What Is Tractive Force?

Tractive force is the force generated at the wheels that pushes a vehicle forward.

It comes from:

1. Engine torque
2. Transmission and gear ratio
3. Wheel radius
4. Available traction between tire and road

In basic terms:

Engine torque × gear ratio ÷ wheel radius = tractive force

But real-world driving is more complex. Air resistance, rolling resistance, road grade, and traction limits all reduce usable force.

That’s why a complete tractive force calculator includes much more than just torque.

Why Tractive Force Matters

Tractive force determines:

• Acceleration
• Towing capacity
• Hill climbing ability
• Off-road performance
• Maximum speed potential
• Wheel slip risk

For example:

• A heavy truck climbing a 10% grade needs high tractive force.
• A sports car on dry asphalt may have excellent acceleration but limited traction on ice.
• A loaded SUV will behave very differently from the same SUV empty.

Understanding tractive force helps you predict performance before you drive.

Key Inputs in a Tractive Force Calculator

A well-built calculator includes multiple performance variables. Let’s go through them one by one.

1. Vehicle Type and Weight

Vehicle weight directly affects:

• Rolling resistance
• Grade resistance
• Adhesion limit
• Acceleration

Typical presets include:

• Passenger car
• Sports car
• SUV
• Light pickup
• Heavy truck
• Bus
• Train
• Locomotive

Heavier vehicles need more force to move. That’s simple physics.

2. Engine Torque and RPM

Engine torque (lb-ft) is the twisting force from the engine.

More torque usually means:

• Stronger acceleration
• Better towing power
• Better grade climbing

Engine RPM determines engine power:

Engine Power (HP) = (Torque × RPM) ÷ 5252

Higher RPM increases power, but torque at the wheels depends on gear ratio.

3. Transmission Type and Efficiency

Different transmissions transfer power differently.

• Manual: ~95% efficiency
• Automatic: ~90% efficiency
• CVT: ~85% efficiency
• Dual-Clutch (DCT): ~92% efficiency

Efficiency reduces usable torque at the wheels.

4. Gear Ratio

Gear ratio multiplies torque.

• 1st gear gives high torque, low speed
• Higher gears give lower torque, higher speed

Example:

A 3.5:1 first gear multiplies engine torque 3.5 times.

But torque at wheels is divided by gear ratio in angular calculations, so the system balances speed and force.

5. Tire Size

Tire diameter affects:

• Wheel radius
• Tractive force
• Acceleration

Smaller tires increase force at the ground.
Larger tires reduce force but increase speed potential.

Wheel radius (feet) = Tire diameter ÷ 2 ÷ 12

6. Drive Type

Drive type affects traction.

• 2WD
• 4WD
• AWD
• 6×6
• 8×8

More driven wheels = better traction.

That increases usable tractive force before slipping.

7. Surface Type and Friction

Surface condition is critical.

Maximum tractive force is limited by adhesion:

Max Tractive Force = Adhesion Coefficient × Vehicle Weight

If tractive force exceeds this limit, wheels slip.

8. Road Grade

Grade (%) measures slope.

• Positive = uphill
• Negative = downhill

Grade resistance increases rapidly on steep slopes.

Grade Resistance = Vehicle Weight × sin(arctan(grade/100))

Even a 10% grade significantly increases resistance.

9. Speed and Air Resistance

Air resistance increases with the square of speed.

Air Resistance = 0.5 × Air Density × Drag Coefficient × Frontal Area × Velocity²

This is why:

• Doubling speed more than doubles drag
• High-speed performance requires much more power

10. Environmental Factors

Altitude reduces air density.

Temperature affects air resistance.

Higher altitude = lower engine power and lower drag.
Hot air = less dense air = reduced performance.

What the Tractive Force Calculator Calculates

A complete calculator provides multiple outputs.

1. Tractive Force at Wheels

The raw pushing force from wheel torque.

2. Net Tractive Force

Net Force = Tractive Force − Total Resistance

If this is positive, the vehicle accelerates.
If zero, it maintains speed.
If negative, it slows down.

3. Maximum Tractive Force (Adhesion Limit)

Shows whether traction limits performance.

If net force exceeds this, wheel slip occurs.

4. Adhesion Utilization

Percentage of available grip used.

• Below 50% = safe margin
• 80–95% = high risk
• Above 95% = likely slip

5. Power Analysis

Includes:

• Wheel power
• Engine power
• Power required
• Power-to-weight ratio
• Efficiency index

These metrics show how effectively power is delivered to the road.

6. Gradeability

Gradeability shows maximum slope the vehicle can climb.

This matters for:

• Trucks
• Off-road vehicles
• Heavy loads

7. Acceleration and Stopping Distance

Acceleration = Net Force ÷ Vehicle Mass
Stopping Distance depends on braking force and speed.

Higher speed dramatically increases stopping distance.

8. Stability and Slip Risk

Includes:

• Slip threshold
• Wheel slip percentage
• Dynamic stability
• Traction control effectiveness

This helps evaluate safety under different surfaces.

How to Use the Tractive Force Calculator

Follow these steps:

1. Select vehicle type
2. Enter vehicle weight (if different from default)
3. Input engine torque and RPM
4. Choose transmission type
5. Select gear ratio
6. Choose drive type
7. Select tire size
8. Choose surface condition
9. Enter grade (%)
10. Enter speed (mph)
11. Add altitude and temperature if needed
12. Select calculation type
13. Click Calculate

The calculator will display full performance analysis.

Example Scenario

Let’s say:

• SUV
• 4500 lbs
• 300 lb-ft torque
• 1st gear
• 4WD
• Wet asphalt
• 5% uphill
• 60 mph

You may see:

• Moderate tractive force
• High resistance from grade
• Reduced adhesion on wet surface
• Increased stopping distance

Small changes like switching to dry asphalt can dramatically increase usable force.

Who Should Use a Tractive Force Calculator?

This tool is useful for:

• Automotive engineers
• Car enthusiasts
• Truck operators
• Fleet managers
• Off-road drivers
• Motorsport teams
• Students learning vehicle dynamics

It turns complex physics into practical insight.

Key Takeaways

• Tractive force is the pushing force at the wheels.
• Torque, gear ratio, and tire size determine raw force.
• Air resistance, grade, and rolling resistance reduce usable force.
• Adhesion limits maximum traction.
• Speed dramatically increases resistance.
• Environmental conditions matter more than most people expect.

A tractive force calculator gives you a complete picture of vehicle performance, not just horsepower.