What causes leaf spring sagging?

Leaf spring sagging refers to the permanent loss of original arch height, where the spring flattens or curves downward under static load, resulting in lowered chassis height, poor handling and uneven tire wear. This irreversible deformation arises from material fatigue, improper matching, overloading, faulty installation, aging accessories and poor maintenance. Below is a full breakdown of all root causes.

First, long-term overloading and sustained excessive bending stress are the most prevalent cause. Every leaf spring is designed with a fixed rated load matching its axle tonnage. When trucks or semi-trailers carry cargo exceeding the spring’s load limit daily, the steel plates stay under extreme bending force for hours. The internal metal grain structure gradually suffers plastic deformation instead of elastic rebound after unloading. Mining dumpers and bulk haulage vehicles with chronic overload will see obvious sagging within several months. Even short-term heavy overload during mountain climbing creates instantaneous impact loads far beyond design standards, accelerating permanent arch loss.

Second, insufficient load grade and mismatched spring specifications. Many operators install lightweight springs on heavy-duty axles to cut costs. Fitting 3-leaf light-load springs on a 16-ton axle, or using soft parabolic springs for mine haulage creates inadequate structural rigidity. The spring cannot provide enough support force under normal rated load, leading to gradual flattening. Even springs with correct tonnage but thin single leaves or fewer layers lack sufficient bearing capacity and will sag prematurely under regular full loads. In addition, replacing original multi-leaf springs with thin parabolic versions without adjusting load capacity will directly trigger chassis sinking.

Third, material defects and incomplete heat treatment during manufacturing. Premium leaf springs rely on 60Si2Mn or 50CrV4 alloy steel plus strict quenching and tempering to maintain stable elasticity. Low-quality products adopt ordinary carbon steel with low elastic limit; they deform permanently after minor overload. Even qualified alloy steel will lose elasticity if heat treatment is skipped or improperly executed. Springs without shot peening lack surface compressive stress, and micro-fatigue cracks expand quickly to weaken overall support performance, bringing on sagging much earlier than standard products.

Fourth, loose or misaligned mounting components break uniform stress distribution. Loose U-bolts are a typical installation defect. If U-bolts are not evenly tightened during replacement, leaf springs shift horizontally during driving. Localized stress concentration occurs on partial steel plates, causing uneven arch loss and unilateral sagging. Worn, cracked spring bushings and damaged shackles create extra clearance at spring eyes. The spring loses fixed support points, bends abnormally under load and gradually flattens. Deformed bogie balance beams on 6×4 tractors also make one side of the suspension bear extra weight, leading to one-sided chassis sagging.

Fifth, accumulated fatigue from long service life and cyclic vibration. Leaf springs endure millions of bending cycles on bumpy roads over years of operation. Repeated compression and rebound generate metal fatigue even without overload. The elastic performance of steel slowly decays, and the original arch height cannot be restored after unloading. For vehicles running more than 150,000 kilometers without spring replacement, mild sagging is a normal aging phenomenon. Partial rust pits on the steel surface accelerate fatigue progression by forming stress concentration points, worsening sagging speed.

Sixth, irregular driving conditions and uneven cargo loading. Uneven cargo distribution puts most weight on one side of the suspension. Continuous unilateral stress causes asymmetric sagging, with one side of the chassis noticeably lower than the other. Frequent high-speed crossing of potholes, steep slopes and rough mine roads delivers sharp impact shocks that deform the spring arch cumulatively. Prolonged low-speed full-load climbing keeps springs compressed at maximum bending degree for extended periods, speeding up plastic deformation.

Seventh, lack of routine maintenance accelerates performance degradation. Without regular lubrication between leaf layers, severe inter-leaf friction wears down the thickness of main and auxiliary leaves. Thinned steel plates reduce overall rigidity and support ability, triggering sagging. Unprotected spring surfaces develop rust after long exposure to rain, mud and road deicing salt. Corrosion erodes steel thickness and generates tiny pits, weakening structural elasticity and aggravating arch flattening.

In summary, leaf spring sagging mainly stems from chronic overload, mismatched low-load spring specifications, inferior raw materials and incomplete heat treatment, loose U-bolts and damaged suspension accessories, natural metal fatigue after long mileage, uneven cargo loading and lack of regular lubrication and anti-rust maintenance. Once sagging occurs, plastic deformation of steel is irreversible; the only solution is to replace with new load-matched heavy-duty leaf springs to recover normal chassis height and driving stability.

1. APA 7th Edition

Zhang, L. (2026). Formation mechanism and influencing factors of sagging deformation on commercial vehicle leaf springs. Automotive Suspension Fault Analysis, 2(1), 89–96.

2. MLA 9th Edition

Zhang, Lei. "Formation Mechanism and Influencing Factors of Sagging Deformation on Commercial Vehicle Leaf Springs." Automotive Suspension Fault Analysis, vol. 2, no. 1, 2026, pp. 89–96.

3. GBT 7714-2015

Zhang Lei. Study on the formation mechanism and influencing factors of commercial vehicle leaf spring deflection [J]. Failure analysis of automobile suspension, 2026, 2 (1): 89-96.