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Why Do Semi Trucks Have Flaps On The Back? (Explained)

Have you ever wondered why some semi trucks have flaps on the back and panels on the sides of the trailer and others don’t?

Perhaps you have been wondering how you can save some fuel costs on your semi truck.

In this article we will be exploring aerodynamic panels and how they can help your semi truck, read on to find out more…

Here’s a quick answer to Why Semi Trucks have Flaps on the Back:

The flaps on the back of a semi truck help to reduce drag by allowing the air to flow more gently through the back of the truck. This makes the semi truck more aerodynamic. The overall objective of these flaps is to increase the fuel efficiency of the semi truck.

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What are the Flaps on the Back of Semi Trucks Called?

Not many people know what to call the flaps on the back of a semi truck. It’s a strange concept, and it can even be difficult to describe to someone who has never seen them.

The flaps on the back of a semi truck have several different names. As with many parts on semi trucks, they often get their own nicknames.

Some of the different names for these flaps are rear tail fairings, trailer tails, and sometimes boat tails.

Why do Semi Trucks have Flaps on the Back?

The goal of the flaps on the back of a semi truck is to reduce drag. A reduction in drag will make the semi truck more fuel efficient overall.

Reducing drag is done by making the semi truck more aerodynamic. Aerodynamics changes how easily the vehicle travels through the air.

Adding the flaps at the back of the semi truck might not help it to cut through the thicker air in front of it though.

However, the flaps at the back help by allowing the air to connect back together more gently behind the truck. This means that there is build up of air pressure in the vacant space at the back of the semi truck.

With the flaps bolted on, the density and pressure underneath the trucks is more similar to that of the ambient air around it. Therefore, there is less suction on the rear of the semi truck.

How Aerodynamic is a Semi Truck?

The majority of semi trucks are not aerodynamic at all. Most semi trucks are designed for practicality and to carry massive amounts of cargo rather than hitting an impressive top speed.

However, as we have begun to understand the importance of aerodynamics, we can see that it has a benefit beyond just making a vehicle faster. It also helps with the fuel efficiency of the vehicle.

Modern semi trucks have become more aerodynamic. The most aerodynamic semi truck will be the Tesla Semi when it is released.

The Tesla Semi has a more rounded tractor with a sharp nose at the front as opposed to a square tractor with a flat nose at the front.

How do the Flaps on the Back of Semi Trucks Work?

The flaps on the back of a semi truck make the vehicle more aerodynamic by allowing the airflow to reconnect more gently behind the truck.

Without the flaps, there is a large open space at the back of a semi truck. When the airflow reaches this space, it rushes into it chaotically, which causes more drag.

The flaps on the back of a semi truck allow the airflow to be channelled or streamlined through that space and towards the back of the truck.

This reduces the amount of drag at the back of the semi truck and therefore makes it slightly more aerodynamic.

Are Truck Wings and Aerodynamic Panels the same thing?

Truck wings ae different to aerodynamic panels. While aerodynamic panels are mostly fitted on the trailer of the truck, specifically on the sides and the rear, the truck wings are often found on the tractor.

Truck wings are often on the roof of the tractor. They look like a fin or spoiler that blocks the gap between the trailer and the tractor.

They serve the same purpose, which is to make the semi truck more aerodynamic and reduce fuel usage.

However, truck wings aim to make the semi truck more aerodynamic from the front by making the semi truck more streamlined, allowing it to cut through the air more cleanly.

Truck wings can be fixed or moving parts. Moving truck wings will automatically deploy once you drive on the highway.

Are Aerodynamic Panels Activated Manually or Automatically?

Aerodynamic panels found on semi trucks can’t be autromatically activated. These panels are fixed onto the semi truck.

This means they need to be manually added or removed from the semi truck. In some cases, they get in the way when loading and unloading cargo.

In these cases they will need to be removed and temporarily stored during the loading process so that they are not damaged.

Can Aerodynamic Panels or Flaps be Fitted on a Trailer?

Aerodynamic flaps found on the back of a semi truck can be fitted to either the tractor or the trailer depending on where they are needed.

However, the aerodynamic panels are much longer and need to be fitted to the trailer in between the axles.

Where on a Semi Truck or Trailer can Aerodynamic Panels be Installed?

Aerodynamic panels are quite large length wise and need to be fitted to the sides of the trailer in between the axles.

This space is where the air flow becomes disrupted which can cause excessive amounts of drag. Installing the panels on the trailer will allow the air to run smoother along the body of the trailer.

Are Aerodynamic Panels for Semi Trucks Expensive?

Depending on the quality of the aerodynamic panels and the model of semi truck you are using, the price for these panels varies.

In general, the flaps at the rear of the semi truck costs around $2000 per set. The side skirts cost around $2500 per set.

How Feasible are Aerodynamic Panels and how Much Fuel do they Save?

Aerodynamic panels are becoming more common on semi trucks today because of the amount of money that can be saved on fuel.

Due to better aerodynamics and smoother airflow over the body of the semi truck, fuel efficiency can increase by around 5% for each one (side skirts or tail fairings).

Using both can save up to $10,000 in fuel costs per year. This means that their initial cost is a small investment that would pay for itself within a year of driving.

Which is the Best Configuration of Aerodynamic Panels for your Semi Truck?

The best option is to make use of both of the aerodynamic panels. This will have the greatest effect on aerodynamics and result in the most fuel saving.

However, if you had to choose just one, the tail fairing are easier to use, smaller and they do not get in the way too much.

At what Speeds do Aerodynamic Panels Work the Best?

Aerodynamics are most effective at high speeds. In order for aerodynamics to properly take effect, you need air to flow over and around the vehicle.

This means that the aerodynamic panels will be most effective at highway speeds when the vehicle can cut through the air with less effort.

Do Aerodynamic Panels on your Semi Work for Short Hauls?

Aerodynamic panels are recommended for long hauls, which is where they will be most effective when travelling at higher speeds.

At lower speeds and shorter distances, they may still save you some costs on fuel, however, the overall effect will be less impactful than the estimated 5-10% in fuel saving.

How do you know if Aerodynamic Panels on your Semi Truck will Benefit you?

It can be difficult to judge whether aerodynamic panels will benefit you. Especially since we are talking about something that can not be seen with the naked eye: air flowing over the semi truck.

However, in most cases, aerodynamic panels will work if your semi truck regularly travels over 50 miles per hour.

At this speed, the panels have a chance to do their job more effectively and prevent turbulent airflow from causing drag.

Where can you get Aerodynamic Panels for your Semi Truck Fitted?

It is best to seek the advice of a professional when you work with aerodynamic panels for the first time. They can be tricky to get fitted on properly.

There are several workshops that offer this service, although you should expect to pay a labor cost when having the panels fitted as well.

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