An air strut combines a shock absorber and an air spring into one compact unit. Commonly used on the front suspension of luxury SUVs and passenger cars, the air rubber bladder wraps around the upper half of the damper shaft. This single assembly handles both load support (ride height) and shock dampening. Many air struts also feature electronic valving inside the shock to adjust stiffness on the fly.
Replacing air struts is expensive because of the complex design. When the rubber bladder leaks, you must replace the assembly, or at least rebuild the strut seal. Mechanics recommend replacing them in pairs to maintain balanced handling and braking response across the axle.