Truck Cab Air Springs: Selection and Replacement Guide

A truck cab air spring and a truck chassis air spring look superficially similar but solve different problems. Conflating them — sourcing one as if it were the other — leads to a part that does not fit, does not last, or rides badly. This guide explains what a cab air spring actually is, how to identify the right one for the major truck platforms, and how the replacement process differs from a chassis spring.

What a truck cab air spring is

A truck cab air spring isolates the driver's cab from the chassis. It is mounted between the cab frame and the chassis rails at two or four points (typically two at the rear of the cab, sometimes two more at the front).

In design, cab air springs are almost universally **convoluted** — one or two lobes, short stroke, high load capacity for their size, strong vibration isolation. Some platforms use compact rolling-lobe or sleeve units for the front cab mounts. The design choice is driven by the requirement to isolate high-frequency chassis vibration over a short travel range — exactly what convoluted springs do best. The rolling lobe vs convoluted comparison covers this trade-off in detail.

Compared to a chassis air spring, a cab air spring typically has:

• Lower load capacity — a typical cab spring carries 1,000–2,500 kg per corner, versus 9,000–12,000 kg for a chassis spring.
• Shorter stroke — often less than 80 mm of useable travel.
• Smaller envelope — has to fit between the cab and the chassis without intruding into the engine compartment.
• Higher cycle count — every chassis bump triggers a cab response.

Why cab air springs fail

Cab air-spring failure modes differ from chassis air-spring failure modes:

• Rubber aging. Cab springs sit near the engine bay and absorb radiated heat. UV exposure on the visible side accelerates ozone cracking. Over 6–8 years the rubber simply loses its elasticity.
• High-cycle fatigue. Every chassis bump cycles the cab spring. Long-haul drivers on rough roads see millions of cycles over the life of the part.
• Driver-weight cycling. Each time the driver climbs in and out, the spring compresses and rebounds. Over a vehicle's life this is a non-trivial cycle count.
• Contamination. Oil leaks from the engine, hydraulic fluid leaks from steering or brake lines, or coolant from a heater core can land on the bellows. All three degrade rubber faster than time alone.

The good news: cab springs are generally well-isolated from road shock and full-load conditions, which is why they outlast chassis springs in calendar years. The bad news: when they fail, the failure mode is often slow rubber degradation rather than a single dramatic event, so it is easy to miss.

How to identify the right cab air spring

The starting point is always the truck make and model. Each major truck manufacturer has its own cab air-spring conventions:

• Volvo FH / FM / FMX / FE — multiple suppliers; Volvo OEM numbers begin with a sequence specific to the model. The FH series cab uses different parts than the FM series.
• Mercedes-Benz Actros / Antos / Arocs — historically ContiTech is a primary supplier; specific part numbers vary by cab type (StreamSpace, GigaSpace, BigSpace) and by Actros generation.
• MAN TGX / TGS / TGM / TGL — multiple suppliers; the heavy-duty TGX uses different cab parts than the medium TGM.
• Scania R / S / G / P — Scania OEM and OES numbering; specific to model and to cab variant (CR, CS, CG, CP).
• Iveco S-Way / Stralis / Eurocargo — multiple suppliers across the platforms.
• DAF XF / XG / CF — DAF OEM numbering; new-generation XG cab has different parts from the previous XF.
• Renault T / C / K range — shared platform with Volvo on many cab parts.

The part number is moulded onto the bead plate or the bellows of the spring currently fitted. Search that number on the OEM cross-reference to find the equivalents — Firestone Airide, ContiTech, Phoenix / Vibracoustic and major Turkish manufacturers all produce cab air-spring equivalents under their own numbering.

Selection criteria

Once you have candidate equivalents from the cross-reference, the standard six dimensional and load criteria apply — see the OEM cross-reference guide for the full checklist. A few cab-specific notes:

• Convoluted design type must match — single, double or triple convolution. A double cannot substitute for a single without changing spring rate and stroke.
• Mounting plate orientation matters more than on chassis springs because the cab frame's bolt access is often constrained.
• Air-port location is often top-side or angled to clear the cab floor and the chassis rail; verify the angle against the candidate.
• Maximum bellows diameter at maximum pressure must clear the cab floor and the chassis frame rail.

A reputable supplier publishes a cab-specific datasheet — not just a generic catalogue page. If the supplier cannot tell you which truck platform the part fits, do not buy it for a cab application.

Diagnosing cab suspension issues

The symptoms of a failing cab air spring overlap with — but are not identical to — chassis air-spring failure. Watch for:

• Cab lean when parked. One corner of the cab sits lower than the others.
• Cab roll on cornering. The cab feels disconnected from the chassis, swaying noticeably under steering input.
• Harsh cab ride that was not there before. A failed spring rests on its internal bump stop — every chassis bump translates directly to the driver.
• Audible hiss from cab mount area. A persistent leak.
• Visible damage on the bellows. Cracking, weeping rubber, or chafing marks against a chassis bracket.

The diagnostic process — soap test, pressure decay, visual inspection — is the same as for chassis springs. The signs of a failing air spring guide covers it step by step.

Replacement: a safety note on cab raising

Replacing a cab air spring requires the cab to be raised or tilted to access the rear cab mounts. This is a safety-critical procedure:

• The cab must be supported by the manufacturer's specified cab-tilt mechanism or by certified support stands.
• The air system must be fully depressurised at the spring before unbolting.
• Hydraulic or electric cab-tilt mechanisms have their own safety locks — engage every lock.
• Never work under a cab supported only by air springs.

This guide does not provide a step-by-step replacement procedure. Cab air-spring replacement should be performed by a qualified commercial-vehicle service technician with the truck manufacturer's specific procedure for the cab platform involved.

Sourcing path

The practical sourcing sequence for cab air-spring replacement, whether for one truck or a fleet:

• Photograph the failing spring with its OEM part number visible.
• Search the part number on the OEM cross-reference and capture the equivalents.
• For a fleet, prefer suppliers with IATF 16949 certification and bus/truck-cab catalogue coverage. The supplier directory and manufacturer directory list certifications.
• Send an RFQ with the part number, the truck make and model, and the cab variant. Verified suppliers respond with datasheet and price.
• Always order in pairs per axle / per mount group.

For terminology — cabin air spring, convoluted, sleeve air spring — see the air spring glossary.