Trailer Leaf Springs

Parabolic vs Multi-Leaf: Choosing the Right Leaf Spring

A parabolic leaf spring uses two to four tapered leaves, each thickest at the center bolt and thinning toward the eyes, so the whole pack shares stress evenly across its length. That taper cuts inter-leaf friction almost to zero, which gives a noticeably softer, quieter ride and shaves several kilograms off unsprung weight compared to a multi-leaf pack rated for the same load. It's the configuration we recommend for highway reefers, tankers, and any fleet where ride quality and fuel economy matter more than field-repair simplicity.

A multi-leaf spring assembly stacks five to nine leaves of constant cross-section, from the longest master leaf down to progressively shorter leaves clamped underneath. The leaves rub against each other under load, which adds interleaf friction and a stiffer, more damped response — exactly what's wanted on broken haul roads, mine sites, and overload duty where a bent or cracked leaf needs to be swapped out with basic tools rather than a full assembly change. Most of what we ship into African and Middle Eastern markets is multi-leaf for this reason: any welder with a bench vise can pull a leaf and replace it.

Both types pair with the same trailer suspension hangers and equalizer beams once eye spacing and center bolt position match the original spec, so a fleet running mixed trailers doesn't need to standardize on one type across the yard.

Steel Grade, Leaf Count, and Capacity Rating

We cut leaves from 60Si2Mn or 55CrMnA silicon-manganese spring steel, both of which hold up under repeated flex without losing set. After forming, each leaf goes through quench-and-temper heat treatment to bring hardness into the HRC 42-48 range, then shot peening to induce compressive surface stress that delays fatigue crack initiation — the single biggest factor in how many loaded miles a leaf spring assembly survives before a leaf snaps.

Leaf count and capacity scale together: a 3-leaf parabolic pack in 90 mm width typically carries 3,000-4,000 kg per spring, a 5-leaf multi-leaf pack in the same width runs 4,500-6,000 kg, and a 7 to 9-leaf stack pushes past 6,500-8,000 kg for tri-axle tippers and heavy lowbeds. We size leaf count and thickness against the actual axle rating rather than defaulting to a stock catalog number, since an under-rated semi trailer leaf spring sags permanently within months on overloaded routes while an over-rated one rides harsh and wears bushings faster than it should.

Corrosion protection is matched to the trade lane: black e-coat is standard for dry inland routes, hot-dip galvanizing goes on anything destined for coastal, monsoon, or de-icing-salt regions where an unprotected leaf spring can lose 20-30% of cross-section to rust within a few seasons.

Eye Types, Center Bolt Sizing, and Shackle Compatibility

Each leaf spring assembly needs a fixed rolled eye on one end and, depending on the suspension geometry, either a second rolled eye or a slipper (sliding) end on the other. The fixed eye bolts straight to the front hanger bracket; the rear end either pivots on a shackle through a rolled eye or slides directly against a slipper pad cast into the rear hanger, which lets the spring's effective length change slightly as it flexes without inducing binding stress. Getting eye diameter, eye spacing, and slipper pad width wrong is the most common reason a replacement leaf spring for trailer use won't seat in existing hangers, so we confirm all three dimensions against the original part before quoting.

Center bolts run M12 to M16 in grade 8.8, sized to clamp the leaf stack tight enough that the pack acts as a single unit under load rather than leaves shearing sideways against each other. Bolt head orientation matters too — it has to clear the axle seat and U-bolt plate, which is why we ask for the mounting orientation on custom orders. The U-bolts and center clamp plates that secure the finished leaf spring assembly to the trailer axle are sized to the same spring width and axle seat diameter, so we quote them together to avoid a mismatched clamp width showing up after the parts have already shipped. Buyers often search for a u bolt for leaf spring or a shackle for leaf spring replacement without realizing the two need to be matched to the same spring width and eye diameter as the spring itself — ordering them separately from different sources is the most common cause of a clamp that doesn't seat evenly or a shackle pin that binds.

Camber, Fatigue Life, and Field Inspection

Free camber — the upward curve built into an unloaded leaf spring — is set during heat treatment to a target dimension, typically 90-180 mm depending on spring length and rated capacity. Camber determines ride height at both empty and loaded conditions; a spring that's lost more than about 15-20 mm of camber compared to spec is fatigued and riding on reduced travel even if no leaf has visibly cracked yet. We check camber on every batch against the drawing before packing, since a spring that ships flat or over-cambered will throw the trailer's ride height off on installation.

Expected service life for a properly rated leaf spring assembly running within its load rating is 300,000-500,000 km on paved routes, less on constant off-road or overload duty where cyclic stress accumulates faster. The earliest failure signs are usually a cracked or snapped leaf near the center bolt, visible gaps opening between leaves under load, or a trailer that visibly sags to one side — any of which means the assembly needs replacing rather than just re-shimming. We recommend visual inspection at every service interval and immediate replacement of any leaf spring showing a crack, since a single failed leaf overloads the remaining leaves and accelerates their failure too. A worn leaf spring and shackle combination is usually replaced together for exactly this reason, even when only one of the two shows visible wear.