Vehicle Performance (1)

Vehicle Performance (1)

 

When I was a very young boy, I liked to watch vehicles on the road, specific interest was given to the speed limit of the vehicle which was written on the right most side of the speedometer.  The young boy thought that the number of the speedometer expressed the real maximum speed of the vehicle, which was not true (I have realized after joining a automotive manufacturing company). Therefore the vehicle with a higher number in the speedometer was better vehicle to the boy.

 

Anyway, how fast can we run with a car ? Does the speed of a car increase unlimited as far as we push the acceleration pedal?  We know already from our insight that it is practically not possible. Then, why it is not possible?  Why the speed of vehicle cannot be increased more if it reaches a certain level ? Let’s have more precise look on it. Imagine you are in the seat of a car running on the very straight road without end, and you have pushed acceleration pedal down to floor. Again imagine each component of forces applies to vehicle body from front, from wheel or from back.

 

Traction Force

Because the car is being accelerated, a certain force is propelling the vehicle body. Let’s call this force a “traction force”. The only point in a vehicle power transaction takes places is the contact point between tire and road surface, as far as the vehicle is not using a jet engine. When the vehicle is being accelerated, the road is pushing tire to rotate as a reaction force. (In fact, the tire is being force to turn through power train of the vehicle and this tire pushed road surface, but the paved road surface is too hard and tight to move away, in consequence the tire is being pushed by the road surface.)

 

Resistance Force

The concept of resistance force is exactly opposite concept of traction force which means the force against vehicle running. The origin of traction force is only one which is power plant in the vehicle, but the origin of resistance force is not one thing but several origins like described below.

 

Aero Dynamic Resistance

 

Everything moves in the air will confront a resistance force created by the friction between the thing and air. The force is proportional to the square of moving velocity of the thing and a certain factor which is decided by the shape of the thing.  If the thing is shaped like fish, for example, then the aero-dynamic drag will be very low. Therefore, the vehicle shaped like falling water drop will experience lowest aero-dynamic resistance, and the vehicle running with 100 km/h will have four times higher aero-dynamic resistance than the vehicle runs with 50 km/h. If we can express this relationship is formula, then this will be like below.

 

Aero Dynamic  » Drag  x  1/2 x  Air       x  Projected x (Speed)*2

Drag                 Coefficient      Density     Area

 

To be continued at part (2)

 

▶ Back to Front Page ▶