Front axle types and stub axle

Why it matters

The front axle carries the front of the vehicle, takes braking torque and road shocks, and at its ends holds the stub axles about which the wheels are steered. Its section shape, material and the way the stub axle is pinned to it decide strength, steering geometry and ease of service — and stub-axle types are a favourite viva and objective question.

Key ideas

Live and dead front axles.

• A dead front axle carries load and steering but transmits no drive. It is the usual front axle of rear-wheel-drive trucks, buses and older cars with a rigid (beam) front axle.
• A live front axle also transmits drive to the front wheels. Four-wheel-drive trucks and SUVs with a rigid front axle use a live axle housing with a differential and constant-velocity (CV) joints at the steering knuckles so the wheels can be driven while steered. Front-wheel-drive cars with independent suspension have no axle beam at all: the drive goes through two drive shafts with CV joints to hubs carried in steering knuckles.

Rigid (beam) dead front axle construction.

• Drop-forged from medium-carbon or low-alloy steel (for example 0.4 % C or Ni–Cr steel), heat treated.
• The middle part, between the spring pads, is an I-section — efficient in vertical bending, which is the main load there.
• The ends, between the spring pads and the kingpin bosses, change to a circular or elliptical section because braking torque twists this part, and closed round sections resist torsion best.
• The centre is usually dropped (drop-centre axle) so the engine and frame can sit lower.
• Spring pads carry the leaf springs; the ends have a boss or a yoke for the kingpin.

Stub axle (steering knuckle). A short axle that carries the wheel hub on two taper-roller (or ball) bearings and pivots on the kingpin (swivel pin) at the axle-beam end. It also carries the steering arm (and track-rod arm) and, often, the brake back plate or caliper. Bronze bushes take the radial load on the kingpin and a thrust bearing takes the vertical load. In independent suspension the kingpin is replaced by two ball joints and the stub axle becomes part of the upright or knuckle.

Types of stub-axle joint.

• Elliot type — the end of the axle beam is a yoke (fork); the stub axle has a single eye that fits inside the yoke, and the kingpin passes through all three.
• Reverse Elliot type — the reverse: the axle beam ends in a plain eye and the stub axle carries the yoke. This is the most widely used type.
• Lamoine type — no yoke; an L-shaped stub axle pivots in a single boss at the axle-beam end.
• Reverse Lamoine type — the Lamoine arrangement reversed (the L-shaped member and boss are on the opposite parts). Lamoine types are rare in current vehicles.

Loads on the front axle beam.

• Vertical: the frame pushes down at the spring pads, the wheels push up at the kingpins. Between a wheel and its spring pad the bending moment rises linearly; between the spring pads it is constant.
• Braking: the brake torque on the wheel is passed through the stub axle into the beam as torsion between the kingpin and the spring pad, and the braking force bends the beam horizontally.
• Cornering: side forces at the tyre contact patch bend the beam and load the kingpin bushes.

Formulas

W_w = W_f / 2

• W_w = vertical load per front wheel (N), W_f = front axle load (N). Static, level road, symmetric loading.

M = W_w · c

• M = vertical bending moment in the beam between the spring pads (N·m), c = horizontal distance from the wheel (kingpin) centre line to the spring-pad centre (m). The moment is constant between the pads.

F_b = μ · W_w, T_b = F_b · r

• F_b = braking force at the tyre (N), μ = tyre–road friction coefficient, r = rolling radius (m), T_b = brake torque carried into the beam end as torsion (N·m).

σ = 32·M / (π·d³), τ = 16·T / (π·d³)

• Bending and torsional shear stress on a solid circular section of diameter d (m).

τ_max = (16 / (π·d³)) · √(M² + T²)

• Maximum shear stress under combined bending and torsion (maximum-shear-stress theory) — used to check the round part of the beam near the kingpin.

Worked examples

Example 1 (standard) — I-section between the spring pads. A truck has a front axle load of 30 kN. The spring pads are 0.30 m inboard of the wheel centre lines. The I-section of the axle has a section modulus of 60 × 10³ mm³. Find the bending stress between the pads.

1. W_w = W_f / 2 = 30 / 2 = 15 kN.
2. M = W_w · c = 15 000 × 0.30 = 4500 N·m = 4.5 × 10⁶ N·mm.
3. σ = M / Z = 4.5 × 10⁶ / 60 × 10³ = 75 N/mm². σ = 75 MPa (constant between the spring pads).

Example 2 (GATE level) — combined bending and torsion near the kingpin. For the same axle under braking, the wheel load (including weight transfer) is 15 kN, μ = 0.6 and the rolling radius is 0.45 m. Check a solid circular section of diameter 70 mm located 0.15 m inboard of the wheel centre line.

1. Braking force: F_b = μ · W_w = 0.6 × 15 000 = 9000 N.
2. Torque on the beam end: T_b = F_b · r = 9000 × 0.45 = 4050 N·m.
3. Vertical bending at the section: M_v = 15 000 × 0.15 = 2250 N·m. Horizontal bending from the braking force: M_h = 9000 × 0.15 = 1350 N·m.
4. Resultant bending moment: M = √(2250² + 1350²) = 2624 N·m.
5. πd³ = π × 70³ = 1.0776 × 10⁶ mm³.
6. σ = 32 × 2.624 × 10⁶ / 1.0776 × 10⁶ = 77.9 MPa; τ = 16 × 4.05 × 10⁶ / 1.0776 × 10⁶ = 60.1 MPa.
7. τ_max = (16/(π·d³))·√(M² + T²) = 16 × √(2624² + 4050²) × 10³ / 1.0776 × 10⁶ = 16 × 4.826 × 10⁶ / 1.0776 × 10⁶ = 71.7 MPa. τ_max ≈ 72 MPa. Torsion dominates here, which is why the beam ends are made round rather than I-section.

Common mistakes

• Saying the front axle of a rear-wheel-drive vehicle is live; it is dead. Front-wheel-drive and 4WD vehicles drive the front wheels.
• Swapping Elliot and reverse Elliot: in the Elliot type the yoke is on the axle beam; in reverse Elliot it is on the stub axle.
• Saying the axle "absorbs shocks" — springs and dampers do that; the axle carries and transmits loads.
• Ignoring the torsion from braking and checking the beam end only for bending.
• Forgetting horizontal bending from the braking force when combining moments.
• Writing σ = 16M/(πd³) (that is the torsion constant) for bending.

For GATE ME

Expect identification questions (live versus dead axle, stub-axle types), and strength-of-materials style numericals: bending of the beam between the spring pads, combined bending and torsion at a round section using maximum-shear-stress or maximum-normal-stress theory. Practise drawing the bending-moment diagram for a beam loaded at two pads and supported at the two wheels.

Quick check

1. Which part of the stub-axle assembly carries the yoke in the reverse Elliot type?
2. Why is the axle beam an I-section in the middle but round near the ends?
3. Is the front axle of a rear-wheel-drive truck live or dead?
4. What is the bending moment between the spring pads if each wheel carries 12 kN and c = 0.25 m?
5. What carries the vertical load on the kingpin?

Answers: 1. The stub axle. 2. The middle carries mainly bending; the ends also carry braking torque, which a round section resists better. 3. Dead. 4. 12 000 × 0.25 = 3000 N·m. 5. A thrust bearing between the stub axle and the axle-beam boss.