Motorcycle testing and measurement

Two motorcycles at a ¼ mile (402 m) dragstrip.

Motorcycle speed tests, especially at high speeds, are prone to variation due to human error, limitations in equipment, and atmospheric factors like wind, humidity, and altitude. The published results of two otherwise identical tests could vary depending on whether the result is reported with or without industry standard correction factors calculated to compensate for test conditions. Rounding errors are possible as well when converting to/from miles and kilometers per hour.[1]

With power typically being the product of force and speed, a motorcycle's power and torque ratings will be highly indicative of its performance. Reported numbers for power and torque may however vary from one source to another due to inconsistencies in how testing equipment is calibrated, the method of using that equipment, the conditions during the test, and particularly the location that force and speed are being measured at.[2] The power of the engine alone, often called crankshaft power, or power at the crankshaft, will be significantly greater than the power measured at the rear wheel. The amount of power lost due to friction in the transmission (primary drive, gearbox and final drive) depends on the details of the design and construction. Generalizing, a chain drive motorcycle may have some 5-20% less power at the rear wheel than at the crankshaft, while a shaft drive model may lose a little more than that due to greater friction.

• 
  Preparing for a dyno test – note the fan used for engine cooling
• 
  Motorcycle rear wheel on a dyno roller.
• 
  Graph of power and torque.

While the crankshaft power excludes these transmission losses, still the measurement is often made elsewhere in the drive-train, often at the rear wheel. A correction for the transmission losses is then applied to the measured values to obtain the crankshaft values. For motorcycles, the reported power and torque numbers normally pertain to the crankshaft. In directive 92/61/EEC of 30 June 1992 relating to the type-approval of two or three-wheel motor vehicles,[3] it is referred to as "maximum engine power", and manufacturers use similar terms. Historically, this convention may have come from the pre-unit construction, wherein the crankshaft was directly accessible for measurements, and the gearbox might have come from a different manufacturer. However, when the engineering details of the transmission are known, the losses therein can be accurately quantified & corrected for. Explicit guidance on the homologation measurements and transmission corrections is given in directive 95/1/EC.[4]

A main source of ambiguity and differences comes from the conditions the test was done at. These conditions include details like atmospheric conditions (temperature, pressure, humidity), tire pressure, how the motorcycle is secured against the dyno drum, but most importantly: the conditions of the motorcycle itself. Examples thereof are: was the alternator fitted?; was the air filter fitted?; what exhaust system was fitted? One would hope that manufacturers would test their motorcycles in normal running order, so the condition that they are sold in, and for which they obtained type-approval, but this is not always the case. Ducati, for instance, has chosen to publish more positive values, stating that "Technical data referring to power and torque was measured on an engine test stand at Ducati".[5] Their published values are typically 5% higher than the homologation values, in normal running order.

Motorcycle weight is expressed in three ways: gross vehicle weight rating (GVWR), dry weight and wet weight. GVWR is the maximum total weight of the motorcycle including all consumables, the rider, any passenger, and any cargo. It is generally well-understood and standardized, being defined by law and overseen by agencies such as the US Department of Transportation.[6] In contrast, wet and dry weight are unstandardized measurements that refer to the weight of the motorcycle without rider, passengers or cargo, and either with (wet) or without (dry) a varying set of fluids such as fuel or lubricants, and the battery.

Wet and dry weight are often used to make comparisons between different motorcycles, because all else being equal, a lighter motorcycle will generally perform and handle better than a heavier one.

The difference between GVWR and wet weight is how much the motorcycle can safely carry, including the rider, fuel, and other load.

Automotive aerodynamics of motorcycles are not as frequently measured by third parties or reported by manufacturers as other figures like power.[14] The dimensionless measure of drag coefficient, C_d, varies from .55 to .65 (comparable to a pickup truck), vs C_d .29 for many sports cars and even .20 for high efficiency cars. However a more relevant figure of merit, C_dA, factors in the vehicle's frontal area, and thus actual power required to overcome wind resistance.[15] A typical projected frontal area for motorcycles is in the neighborhood of 2.9–3.9 sq ft (0.27–0.36 m²). When the C_dA figure is reported, it is sometimes in nonstandard units making it hard to compare between motorcycles and automobiles.[16]

C_d could be reduced through the use of "dustbin fairings" or fully enclosed and streamlined fairings (like the Vetter Streamliner prototype) to reduce wake turbulence and flow separation, but these were banned in racing in the 50s and so are out of fashion among street motorcycles.[17] The adherence to "styling aerodynamics" (as opposed to measurably improved aerodynamics) and ergonomic considerations have affected production motorcycle design.[18]

Another factor making aerodynamic measurements relatively uncommon in the popular press is the lack of access to research-quality wind tunnels by journalists.[19]

Complete analysis of motorcycle aerodynamics would include measurement of drag, lift and side force at various speeds, wind angles, and rider postures; as well as pitching moment, rolling moment, and yawing moment (see yaw, pitch and roll for descriptions).[19]

The following have been commonly used at one time or another to describe motorcycles. Not all of them apply to every motorcycle, and most sources publish only a subset of this list, while others have unique measurements not seen elsewhere.

• Conversion of units
• Curb weight - the car and truck equivalent of motorcycle wet weight
• Dry weight (automobile)
• Rounding
• Speedometer error
• Vehicle metrics