Leaf Spring

Leaf springs, commonly referred to as multi-leaf springs or simply leaf springs, are among the oldest suspension components in automotive history. Composed of several stacked spring steel leaves of varying lengths and matching curvatures, they rely on inter-leaf friction and natural elastic deflection to bear loads while cushioning road shocks.

Structure and Working Principle

Multi-leaf construction: The leaf pack is secured by a centre bolt and mounted to the chassis frame at both ends via suspension shackles. 

Friction damping: As the vehicle carries a load or navigates uneven surfaces, slight relative sliding occurs between the individual leaves. This inter-leaf friction inherently provides a damping effect by dissipating energy, which allowed early suspension setups to reduce their reliance on dedicated shock absorbers. 

Load distribution: As payload increases, the effective working length of the leaf spring shifts and the spring rate progressively stiffens (non-linear elasticity), ensuring adequate load-bearing support under heavy hauling. 

Why Are They Rare in Modern Passenger Cars?

While leaf springs have virtually vanished from passenger cars and modern passenger SUVs, they remain dominant in the commercial vehicle segment:

Advantages (Why commercial vehicles rely on them):

High load capacity: Simple and robust, it handles immense vertical payloads reliably without easily sustaining damage.

Cost-effective and low maintenance: The minimalist design makes servicing straightforward and replacement parts highly affordable.

Simple packaging: Acting as an integral suspension member, it combines axle location and springing duties into one, cutting down the need for extra control arms and linkages. 

Disadvantages (Why passenger vehicles avoid them): 

Poor ride comfort: Due to inherent inter-leaf friction and substantial self-weight (high unsprung mass), it barely filters out road imperfections, leading to a notably harsh and bumpy ride. 

Heavy weight: Adds significantly to the unsprung weight, compromising dynamic handling and fuel efficiency.

Vague steering and handling precision: Difficult to dial in refined suspension geometry for precise cornering response.

Applications

Heavy-duty 4x4s and off-roaders: Utilised in classic Toyota pick-up trucks and rugged 4WDs for their sheer durability and torsional strength.

Heavy-duty trucks and lorries: The primary home ground for leaf springs, built specifically to tackle multi-tonne commercial payloads.

Common Issues and Maintenance

Inter-leaf wear and squeaking: The most common symptom is a squeaking friction noise, usually caused by dried-out grease or road grit trapped between the leaves.

Maintenance tip: Periodically clean the leaf pack and apply dedicated graphite grease between the spring leaves.

Spring sagging or snapping: Prolonged overloading causes metal fatigue, leading to vehicle sagging/uneven ride height on one side or a snapped spring leaf. 

Repair advice: A broken leaf must be replaced immediately; never attempt to weld it, as welding compromises the heat-treated metallurgical structure and is hazardous. 

Fastener inspection: The centre bolt and end shackles are high-stress points; regularly checking their torque is essential to prevent axle misalignment.