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The variation in the decoupling effect of exhaust flexible couplings under a vertical preload caused by changes in the direction of the exhaust pipe routing was investigated. Both self-supporting and underbody flexible couplings were tested. The results indicate that, in general, a preload decreases the decoupling efficiency of both types of flexible couplings. In addition, the results indicate that the efficiency of the flexible coupling is effected by the following three conditions: the direction of preload with respect to gravity, the location of the preload relative to the coupling, and the stiffness of the various components of the flexible coupling.

The effect of adjusting the kick-off speed to attain a “worst case vehicle configuration” on drive-by noise measurements using test procedure SAE J986OCT88 was investigated. It was determined that similar noise results were obtained for both the “forced worst-case” vehicle and the actual worst-case vehicle. This information is a useful development tool due to the fact that a wide variety of axle ratios, tires, and transmissions are available as options on light duty trucks, making it difficult to determine or obtain the actual worst-case vehicle. Forcing the worst-case configuration allows for the testing of vehicles when the exact worst case vehicle is not available.