Short Answer:

Tractive force is the pulling or driving force applied by a vehicle to move it forward on a surface. It is the part of the engine power that is actually used to overcome road resistance and provide motion. In simple words, tractive force is the force that helps the wheels of a vehicle to grip the road and move without slipping.

Tractive force depends on the friction between the driving wheels and the road surface. If the tractive force is higher than the resistance forces like rolling resistance or air drag, the vehicle will move forward smoothly. It is an essential factor for the performance and stability of vehicles.

Detailed Explanation:

Tractive Force

Tractive force is the force that allows a vehicle to move forward by transmitting engine power to the road surface through the wheels. It is also called the driving force of a vehicle. The main function of tractive force is to overcome all types of resistances that act on the vehicle during motion, such as rolling resistance, air resistance, gradient resistance, and frictional losses.

In a vehicle, the engine produces torque which is transmitted through the transmission system to the wheels. The wheel then pushes backward on the road surface. Due to Newton’s third law of motion (every action has an equal and opposite reaction), the road pushes the wheel forward with an equal and opposite reaction. This reaction force is called the tractive force. Thus, the tractive force is the useful part of the engine power that causes motion.

Generation of Tractive Force

The tractive force in a vehicle is generated through the interaction between the wheel and the road surface. The steps are as follows:

The engine develops torque.
This torque is transmitted to the driving wheels through the clutch, gearbox, propeller shaft, and differential.
When the wheel rotates, it exerts a backward force on the road due to friction.
The road surface reacts with an equal and opposite forward force on the wheel.
This forward force is the tractive force that moves the vehicle.

The amount of tractive force depends mainly on the coefficient of friction (μ) between the tire and the road, and on the normal reaction (W) acting on the driving wheel. It can be expressed as:

Where,
F = Tractive force
μ = Coefficient of friction between wheel and road
W = Normal load on the driving wheel

If the tractive force exceeds the available frictional force, the wheel will slip and lose traction, reducing the vehicle’s ability to move efficiently.

Factors Affecting Tractive Force

Type of Road Surface:
Smooth surfaces like concrete provide less grip compared to rough surfaces. Rougher roads generate higher friction and therefore more tractive force.
Condition of Tires:
Worn-out or under-inflated tires have less grip, which reduces the available tractive force. Proper tire maintenance ensures efficient traction.
Weight on Driving Wheels:
A heavier load on the driving wheels increases the normal reaction and improves tractive force, but too much weight can cause excessive rolling resistance.
Coefficient of Friction:
The coefficient of friction varies for dry, wet, or icy roads. Dry roads have a higher friction coefficient, thus providing better tractive force.
Engine Power and Torque:
The tractive force also depends on how much torque the engine produces and how efficiently it is transmitted to the wheels through the transmission system.

Types of Tractive Force

Maximum Tractive Force:
This is the highest possible tractive effort that can be developed without wheel slip. It depends on the coefficient of friction and the weight on the driving wheels.
Actual Tractive Force:
This is the force that is actually available to move the vehicle after overcoming the resistance forces. It is always less than the maximum tractive force.
Limiting Tractive Force:
It is the maximum value of tractive force that can be developed before the wheel starts slipping. Beyond this limit, the tires lose grip on the road.

Importance of Tractive Force

Tractive force plays a key role in the performance of vehicles. Some of its main functions are:

To start the vehicle: Without tractive force, the vehicle cannot overcome its inertia and start moving.
To maintain motion: It keeps the vehicle moving by balancing the resistance forces such as air drag and rolling resistance.
To climb slopes: The tractive force helps to overcome gradient resistance when moving uphill.
To achieve acceleration: More tractive force leads to better acceleration of the vehicle.
To ensure safety: Proper tractive force prevents wheel slipping or skidding, ensuring stable vehicle control.

Applications of Tractive Force

In automobiles, it helps vehicles move forward efficiently on roads.
In railways, tractive force is developed by the adhesion between locomotive wheels and rails.
In tractors, tractive force helps to pull loads or agricultural equipment in fields.
In construction vehicles, it ensures smooth movement on rough or uneven terrain.

Relation Between Tractive Force and Vehicle Performance

The tractive force directly influences the vehicle’s ability to move and accelerate. If the tractive force is higher than the total resistance acting on the vehicle, it will accelerate. If it equals the total resistance, the vehicle will move at a constant speed. If it is less, the vehicle will slow down or stop.

The vehicle designers focus on maximizing the tractive force without causing tire slip. This is achieved by improving tire design, weight distribution, and road adhesion.

Conclusion:

Tractive force is the essential driving force that moves a vehicle forward by converting the engine’s torque into useful motion through the wheels. It depends on the friction between the wheel and road surface, the load on the driving wheel, and the road condition. Proper tractive force ensures smooth acceleration, safe driving, and better performance. Thus, it plays a vital role in all types of vehicles, from automobiles and tractors to trains and heavy machinery.