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How do you calculate G/force?

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  • How do you calculate G/force?

    How do you calculate the G/force created by a car on a skid pad from the measurements that are taken of a car at it’s maximum speed?

    These are the measurements; weight 74 grams, elapsed time 0.725 seconds, radius of the skid pad 10.25 inches.

    Are the centripetal and centrifugal forces the same value?

    Please show the formula and the process that you used to arrive at the G/force of this car.

    Dave

  • #2
    You would use at least two equations.

    The first being the velocity equation.

    V=2 pi R/T

    R = 10.25 inches or 26.035 cm

    T = 0.725 Seconds

    Therefore:

    V= 225.63 cm/sec (7.4 ft/sec)

    The second equation is the Acceleration equation for radial acceleration

    a = V^2/R

    Therefore:

    a = 225.63^2/26.035 = 1955.4 cm/sec^2

    Divide by 9.8 and you get 199.53 gs

    or you can google it

    http://science.howstuffworks.com/question633.htm
    Last edited by Mudcat; 01-15-2009, 09:18 AM.

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    • #3
      1st, let's please get rid of the idea of 'centrifugal force'. There's no such thing, and never will be.

      Mudcat has the right equations, but the units are tripping us up. Coming up with a value in centimeters then dividing by 9.8 meters per second squared ain't so good. You'll get a meaningless value for acceleration.

      V = C/T (C = circumference = 2*pi*R)
      V = 2*3.14*0.854ft/0.725s = 7.4 ft/s

      a = V^2/R
      a = (7.4 ft/s)^2 / 0.854 ft = 54.8 (ft/s)^2 / 0.854 ft = 64.1 ft/s^2

      1 G = 32 ft/s^2, so 64.1 ft/s^2 / 32 ft/s^2 = 2.0 G's
      Last edited by ElSecundo; 01-15-2009, 12:49 PM.

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      • #4
        So the error is in the decimals. I saw that 199.53 G's and thought, no, not 200 times the force of gravity. But twice, I can buy.

        I also recall learning in high school physics that there is no such thing as centrifugal force (outward force on a rotating object) because all you really have is the inertia of an object that wants to go straight. The force that pulls the object into a curved path is centripetal force, which is recognized by physics as the real force. That is the same force that pulls the nose of your slot car sideways by the guide, exerted by a curved slot.

        I learned all this stuff before metrics. I recall the acceleration due to gravity is 32 fps per second. What is the acceleration due to gravity in metric?
        Last edited by Robert Livingston; 01-15-2009, 11:42 AM.

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        • #5
          metric

          Robert,

          9.8 METERS/sec squared.

          Paul

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          • #6
            So to sum it up:

            Radius = 0.854 feet
            Lap length (2*r*pi) = 5.36 feet
            Lap time = 0.725 seconds
            Speed (Lap length / lap time) = 7,4 feet per second
            G-force = (7.4^2 / 0.854)/32 = 2 Gs

            But I continued to read that "how stuff works" page and they also show how to translate banking into downforce once you know the G-force

            So how much tire pressure would you get if you added, lets say a 5% banking to the turn, which is so little that it can barely be noticed unless you know about it.

            2 Gs x sin5° = 0.17 Gs * 74 gram = 12.6 gram

            So by adding a barely noticeable banking you get noticeable better grip by adding 12 gram extra "weight" on the tires. For a track builder these are very interesting calculations.

            To complete the calculation you can also add the gravity by the car itself:

            1 G x cos5° = 0.996 Gs

            So the total weight (downforce) of that car in that turn is (0,17 Gs + 0,996 Gs) * 74 gram = 86.3 gram

            Who wants to calculate the added effect by shifting weight caused by floating bodies ?

            Last edited by 356speedster; 01-15-2009, 12:14 PM.

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            • #7
              Dam, I can't believe I screwed up the units like that!

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              • #8
                Originally posted by Mudcat View Post

                Dam, I can't believe I screwed up the units like that!

                Well the good news is that tried to answer the question. I have found if you make no errors, then you are not doing a thing. Me, I make lots of errors!

                You didn't screw up the units, just the placement of the decimal point is all.

                Of course, almost 200 times the actual weight should have been a dead giveaway that your answer was not correct.

                Besides, we were not getting ready to detonate a thermonuclear devise! Thankfully!

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                • #9
                  Thanks for all the input, I had the same answer of 2 Gs, but thought that I was making a mistake.

                  The G/meter was registering between 2.3 and 2.4Gs, but the calculation showed 2Gs

                  Because the front of a slot car is running in a slot with a radius of 10.25 inches at 0.725 seconds it is making 2Gs, the rear wheels aren’t running in the same radius.

                  With the rear wheels running in a 11.25 inch radius at 0.725 seconds, the rear wheels have a velocity of 8.125 f/sec and 2.2Gs

                  With the rear wheels running in a 12.25 inch radius at 0.725 seconds, the rear wheels have a velocity of 8.840 f/sec and 2.39Gs.

                  Dave
                  Last edited by davejr; 02-04-2009, 07:01 AM.

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                  • #10
                    What were the 1:1 group C cars pulling in G's?

                    I know modern day prototypes can easily pull 2g's along with F1's.

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                    • #11
                      The information I have from G-Tech is a modern day open wheel car will pull between 1.4 and 1.7 continuous Gs on a skid pad.

                      Due to the relatively low speed of the car and little aerodynamic down force, the tire grip is mostly mechanical.

                      Dave

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                      • #12
                        Crazy Talk!

                        Originally posted by Mark View Post
                        What were the 1:1 group C cars pulling in G's?

                        I know modern day prototypes can easily pull 2g's along with F1's.
                        I remember when they were running the Jaguar XJR-14, the one with the same V-8 Ford some guy named Michael won the F1 World Championship...

                        The TV announcers said it was a 2,000 lb car that generated 9,000 lbs of downforce!!!

                        In the late 80s, Price Cobb drove reporters around Portland in an XJR-9 and without getting close to their regularly 3+ Gs, the reporters could bearly handle a lap.

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                        • #13
                          I saw an interesting medical research paper comparing physiological changes in F1 drivers, and the comparison was between G. Villeneuve and Didier Pironi. They were discussing 4 G's. If the Group C cars weren't as severe, it wasn't by much.

                          Kerry, the 9,000 pounds of downforce they are referring to isn't the same as lateral grip. The car experiences the equivalent of 4.5 G's on the straight at top speed, but the driver doesn't. That does mean, though, that in a high-speed bend, we could expect something approaching 4 G's of lateral acceleration. That's nasty stuff!

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                          • #14
                            Still amazing

                            That was the last year before pit lane speed limits and Davy Jones had the first pit...

                            Dad and I were standing down by the pit exit during practice when he went by somewhere near 170...

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                            • #15
                              mass, weight, and Gs

                              ElSecundo,

                              On a level road at top speed, a car will experience 1G. The car has the equivalent weight of 9,000 pounds (weight of car plus downforce), but a G meter would only read "1."

                              If an airplane were pulling 4.5 Gs, the plane, G meter, and the crew would all feel 4.5 Gs, and the airplane would rapidly change direction.

                              Cheers!

                              Paul

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