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In an effort to reduce the dry stopping distance required for heavy trucks, it is imperative to increase the effectiveness of the foundation brake systems. Where brakes are torque limited, increasing the brake output can be obtained by increasing brake size, chamber size, slack length, and friction of the braking materials. Looking just at the aspect of foundation brakes, the majority of current tractor and trailer brakes are of the S-Cam and Drum type. Two commercially available alternatives that produce higher output are Air Disc brakes and larger sized S-Cam brakes. Using one type, or a combination of these brakes (discs and drums on different axles) warrants a comparative study. The goal is to improve the effectiveness of the brake system, while maintaining or improving upon vehicle stability during braking. NHTSA's Vehicle Research and Test Center recently completed a brake test study of the effectiveness and stability characteristics of tractor and trailer combinations.