Manufacturers use many terms to describe the performance of vehicles — horsepower, torque, gradeability, turning radius, side-slope stability and others. Here is a brief guide to help users understand what some of these terms really mean in everyday operation.

Horsepower. Engines are rated in brake horsepower produced at a specific engine speed. This is called the rated horsepower. The maximum horsepower usually is the same as the rated horsepower, although it may be higher. The governed horsepower is measured at the governed engine speed and may be lower than the rated or maximum horsepower. Horsepower helps determine how fast the vehicle can go with a specific load under specific operating conditions such as grade, rolling resistance and head winds. The parasitic horsepower is the amount of power diverted, or lost, to various components before the engine horsepower reaches the driving tires. Fans, alternators, air compressors, and other engine-driven components divert horsepower. Friction in the transmission and rear axle gearing does, too. Pumps operated in the pump-and-roll mode divert horsepower. The more horsepower that gets diverted to run components, the less horsepower there is to move the vehicle.

Torque. Engines also are rated in the maximum foot-pounds of torque produced at a specific engine speed. This is called the peak torque. The peak torque occurs at an engine speed that is much lower than the governed engine speed. Torque helps determine the grade the vehicle can climb with a specific load under specific operating conditions such as tire size, total gear reduction and rolling resistance. This is called gradeability. The vehicle speed while climbing that grade is determined by the engine rpm, total gear reduction and tire size. To climb a steeper grade at the same speed, the vehicle needs more engine torque. To climb a steeper grade at a faster speed, the vehicle needs more engine torque delivered at a higher engine rpm, which means more engine horsepower. The old trucker’s rule of thumb is “torque determines the grade, horsepower determines the speed.”

Engine speed. Engine speed is measured in revolutions per minute. The governed rpm is the maximum allowable engine speed. The cruising rpm is lower than the governed and is usually the point of best fuel economy and all-around engine response. It is the recommended engine rpm for prolonged driving. The peak torque rpm is the point of maximum torque and is usually the lowest recommended engine speed when the vehicle is moving. In general, if two engines have the same horsepower, the one with significantly higher governed rpm will have a lower peak torque. Be aware of this when comparing engines and be sure to check both horsepower and torque.

Turning radius. The measurement of how tightly a vehicle will turn is usually specified by the turning radius with the front wheels angled fully to one side or the other. Sometimes it is specified by the turning circle or diameter, which is twice the radius. The curb-to-curb radius is measured from the center of the circle to the outer edge of the outer front tires. This is an indication of how tightly a vehicle can turn on a street without hitting the curbs. The wall-to-wall radius is measured from the center of the circle to the forwardmost and outermost edge of the cab or extended bumper. This is an indication of how tightly a vehicle can turn on a street or alleyway without hitting the walls, telephone poles and other objects adjacent to the roadway. The wall-to-wall radius is usually the greatest and is the one to use. Both are determined by the outside front wheel cut in degrees and the wheelbase of the vehicle in inches — the larger the outside wheel cut and the shorter the wheelbase, the tighter the radius. Most vehicles have different wheelcuts to the left and right and will therefore have different turning radius to the left and right. Consider both.

Side-slope stability. When a vehicle drives on a side slope, where one side of the vehicle is higher than the other, the center of gravity shifts toward the downhill side. If the center of gravity shifts beyond the point where the downhill tire contacts the ground, the vehicle will tip over. The best way to maintain stability is to keep the center of gravity low. Tankers/tenders and aerials have naturally higher centers of gravity than other vehicles and need careful design. NFPA 1901 and 1906 establish requirements for vehicle stability, weight distribution and load distribution. These are good items to include in apparatus specifications.

Angles of approach and departure. When a vehicle encounters a sharp grade change, the overhanging front or rear portions of the chassis and body can hit the roadway. Going up steep residential driveways, crossing severe dips in the road, or even entering and exiting to the street from an angled station apron can cause damage to the vehicle. To avoid these problems, the NFPA 1901 and 1906 specify minimum angles of approach and departure for apparatus. The angle of approach is the angle between the ground and the lowest portion of the vehicle ahead of the front tires when measured in a vertical plane from the point of contact between the front tires and the ground. To visualize this, imagine sliding a sheet of plywood along the ground under the front of the vehicle until the long back edge hits the front tires, then raising the front edge until the plywood hits some part of the vehicle. The angle formed between the ground and the raised plywood is the angle of approach. The angle of departure is the same, only between the rear tires and the rear of the vehicle. Vehicles with short wheelbases and long rear overhangs are the most vulnerable. Wildland vehicles need much larger angles of approach and departure than others.

There are lots of other terms to describe vehicle performance. Some are clear, while others are not. The NFPA standards contain good definitions and explanations to help you understand many terms. If a particular vehicle manufacturer uses a term you don’t understand, ask for an explanation. If you are interested in a particular value — for example, the left and right curb-to-curb turning radius — ask the vehicle manufacturer for that information. If the manufacturer cannot give you the answer, maybe you should be dealing with one that can.