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Rolling Road Wind Tunnel To Improve Aerodynamics of Class 8 Trucks

Cd comparison of truck models with and without rolling wheels, with the road on and off. ARC concludes wheel rotation effects are responsible for a 7.8% increase in drag. The results show that while improvements in the general underbody of the truck will show an improvement in general, improvements around the wheels will give a much bigger benefit. Click to enlarge.

Concerns stemming from oil prices and greenhouse gas emissions have re-intensified efforts to design acceptable aerodynamic designs for long-haul Class 8 trucks that optimize fuel economy. In this effort, wind tunnels and models designed for tractor-trailer combinations play an important role.

Auto Research Center, Inc. (ARC), an independent aerodynamics research team that is based in Indianapolis and used by vehicle manufacturers, NASCAR and other racing teams, has developed a rolling road wind tunnel and truck model for Class 8 trucks that it says provides better insight into the on-road aerodynamic impact of design decisions.

Aerodynamics as a competitive differentiator. The Freightliner Cascadia (earlier post) was recently assessed as more aerodynamic than other Class 8 commercial vehicles, according to Freightliner in consultation with ARC.

ARC researchers compared the Cascadia with four other similarly spec’d Class 8 vehicles—the International ProStar, Kenworth T660, Peterbilt 386 and Volvo 780—using Freightliner’s wind tunnel, the only aerodynamic testing facility built specifically for Class 8 vehicles.

Testing took place in June and July. Researchers measured wind drag on the front, sides and back of the tractor, as well as the front of the trailer. There was a tractor-trailer gap of 48 inches. The amount of drag for the Cascadia was consistently lower and better than competitors. The ProStar recorded 7.8% more drag than the Cascadia; the 780 showed 9.6% more drag; the T660 showed 18.8% more drag; and the 386 showed 22.9% more drag.

My overall conclusion from this test is that based upon the results, the Freightliner Cascadia is the most aerodynamic of the five tractors tested.

—Mike Camosy, ARC operations manager

Using anticipated fuel costs and differing applications, the Cascadia’s efficiency could save customers as much as $950 to $2,750 a year per truck, according to Freightliner. Fuel savings were calculated assuming each truck was driven 144,000 miles per year, with fuel at $3 per gallon, driving 60 mph.

Rolling Road. ARC, which operates one of only 3 moving ground wind tunnels in the US, has built the US’ first rolling road semi truck model for aerodynamic development (wheels rolling on a moving ground plane in the wind tunnel).

The model is 1/8th scale, with instrumentation to record areas of high/low drag under different configurations.

At the recent Aerodynamics of Heavy Vehicles II Conference, ARC presented a technical paper—“Advanced Experimental Methods for the Analysis and Aerodynamic Design of Heavy Vehicles”—describing its experimental findings in comparing rolling road with non rolling road results, using two different models based on the NASA Generic Conventional Model (GCM): one with rolling wheels, the other with non-rolling wheels.

For both model configurations, NRW [non rolling wheels] and RW [rolling wheels], the road off conditions gave closely matching results. However, the road on condition revealed quantitative differences between the road off conditions with respect towards each model configuration as well as differences between these configurations themselves while tested in the road on condition.

The ARC detailed model tested in the tunnel represented a more “real world” class 8 truck. Multiple pieces to this model were systematically fitted to build it to a final more accurate specification as individual test runs. While fitting these basic parts, several of them caused a reversal of force trends between the road off and road on conditions. A combination of three of these basic parts, (landing gear, air tank, spare wheel), actually recorded a 0.42% drag increase in the road off condition, while recording a 0.95% drag decrease in the road on condition representing a 1.4% variance.

Three development items were chosen to represent underbody changes that would be influenced by rotating wheels as well as a rotating ground plane. The test items were a tractor rear wheel deflector, a rear trailer diffuser and a set of wheel covers. Two of these three development items showed a trend reversal of the forces.

The tractor rear wheel deflector gave a 1.04% drag increase in the road off condition, while giving a 1.52% drag decrease in the road on condition representing a total variance of 2.56%. The rear trailer diffuser gave a 0.82% drag decrease in the road off condition, while giving a 0.93% drag increase for the road on condition representing a variance of 1.75%. Although the wheel covers gave the same force trend for both the road on/off conditions, this change was much larger in the road on condition (1.33% greater change in road on).

ARC concludes that without rolling road testing, current development efforts to increase truck aerodynamics may be misleading.


Any good reasons why this was not done decades ago? Wind tunnels have be around for a long time.

I guess that the same could be done with samller trucks, pick-ups, 4 x 4, Hummers, SUVs, cars and other gas guzzlers with drag figures over 25 and sometimes as high as 40+.

Cameron Dell

I stead of playing around with 1 or 2% why don't they just mount the engine transversally like a lot of buses only in reverse. That should give them 20 to 25% gain in fuel and power. With no hypo gears their should be great gains in power and economy. Same design as a GT 40 sports car truckers are going to love it.


I doubt it's that straightforward, Cameron. A semi truck that spends most of its time at highway speeds is not a stop-and-go bus. It has a very different duty cycle. If it was as "simple" as changing the way the engine was mounted, they would have done so a long time ago. So I'm pretty sure these engineers know what they're doing.

A 1-2% efficiency improvement over the lifetime of a semi truck that drives millions of miles in its lifetime is still many thousands of gallons of diesel fuel saved.



Wind Tunnels have been around since they were invented by the Wright Brothers. But moving road wind tunnels have not. And as indicated in the resuts, aerodynamic testing of road vehicles requires the moving road wind tunnel to avoid introducing actual drag by the modifications.

1 % here and another there and there can add up. The Freightliner truck was almost 8 % better than the nearest rival, according to the article. That is probably 4 % less diesel consumed on a level run.

Eventually, somebody is going to do a radical change, and its going to take a moving road wind tunnel to prove to the money-men that it will be worth it.

As to diesel pushers, a tractor pulls a trailer. And the drive (rear) wheels are under the trailer, not under the engine. So even if you put the engine under as far back as it could go, transverse mounting would still require a right angle gear set. Unlike a diesel pusher bus.

Cameron Dell

Well I'm not sure at all that those engineers know what their doing. Most of the engineering is done. If the back of a bus was spun around the engine would be between the sets of wheels ( the front and the rear tandem axles). You might lose 3000 lbs of load capacity because the engines farther back but the gains economy and power would far out way that.

gavin walsh

"I guess that the same could be done with smaller trucks, pick-ups, 4 x 4, Hummers, SUVs, cars and other gas guzzlers with drag figures over 25 and sometimes as high as 40+."

you don't need a wind tunnel or a degree in aerodynamics to know these shapes are going to have massive drag. they're basically brick-shaped. They don't design them aerodynamically because your average Hummer buyer just doesn't care.


Cameron Dell: What makes you think that a transverse engine will save any fuel, especially the totally unrealistic 20-25% increase you mention? In reality, rear bus engines have an additional angle drive from the engine/transmission, which are an in-line unit, at the extreme rear of the bus, to the hypoid drive axle,via a drive shaft, which is many feet further ahead. The truck layout eliminates that additional angle hypoid gear, and is more direct and efficient than the bus.

The typical bus drivetrain layout is nothing like a FWD car powertain layout. And even in FWD cars, the efficiency/power gain from their transverse layout is minute; nothing vaguely like amounts you cite.


Would the cost to modify and service a vehicle be greater or lesser than the cost of fuel saved?

Eric S

Gary -

That depend on what modifications need to be done. The beauty about the trucking industry is that these tractors are not on the road all that long in high mileage fleets. New technology can be equipped the next time a company orders new tractors. The fuel savings is only half of the story though. The trucking industry is preparing for federal regulation of carbon emissions. At this point it is not a question of if, but when.

In general - these new technologies pay for themselves within two years at current ($2.80+ / gallon) fuel prices.

I am working on several of these projects in a substantial fleet and have found that there are many simple, and fairly inexpensive changes a company can make to a truck/trailer combo and see immediate and substantial results. Contact me if you are interested in discussing further.


I can hardly bear to read this. Why are tractor trailers still designed like, yes, bricks, with a gap between the two, cab below height of trailer and square ends to the trailer, not rounded? That alone would greatly reduce drag and save these cost conscious truckers, on whom we all depend for our lives, megabucks.

See the papers by Robert Englar on modifying trucks with active blowing to achieve dramatic fuel savings.

Heavy trucks is the sector we really need some attention on - everything is transported by truck now. How are we going to get EVs in here? Sensible aerodynamics should be an absolute mandatory priority,perhaps with series hybrid operation - on those long motorway runs, the engine can just chug along as a generator at optimum efficiency to power the electric motor. Or use a turbine - even better, a Tesla bladeless parallel disk turbine. Then we have a sensible APU.


How can we eliminate the tractor-trailer's reason for existence? You achieve 100% fuel savings that way.



I tried to get PACCAR interested in improving aerodynamics 25 years ago to no avail. These companies hire "stylists" to make their trucks look good, not perform better. Our investigations provided the path to better fuel economy WITHOUT moving wind tunnels and our vehicles did .4Cd back then tested WITH a trailer.
%.8 improvement can be achieved by eliminating mirrors.
It is time to re-think design based on the premise thta "Form follows function" FLW and by taking into account that you "cannot solve problems by using the same logic that created them" AE www.alateinc.com

Thomas M. Tito

I have a 2005 4x4 Chevy Silverado Ext Cab Short Box pickup truck. In which I tow an 18' work an play enclosed trailer it is a very light trailer but still acts like a parachute. Can you help me.
My info is:
E-Mail tmtusnsarc@verizon.net
Cell (570)233-3278
Address 119 W. 22nd St.
West Hazleton, Pa. 18202

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