know that one can calculate estimated speed by using the following formula:

    mph = .00297432*(tire_size/gearratio)*RPM

    Where does torque fit into this equation?

    Look at the older Zircon's stats: Zircon (9x2) RPM 41,000 and compare that to the newer Cryptonite (9x2) RPM 31,400 and Power 130 .

    Any ideas? I would like to be able to calculate the speed including all of the figures.
    Jason

    RC Cars: Stadium Blitzer, RC10T (Original), Super Sabre, TC3, RC10B4 & Stampede

  2. #2
    RC Turnbuckle Jr.
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    Trying to calculate your speed using that will not be accurate at all. What you describe is called "rollout speed". This is the theoretical highest speed you could obtain under optimum conditions.

    However the RPM figures usually used are inaccurate. This is because most figures are with the motor unloaded. Once the motor is actually doing some work, the RPM figure will go down dramatically. Torque affects how far that figure does go down. Even if you had an accurate figure, that formula still wouldn't work well. Factors such as driveline drag, motor condition, terrain, and even the weather could affect you to an extent.

    The best way to determine speed is use a GPS, radar, or speed trap. Anything with that many variables can never be accurately calculated with a formula.
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  3. #3
    RC Turnbuckle Jr.
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    Re: Calculating Speed

    Originally posted by tibbar110
    Look at the older Zircon's stats: Zircon (9x2) RPM 41,000 and compare that to the newer Cryptonite (9x2) RPM 31,400 and Power 130 .
    The reason for the big difference between the two is that Trinity now tests motors at 5 volts. They used to test them at a 7.5 volt simulation on the dyno. (That's why the older motor has the higher RPM figure)
    Speed costs money, how fast do you want to go?

  4. #4
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    I just like to measure out a 100' of a flat surface then start ~50' before the start of the 100' then once it hits the line I have someone time it

    then I take 100/the time i got

    just take this simple math and plug in the #'s

    disance/time = how fast you were going

    that reminds me I still need to get a reading on my new 17t amber
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  5. #5
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    ohh yea for the best reading use the measurement in meters cause thats what the formula is for which is meters per second (that can be converted to mph but I cant remember that right equation from my physics class)
    click www (down there)
    If your not part of the problem there is great money to be made in prolonging the problem

  6. #6
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    Quote Originally Posted by tibbar110
    I know that one can calculate estimated speed by using the following formula:
    mph = .00297432*(tire_size/gearratio)*RPM
    Where does this come from: mph = .00297432 ??
    Aint this simpler, i.e. my slash ? :

    Primary: (spur / pinion) 90 / 17 = 5.294
    Internal: (diff / center) = 2.72
    FINAL DRIVE RATIO 5.294 x 2.72 = 14.34

    Now my lrp brushless 7.5 t 42.480 (at 7.2 volt ran free) is lets say about 40.000 rpm under load, on flat tarmac and a fresh lubricated drivetrain, little wind in the back.

    So: 40.000 / 14.34 = 2789 rotations per minute
    The tires are 2.2 inch = 5.632 centimeters or about 17 cm travel per rotation
    is 5.632 x 2789 = 15707 cm per minute
    or: 1.57 kilometer per minute
    1,57 ,times 60 makes 94,2 kmph, or about 59 mph.

    EDIT and update

    Hmm I must be making a mistake, diameter is 2.2 inch, so per rotation about 17 cm travel... and my slash is fast but not 90 + kmpu...
    Last edited by new2slash; 06-23-2009 at 10:59 AM.

  7. #7
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    Quote Originally Posted by bestshibby
    ohh yea for the best reading use the measurement in meters cause thats what the formula is for which is meters per second (that can be converted to mph but I cant remember that right equation from my physics class)
    1 mile = 1609,344 meter
    1 kilometer = 0,621371 mile
    http://nl.wikibooks.org/wiki/Omreken...enheden/Lengte

  8. #8
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    On http://wahiduddin.net/calc/calc_speed_rpm.htm
    What should one fill in there for the slash, at:

    Transmission Gear Ratio ??
    Differential Gear Ratio ??

  9. #9
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    Quote Originally Posted by new2slash
    On http://wahiduddin.net/calc/calc_speed_rpm.htm
    What should one fill in there for the slash, at:

    Transmission Gear Ratio ??
    Differential Gear Ratio ??
    The tranny is a Magnum 272, meaning 272 is the ratio.
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  10. #10
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    Quote Originally Posted by GrimeFighter338
    The tranny is a Magnum 272, meaning 272 is the ratio.
    so with the standard 12t (24.000 rpm?) , the wheels make 8824 rpm
    with 2.2 inch wheels that is 6,9 inch per rotation
    6,9 times 8824 = 60806 inches per minute
    x 60 = 3648360 inches per hour...

    thats fasssttttt !

    Would be GREAT if traxxas makes an application online, a calculator for theoretical speeds.
    Fill in tire diameter, motor rpm, spur t, pinion t, and such and see outcome.
    With then also what each model has or for each model 1 !
    Last edited by new2slash; 06-23-2009 at 12:23 PM.

  11. #11
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    Found a great site for this !!

    http://dhrc.rchomepage.com/RolloutCa...Item=MotorRPM&

    The link above opens a page with as an example of my setup, the 1 i used today for offroad short course. The used motor is LRP x11 7.5t 4600 kv.
    Last edited by new2slash; 06-23-2009 at 05:00 PM.

  12. #12
    RC Turnbuckle Jr. streetdemon's Avatar
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    Somethings not right with that. It's telling me that if I use a VXL with 2s LiPo in my rustler with 3 in tall tires I am only going 24.7 mph.

    By the way everyone using 2.2 as a measurement, that's the rim size, the tires are much larger than that.
    Some like R/C's that are fast, others like nitro

  13. #13
    RC Turnbuckle Jr. 31794ty's Avatar
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    Um this thread is 5 years old....
    Stampede VXL Mini-T Nitro Stampede RC10T RC10B4 FT

  14. #14
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    Five years old or not, we can help. What you want is here: http://scriptasylum.com/rc_speed/_top_speed.html

    Differential Ratio: 1
    Transmission Ratio: 2.73
    Other Ratio: 1

    Stock Stampede tires are 4.9" diameter, if that's what you're using.

    Quote Originally Posted by streetdemon
    Somethings not right with that. It's telling me that if I use a VXL with 2s LiPo in my rustler with 3 in tall tires I am only going 24.7 mph.
    That sounds about right. Rustler tires are 4.1" tall.

  15. #15
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    Site vs real measurement by garmin nuvi 660t GPS

    right column, 47.1 km p/h

    So the site is pretty accurate i.m.o.
    my setup calculated on 53.1 real life difference about 6 km p/h or not because: this was in a street with 2 elevated crosspoints, i only had about 20 meters, cars on both sides so ride very carefully in straight line, so i guess when he realy spins up, it will get 50 km hour or even a tiny bit more. Also the original esc is still mounted also. The nimh 4000 was charged 1 hr. before but only to about 80%
    For 17t pinion and 90t spur that is good enough offroad speed for me :-) speed is power to, more speed might breaks things, that with lesser speeds wil hold.


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