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Short Trailer Skirts Expose Wheels to Crosswinds:

Lately, many fleets have been utilizing much shorter trailer skirts disposed far forward on the trailer, likely in order to reduce invested costs over the more expensive full-length trailer skirts. However, if these semitrailers are operating in even somewhat windy conditions, then the rear wheels are actually very exposed to headwinds, producing considerable vehicle drag that largely negates any fuel savings gained under the null wind conditions.

(Even under null wind conditions, the outer wheel and upper sidewalls are still exposed to headwinds when using these shorter trailer skirts.)

Industry practice is to conduct fuel economy road tests under near null wind conditions, since producing repeatable winds in more windy conditions is virtually impossible to achieve. Thus, road tests do not account for more real world windy conditions often present in many parts of the country.

As a result, fleets choosing to employ these shorter trailer skirts while operating in windy conditions based on the measured performance gains obtained solely under null wind condition road tests are largely misjudging the actual gains attainable. However, by simply adding our deflector panels for use in combination with these shorter trailer skirts, fleets can then enjoy an optimum aerodynamic configuration for use in real world windy conditions. 

And for LTL semi-trucks where trailer skirts are simply too expensive to yield a reasonable payback period, our deflector panels also offer a simple, low-cost alternative to still save considerable fuel.

Industry Testing Protocols Further Explained:

As mentioned, fleets typically road-test trailers only under near no wind conditions, since no wind conditions are at least somewhat repeatable from test-to-test. And no wind conditions happen to show that short trailer skirts can be somewhat effective, since the rear wheels are still at least somewhat shielded from headwinds in the absence of any crosswinds. 

But testing in the windy conditions under which many fleets actually operate is quite problematic, since windy conditions are very unrepeatable from test-to-test, and is therefore avoided in standard testing protocols. And since shielding the upper wheel is much more effective in windy headwind conditions than under null or no wind conditions, standard industry road testing protocols (including wind tunnel protocols) simply will not show just how effective our deflectors are under real world windy conditions.

Therefore, fleets should not rely solely on standard low-wind road tests of shorter trailer skirts in determining just how effective these skirts are in real world windy conditions, since the rear wheels being exposed to crosswinds become very high drag inducers on the vehicle, largely negating much of the benefit of these short skirts gained under null wind conditions. However, adding our deflector panels to these trailers already having shorter trailer skirts can then correct for this savings degradation by also shielding the upper wheels under windy conditions. In fact, this combination is likely the most efficient and cost-effective deployment of both technologies under windy conditions.

Furthermore, wind tunnel testing also will not show the magnified effect that wheel drag has on the vehicle, since the complex mechanics of the freely propelled vehicle on the road is negated by the wind tunnel model being instead fixed to the ground by attachment to a force-measuring stinger. Rather than power dissipation being directly measured inside the wind tunnel, drag forces acting on the body of the vehicle are instead measured to only infer what effect measured vehicle body drag has on overall vehicle efficiency. Actual vehicle efficiency is then simply estimated from measured vehicle body drag by using a rough 'rule of thumb' estimate, as explained by test engineers at ARC while we were testing our Inner Wheel Skirt invention. While this estimate can be useful for vehicles with covered wheels, it becomes quite inaccurate for vehicles with open wheels exposed to headwinds.

(This reliance on simple vehicle body drag measurement protocols in the wind tunnel at ARC in Indianapolis was quite surprising to us when we tested our Inner Wheel Skirt invention in 2021, since we assumed ARC would be a bit smarter in measuring the efficiency of open wheeled vehicles. But the racing industry has likely grown accustomed to this comparative approach of body drag measurements, apparently satisfied in ignoring the highly magnified drag effects of the exposed wheels.)

As a result of these testing limitations being largely inaccurate for measuring the effect of shielding only the upper wheel, Null Winds Technology invented a more accurate method for vehicle wind tunnel testing that instead has the vehicle being self-propelled and unrestrained on the rolling road inside the wind tunnel, just as it would be on the actual open road, in order to directly measure the power being dissipated in drag on the entire vehicle, including the wheels.

In this patented method, rather than relying on any estimated effect based on body drag forces on a restrained vehicle that is effectively attached to the ground via a force-sensing stinger, the total power being dissipated in drag including the otherwise unmeasured magnified drag loss on the wheels is measured directly through the power being delivered through the wheels by the rolling road. This power being delivered is then equivalent to total drag power being dissipated on the vehicle.

This new wind tunnel testing method can then capture the true effect of shielding the upper wheel, whereas standard wind tunnel protocols measuring only drag forces on the vehicle body will not. However, employing this patented method would require a retrofit of the rolling road controller, an expensive modification wind tunnels have yet to adopt for increased measurement accuracy specifically for testing open-wheeled vehicles.

We confirmed these standard protocol testing limitations while testing in the ARC wind tunnel in Indianapolis in 2021, where we also confirmed the enhanced crosswind gains produced by our Inner Wheel Skirt invention. We tested our Inner Wheel Skirts on both a semitruck model and on a pickup truck model using standard wind tunnel drag force sensing testing methodology.

Both tests showed dramatically enhanced gains in crosswind yaw angle of only four degrees, reducing vehicle drag up to 10X more than in simple headwind conditions.  We expect those gains to increase even further in larger yaw angles often encountered under windy conditions, since rear wheel sets directly facing headwinds become major drag inducers on the vehicle.

Factors Affecting Vehicle Drag on Trucks:

Null Winds Technology has road tested numerous aerodynamic concepts with MVT Solutions in Pecos, Texas over three separate occasions between 2017 and 2021. As a result, we have honed in on numerous factors affecting vehicle efficiency of semi-trucks. While some concepts proved viable, unexpectedly several did not. From these tests and including additional testing in the ARC wind tunnel in 2021, several basic concepts can be concluded:

1. Since power lost in drag results from both friction (skin) drag and form (pressure) drag, a balance must be obtained between losses from both of these sources of drag, an aerodynamic systems problem.

2. A moving vehicle must push relatively heavy air out of the way, requiring considerable power at highway speeds where losses are maximized. Minimizing the displacement of air by the moving vehicle can then minimize overall vehicle drag.

3. Turbulent air includes losses due also to the rotational displacement of the air (creating eddies), further increasing the power lost through even further displacement of the air over simple translational displacements in laminar air flow. Where possible, any displacement of air should be limited to be mostly laminar flow.

4. Inducing air to flow laterally inward to flow from the outside to between the wheel sets actually increases vehicle drag, since it also increases the displacement of air by the moving vehicle.  This displaced air also disturbs the otherwise relatively static air naturally passing under the vehicle between the wheels. Static air is higher in pressure (not having a momentum component), and therefore should be preserved as static behind the vehicle as much as possible.

5. Blocking air flowing in-between the wheel sets is also counterproductive, since diverting this airflow also increases the displacement of air by the moving vehicle, requiring increased power to do so, while also reducing the static pressure developed behind the vehicle, also increasing vehicle drag. This unexpected result was observed from all road tests that employed a shield disposed immediately in front of the rear axle. It is only the wheel portion of the undercarriage that should be shielded, and preferably only on the uppermost portions thereof.

6. Trailer skirts perform well by inhibiting the lateral displacement of air under the vehicle, thereby improving the static air pressure developed behind the vehicle, while also minimizing the overall displacement of air by the moving vehicle, minimizing the input power required.

7. Exposed rear wheels in crosswinds dramatically increase vehicle drag, since both wheel drag is highly magnified at the top of the wheel, and crosswinds increase turbulent air developed behind the vehicle. Only the uppermost portion of the exposed wheels should be shielded. Short trailer skirts expose wheels to crosswinds, largely negating their effectiveness under windy conditions.

8. Inner Wheel Skirts inhibit the lateral displacement of air by the forward moving wheels toward the inside of the wheel sets, thereby stabilizing the relatively static central air column passing between the wheels under the axle.  Increased static air passing under the vehicle increases the static pressure developed immediately behind the vehicle, to thereby decrease overall vehicle drag. While this reduced vehicle drag effect is minimal in null wind conditions, the effective reduction in vehicle drag is greatly enhanced in the presence of minor crosswinds, by a factor of 10 or more on both a semitrailer as well as a pickup.