Making sense out of Motorcycle Specifications
The following provides definitions of terms to demystify motorcycle specifications as usually listed in the motorcycle magazines and on Websites. These definitions of terms should be easier to understand by riders who would rather ride than learn the technical complexities of their machines. If you have the stats for your bike in front of you and still ask, “What do they mean?” then here is a translation that lies somewhere between technical jargon ‘it goes really fast when you turn the throttle’ and what appears in those specs.
Bear in mind that whole books could be written about each term, this is merely the tip of the spark plug.
No. of Strokes: Engines in motorcycles discussed on this site are all four-stroke. There are two-stroke engines as well. Also, don’t confuse four-stroke with four-cylinder or two-stroke with two-cylinder. They are totally different. The four strokes in a four-stroke engine are defined as follows:
- Stroke 1 – Intake valve(s) open, piston moves down, sucking in the air/fuel mixture.
- Stroke 2 – All valves closed, piston moves up, squeezing the mixture.
- Stroke 3 – All valves still closed, spark plug ignites the fuel/air mixture, pushing the piston down.
- Stroke 4 – Exhaust valve(s) open, piston moves up, blowing the old charge out the exhaust valves.
- A short version is Suck, Squeeze, BANG, and Blow.
Cooling: An engine can be liquid-cooled by running coolant through the engine and exchanging the heat via a radiator like a car. It can also be air-cooled by airflow over fins on the engine. Another variation is oil-cooling where engine oil is circulated in the engine and its heat exchanged by a small radiator. There are even oil/air/fan cooling systems.
Valves: Valves are used as mentioned above. There may be one or more valves used for intake and exhaust. The specs may indicate how many valves are used per cylinder or for the whole engine. Terms such as SOHC (Single OverHead Cam) and DOHC (Double OverHead Cam), etc. are used to indicate how the valves are opened and closed. There are other methods to operate the valves as well.
Number of Cylinders: Engines can have one to six cylinders depending on motorcycle type.
A single-cylinder engine, sometimes called a thumper, is used in small bikes.
Most bikes have engines with two-cylinders arranged in many configurations including: V-twin where the cylinders are spaced at a particular angle to each other; Parallel twin where the two cylinders are right next to each other in a vertical position; and Flat-twins where the two cylinders are opposed such as in BMW Boxers.
Engines may have more than two cylinders such as: Triples where three cylinders are lined up next to each other in a vertical position; In-line fours similar to a triple except with another cylinder added; V-4 engines such as used in Honda ST1300; V-6 engines such as used in Boss Hoss; Flat-six engines such as used in Honda Gold Wing 1500/1800s.
There are other engines not described here.
Displacement is the volume displaced in the cylinders of an engine as the pistons move from their bottom position to their highest position in the cylinders. Displacement is measured in either cubic centimetres (CC) or cubic inches (CI).
Bore and Stroke: Bore is the diameter of the cylinder in the engine in which a piston moves up and down. Stroke is the distance that the piston moves up and down inside the cylinder.
Fuel System: Either carburetor(s) or fuel injection is used to control the mixture of air and gasoline and prepare it to be sucked through the intake valves and into the cylinders during Stroke 1, described above. Specs usually identify the carburetor name and how many carburetors are used. Fuel injection comes with different names such as Fuel Injection (FI), Digital Fuel Injection (DFI), Electronic Sequential Port Fuel Injection (ESPFI), Programmed Fuel Injection (PGM-FI), etc.
Compression Ratio: Think of Compression Ratio this way: When the piston is at the bottom of the cylinder, say you could pour 100cc of water into the sparkplug hole (both valves closed) and it would be full. When the piston is at the top of its stroke, you can only pour 10cc of water into the hole to fill it. The compression ratio would be 100 to 10 or 10 to 1. That’s about as easy to understand as I can make it. Oops, forgot to say — Higher compression ratios (in general) will let the engine make more power, require higher octane gas and be harder for the starter to turn over.
Maximum Torque: Torque is described as twisting force. This is the maximum amount of twisting force the engine can put out and at what value of engine Revolutions Per Minute (RPM). Put simply, torque is the “grunt, stretch your arms, push your eyeballs into their sockets” quality when you aggressively use the throttle and accelerate hard. The higher the number, the more forceful that feeling is.
Maximum Horsepower: This is the maximum amount of horsepower delivered by the engine. It occurs at a particular engine speed. Horsepower (HP) and torque are interrelated with RPM by the formula HP = TORQUE X RPM / 5252.
The following provides definitions of terms to demystify motorcycle specifications. These definitions of terms should be easier to understand by riders who would rather ride than learn the technical complexities of their machines. Bear in mind that whole books could be written about each term. We are only touching the basics here.
Transmission: The specs may show the transmission as 4-speed, 5-speed, 6-speed, or an automatic transmission such as found in some bikes and most scooters.
Final Drive: The final drive is what connects the engine and transmission to the rear wheel. Options are a chain, belt, or driveshaft.
Brakes: There are many brake systems given in motorcycle specs. The usual system is a disc front brake controlled by the right handlebar brake lever and a rear disc or drum brake controlled by the right-side foot brake pedal. There are also Linked Braking Systems (LBS) where use of either the front brake lever or the foot brake will activate both front and rear brakes. A variation on LBS is the Integrated Braking System (IBS) where the foot brake also controls a portion of the front brake. In some integrated braking systems, the front brake will also control a portion of the rear brake.
Disc brakes are made of a rotor that is attached to the wheel and a calliper that is fitted over the edge of the rotor. The calliper contains one or more pistons under hydraulic pressure that, when activated by a hand brake lever or foot brake pedal, will push against the brake pads that then contact the rotor to create friction that stops the bike. There are fixed rotors and floating rotors. The floating rotors can move and expand and contract thereby allowing less heat to be generated and braking performance to be increased.
Anti-Lock Braking Systems (ABS) may also be part of a motorcycle braking system. ABS uses computer controls to keep the brakes from locking by applying and releasing the brakes very rapidly.
Front Suspension: Springs and shock absorbers are used in various configurations to provide a comfortable ride. The concepts used for both front and rear suspensions are similar but the configurations of springs, dampers, and links are quite different.
Spring pre-load is the amount of compression already in the spring before applying any load. Many systems allow you to adjust the pre-load to suit the expected road conditions or rider preferences. Most motorcycles use a telescopic front suspension consisting of two fork tubes containing springs and some method to control their motion. When brakes are applied, the front end tends to dive because of weight transfer. When the bike hits a bump, the spring also compresses to absorb the shock. To keep the spring from bottoming, some form of compression damping is used. As the spring comes back up, rebound damping is used to control its travel. Without damping, the spring would propel the rider up and down in pogo stick fashion. Many bikes have methods to control and adjust compression damping and rebound damping and these are spelled out in the motorcycle specs. Most telescopic front forks have a smaller top portion that is pushed into a larger lower portion. A variation on this is the inverted fork where the fork tubes are essentially turned upside down with the upper portion sliding down outside the lower portion.
BMW uses a Duolever front suspension that still has telescopic front forks but the damping is provided by a shock absorber that is placed in back of the forks. This configuration effectively separates braking, steering, and suspension forces and eliminates dive when braking.
Rear Suspension: Today, most bikes use a swingarm system that is laterally fastened to the frame in front of the rear wheel and moves up and down in an arc. Two arms extend back and hold the rear wheel. One or two shock absorbers connect between the frame and the swingarm to provide spring action and appropriate damping. Some bikes use a single-sided swingarm.
Older bikes had no rear suspension and were known as hard tails. Many bikes today have hidden rear springs and shocks to give a hard tail look but a soft tail ride. The Harley-Davidson Softail line is an example. Use of springs that become progressively more widely spread from one end, allows a spring to give a good ride on normal surfaces but also handle sharp hits from the road as well. These are called progressive springs. Another approach is to use a standard single rate spring but connect it to the wheel using linkages that change the leverage on the spring thus making it into a progressive type system. So called link-type rear suspensions, usually using a single shock, are examples.
BMW uses a Paralever rear suspension that employs two universal joints on its driveshaft to allow the suspension to move through two angles in a parallelogram fashion.
Tyres: The first two numbers in a tyre size indicate the width of the tyre and the height of the sidewall. For example, 240/40 indicates that the tyre width is 240mm across the tread and the aspect ratio is 40. The aspect ratio, expressed in percent, when multiplied by the width gives the height of the sidewall. Thus, the example tyre size tells us that the tyre is 240 mm wide and the sidewall is 96mm high.
Seat Height: Seat height may be given as measured with a rider aboard (laden) or it may be given unladen. Riders may be interested in the seat height to tell them how easy it will be for them to get their feet flat on the pavement when stopped. See my Seat Heights article.
Rake and Trail: Rake is the angle of the fork away from vertical toward the rider. Trail is the distance on the ground between a vertical line dropped straight down from the center of the wheel and a projection of the fork extended until it touches the ground. As the rake increases, the trail increases. The more rake, the more stable the handling at speed. As rake decreases, handling becomes easier at low speeds.
Wheelbase: This is the number of inches measured from front axle to rear axle.
Weight: Weight may be given as wet weight with all lubricants, liquids and gasoline added or it may be dry weight with nothing added.
Fuel Tank Capacity: Capacity is usually given including reserve tank.