Braking with a Trailer and Mountain Pass Descent

A truly strange - but very interesting - juxtaposition of thought occurs when considering customer’s deceleration needs for towing heavy trailers in mountainous regions, and the seemingly very different area of sizing brakes for Battery Electric Vehicles (BEV) and other regenerative braking-intensive vehicle applications, versus brakes for heavy-duty trucks and other vehicles rated to tow heavy trailers. The common threads between these two very different categories of vehicles include (a) heavy dependence on the powertrain and other non-brake sources of energy loss to control the speed of the vehicle on the grade and ensure adequate capacity of the brake system, (b) a need to consider descent conditions where towing a heavy trailer is feasible (in the case of heavy trailer towing) or initiating a descent with a full state of charge is realistic (in the case of BEVs), which forces consideration of different descents versus the typical (for brake engineers) mountain peak descent. In this paper, a vehicle operator-based requirement is derived that integrates engine (or motor), transmission (or gearbox), driveline, brake system, brake cooling, trailer brakes, and for BEVs, battery and regenerative brake capacity, and influences these parameters to enable combination vehicles (tow vehicle plus trailer) of any propulsion type to meet customer expectations for deceleration performance. Case studies are used throughout to illustrate the methodology, and physical (vehicle and chassis dyno) tests are shown to validate it. Additionally, industry standards (such as SAE J2807) that affect straight-line braking of combination vehicles are reviewed for what they really mean to the tow vehicle design. Enhancements to these standards are proposed, and when merged with the mountain pass descent case, form a comprehensive means of specifying brake systems and influencing supporting systems to ensure brake performance for a diverse range of vehicles.