Bilstein B4 OE Replacement Shock Absorber REAR SINGLEFor an optimum driving experience and safety. Over 60 years ago, we developed the monotube gas pressure shock absorber for series production. The BILSTEIN B4 OE replacement series is available for most passenger cars in both monotube and twin tube shock absorber versions. Technology: BILSTEIN gas pressure technology Traditional oil shock absorbers, when put under extreme stress, can lead to oil foaming which causes shock fading and results in damping