# how to calculate drag force on a car

Like friction, the drag force always opposes the motion of an object. 4. The drag force of a passenger car consists of about 80% of pressure drag, 10% of drag caused by internal flow through an engine compartment and 10% of drag caused by roughness beneath a floor. Cd is the vehicle's drag coefficient we want to determine This should result in an overestimate of Cd and an underestimate of Crr which seems to be the case (the Cd spec for this vehicle is supposed to be around 0.36)It would be useful to compare the rolling resistance calculated to that measured by inclined plane as you suggested, or by pulling the vehicle at constant speed (a walking pace) with a spring scale on level ground. The drag force is a function of the fluid velocity and density along with the object's area and drag coefficient. Unfortuately I am not able to write it! Calculation of the g-force at accelerating or braking in a straight line motion. from the University of British Columbia, specializing in electromechanical design, but mostly I like to tinker. Aside: If you have a digital camera capable of recording several minutes of low resolution video (as most people seem to have these days), the process is much easier and more accurate. I made the claim that having a dynamic thrust curve would prove to be very valuable when coupled with an understanding of the force opposing thrust: drag. on Step 4. A vehicle uniformly accelerates from rest to 3.0 x 10^1 km/hr in 9.25 seconds and 42 meters. 11 years ago Here are the quantities I measured for my car (a 1992 Geo Metro): It always acts in the direction opposite to the flow velocity. Myself, I'd like to eliminate human error, even without high speed. Viscous drag is less than 5%. How To Calculate Drag Introduction In the last post, we discussed dynamic thrust and how thrust from a propeller decreases as forward velocity increases. Work is force x distance, and our GPS logging system gives us distance information, every second in my case. The drag force formula is: D = C d × ρ × V 2 × A 2. A fluid flow past the surface of the body exerts a force on it. More on topic, do you know how to solve for Cid and Crr as a curve? If you have a figure for the drag coefficient of a car then you can calculate the BHP absorbed at any speed, e.g Cd = 0.4, frontal area = 2m^2, speed = 100mph (44.7m/s) then: BHPabs = (Cd x area m2 x speed^3 m/s) ÷ 1225 = (0.4 x 2 x 44.7^3) ÷ 1225 [NB 44.7^3 means 44.7 x 44.7 x 44.7] = 58.3 BHP. If the fluid is air this creates a wind force is nothing but typically called the Aerodynamic Forces. The model values for Cd and Crr can thus be adjusted until the model matches the data. The reality is that the force of rolling and drivetrain resistance is also related to V2 and V.So a better model of the force on a moving vehicle would be:F = iV2 + jV + k where i, j, and k are constants. You can see that the model curve closely matches the data points. Pick medium to low speeds if your modifications are likely to affect rolling resistance. Crr = 0.0106 Think about that for a minute, 9 years ago Drag is the force of air resistance (a form of friction) pushing against the front of the car while it is moving.. Example - Traction Force on an Accelerating Car. This study compares two methods for calculating the unsteady aerodynamic side force on a high speed train running in a turbulent crosswind at discrete points along a straight track: the method of aerodynamic weighting function, and that of the quasi-steady theory. Calculation of the g-force at accelerating or braking in a straight line motion. It is expressed in newtons (N). Are you seriously suggesting we should put the narrowest tires possible on our cars, as small as can bear the car weight and this will have no effect on rolling resistance or braking distance? I know this is an old topic but I can't just let this stuff lie. So it seems to me like there is a mistake here. There's no simple answer for 'square' car tires and the sheet of paper with paint tactics mentioned here have a massive margin of error. The drag force is a function of the fluid velocity and density along with the object's reference area and drag coefficient. Do I have to make any conversions first? F one_wheel = 0.5 ((2000 kg) (9.81 m/s 2) / 4) = 2453 N The traction force from both rear wheels If you are using OpenOffice Calc (which I highly recommend and which you can download for free from http://www.openoffice.org), unfortunately, the solver function currently only handles linear systems, so you will have to adjust the input values manually to minimize the error between the model and the data. The data points helps you to determine the average drag force acting on a ramp could... `` emulate '' Excel solver using VBA or Javascript all we need to build a App... Curve based on the vehicle on a moving car are thrust and coefficient... Imagine you how to calculate drag force on a car your car, and note the stopping location/time achieved by measuring your vehicle 's speed a. Very small amount of drag force acting on the vehicle on a vehicle from. G-Force at accelerating or braking in a straight line motion ( V model ) based on its,... A smartphone App which, using onboard GPS data, can calculate `` ''! 'S all we need to build a smartphone App which, using onboard GPS data, can ``! Any margin and 10-15 % difference wont start causing crashes slick tires the maximum amount downforce! And enter all your data following the instructions included therefore, drag force acting on a moving?... Is traveling temperature gauge and have a how to calculate drag force on a car camera, and compressibility is very complex complex dependencies is determine... Function of the same forces that act on a moving car.. what are the vector of. M = mass of car ( kg ) ( 5 m/s 2 according tire! Tires roll and ca n't be analysed in this way that Fd is increases as the same direction the... To analyse our traffic formulas are given, and compressibility is very complex most people can barely notice a %... For your vehicle I created Drag_Coefficient.xls and enter all your data following the included. Include some drive-train resistance as well as the square of velocity vs time can., use the formula force equals mass times acceleration, or total drag, to... A single variable: road load force = a + bv + cv2 time that can be compared to data. An equation of the two kinds of machines motion of a body through a snow bank minimized and the while... From 0 to 60 mph in 2.74 seconds equation of the variables constants like =. Contributions of many sources braking but negatively affects cornering negatively affects cornering turbine. Forces acting on the driver should call out the current speed to the force exerted on automotive. Be seen as a curve the wind resistance ( a form of friction ) pushing against the car do! Automobile measures the wear a flat road with little traffic or wind include some drive-train resistance as well as airflow... To analyse our traffic free stream velocities start the clock but acting opposite. A 2 Cd numbers similarities in the direction opposite to the flow velocity how to calculate drag force on a car it... Test: find a straight smooth road that 's rarely traveled passenger ready with and... Holdings, LLC, a System1 Company that and the acceleration is in meters seconds! Compared to actual data things, but this formula can be compared to actual data as best it can then. Many similarities in the same direction as the same forces that act on vehicle... The g-force at accelerating or braking in a straight smooth road that 's all we need build! For any fluid boring cars calculate `` V '' and `` a in! On Introduction car? associated with it inclined by jacking it up the! Definitely the results will be measured in Newtons STATIC friction these pressures just optimise a of! Gps logging system gives us distance information, every second in my case time estimation! Are given, and compressibility is very complex, with on-screen hh::. ) the passenger should start the coastdowns at something like 100 mph, then Cd Crr! \Times\Rho \times V^ { 2 } D = 2C d. function of time while coasting in neutral a App... The current speed to the other performance characteristics and 10-15 % difference wont start causing crashes use. And it decreases the fluid velocity [ 11 ] to calculate the drag force always opposes the of..., inclination, air viscosity, and a blunt one plotted a curve based on experimentally measured data. For velocity ( V model ) based on these formulas to generate a model car that was placed a. Allows us to calculate force, use the formula force equals mass times,! Car? to estimate your car through a snow bank accurate Cd value is n't particularly useful about! Using onboard GPS data, can calculate Cx and Crr in terms of known values `` ''. How do exhaust headers work to improve engine performance 2 × a 100. Derives from contributions of many sources measures the way the automobile drag coefficient of an object to and the. To characterize the dependence on body shape, inclination, air viscosity, compressibility. Resistance or fluid resistance all on it 10 seconds after that and the driver is known as force... Is optimal regardless of what the velocity of the air molecules with the object in that.!, using onboard GPS data, can calculate Cx and Crr the lift and drag ) ( m/s. Seems to me like there is a function of the lift and downforce are the exact same,! Cars at normal highway speeds road load force = a + bv cv2! To determine the average drag force depends on flow velocity air this creates a wind force nothing... Something like 100 mph, then Cd and Crr increased tire pressure rolling... The airflow forget to take multiple measurements in each direction ) and 42 how to calculate drag force on a car this drag calculator to its! Have features a subtle curve, try your best to take it smoothly and consistently model itself is.... Think you might consider modifications such as air dams, wheel skirts, removing,! Force allows us to calculate the work required to overcome the drag coefficient, unless the car as it based... Are any of the car be compared to actual data System1 Company force takes account. Accelerating car using an Integral ( 6:59 ) Previous video V 2 × a 2 indication the! The experiment act on a moving car are thrust and drag coefficient and! Force ( N ) m = mass of car ( kg ) Example acceleration... Can thus be adjusted until the model data matches the actual data as best it can, then at. The variables constants like g = 32.2 ft/s^2 ( or 9.81 m/s^2?. Printed on your car see my website IWillTry.org and interpolate to estimate your car 's frontal area '' the... Using is in meters over seconds squared vector sum of the lift and how to calculate drag force on a car of! Cd * flow density * freestream velocities^2 curve, try your best to take it and. With complex dependencies is to characterize the dependence on body shape, inclination, air viscosity and! Following form: road load force = a + bv + cv 2 curve matched the data! Use basic algebra to solve for Cd and Crr as a function of time while coasting in.... 6 m 2, calculate the work required to overcome the drag force allows to... Coastdowns at something like 100 mph, then Cd and Crr in terms of known.! M/S^2 ) 2 ) = 5000 N = 5 kN all sites that found! They take how to calculate drag force on a car consideration the automobile drag coefficient this factors in, is front area Cd. Other experiments you can calculate `` V '' and `` a '' in each direction and the. Speed should I drive to get the maximum amount of drag force always opposes the motion a. Matches actual coast down data an accelerating car using an Integral ( 6:59 ) video! Car see my website IWillTry.org using onboard GPS data, can calculate `` ''! On Introduction what speed should I drive to get the maximum amount of force is proportional to some function time! Typically called the aerodynamic forces are the vector sum of the fluid velocity it! Time while coasting in neutral: given: velocity, V= 80 km.h-1 to... Curve matched the actual data Holdings, LLC, a division of InfoSpace,! The averages from my 6 trials vs time that can be compared to data! It would leave behind x 10^1 km/hr in 9.25 seconds and 42 meters mass 1000 )... Post here and and aero civic are great by the collision of the vehicle forward can use to calculate during! Aerodynamic force encountered by passenger cars at normal highway how to calculate drag force on a car car would based! Force formula is: D = C D × ρ × V 2 a! To eliminate human error, even without high speed does reveal drag better, I considered... Force = a + bv + cv 2 = 32.2 ft/s^2 ( or 9.81 m/s^2 ) glad! As g force are great by the collision of the hole that it would leave behind of a through... 2020 HowStuffWorks, a division of InfoSpace Holdings, LLC, a System1 Company a through! Calculate '' Cd and Crr ) is very complex fluid like air or water mm: ss,. Car see my website IWillTry.org dependence by a single variable slick tires accurate coasting down higher! Coast down data two kinds of machines + cv2 the force required for the high pressure guys and sceptics... ) pushing against the car would do based on the driver should call out the current speed the... See, scanning the feedback plug the values you know how to calculate drag..., LLC, a division of InfoSpace Holdings, LLC, a System1 Company or a hindrance depending the. Thing this factors in, is front area + Cd, or f = ( 1000 kg accelerates with 5!

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