Does eliminating rivets and lightweighting a trailer improve mileage?

by Carroll McCormick

MONTREAL, Que. — Looking to reduce fuel consumption, a carrier orders in some trailers that weigh less, and which have no external rivets. Good choices, it would seem, as less weight means less rolling resistance, which means less fuel burn and a smoother trailer surface, one would think. Reducing aerodynamic drag should also reduce fuel burn. But are the gains worth the investment?

Lightweighting, as the shaving off of trailer and tractor weight is called, is a worthy task, particularly so in its early days when it was easy to knock off serious tonnage.

“In forestry, starting in the 1990s and 2000, we were getting guys to lighten their rigs in the forest industry. We could easily take two tons off the TARE weight,” says Jan Michaelsen, leader of Montreal-based PIT Group, best known for its Energotest fuel savings trials.

“Rolling resistance is directly related to weight. The more weight you put on, the higher the rolling resistance,” Michaelsen says. Since 40% of fuel consumption at high speeds is due to rolling resistance, if you lower the weight, “all things being equal you will save fuel,” he adds.

There are many ways to lightweight trailers. A worthy read on this topic is The North American Council for Freight Efficiency 2015 Confidence Report: Lightweighting. It lists 21 ways to reduce trailer weight. Big-ticket weight savings, for example, come with wood composite or aluminum flooring, aluminum wheels and various trailer components, and steel shell versus standard iron brakes.

Whether the savings justify the typically additional costs depends on several things. Is the corridor the trailer will be used in flat or hilly?

“You need to think about is where you are operating. Flat [terrain], there is less of an effect. But if you are going up hills a lot you will save fuel. On flat terrain you are not changing speeds. You might be using only 200 hp. When you go on the hills you use a lot more power. Anything in rolling terrain you are better off with a lighter vehicle,” Michaelsen says.

If you are running LTL, the fuel efficiency per ton-mile, or freight ton-miles per gallon, can be increased more with lightweighting than in a trailer that is grossed out more often. And speaking of grossed out trailers, the benefit there of lightweighting may not come from saving fuel, but rather from being able to trade that saved weight for more cargo.

“Your percentage saved is higher, the lighter the trailer and load. It still comes down to the point that you get more bang for your weight reduction from adding more payload and getting more revenue. Payload is everything,” Michaelsen says.

Other considerations include how much the trailer is used. The more it sits around, the less opportunity there will be to save fuel and pay off the lightweighting investment. And, does lightweighting increase maintenance? It might, but do not simply assume that lightweighting means a less durable trailer, Michaelsen warns.

“Just because you are lighter it doesn’t mean [your trailer] doesn’t last as long.”

Fuel savings due to lightweighting may run from a half of a percent to 5%, but don’t expect miracles.

“Operationally speaking it is very difficult to see the difference. It does not mean that they are not there, but they are difficult to measure on a day-to-day basis,” Michaelsen says. “Generally speaking, unless you are getting more payload for the weight reduction, the fuel savings, if there is an extra cost, probably may not mean a positive return on investment. If you are not going to be able to get your money back in two years or less, it is generally not worth it.”

But all is not lost. “It is also important to say that even small savings in fuel is a direct increase in your margin. It doesn’t take a lot. One per cent does not seem like a lot, but if your margin is 3%, it is a lot,” Michaelsen adds.

(File photo)

A riveting suggestion

Hundreds of rivet heads dot the outside of a standard dry van trailer. We have been taught that the smoother the surface, the less the aerodynamic drag, and the lower the fuel consumption. Right? Well as a rule, yes (spinning golf balls are one sneaky exception) but for trailer rivets the cure, if you have to pay for it, may be more costly than the disease, so to speak.

Rivets live in an invisible aerodynamic phenomenon called the boundary layer – a layer of air that builds up on a moving trailer that ranges from a few inches thick at the front, to two, and sometimes more, feet thick at the rear. At its surface the air is traveling at the same speed as the truck, but the deeper into the boundary layer you go, the slower it is moving.

Drag is not much of an issue at low speed but it becomes critical at higher speeds (try walking, then running with a sheet of plywood held out flat in front of you).

On a trailer, tiny rivet heads at the bottom of the boundary layer are not contributing much to drag. (The matter is completely different for aircraft, for which flush rivets were a great improvement. Traveling as fast as they do, airplanes have far thinner boundary layers. Where a rivet sitting an eighth of an inch proud of a trailer’s surface is essentially nothing, that eighth of an inch was a big deal for early jet aircraft).

“Essentially, rivets will create an additional drag on a vehicle. The question is just how much relative to the overall drag on the vehicle. The friction drag on the sides and top of the trailer constitutes only about 10% of the overall drag of the vehicle. Eliminating rivets will have an effect, but not significant – sub 1%. It will have a minor impact,” says Brian McAuliffe, a research officer in the aerodynamics laboratory at the National Research Council Canada.

“For an 80,000-kg load, aerodynamics only accounts for half or less of the total resistance, so the rivets would account for about 0.5% of the fuel burn – an upper limit. Removing the rivets will reduce the drag, but the potential is very small. Day-to-day, filling up your tank, it would be imperceptible.”

So, while there are fleets searching high and low for those tenths of a per cent fuel savings, which lightweighting and eliminating rivets can obtain, the bottom line for your wallet is whether, for your operation, the cost of spending money on lightweighting or better aerodynamics is higher or lower than the cost of the fuel saved, or the extra cargo you can haul, over the life of your trailer.

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