Top Motorcycles:
. Honda, Yamaha, Kawasaki, Suzuki
. Harley Davidson, Triumph, Indian

On one side of the piston is the combustion chamber, where the piston is compressing the air/fuel mixture and capturing the energy released by the ignition of the fuel.

On the other side of the piston is the crankcase, where the piston is creating a vacuum to suck in air/fuel from the carburetor through the reed valve and then pressurizing the crankcase so that air/fuel is forced into the combustion chamber. Meanwhile, the sides of the piston are acting like valves, covering and uncovering the intake and exhaust ports drilled into the side of the cylinder wall.

It's really pretty neat to see the piston doing so many different things! That's what makes two-stroke engines so simple and lightweight.

With 2003 being the 100th anniversary of Harley-Davidson motorcycles, it seems only appropriate to write the article "How Harley-Davidson Works" to celebrate. But how do you write about Harley-Davidson? When someone mentions the words "Harley-Davidson" several things come to mind:
The company that produces motorcycles.
The actual motorcycles that the company produces.
The history and tradition that surround the company and the products.

And, there's something else, too -- the aura of Harley-Davidson. Some call it Harley mania. Others refer to it as Harley-culture. If you're having a hard time understanding this concept, just consider this: Every year, hundreds of thousands of Harley-Davidson enthusiasts converge on towns like Sturgis, SD (August), Myrtle Beach, SC (May) and Daytona Beach (March) for an entire week of partying and vendor demos. These folks ride their bikes hundreds if not thousands of miles just to participate.

Unlike any other major brand of bike, there's a certain something that Harley-Davidsons carry with them. It could simply be the sound of a large V-Twin engine through straight pipes at full throttle -- this has certainly been glamorized in dozens of movies. But we think it's more than that. Call it mystique if you like. There is definitely something to it.

With growing engine sizes, motorcycles became much faster and could travel much longer distances. Higher speeds meant the addition of a transmission. Rider comfort in the form of bigger tires, suspension systems, seats and so on were added to make the ride better. Additional speed also makes the tires more important for performance, so the tires got wider. You can see all of these advanced motorcycle features in the motorcycle pictured below.

You find two-stroke engines in such devices as chain saws and jet skis because two-stroke engines have three important advantages over four-stroke engines:

Two-stroke engines do not have valves, which simplifies their construction and lowers their weight. Two-stroke engines fire once every revolution, while four-stroke engines fire once every other revolution. This gives two-stroke engines a significant power boost. Two-stroke engines can work in any orientation, which can be important in something like a chainsaw. A standard four-stroke engine may have problems with oil flow unless it is upright, and solving this problem can add complexity to the engine. Horsepower

These advantages make two-stroke engines lighter, simpler and less expensive to manufacture. Two-stroke engines also have the potential to pack about twice the power into the same space because there are twice as many power strokes per revolution. The combination of light weight and twice the power gives two-stroke engines a great power-to-weight ratio compared to many four-stroke engine designs. You don't normally see two-stroke engines in cars, however. That's because two-stroke engines have a couple of significant disadvantages that will make more sense once we look at how it operates.

You can understand a two-stroke engine by watching each part of the cycle. Start with the point where the spark plug fires. Fuel and air in the cylinder have been compressed, and when the spark plug fires the mixture ignites. The resulting explosion drives the piston downward. Note that as the piston moves downward, it is compressing the air/fuel mixture in the crankcase. As the piston approaches the bottom of its stroke, the exhaust port is uncovered. The pressure in the cylinder drives most of the exhaust gases out of cylinder, as shown here:

As the piston finally bottoms out, the intake port is uncovered. The piston's movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel, as shown here:

Now the momentum in the crankshaft starts driving the piston back toward the spark plug for the compression stroke. As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum opens the reed valve and sucks air/fuel/oil in from the carburetor. Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle. It's called a two-stoke engine because there is a compression stroke and then a combustion stroke. In a four-stroke engine, there are separate intake, compression, combustion and exhaust strokes.

You can see that the piston is really doing three different things in a two-stroke engine:

If you have ever used a two-stroke engine, you know that you have to mix special two-stroke oil in with the gasoline. Now that you understand the two-stroke cycle you can see why. In a four-stroke engine, the crankcase is completely separate from the combustion chamber, so you can fill the crankcase with heavy oil to lubricate the crankshaft bearings, the bearings on either end of the piston's connecting rod and the cylinder wall. In a two-stroke engine, on the other hand, the crankcase is serving as a pressurization chamber to force air/fuel into the cylinder, so it can't hold a thick oil. Instead, you mix oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls. If you forget to mix in the oil, the engine isn't going to last very long!

What is a motorcycle?
At its very simplest, a motorcycle is a bicycle with an engine. If you go all the way back to the beginnings of Harley-Davidson Motorcycles in 1903, a motorcycle is exactly that. The first production Harley-Davidson was a bicycle with a slightly modified frame to make the engine easier to mount. A leather belt carried the power from the engine to the rear wheel. This motorcycle had pedals, so you could pedal it like a bicycle if you wanted to. It also had a normal coaster brake in the rear hub that you could apply with the pedals (by pedaling backwards), just like you would on a normal one-speed bicycle today.

Copyright Harley-Davidson Archives
Photo courtesy Harley-Davidson Motor Company Archives
The first Harley-Davidson was basically a motorized bicycle.

That original Harley-Davidson motorcycle had a 24.75 cubic inch (405 cc), single-cylinder, air-cooled engine with an F-head valve configuration. The engine weighed 49 pounds.

Today a motorcycle does not really look at all like a bicycle. Both vehicles have two wheels, but that is where the similarity ends. Two things worked in concert with each other to make motorcycles look so large today:

The engine grew bigger and bigger, so the motorcycle grew bigger to hold the engine and support the weight. As the motorcycle grew heavier, the wheels and tires got bigger and stronger to support the weight, as did the brakes, the frame, etc. The fuel tank also expanded to give the larger engine the fuel it needs.

Today's motorcycles are very distant relations to the motorized bicycles of the past.

This motorcycle is not a Harley -- it is a Harley-like custom motorcycle that strips a modern motorcycle down to its bare essentials. For example, this motorcycle has an engine and a transmission:

88 cubic-inch (1,450 cc), two-cylinder V engine

The stainless steel exhaust pipes are connected directly to the engine's cylinders.

In this case the engine is a 88 cubic-inch (1,450 cc), two-cylinder engine. The cylinders are arranged in a V configuration with a 45-degree angle to the V. The engine uses dry sump lubrication, and the external tank for the oil mounts above the transmission. Exhaust is simply a pair of stainless steel pipes connected to the exhaust port of each cylinder. There is no muffler or catalytic converter of any kind.

The transmission sits just behind the engine. It is a 6-speed sequential manual gearbox. The engine transmits its power from the crankshaft to the transmission through a primary drive. The power then moves from the transmission to the rear wheel through the final drive, which in this case is a chain as seen in this photo:

The primary drive on this motorcycle is a very wide Kevlar/rubber belt. The large chain is the final drive.

All the engine's power ends up going to the back wheel.

The engine's power ends up going to this extremely wide back wheel.

You can also see the rear disk brake, as well as the back portion of the frame. In the above photo, as well as the photo below, it's obvious that motorcycle frames can be extremely simple.

This motorcycle frame has a simple design. Because it has no rear suspension, it is called a "hard tail."

The frame is just a few steel tubes bent and welded together. See this page for photographs of several different frames. This motorcycle has no rear suspension and is therefore called a hard tail.

The front fork, the only suspension component on this bike, is located at the front end of the motorcycle.

The front fork is the only suspension component on this bike. It can compress to act like a shock-absorber.

You can see that the front wheel is much smaller than the rear. There is a single disc brake at the front. The front fork can compress to absorb shock. At the top of the front fork are the handlebars and headlight.

Certainly this motorcycle is not as simple as the first Harley-Davidson in 1903, but it is not super-complicated either. This is minimalist gasoline-powered transportation. So, how does a Harley compare to this basic-bike? Let's take a look...

What is a Harley?

Photo courtesy Harley-Davidson Motor Company
Harley-Davidson 100th Anniversary Tank Emblem with Chrome Bar & Shield

You can get all kinds of motorcycles today, mostly from Japanese manufacturers like Honda, Yamaha and Kawasaki. But, most folks would argue that none of them have the aura, tradition, name recognition or mystique of the motorcycles from Harley-Davidson. You're probably wondering what it is that is so unique about a Harley, let's start with the engine.

Here are some of the distinguishing characteristics of the engine:

Until 2001, Harley-Davidson has been devotedly married to the two-cylinder V-twin design with a 45 degree angle between the cylinders.
The engines are air-cooled.
The engines have overhead valves that are activated by camshafts in the crankcase.
Oil comes from a dry sump lubrication system.
Harley engines have a long stroke. This means that the engines are low-revving and have lots of torque. Redline is typically in the 5,000 RPM range.
Harley engines have a single-pin crankshaft, giving these engines a unique sound. We'll have more on that distinctive sound in the next section.
In 2001, Harley made what was for it a radical move. A new engine called the Revolution™ engine was introduced, to be used in the upcoming 2002 VSRC. The Revolution is still a V-twin but it has a 60-degree V, is water-cooled, has 4 overhead camshafts and is high-revving (9,000 RPM redline).

Photo courtesy Harley-Davidson Motor Company
The Revolution™ engine is currently used on only one Harley production model -- the VSRC. Both the 2003 VRSCA V-Rod™ (shown here) and the 2002 VSRC sport the Revolution™ engine.

Beyond the engine, Harley motorcycles themselves have their own look and feel. Because of the big engines, Harleys tend to be big bikes. The biggest Harleys weigh close to a thousand pounds (453.6 kilograms) and tend to incorporate retro styling. We'll discuss the new engine in more detail a little later on in this article.

Harley in the Movies
From "Easy Rider" to "The Terminator 2" to "Mission Impossible 2" -- motorcycles have been making the movie scene for decades.
Photo courtesy

In fact, Harley-Davidson gets top billing in the 1991 movie entitled "Harley Davidson and the Marlboro Man." Featuring two main characters: one named for the famous motorcycle company and the other a popular cigarette brand, this movie continues to have a following. Some other products are also given a nod in the film -- there's a character named "Virginia Slim" (like the cigarettes) and another named after the famous whiskey "Jack Daniels."
None of this, however, really explains the mystique. Where does the Harley mystique come from? Think about a Marlboro cigarette. It's simply a bit of tobacco wrapped in paper. There are hundreds of other brands of cigarette that are nearly identical, but the Marlboro brand has mystique. Part of it comes from image advertising, but the other part is cultural. The people who smoke Marlboro cigarettes tend to be a certain type of person. Because they advertise and reinforce the brand image, they in turn attract more of their kind to the brand -- and so on.

In the case of Harley-Davidsons, although some amount of advertising is done, far more of the brand image comes about in other ways:

The use of Harleys in the movie and television industry.
The huge Harley rallies that can attract over 100,000 bikers.
The people who ride Harleys, and the image they project in public.
These elements along with a few others create a resonating, grassroots type of promotion that is extremely rare, but fascinating when it happens. This begins to convey the Harley mystique, now let's discuss something else that resonates -- the tell-tale Harley sound.
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