There are many classic motorcycle models that feature a four-stroke engine. In this article, we'll look further into the history of four-stroke engines and highlight key models. We'll also discuss the main benefits of riding a classic four-stroke motorbike.
What Is A Four-Stroke Motorcycle?
Four-stroke motorbikes feature an internal combustion engine that operates on four strokes per cycle. Technically, the crankshaft moves twice, while the piston moves four times.
A four-stroke engine is heavier and takes longer to complete a cycle. Yet, 4-stroke engines do have many benefits. Four-strokes are fuel-efficient and are more comfortable, with less vibration.
Unlike two-stroke engines, four-stroke ones do not need oil to be mixed into the petrol.
You can find a 4-stroke engine on all contemporary models of motorcycles and dirt bikes today. The reason for this is that two-strokes are no longer street legal. Two-stroke engines were banned due to their high level of emissions. Four-stroke are street legal and are more environmentally friendly.
Four-Stroke Engines: A Brief History
During the peak of motorcycling popularity, two-stroke was undeniably the dominant engine type. Models such as the Yamaha RD350 and the Kawasaki 500 H1 defined the motorcycle boom of the 1970s and 80s. There were also several iconic 4-stroke bikes released during this time. Classic examples include the XT500 Thumper and the Honda Super Cub.
The 4-stroke story dates a lot further back than that, with the first being constructed in 1939. The Gilera 500 Rondine was an Italian model and one of the fasted bikes of the era. It was also one of the most inventive, with overhead camshafts and a water-cooled engine.
Yet, it wasn't until the 1970s that the benefits of 4-stroke engines were fully realised. As two-strokes peaked in popularity, the environmental impact of these bikes was first noted. This was evident in the thick smog that would exit the exhaust valve of such bikes like the Kawasaki H2.
Following this, EPA (Environmental Protection Agency) regulations were tightened. This culminated in the outright ban on two-strokes in 1984 in the US. The United Kingdom followed in banning these motorbikes a lot later, in 2004.
At the same time, 4-stroke engines were noted for releasing relatively lower emissions. This encouraged the leading motorcycle manufacturers to move away from two-strokes. By the 1990s, four-stroke engines had overtaken two-stroke manufacturing numbers worldwide.
How Does a Four-Stroke Engine Work?
The difference between four and two-stroke engines is the movement and function of the piston. Four-stroke engine pistons complete a cycle within four movements. A two-stroke engine does the same thing within two movements, produce twice the number of power strokes per revolution.
During an engine cycle, the piston moves down and up twice. This makes four movements in total, which is why it is referred to as a 4-stroke engine. During this cycle, the crankshaft turns twice. The engine draws in air and expels exhaust using intake and exhaust valves. The four moves of the piston are intake, compression, combustion, and exhaust.
The pressure inside the cylinder is reduced by the first downward movement of the piston. This opens up the intake valve, which lets in a fuel and air mixture. This is known as the 'intake stroke'. As the piston travels back to the top, it compresses the fuel and air mixture. This is known as the 'compression stroke' or 'power stroke'.
The spark plug then sparks this compressed air within the combustion chamber, which forces the piston down a second time. This is known as the 'combustion stroke'. Once the piston has reached the bottom, the exhaust valve opens up. The piston then makes its final move to the top, pushing the exhaust out of the open valve. This is known as the 'exhaust stroke'.
What's the difference between four-stroke and two-stroke engines?
The main difference between a four-stroke and a two-stroke engine comes down to the number of movements that the piston makes per cycle. Two-strokes work twice as fast.
The piston in a two-stroke engine consolidated all that the four-stroke engine does into two strokes per cycle. The piston performs both intake and compression during the first stroke. It then performs combustion and exhaust in the second. To draw in air and expel exhaust, small holes known as scavenging ports are used.
A two-stroke engine has three ports: transfer, outlet, and inlet. A 4-stroke engine has at least two valves per cylinder head. One valve for intake, and another for the exhaust port.
Fuel is another factor that differentiates a four-stroke engine from a two-stroke. Four-strokes run on either petrol or diesel. Two-stroke internal combustion engines require correct ratio of engine oil mixed into the petrol before it's filled into the fuel tank. The majority of diesel engines are designed for cars, there aren't many diesel powered motorcycles with exception of Sommer 462, Track T800CDI and some Royal Enfield models.
Types Of Four Stroke Motorcycle
You can find single-cylinder engines on small motorcycles and dirt bikes. It only has one cylinder, so this type of engine is notably lighter than its counterparts. This lightness makes them good for trail riding.
Single-cylinder engines contain just one piston, one valve set, one connecting rod, and one base gasket. Due to their reduced features, single-cylinder engines tend to be the cheaper option.
However, single-cylinder engines have some disadvantages. They are less durable than other engines and have greater vibrations.
The engine capacity of a single-cylinder tends to fall between the 50 and 750cc range. Classic examples of a single-cylinder motorcycle would be the Honda C90 or Yamaha XT500. Another classic 4-stroke, single-cylinder bike is the 1950 Norton Manx. The 1950 Norton Manx is still a widely sought-after bike and is a favourite for off-road biking.
A two-cylinder engine can provide you with balance and stability. Two-cylinder engines consist of dual cylinders. These cylinders can be found either horizontally on either side of the vehicle or vertically, one behind the other. The weight distribution provides a steady balance when riding. Two-cylinder engines are favoured over one-cylinder as they produce less vibration when riding. They are also durable and easy to maintain.
There are several different two-cylinder engines found on classic bikes. One example is the flat-twin style, which can be found on such classic BMW models as the 4-stroke R32. Another type of two-stroke engine is the straight-twin. Straight twin engines can be found on such 4-stroke bikes as the Kawasaki Ninja 250R and the Triumph TR6 Trophy.
Two-cylinder engines can be found on BMW models, including many of its legendary GS models. Other types of two-cylinder engines include the inline twin and the v-twin such as those on Yamaha Virago series.
Three-cylinder engine models are more fuel-efficient than two-cylinder engines. They are sleeker than a four-cylinder and more powerful than a two-cylinder.
Examples of classic bikes featuring three-cylinder engines include the Triumph Rocket 3. This bike boasts an engine capacity of 2500cc - the largest production motorcycle engine in the world.
The two main subcategories of three-cylinder engines are straight three or V-3. An example of a straight three, 4-stroke engined motorcycle would be the Triumph Trident 750 or Yamaha XS750.
Four-cylinder engines provide a smooth, balanced ride. This type of engine is good for 4-stroke motorbikes as vibration is kept to a minimum. With broad power and good fuel efficiency, four-cylinder is favoured for racing.
There are various subcategories of four-cylinder engines. The main types include the straight four, the V-4, flat-four, and the square four. An example of a straight four motorbike would be 1971's Kawasaki Z1. V-4 four-stroke bikes include the Ducati 1200 V4 Apollo.
There are many other configurations of four-stroke engines. A comprehensive guide is available at Visordown - "Every Type of Four-stroke Bike Engine Ever Made".
The Most Iconic Four-Stroke Models
New motorbikes are being designed every year. Many of which feature new, innovative approaches to the 4-stroke engine. However, new is not always better. When deciding what to pick for your first motorbike, you should first consider classic 4-stroke models. Here are some that are worth taking note of:
The Most Popular Classic
The Honda Super Cub set the bar high back in 1958 when road bikers first revved it up. With a single-cylinder engine and a timeless design, the Super Cub remains a go-to for urban riders. Sold in over 160 countries, the Super Cub is as ubiquitous as the Volkswagen Beetle.
Not only is it the most popular 4-stroke bike in the world, but also the best-selling motorbike ever. The latest model of the Super Cub, the C125, comes in matte black and features an eye-catching red seat.
The revolutionary 1939 Gilera 500 Rondine is the oldest 4-stroke motorbike. It was ahead of its time in many ways. With a top speed of 140mph and a capacity of 492cc, the Italian Rodine was the most powerful motorcycle of its day. It was one of the first to feature a four-cylinder engine. This engine was water-cooled instead of air-cooled and came with a double overhead camshaft.
Notably, it was used by Italian rider Dorino Serafini. Serafini used it to win the 500cc class of the 1939 European motorcycle championship.
Today, the fastest 4-stroke motorbike is the Dodge Tomahawk, which can reach a top speed of 350 mph! However, the Dodge Tomahawk is technically not a motorbike. It has four wheels and is not street legal, so should not be considered the fastest.
The Suzuki Hayabusa is the official fastest 4stroke motorbike. Boasting a top speed of 248 mph, the vehicle can impressively go from 0 to 60 mph in just 2.47 seconds. The four-cylinder engine of this vehicle has a capacity of 1,340cc. It also features 16 valves and is liquid-cooled. Unlike the Dodge Tomahawk, it is street legal.
If you are just learning to ride and wondering which is the fastest 4 stroke 125 cc bike, you might be dissapointed with the answer. Many learner bikes have limits on speed and generally speaking, you will be lucky to go ove 80km/hour on 125cc. Here are the top 7 fastest 125cc learner bikes.
Advantages Of A Four Stroke Motorcycle
Four-strokes hold many advantages over other engine types, including the following:
Four-stroke dirt bikes display greater fuel efficiency because the engine fires less frequently. This is unlike two-stroke engines, which have a 10-15% higher fuel consumption.
Four-strokes are better for the environment than other engine types. They emit less carbon dioxide (CO2), particulate matter (PM), volatile organic compounds (VOCs), and hydrocarbons (HCs).
Easy to control
A 4-stroke engine only fires on every second crankshaft revolution. This means that the bike's power delivery is consistent and easy to control. Modern powerbands ensure a linear power curve. This makes the bike predictable, reliable, and easy to handle for beginners.
Motorbikes that feature a 4-stroke engine generally tend to have more torque. Torque is the amount of pulling power that a bike possesses.
When To Consider A Four Stroke Motorcycle
You should consider a four-stroke motorcycle if you need a street bike. Four-stroke engines are the only engine type that is street legal in the UK today. It is also the only engine style that most leading motorbike manufacturers fit onto new motorbikes.
If you're looking for a dirt bike to use for trail riding, a 4-stroke motorcycle could also be an option. Four-strokes come with a smooth powerband, which makes it easier to control the bike over rugged terrain. For this reason, a four-stroke dirt bike is also the ideal starting vehicle for beginners.
Four-stroke motorbikes came to the forefront after the phasing out of two-stroke engines. However, they've always played a vital part in motorcycle and dirt bike history.
Four-stroke engines have changed the capabilities of motorcycles for the better. The pros of 4-stroke include fuel efficiency, added torque, and better vehicle control. They have also helped to lower motorcycle fuel emissions.
You can find 4-stroke engines in some of the most iconic motorcycle models. Such models include the Honda Super Cub, Norton Manx, BMW R32, Yamaha SR500 and the Kawasaki Ninja 250R.
Which is better, a two-stroke or four-stroke motorcycle?
That depends on what you expect from your motorbike. Particularly dirt bike riders argue whether the intense power of a two-stroke is better than the stability of the four-stroke engine.
Despite many nostalgic factors of owning a two-stroker, it could be argued that 4-stroke motorcycles are better. They are easier to control, need less fuel, and produce more torque. They also emit far less carbon dioxide, amongst other pollutants. They are the only engine type that is currently road legal in the UK and US.
Is a 4-stroke a good beginner bike?
Whether you are looking for your first dirt bike or a learner street bike, a small displacement 4-stroke is a great place to start for a beginner. They are easy to control due to their wide powerband. The steady power delivery makes each ride on a 4-stroke bike smooth and predictable. You won't get any unexpected bursts of power with a 4-stroke dirt bike.
Do four-stroke dirt bikes require fuel pre-mixing?
A four-stroke bike does not require oil as the crankshaft already sits on an oil bath. Mixing oil into the fuel can actually be damaging to the bike.