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Drag Mechanics of Trucks/Trailers with Open Wheels

Truck Drag Mechanics Further Explained (For those with a technical interest).

Our numerous patents show that all open-wheeled vehicles needlessly waste fuel by exposing the top portion of the wheel whose surface necessarily moves forward directly against the wind at twice the vehicle speed. (It is only the outline of the wheel that appears to move forward at the vehicle speed. The wheel surfaces move forward anywhere between zero at the stationary ground to up to twice the vehicle speed at the top of the wheel.) Thus, if the vehicle speed is at 65 mph, the top forward-moving surface of the open wheel is exposed to 130 mph winds plus any additional headwind that may be present at the time.

And most importantly, since power loss due to drag force is actually proportional to the wind speed cubed—a very sensitive exponential relation—the much faster surface at the top of the wheel is dissipating at least 8X more power than any equivalent slower surface on the deflector panel. Moreover, this highly magnified power loss on the exposed wheel increases rapidly with any additional external headwind present. For this reason alone, our minimally sized deflector panels become far more efficient in windy conditions, a non-obvious relation supporting our numerous patent allowances.

The wind shielding deflector panel is simply fixed directly to the vehicle frame, and is therefore moving forward at only the vehicle speed, dissipating much less power than equivalent uppermost wheel surfaces. Thus, it is only logical to shield the drag-sensitive top of the wheel using a slower-moving deflector panel in order to shift the magnified drag otherwise induced from the faster upper wheel surfaces onto the slower vehicle frame, thereby reducing the effective drag on the vehicle considerably, especially in headwinds. As a result, our deflector panels are optimally sized to shield only the uppermost wheel and should always be used in order to minimize vehicle drag. 

Furthermore, since the lower wheel rolls easily over any bumps in the road (just push along a toy model while resisting first on the lower and then upper wheel in order to demonstrate the relative mechanical effects), it likewise rolls just as easily through any headwind, much more easily than any lower wheel deflector panel plowing through the wind at the vehicle speed. Thus, the lower wheel should ideally remain exposed to headwinds, a non-obvious relation not yet appreciated even in auto racing where airplane wind tunnel testing protocols are erroneously applied to open-wheel vehicles, thereby having misled an entire industry for many decades.

And making the deflector panel much bigger also defeats the savings potential, since larger panels induce too much drag on the vehicle without providing any gains from reducing drag on the wheel, since the lower wheel should ideally remain exposed to headwinds. Our panels are optimally sized to shield just enough of the critical upper wheel surface to yield a substantial reduction in vehicle drag even under our worst case null wind conditions.

And as discussed above, whenever headwinds are present, vehicle efficiency actually increases rapidly even further, this predicted effect having been confirmed through numerous road tests on bicycles. So for vehicles operating frequently on the highway under windy conditions, fleets can expect even more savings, reducing payback time considerably.

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 savings gained under the null wind conditions in which fleets often road test in order to confirm the performance of these much shorter skirts. 

As a result, fleets employing 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 save fuel.

Industry Testing Protocols Further Explained:

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 not so exposed to potential 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 deployment of both technologies in 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 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 this estimate can be useful for vehicles with covered wheels, it becomes quite inaccurate for vehicles with open wheels exposed to headwinds.

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. It showed dramatically enhanced gains in crosswind yaw angle of only four degrees. 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 confirmational 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 induced to be mostly laminar.

4. Inducing air to flow inward between the wheel sets increases vehicle drag, since it also increases the displacement of air by the moving vehicle, while also disturbing the otherwise relatively static air passing between the wheels. Static air is higher in pressure, and should be preserved static behind the vehicle as much as possible.

5. Blocking air flowing in-between the wheel sets is also counterproductive, since this airflow increases the displacement of air by the moving vehicle, requiring increased power, while also reducing the static pressure developed behind the vehicle, 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 wheel drag is highly magnified at the top of the wheel. Only the uppermost portion of the exposed wheels should be shielded.

8. Inner Wheel Skirts inhibit the lateral displacement of air by the wheels toward the inside of the wheel sets, thereby stabilizing the relatively static central air column passing between the wheels and thereby increasing the static pressure developed behind the vehicle to decrease overall vehicle drag. This effect is greatly enhanced in the presence of crosswinds.