Running resistance

　・In Japanese

■What is running resistance?

The running resistance of a vehicle is determined by the air resistance, and the rolling resistance that occurs on the contact surface between the tire and the road surface.

＜Air resistance＞

Air resistance is expressed by the following formula. Other than the vehicle speed, it is a fixed value, and the air resistance is proportional to the square of the vehicle speed.



The Cd value (Constant drag) is an air resistance coefficient, which indicates the ease of air flow. The Cd value becomes lower when the shape of the vehicle is streamlined. Strictly speaking, the Cd value changes depending on the vehicle speed.

The part of the shadow projected on the wall when the vehicle is exposed to light from the front is called the front projected area. The smaller this area, the smaller the air resistance.

＜Rolling resistance＞

Rolling resistance can be expressed by the following formula. "c" is the rolling resistance coefficient, which is determined by the tire material, air pressure, hardness, and temperature. The higher the temperature, the smaller the rolling resistance. Strictly speaking, it also changes depending on the vehicle speed. Rolling resistance on asphalt is said to be about 0.01 to 0.015.

■Calculate the running resistance by an approximate formula

＜Coasting method＞
When an automobile manufacturer measures a fuel consumption value, it is necessary to measure the running resistance in advance in order to set the chassis roller load, and one of the measurement methods is the coasting method. The coasting method calculates the running resistance by driving the vehicle on the road, setting the transmission to the neutral state, and measuring the deceleration of the vehicle.



The relationship between running resistance and vehicle deceleration d is as follows according to Newton's law. By acquiring data at several points, it can be approximated to the square of the vehicle speed, so the coefficients A and B can be obtained respectively.



However, strictly speaking, the rolling resistance coefficient c and the air resistance coefficient Cd change depending on the vehicle speed as described above, so the running resistance can be more accurately approximated by the following approximation formula.

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