How It Works: Truck Guards vs. Truck Skirts. What you think is a truck guard probably isn't.
by JIL MCINTOSH | FEBRUARY 20, 2019
When pedestrians or cyclists are hit by large trucks, they’re often pulled under and run over by the rear wheels. Many people ask why all trucks aren’t equipped with the side guards they’ve seen on tractor-trailers, to help prevent such tragedies from happening.
But what they’re describing aren’t truck guards – instead, they’re aerodynamic skirts. Truck side guards are different and aren’t used on tractor-trailers. They are required on trucks in several countries, but they’re still very rare over here.
Fuel is a major expenditure for trucking fleets, and companies are always looking for anything that can save even a small amount, because it will add up over the number of trucks, and the kilometers they rack up each year. Manufacturers are adding aerodynamic features to their trucks, but it’s harder to apply wind-cheating designs to the trailer. It has to be a rectangular box because that maximizes the interior space needed to turn a profit when hauling goods.
The skirts are attached to the trailer ahead of its wheels, where they divert air to reduce the turbulence and drag that wastes fuel. They’re usually made of materials such as aluminum, rubber, or fiberglass. They have to be firm enough to do their job, but they have to have some flexibility, so they won’t break off if the trailer hits something like a snowbank.
While they add weight to the trailer – between about 60 to 160 kg, depending on their construction – they still save between 4 to 7 percent in fuel. However, they don’t become effective until the truck is traveling above 70 km/h, which is why they’re found almost exclusively on highway haulers. They can be used alone or with other aerodynamic add-ons, such as fairings that close the gap between the truck and trailer, and rear trailer fairings with panels that fold out from the back of the trailer.
These flexible trailer skirts are primarily for fuel savings and are not meant for pedestrian protection. Side guards are. They are usually used only on “straight” or “single-unit” trucks – those with all their axles on a single frame, such as garbage or dump trucks, rather than a tractor-trailer, where the truck pulls a separate trailer. As opposed to highway trucks, delivery-style straight trucks are more likely to be used in downtown areas where there are more pedestrians.
The guards fit between the truck’s front and rear wheels and are usually made of steel, so they won’t bend or flex if a pedestrian or cyclist makes contact with them.
Skirts and guards aren’t used together, although some skirt manufacturers are working to develop new skirt designs that would be strong enough to help protect pedestrians. While Europe, Japan, and the United Kingdom have mandated side guards, not every jurisdiction does. They’re not federally required in Canada or the US, although some private or municipal fleets, in both countries, require them to be installed on their straight trucks.
Tractor-trailers are required to have rigid rear-guards, known as underride guards, which help to prevent vehicles from going under the trailer in a rear-end crash. If there’s no rear guard, vehicles may slide under far enough to crush the cabin.
Advocates say adding rigid side guards to the trailer could help prevent cars from going under it if the truck jackknifes, or if a car driver sideswipes the truck, in addition to protecting pedestrians. Studies are being done on them, but one issue is that trailer sides – which are relatively lightweight to maximize payload – might not be able to support a guard capable of deflecting a vehicle in a crash.
The US Insurance Institute for Highway Safety (IIHS) conducts tests on the mandated rear underride guards, crashing vehicles into them at full width, and at 50 and 30 percent overlaps, rating their manufacturers as it does automakers for their crash protection. Not all underride guards meet the IIHS’ top safety standards for them. The US National Highway Traffic Safety Administration (NHTSA) announced in 2013 that it was collecting heavy-vehicle crash data, with a plan to create a computer model to test underride guard designs, but has not put out any updates since then.
Proponents of side guards say that they could help prevent pedestrians and cyclists from being caught between the wheels of a truck if they collide as the truck is turning right. Those who disagree point out that pedestrians have been killed by falling under the rear wheels of buses, which have flat sides that are lower than a side guard would be.
Studies show that most people killed by trucks in urban settings are hit by the front of the vehicle. Many of these collisions might be avoided by using convex crossover mirrors, which bolt onto the front fenders and give the driver a view of anyone who is beside or in front of the truck. These mirrors are mandatory on all school buses in Canada but are not required on trucks. And while side skirts will probably increase in popularity to save fuel costs for fleets, it’s not likely that side guards will be required anytime soon.