2-Stroke
An internal-combustion engine that requires only one revolution per cylinder or two piston strokes (up and down) to achieve a power stroke. Rarely used in automobiles.

4-Stroke
An internal-combustion engine that requires two revolutions per cylinder or four piston strokes to achieve a power stroke internal stroke, compression stroke, power stroke, exhaust stroke. More efficient than the two-stroke-cycle engine. Also called Otto cycle.

The Shovelhead
engine (V-Twin, produced from 1966 – 1984)

Soft Tail
A Soft Tail is a motorcycle that appears to have a Hard Tail rear suspension but in reality has shock absorbers that are hidden from view. Harley-Davidson has many Soft Tail models but they call them Softails.

SOHC
Single Over Head Cam. A single cam shaft found in the head or top of the engine that activates the valves.

SOLENOID
Solenoids are electromagnetic switches. When electricity is sent to the solenoid, a magnetic field forms. The magnetic field moves a metal piston, which is connected to a mechanism that performs a function. When the metal piston moves, the application is made. The application may be the moving of a valve, switch, or mechanical linkage.

SPARKPLUG
A sparkplug is a device that provides a fixed air gap for an electrical current to jump across, in order to create an electrical spark. It is this spark which ignites the air/fuel mixture that has been injected into the combustion chamber. And it is the energy from this mini-explosion that creates the power stroke (pushes the piston down), powering the vehicle. Sparkplugs can get fouled (dirty) from various conditions: (1) bad sparkplug wires not delivering a hot enough spark (2) excessively rich fuel mixtures and (3) oil finding its way into the engine for any number of reasons. When a sparkplug gets dirty, it becomes ineffective (won’t produce a hot enough spark to effectively and efficiently ignite the air-fuel mixture in the combustion chamber). This results in reduced fuel economy and poor power.

Sportbike
A focused motorcycle designed for speed and handling. These machines are usually equipped with aerodynamic bodywork.

Sportster
Harley-Davidson's entry level model is a popular model for both men and women riders.

Sport-tourer
A motorcycle that combines some of the handling and power of a sportbike, with some of the amenities of a touring bike, like saddlebags, comfortable ergonomics, etc. Not as focused as either a pure sportbike or a pure tourer.

Standard
A motorcycle intended for general, all-around street use, typically with an upright seating posture and higher handlebars.

STOCK
A motorcycle set up to OEM specifications with no alterations.

Supercharger
A supercharger is used to increase the volume of air that an engine can take in, resulting in more power. It is run directly off of the engine by a drive belt.

Supermoto
Generally, a style of motorcycle usually built around, and looking like, off-road machines with street tires. They tend to be very light, flickable machines, and are used in a new genre of racing that usually encompasses riding on a mixture of pavement and dirt surfaces.

Suzuki
A brand of motorcycle produced in Japan.