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Semi-Truck Aerodynamics

For individual operators and fleet managers alike, optimizing truck performance is essential to saving money. One area that can make a significant difference is in aerodynamics. A truck that isn't aerodynamic enough will experience the adverse effects of drag. On every trip, tractor-trailers work harder than necessary to combat and negate drag, and the worse it is, the more energy it takes to achieve that balance. Increased energy usage translates to unnecessary fuel use and higher operating expenses with every trip.

In the same right, aerodynamics directly impact the truck's miles per gallon (mpg) and fuel efficiency. The size and weight of rigs alone create a large amount of resistance, so it's crucial to create an exterior design that encourages smooth airflow. Aerodynamic 18-wheelers will save money, reduce the number of refueling stops per trip and perform much better overall as compared to standard models.

Responsible parties can reduce the effects of aerodynamic drag and travel expenses in multiple ways. From investing in new truck models to finding the right combination of attachments, every driver and manager has options. This guide will cover the basics of truck aerodynamics and explore how to increase fuel mileage in a semi-truck by minimizing the effects of drag.

 

How Semi-Truck Aerodynamics Work

Tractor-trailer aerodynamics follow the same concepts as other vehicles, but there are more areas where airflow is interrupted. As the truck moves forward, air catches on specific sections of the vehicle's body, effectively slowing the rig and requiring it to compensate with more power. Rigs already aren't particularly fuel-efficient vehicles, and drag can noticeably increase the amount of fuel a tractor needs to use for travel.

Four sections cause the most significant amount of drag — the front of the tractor, the gap between the tractor and trailer, the back of the trailer and the underbody and length of the sides of the trailer. Overall aerodynamic loss from these sections is 53%. Among these four areas, there are two distinctive types of drag — namely, pressure and skin friction.

  • Pressure drag begins with the front of the tractor. As it travels forward, the impact compresses the air particles. Those particles move to the back of the trailer, where they swirl off the end and create turbulent flow. The pressure becomes imbalanced, as the front surfaces of the truck initiate more than the back, which results in drag.
  • Skin friction is less about direct impact. It involves the air particles nearest to the surface of the truck colliding with the outer planes and slowing the rig. While tractor-trailers do experience the effects of skin friction drag, they are very minimal. The weight, size and shape of these trucks make them far more susceptible to and affected by pressure drag.

As an additional detriment, seasons and climate can increase the effects of aerodynamic drag. In cold conditions, the air surrounding the truck is denser, intensifying the friction and decreasing the fuel efficiency. Also, the faster a vehicle travels, the worse the drag gets.

But how much does it affect big rig fuel efficiency? More than you may think. When faced with strong air resistance, plus rolling resistance and engine inefficiency, heavy trucks are forced to burn through more diesel fuel. Aerodynamics improvements for tractor-trailers can significantly reduce that waste and improve fuel efficiency, saving money and time on refill stops.

 

How Truck Manufacturers Are Dealing With Aerodynamic Drag

Manufacturers are actively creating new and improved solutions to combat aerodynamic drag. They've released installable solutions that attach to various areas of tractor-trailers and are working to improve the base designs as well. As companies continue to study the effects of aerodynamic drag and find new ways to combat them, the tech will keep evolving.

The current solutions focus on streamlining the airflow around, over and under the entirety of the tractor-trailer body. Some of them deal with the shape and mechanics of vehicles themselves, but others are attachments that apply to any big rig. However, manufacturers are making a concerted effort to improve aerodynamics on semi-trucks, starting at the base of their upgraded models. Some of the more recent design changes include:

  • Tractor design: The shape of the tractor creates pressure drag, making it one of the most significant areas for improvement. Appropriately, most enhancements involve the tractor design, such as antennas, bumper valances, roof fairings and chassis configurations. New models focus on airflow direction, channeling it up, over and around the front of the tractor, so it creates less pressure. Every detail of the tractor matters to air resistance — even the side mirrors, which some companies have designed in an elliptical shape to reduce drag. Tractors with several of these upgraded elements yield a significant improvement in fuel economy.
  • Narrowing the gap: One area that gets the most attention is the gap between the tractor and trailer. The wider that space is, the more air resistance a truck will experience. Manufacturers suggest the gap between trailer and tractor shouldn't be more than 30 inches, and have been exploring multiple avenues to solve the gap — including minimizing the space in newer models, shifting the fifth wheel forward, producing cab extender fairings and creating a vertical row of vortices called "airtabs" on the body of the cab.
  • Altered bumper shapes: Bumpers can significantly change the direction of airflow, depending on how they're shaped. Multiple companies have focused on designing and installing front bumpers that channel air under and around the tractor, creating a smoother stream. The front end of the cab is the first area to produce drag and cut the air, making it one of the most critical sections of the truck in terms of smooth airflow. An aerodynamic bumper sets the direction for the airflow that will travel over the rest of the truck body.
  • 6x2 power configuration: The standard highway tractor has a 6x4 power configuration. Installing a 6x2 configuration in its place has consistently provided a boost in mileage — a 2.5% increase, comparatively, with improved fuel mileage.
  • Fuel efficiency packages: Some companies offer entire factory-installed packages that target multiple areas on the truck, creating a more streamlined model overall. They can include some or all the above solutions as well as other additions, depending on the manufacturer.

While updated truck models generally have increased overall fuel efficiency, most operators and fleet managers can't afford to purchase entirely new trucks. However, older vehicles are modifiable — manufacturers produce aftermarket accessories that improve 18-wheeler aerodynamics. These solutions are cost-effective, and even though they won't affect the rig on a mechanical level, they'll have a significant impact on fuel efficiency.

 

Ways to Improve Your Truck's Aerodynamics

There are multiple ways to improve the aerodynamics on a semi-truck. From the general shape of the rig down to specific curves and gaps, there are many available additions and components that work to reduce drag. Most of them make a difference by redirecting air in the four major areas that cause drag, as mentioned earlier. By channeling the stream away from gaps and protrusions, these additions prevent air pockets and crosswinds from forming in the first place.

With many different options available for most truck configurations, any operator or company has access to solutions for nearly every area where drag occurs. Among these solutions are:

1. Fairings

Due to skin friction drag, even the long sides of a tractor-trailer can negatively affect fuel efficiency. Attaching fairings in various placements over the body can improve airflow and reduce drag by a significant amount. Fairings are panels meant to redirect the air. There are several varieties for different areas on the truck, including:

    • Side skirts: Side skirts attach to the underside or body of a trailer, reducing drag around the wheels and undercarriage area. They prevent the airflow from traveling under the truck and getting interrupted.
    • Rear tail fairings: Sometimes referred to as a "boat tail," these fairings are fitted to hang off the back end of the trailer, and they reduce the amount of tail drag. Typically, the setup consists of three panels — one for each side and one for the roof — that angle slightly inwards. They minimize the chances of a low-pressure vacuum forming at the back end of the trailer, which is liable to occur at highway speeds. They're all easily detachable for loading and unloading convenience and can noticeably improve fuel efficiency.
    • Cab extenders: Cab extenders are forms of fairings specifically for the cab part of an 18-wheeler. The extenders do more than just fan air outwards — they also prevent the threat of crosswinds. Crosswinds occur when the airflow coming around the cab enters the gap area between the tractor and trailer. Without extenders, there is little else to protect that zone. There are also fairings available that slant outwards to compensate for trailers that are wider than the cabs pulling them.
    • Rear roof extenders: Rear roof extenders have a similar function to cab extenders, but attach to the top of the cab and make up for the height difference between tractor and trailer. They're typically manufactured to the individual specs of trucks, to ensure a proper fit that doesn't get in the way and performs well.
    • Drive wheel fairings: This variety of fairing is made to fit snugly around the drive wheels of the tractor. The fairings fill the spaces between and just behind the tires, and when paired with wheel covers, these panels create a much smoother surface for air to flow over.

All these fairings are compatible with one another, so they function well independently as well as together.

2. Wheel Covers

Standard truck wheels are far from aerodynamic, though they aren't often considered part of the problem. They have large indentations and ridges that catch air as they spin. When the truck travels at high speeds, the air gets stuck in the wheel wells, and the turning motion causes it to swirl, disrupting airflow and slowing the rig.

Applying wheel covers plugs up all the holes and smooths out the ridges, allowing air to flow past the tires uninterrupted. Even if it makes a quarter of a percent of a difference in terms of efficiency, it will still offer savings if applied on a single rig.

3. Vented Aerodynamic Mud Flaps

Most trucks have mud flaps behind their rear tires. Functionally, they serve as barriers to prevent grime, dirt, rocks, water and road debris from collecting on or damaging the undercarriages of trucks. While most mud flaps are solid, vented ones have small slats that allow air to pass without letting dirt through. They serve as a more aerodynamic version of standard mud flaps and may contribute to increased fuel efficiency.

4. Drive Fenders

The front fender of a truck is the first part to cause drag and redirect airflow. If it isn't shaped correctly, the air won't have a chance of passing smoothly around the rest of the vehicle. Aerodynamic drive fenders solve this problem. Adding them to a rig creates a front shape that encourages better aerodynamics, reducing the effects of drag and improving gas mileage.

5. Vortex Generators

While the name "vortex generator" may sound relatively technical compared to the other options, it functions on the same simple concept as the previous solutions — air redirection. A vortex generator consists of a small vane, which easily attaches to the surfaces of vehicles. By affixing them wherever downward airflow separation occurs, the vanes delay the separation and reduce the subsequent drag. They also minimize rear-end drag and turbulent airflow, allowing for better mileage and improved performance.

6. Whale Tails

When air goes over the top of the cab and runs into the boxy trailer, it creates wind drag. A whale tail, also called a truck wing, installs on the top of the cab to help direct air smoothly over the trailer. Truck roof wings create better aerodynamics and reduce wind resistance as well as stabilize the truck in windy conditions. Whale tails are also an excellent way to save on fuel costs since they improve fuel efficiency.

Each of these options has a distinct purpose and placement, but they all work towards the goal of improving truck trailer aerodynamics. That said, they aren't exclusive solutions. Instead, they have a cumulative effect on fuel efficiency. Purchasing and installing multiple components will draw a more significant benefit than choosing a single upgrade. The more you incorporate in your rigs, the more money and time you'll save.

 

Purchase Aerodynamic Solutions From Bud and Tony's Truck Parts

Semi-aerodynamics is an essential science in the trucking industry. It has such a significant impact on tractor-trailer fuel consumption, power efficiency and general expenses. Failing to acknowledge the real effects of drag can cost operators and fleet managers a large sum of money every year. Even small angles and gaps in the truck body that may seem inconsequential — such as holes in the wheels and bumper shapes — can add up to a substantial loss over time.

With the right tractor-trailer aerodynamic improvements, you'll benefit from multiple gains. The truck won't need to work as hard to combat the effects of drag — meaning, increased fuel efficiency, fewer gas station stops per trip and the potential for less expensive maintenance. Each of these advantages contributes to lower transportation costs overall, and the better the aerodynamics, the more those expenses shrink.

If you want to improve your rig with aerodynamic solutions, Bud and Tony's Truck Parts has your back. With a focus on customer service and satisfaction, we'll make sure you have everything you need to make your long haul more cost-effective. We provide reliable additions — including wheel covers, fenders, skirts and vented mud flaps — for the top brands in trucking, such as Kenworth, Peterbilt, Volvo, Mack, Sterling, Western Star and more.

We want you to have the parts you need fast, so Bud and Tony's Truck Parts has a 48-hour turnaround from the time you place your order to shipment. If you want to make a large volume purchase for a full fleet, we can provide you with a free quote ahead of time.

Choose to save money with aerodynamic solutions by shopping our selection of exterior accessories or contact us for more information.

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