Short Answer:

Tractive force is the force applied by a vehicle to move itself forward by overcoming all the resistances acting against its motion. It is the pulling or driving force generated at the point of contact between the wheels and the ground. This force helps the vehicle to accelerate, climb slopes, and carry loads.

In simple words, tractive force is the useful force available from the engine and transmission system that actually moves the vehicle. Without sufficient tractive force, a vehicle cannot start, move, or maintain its speed on a road surface.

Detailed Explanation :

Tractive Force

Tractive force is one of the most important concepts in vehicle mechanics and transportation engineering. It is defined as the force exerted by the driving wheels of a vehicle on the road surface to move the vehicle forward. In other words, it is the force available at the contact point between the tire and the road which overcomes the resistances such as rolling resistance, air resistance, and gradient resistance.

Every vehicle engine produces power, but that power must be converted into a usable force that can make the vehicle move. The tractive force acts as that converting link, turning engine power into motion.

1. Meaning and Basic Concept

When a vehicle engine runs, it transmits torque through the clutch, gearbox, and differential to the driving wheels. This torque, when applied to the wheels, tries to rotate them. The wheels push backward against the road surface, and due to Newton’s third law of motion, the road pushes the wheels forward with an equal and opposite reaction. This forward reaction is known as tractive force.

It is this tractive force that makes the vehicle move ahead. The stronger the tractive force, the faster the vehicle can accelerate or climb an incline. However, the tractive force cannot exceed the frictional force between the wheel and the road, otherwise, the wheels will start slipping instead of rolling.

2. Expression for Tractive Force

The tractive force can be expressed mathematically as:
Ft = (T × ηt) / r
Where,

• Ft = tractive force (in newtons)
• T = torque transmitted to the driving wheels (in newton-meter)
• ηt = transmission efficiency
• r = radius of the driving wheel (in meters)

This equation shows that the tractive force depends on the torque provided by the engine, the efficiency of the transmission system, and the size of the wheels. If the torque or efficiency increases, the tractive force also increases.

3. Factors Affecting Tractive Force

Several factors affect the amount of tractive force available to a vehicle:

• a) Engine Power: A powerful engine produces more torque, leading to greater tractive force.
• b) Gear Ratio: Lower gears provide higher torque at the wheels, increasing tractive effort, which is useful during starting or climbing hills.
• c) Road Surface: Smooth or slippery surfaces reduce friction, lowering the available tractive force. Rough or dry surfaces increase it.
• d) Tire Condition: Proper tire tread and pressure ensure maximum grip and better transmission of tractive force.
• e) Vehicle Load: A heavier vehicle increases the normal reaction between the tire and road, thus improving tractive grip up to a limit.

4. Types of Resistances Opposed by Tractive Force

The tractive force developed by the vehicle must overcome various resistances during motion. The main resistances are:

• Rolling Resistance: The friction between tires and road while rolling.
• Air Resistance: The opposing force of air as the vehicle moves forward.
• Gradient Resistance: The resistance offered when a vehicle moves uphill due to its own weight component acting backward.
• Starting Resistance: The additional resistance encountered while starting from rest.

For a vehicle to move forward, the tractive force must be greater than the sum of all these resistances.

5. Limiting Tractive Force

The maximum tractive force that can be developed without slipping is limited by the coefficient of friction (μ) between the wheel and the road surface. The limiting tractive force is given as:
Ft(max) = μ × W
Where,

• W = load on the driving wheel (in newtons)
• μ = coefficient of adhesion (friction) between wheel and road

If the applied tractive force exceeds this limiting value, the driving wheels will spin without effectively moving the vehicle. This is often seen when vehicles accelerate too quickly on slippery roads.

6. Importance of Tractive Force

Tractive force plays a vital role in the performance of a vehicle. Some key points include:

• It allows the vehicle to start from rest and accelerate smoothly.
• It helps to overcome gradient and rolling resistances while driving.
• It determines the climbing ability of the vehicle on slopes.
• It affects vehicle speed, acceleration, and fuel efficiency.
• It ensures safe and efficient operation under different road conditions.

Thus, the design of the power transmission system and tires is done to ensure adequate tractive force under all driving conditions.

Conclusion

Tractive force is the driving or pulling force generated by the wheels of a vehicle that moves it forward by overcoming all the resistances to motion. It depends on factors like engine torque, gear ratio, friction, and wheel size. Proper tractive force is essential for smooth acceleration, good fuel economy, and stable driving. Insufficient tractive force leads to slipping and poor vehicle control, while excessive force can waste energy and damage components. Hence, maintaining a balance between engine power and available traction is crucial for efficient vehicle performance.