🚗 Wheel Torque & Tractive Force Solver
Enter engine specs, transmission gears, and tire dimensions below
How Engine Torque Becomes Wheel Torque
A common misconception in vehicle dynamics is that engine crankshaft torque is what accelerates the car. In reality, the engine's torque is multiplied by the transmission gear ratio and then further multiplied by the differential's final drive ratio before reaching the axles. This output is known as **Wheel Hub Torque**.
The core equations are:
Linear Thrust: Tractive Effort & Tire Radius
The wheel hub torque is converted into a linear force (thrust) at the tire contact patch where the tread grips the road. This forward push is known as **Tractive Force** or Tractive Effort. To calculate it, divide the wheel torque by the radius of the tire:
Because the radius is in the denominator, smaller tires produce more tractive thrust for the same axle torque, while larger tires reduce force (but increase top speed capability for a given engine RPM).
Drivetrain Layout Efficiencies Comparison
Some power is lost as heat as torque travels through gears, universal joints (U-joints), CV joints, and viscous clutches. Typical efficiency ratings depend on the vehicle layout:
| Drivetrain Layout | Typical Efficiency | Why Energy is Lost | Automotive Example |
|---|---|---|---|
| Front-Wheel Drive (FWD) Transverse | 88% - 92% | Short power path, fewer gear sets | Honda Civic, Toyota Corolla |
| Rear-Wheel Drive (RWD) Longitudinal | 83% - 87% | 90-degree turn in differential, long driveshaft bearings | Ford Mustang, BMW 3 Series |
| Four-Wheel Drive (4WD) / AWD | 78% - 83% | Transfer case drag, front and rear differentials, multiple CV joints | Ford F-150 Raptor, Subaru WRX |
Worked Calculation Example
📐 Example: Calculating Wheel Force in 1st Gear
A sports car produces 300 lb-ft of engine torque. It is in 1st gear (ratio of 3.5:1), and has a rear differential ratio of 3.73:1. The drivetrain efficiency is 85% and the tire diameter is 26 inches.
Step 1: Calculate Gear Multiplication
Total Gearing = 3.5 × 3.73 = 13.055
Step 2: Calculate Hub Torque
Hub Torque = 300 lb-ft × 13.055 × 0.85 = 3,329.03 lb-ft
Step 3: Calculate Tire Radius (feet)
Radius = (26" / 2) = 13" = 1.083 feet
Step 4: Calculate Tractive Force
Tractive Force = 3,329.03 lb-ft / 1.083 ft = 3,073.9 lbs of thrust
Frequently Asked Questions (FAQ)
Why does a car feel faster in first gear than fifth gear?
This is due to torque multiplication. First gear has a high ratio (e.g., 3.5:1), which multiplies engine torque significantly at the wheels. Fifth gear is usually an overdrive gear (e.g. 0.8:1), which reduces axle torque below crankshaft torque, offering higher speed but much less acceleration thrust.
Does tire slippage affect the tractive force?
Yes. The maximum tractive force a tire can transmit is limited by the tire's coefficient of friction and the weight pressing down on the drive wheels. If the calculated tractive force exceeds this grip limit, the tires spin, and traction is lost.
What socket sizes and tools are needed to measure axle torque?
Axle torque is not measured with hand tools; it is calculated or measured using a chassis dynamometer (dyno) or strain-gauge hub sensors on race cars.