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So you were passing a semi truck on the highway the other day and noticed something odd. A huge piece of plastic was attached to the bottom of the trailer, just in front of the back wheels. This increasingly common device, which one redditor described as “A snow shovel blade mixed with a kiddie slide” is engineered to improve a big rig’s aerodynamics.

Why worry about the aerodynamics of an 80,000-pound diesel truck? They only get eight MPG on a good day, so aerodynamics may seem useless. But the truth is that for a vehicle that may haul trailers across the country repeatedly, a fraction of a MPG can make a huge difference.

There are already several ways semi trucks are changing to improve aerodynamics. Most already have a sort of ramp on the cab’s roof to help air pass over the trailer. Some companies are even adding fins on the back of semi truck trailers to reduce turbulence behind the truck and improve MPG. The latest trend is to install an “UnderTray” system beneath the trailers.

How can aerodynamics underneath a vehicle help airlfow? The bottom of a semi truck’s trailer is nearly four feet off the ground. While the truck’s rolling, a ton of air flows in to fill that space. Then that air slams into the rear wheels and axles beneath the trailer. This means the semi truck needs to work extra hard to pull the trailer down the road.

Semi truck trailer with two orange plastic aerodynamic ramps installed underneath.
Semi trailer UnderTray system | SmartTruck

The reason that an aerodynamic trailer “UnderTray” looks like a combination between a snowplow and a slide is…it kind of is. Its job is to divide the air flowing beneath the trailer and force it down and away from the wheels. A full system includes two aerodynamic elements. One in front of the wheels and a second one behind.

SmartTruck claims that the latest generation of its UnderTray system can bump a semi-truck’s MPG more than 10%. That makes investing in it a no-brainer for many truck owners. And it makes investing in better research a no-brainer for SmartTruck. The company’s President is Mitch Greenberg. He said, “The pace at which the industry is embracing the Trailer UnderTray System encourages us to further develop the product and bring additional enhancements to the industry.” It’s no surprise the company is now selling the sixth generation of its UnderTray.

Next, read why some semi trucks have a tiny window by the passenger’s feet, or learn all about the complex aerodynamics of a semi truck in the video below:

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Staff Writer

Henry Cesari

Henry Cesari joined MotorBiscuit in 2021 and brings his deep interest in vintage cars, trucks, and motorcycles to the site. Having restored his first classic car at just 16 years old, Henry has wrenched on everything from overland campers to Japanese motorcycles and even pre-war Bugattis. Henry is also an avid attendee at local car shows and genuinely enjoys connecting with fellow auto enthusiasts.

Henry earned a Bachelor’s in Anthropology and English from the University of Vermont. He has since incorporated his love of the written word with his admiration of classic cars. While Henry is interested in vintage cars, he believes the golden age of cars is yet to come. He is confident journalists will play an important role in this revolution by exposing the industry as it is now and envisioning its possible future.