They may slow us down while we’re driving, but we can’t live without them. No, not cell phones, semi-trucks. Those highway behemoths cloaked in chrome, towing long trailers, are the lifeblood of the international consumer economy. But why do semi-trucks vary in design so much? Let’s grab a gallon of diesel and take a look inside the world of heavy-duty trucking.
Semi-trucks come in two basic types – conventional and cab-over-engine. Semi-truck drivetrain types include 4×2, 6×2, and 6×4 axle chassis-cab configurations. Conventional semi-trucks have larger cabs, while cab-over-engine semi-trucks have smaller cabs and greater payload capacity.
Despite increasing online commerce, 70% of global freight is moved by semi-truck. There are dozens of semi-truck designs and configurations. Semi-trucks in North America, however, look markedly different from their European counterparts.
Basic Types of Semi-Trucks
Semi-trucks service diverse transport operations using various engine, drivetrain, chassis, and cab specifications.
All semi-trucks share one common characteristic – the trailer is hitched to the truck using a fifth-wheel.
In the US: A semi-truck (big rig or 18-wheeler) is a truck-tractor towing a semi-trailer.
In Europe: A semi-truck is the truck-tractor or ‘prime-mover’ that tows a semi-trailer.
The most obvious difference between North American and European semi-trucks is engine placement.
In the US, the most common semi-truck has a front-mounted engine. This design of truck is known as a Conventional Semi-Truck. A hood covering the engine in front of the cab distinguishes this type of semi-truck.
In Europe and Asia, under a flat-faced cab, semi-trucks have the engine above the steering axle. This design of semi-truck is known as a Cab-Over-Engine truck-tractor (or simply a cabover).
They certainly look different, the conventional and the cab-over-engine semi-tractors. The question is, why the divergent designs?
The short answer is – road safety.
How Long are Semi-Trucks?
Around the world, countries have laws governing the length of road freight vehicles, particularly semi-trucks towing semi-trailers.
The safety issue is that long vehicles and trailers have an unusually large turning circle. The trailer will move into an oncoming or adjacent lane on sharp corners and hairpin bends, posing an obvious risk to other road users.
The interesting difference between North American semi-trucks and their European (or Asian) counterparts is based on the topography and roadway characteristics of the respective geographic regions.
The US and Canada have straight highways for thousands of miles, with relatively few tight corners. For this reason, North American semi-truck tractors (not the trailers) can be much longer than Euro and Asian semi-trucks.
On the other hand, Europe is relatively mountainous with narrow, winding roads, demanding a semi-truck be as agile as possible (limited length) without losing its cargo-carrying efficiency.
Asian states are also characterized by mountainous country and narrow roads hence, cab-over-engine design is preferred, as it is in Europe.
In the US, semi-truck tractor length is not limited by legal restrictions. The semi-trailer, however, is. The maximum permissible length for a semi-trailer is 48 feet, but most states have a grandfathered limit that is longer (up to 59 1/2 feet). The semi-truck tractor can be as long as the operator chooses.
In Europe and the UK, legal length regulations govern the total length of the truck-trailer combination. The shorter cab-over-engine design is chosen to allow more trailer load space. The maximum length for semi-trucks with a trailer in Europe is 83 feet.
The upshot is – in the USA, there is no length limit to semi-truck and trailer combinations, whereas, in Europe and the UK, overall vehicle length is restricted.
How Heavy are Semi-Trucks?
A second important legal restriction on semi-truck and trailer combinations is their combined weight when fully loaded. This maximum permissible weight is known as gross vehicle weight rating (GVWR).
Apart from safety issues that arise from overloaded vehicles, semi-trucks under load place immense stress on the road pavement (asphalt), causing road pavement wear.
As a result, semi-trucks towing loaded trailers have to pass through weigh stations (weighbridge) alongside the highway to gain clearance to proceed with the load.
If a semi-truck combination is over the legal GVWR, the operator can be penalized. Excess cargo will have to be removed from the vehicle to continue on its course.
In North America, the GVWR for a semi-truck and trailer combination is 80,000 pounds.
In Europe and the UK, the GVWR for a semi-truck and trailer combination is 97,000 pounds.
The weight of the semi-truck itself (the tractor), known as tare mass, plays a critical part in the load-carrying capacity of the overall big rig. The heavier the tractor, the less load mass is possible on the trailer.
To optimize payload potential, semi-truck tractors in Europe are not only more compact and maneuverable than North American semi-truck tractors, but they also have a lower tare mass. The best European semi-trucks are hard to beat when it comes to payload efficiency.
The longer wheelbase and extra-large cab of the typical American conventional semi-truck are designed for long-haul trucking life on the road, to the extent that many conventional semi-trucks are fully-equipped mobile homes. What they lose in payload efficiency, they make up for in driver comfort.
Types of Semi-Truck Cabs
American conventional and Euro semi-truck cabs look very different, inside and out. While both come in various sizes and levels of comfort, the conventional semi-truck cab is geared for a trucking lifestyle, while the Euro semi-truck cab is geared for trucking as a business.
Conventional semi-truck cabs differ in size, from a small day cab to large sleeper cabs. Roof heights of the cabs fitted to conventional semi-trucks also vary from a flat, low roof on day cabs to raised roofs on sleeper cabs that provide the proverbial ‘headroom’, allowing the driver to stand in the living compartment of the cab.
Conventional semi-truck sleeper cabs have three official categories:
- Flat Roof Sleeper
- Mid-Roof Sleeper
- Raised Roof Sleeper
Conventional semi-truck sleeper cabs include beds or bunks, storage lockers, a refrigerator, a cooker, a toilet, and a shower. These amenities enable long-haul truckers to rest at roadside rest stops, without relying on the hospitality facilities provided by commercial truck stops.
Cab-over-engine semi-truck cabs include day cabs and sleeper cabs with raised roofs. A double bunk, a refrigerator, and adjustable lighting are common features in European cabs and truck stops are the chosen destination for weary long-haul truckers.
Cab-over-engine semi-truck sleeper cabs include three generic types:
- Single Sleeper Cab
- Double Sleeper Cab
- Super Space Cab
Comfortable adjustable seats mounted on air suspension, navigation consoles, and entertainment systems can be found in most leading semi-truck cabs of both types.
Types of Semi-Truck Engines
Almost all modern semi-trucks are powered by a diesel engine. The power output of semi-truck engines generally ranges from around 300 horsepower (hp) to 700 horsepower.
Some semi-trucks conducting extra-heavy haulage often have engines pushing out over 1000 horsepower.
Engine Torque rating is an all-important factor in semi-truck selection, due to the need to move heavy loads from a standstill and up steep gradients. The more torque the engine produces, the better.
American conventional semi-truck engine torque is measured in pound-feet (lb-ft).
European cab-over-engine semi-truck engine torque is measured in Newton-meters (Nm).
For semi-trucks, the best diesel engines deliver maximum torque at the lowest possible engine revolutions (RPM). This improves fuel consumption and engine longevity.
A 500hp semi-truck diesel engine will produce approximately 1,850 lb-ft of torque at around 1,200rpm.
The leading brands of American diesel semi-truck engines include:
- Detroit Diesel
Top European semi-truck diesel engines are manufactured by:
Evermore stringent emissions regulations have forced diesel engine manufacturers to build semi-truck engines that are more fuel-efficient and clean-burning.
Common-rail fuel injection and exhaust gas after-treatment systems are the primary technologies in modern diesel engines assisting in reducing harmful exhaust gasses from semi-truck.
While environmental legislation on diesel engines presents many challenges for semi-truck manufacturers, the new engine technologies produced to limit atmospheric impact do deliver improved fuel economy to semi-truck operators.
Types of Semi-Truck Drivetrains
Outside of engine and cab configuration, the next key differentiator between semi-truck types is the number of wheels and axles on a chassis-cab unit.
While conventional semi-trucks have longer wheelbases than cab-over-engine semi-trucks, both types use the same set of drivetrain configurations to accommodate the weight placed upon the rear axles of the truck-tractor by the fully loaded semi-trailer.
A semi-truck drivetrain includes the transmission (gearbox), prop shaft, differentials, axles and wheel hubs.
Conventional semi-trucks are generally equipped with manual transmission systems.
European cabover semi-trucks are equipped with automated manual and fully automatic transmission systems.
The most common semi-truck axle configurations in both conventional and cab-over-engine semi-trucks are:
- 4×2 – One steering axle with one rear drive axle, and four wheels (not including dual wheels on the drive axle).
- 6×2 – One steering axle with two rear axles, one of which is driven, and six wheels (not including dual wheels on the rear axles).
- 6×4 – One steering axle with two rear axles, both of which are driven, and six wheels (not including dual wheels on the rear axles).
Many conventional semi-trucks used in extra-heavy haulage operations are equipped with a third rear axle that lifts up off the road when the vehicle is not towing a heavy load.
This type of axle is called a lift axle, which helps distribute the weight of the loaded trailer across a greater wheelbase (footprint), effectively reducing the impact of each axle on the road surface.
Types of Semi-Truck Braking Systems
Modern braking systems in semi-trucks include service brakes (foot brake), which are powered by compressed air, and engine brakes, which use the power of compression in the engine to slow down the semi-truck and trailer.
The engine brake is activated when steep declines are negotiated. Use of the engine brake reduces the need for use of the service brakes, which improves safety and reduces service brake wear and tear.
In North America, the most common type of engine brake is the Jake Brake (Jacobs Brake).
In Europe, the most common type of engine brake is technically not an engine brake, but a transmission brake called a Retarder, which uses torque generated by the gearbox to slow the vehicle.
While engine brakes and retarders significantly improve the braking efficiency of semi-trucks, the service brakes (air brakes) are still the only type of braking system that can bring the truck to a complete stop.
Types of Semi-Truck Suspension Systems
Two types of suspension systems exist for semi-trucks – air and mechanical.
Most modern long-haul semi-trucks use air suspension on all axles. This provides a smooth ride for on-highway duty while reducing vehicle tare mass.
Mechanical suspension systems are also known as leaf spring suspension, and are typically used in on/off-road transport applications like logging and mining, where durability takes precedence over ride comfort.
Types of Semi-Trailers
Semi-trailers are the ‘business end’ of the semi-truck, the payload carrier, and come in as many types as there are trucking applications.
Modern semi-truck trailers are built tough but lightweight to optimize payload-carrying potential.
The most common types of semi-truck trailers include:
- Flatbed Trailers – an open load surface with no sides or roof, used to carry rugged equipment and materials.
- Lowboy Trailers – very low loading deck used to transport heavy equipment.
- Reefer Trailers – refrigerated van used to transport perishable goods.
- Curtainside/Conestoga Trailers – a load deck with tarp sides and hard roof, used to carry non-perishable goods.
- Dry Van Trailers – a closed van body used to carry non-perishable products.
- Bulk Liquid Tankers – used to transport fuels and chemicals.
- Dry Bulk Tankers – used to carry animal foods and industrial powders.
- Multi-Car Trailers – used to transport automobiles.
- Tipper Trailers – used to carry mining ore, construction sand, and stone.
- Logging Trailers – used to carry harvested lumber.
Semi-trailers typically have two or three axles but may be fitted with more, depending on the load. Lowboy trailers, for example, can have as many as ten axles.
The World’s Top-Selling Semi-Trucks
Today’s top truck manufacturers have histories dating back to the early part of the 20th century, providing both reputation and technological leadership in markets that new Asian semi-truck models are penetrating.
Leading American conventional semi-truck brands include Freightliner, Kenworth, Peterbilt, Volvo, International, Mack, and Western Star.
Popular European cab-over-engine semi-truck brands include Mercedes-Benz, Volvo, Scania, MAN, Iveco, DAF, and Renault.
Semi-truck manufacturers in Russia, Japan, China, India and South Korea cannot be overlooked. Here are a few brands making inroads internationally:
- UD Trucks
In South America, Africa and Australia, both conventional and cab-over-engine semi-trucks are used for specific applications, both on and off-road.
There it is – a snapshot of the fascinating world of semi-trucks on both sides of the Atlantic. The scope of cab design and the technologies deployed in modern semi-trucks exemplifies the diversity of the world we live in. Simply put, semi-trucks mirror the societies and economies we live in, our regional idiosyncrasies and our cultural heritages. Whether it’s American conventional semi-truck gusto, or Euro cabover efficiency, they’re out there across the world, semi-trucks and their drivers, delivering the goods.