Torque Steer

Torque Steer is a yawing effect caused by unequal stiffness driveshafts, most frequently due to unequal length. A stiffer shaft will turn the wheel further than a softer shaft for the same input torque.

Example Calculation

Using ballpark values, we can calculate an approximate torque steer value for a generalized FSAE car.

Step 1: Polar Moment of Inertia

The moment of inertia (J) for a hollow shaft is found using the formula π/32 * (D⁴-d⁴). Using values from an RCV axle below, we can find J.

Shaft OD (D)	0.8"
Shaft ID (d)	0.5"

Shaft Area of Inertia = 0.034076in2

Step 2: Calculate Torsional Stiffness

Shaft Material: 4130 Steel

4130 Shear Modulus: 80 Gpa or 1.16e+7 psi

Left Shaft Length: 15 in

Right Shaft Length: 19 in

k = G*J / L

Left Torsional Stiffness (k_L): 38.3379023 lb-ft / degree

Right Torsional Stiffness (k_R): 30.26401 lb-ft / degree

Step3: Find Torque

Max Torque from Engine (Launch): 40 lb-ft

Final Drive Ratio: 3.55

Torque Seen by Driveshaft: 142 lb-ft

Step 4: Find Shaft Twist

phi = T / k

Left Shaft Twist (phi_L): 3.704 degrees

Right Shaft Twist (phi_R): 4.692 degrees

Step 5: Find Vehicle Yaw

Tire travel (T) = Shaft twist/360 * Tire Circumference

Yaw = arcsin(T_max-T_min / Track Width)

Circumference of Tire: 56.52 in

Left Tire Travel: 0.582 in

Right Tire Travel: 0.737 in

Rear Track: 36 in

Yaw: 0.247 degrees