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As with most devices, there are compromises that should be considered. In order for side guards to be effective against under run and for side skirts/fairings to be effective in reducing drag they should be mounted as low to the ground as possible. However, very low side guards and skirts/fairings are prone to damage as trailers and trucks break over road disturbances such as rail road tracks. It is generally accepted that most side guards/fairings are mounted between 8 and 16 inches above the ground, depending on the application and the type of material used.
4.6.1 Drag Coefficient
It is clear that the use of complete aero packages (side and gap fairings and boat tails) can drastically reduce a vehicle s overall CD. However, the scope of this study is focused only on side guards, which could play a role in the overall reduction but would certainly not be the only contributor. It is estimated that CD as low as 0.30 (Figure 30, taken from Scania) could be achieved with full aero packages installed on large highway vehicles that currently have CD as high as 0.6.
4.7 Reduced idling due to reduction in collisions A common misconception identified in the literature review is that side guards reduce the number of collisions between vehicles and VRUs and thus reduce the number of times, or at least the duration of time, that traffic is backed up while emergency vehicles attend to the victim(s) and clear the area. However, side guards will not prevent all incidents from occurring but may simply minimize the risk that VRUs will be dragged under the wheels of the vehicle after coming into contact with a heavy vehicle. Therefore since most VRUs who strike a heavy vehicle, with or without side guards, will likely sustain some form of injury and thus require treatment from an emergency vehicle, traffic congestion will not necessarily be lessened by any amount. As a result, it is not clear if side guards will reduce the number of incidents or if the guards will simply cause VRUs to strike the guards and then be ejected or diverted into another lane of traffic to suffer a serious injury as part of secondary event with another vehicle or with the road/sidewalk surface. Evidence supporting this theory may be seen in Figures 16 and 17 where serious injury rates dropped but the proportion of slight injuries rose. Even slight injuries may still require emergency vehicles and cause traffic delays.
Despite these factors, NRC-CSTT attempted to create rudimentary models to calculate the amount of CO2 that is currently released as a result of traffic congestion due to serious side impact injury events. Additionally, attempts were made to predict the reduction in CO2 levels from traffic congestion if all trucks had side guards, thus reducing injury events requiring emergency vehicles and road closures. However, the historical data required for inputs to such a model are scattered and unreliable and, more importantly, the potential reduction of injury and need for road closures cannot reliably be predicted at this time. Therefore, a model based on currently available data is not worth considering at this time. It is not known if side guards would reduce the total amount of CO2 as a result of fewer traffic slowdowns and the actual amount of any potential reduction is impossible to predict. More importantly, even if the amount could be predicted, it is clear that any reduction in CO2 as a result of fewer truck/VRU injury events would be insignificantly small compared to the total amount of CO2 released in Canada every year.
Significant reductions in CO2 emissions from diesel powered trucks could be more easily predicted and achieved via other methods, such as proper tire maintenance and anti-idling strategies.
4.8 Operator Considerations Many aspects of side guards affect other users on the road and potentially the environment.
However, there are certain aspects and side effects that will impact the operators and maintainers of the fleets.
4.8.1 Access to underbody equipment The side guards must not interfere with the normal operation of the landing gear or prevent access to the spare tires, load securement points, twist lock levers, connection points for ferries, the diesel fuel tanks for the refrigeration unit nor should they prevent a driver from adding air to the tires.
Additionally, the side guards should not prevent a driver or mechanic from inspecting the brakes, particularly the brake slack adjusters as operators are currently required to inspect their
air brakes prior to each shift. This is a fairly routine procedure on a typical semi-trailer.
However, performing this inspection on a highway motor coach bus presents serious challenges due to the low body skirting and access panels. The addition of side guards would cause a trailer to look more like a highway coach in this regard; however, as long as access to the underside of the trailer is still achievable from the rear of the trailer, the brake inspections should still be possible. Most manufacturers provide removable access doors within the panels of fairings (Figure 28) that are meant to cover the wheels and tires of the vehicle. This allows drivers and maintainers access to the tires with only minimal effort.
Most van and flat bed trailers have minimal amounts of gear underneath the decking that would be covered by side guards. However, some tanker trailers have significant amounts of piping and valves that must be accessed on a daily basis. There will be a challenge to engineer safe and effective side guards that do not interfere with tanker plumbing. However, if properly designed, the guards would not only protect VRUs from entering under the vehicle, but the valves underneath the tanker trailer will be protected from being sheared off in a collision with another vehicle.
4.8.2 Brake Cooling Many heavy vehicle brakes rely on a volume of air flowing over the drums or disks for cooling.
This must be considered when installing full length flush mouth side guards that could restrict this flow of air over the brakes and wheels.
4.8.3 Added tare weight to trailers and trucks The addition of side guards will inevitably add to the tare weight of the vehicle. For vehicles that tend to maximize load based on volume before weight (i.e. cube out) this may not be an operational issue since they will likely still be lighter than the maximum permissible axle loading, with respect to gross axle weight rating (GAWR) and local weight restrictions. However, this will represent a reduction in the gross payload that may be loaded into, or on, the vehicle/trailer for vehicles that are already loaded to their legal limit with respect to weight. In essence, every additional pound of side guard weight diminishes their value from the operator s perspective.
Regardless of loading configuration, the addition of any weight to a trailer will negatively affect the fuel consumption of the tractor unless the increase in fuel consumption is offset by a sufficient reduction in aerodynamic drag by the device itself.
In order to combat the weight issue, trailer and truck manufacturers may develop strategies to reduce the weight of other vehicular components such that the addition of side guards has no overall weight penalty.
Most European side guards are made of steel. Early versions of side fairings were constructed of lightweight aluminum. However, the aluminum plates tended to bend and deform very easily, particularly in and around the yard and when traveling over rail tracks and other road disturbances. Many newer fairings are constructed of a semi-rigid plastic such as high density polyurethane which is extremely durable, light and impact resistant but also flexible.
Typical complete aerodynamic packages add approximately 159 kg (350 lbs) to a 53 ft semi trailer. However, this includes many fairings and devices that are not part of the side or belly area of the trailer. It would be reasonable to assume a weight penalty of at least 114 kg (250
lbs) for aerodynamically designed belly fairings and the attaching hardware. The added weight of the fairings can be offset with the installation of alloy wheels (unless the trailer is already so equipped) which can reduce a trailer s weight by 116 kg (256 lbs) for a tandem trailer, and more for each additional axle.
4.8.4 Interference with self steer axles
The United States has relatively conservative axle load regulations and thus the vast majority of trailers operated in the US are equipped with a tandem non-steerable axle. This provides a large expanse of the trailer with which to mount a continuous flush mouth side guard (Figure 28). However, Canada s higher allowable axle loads allow the use of tri-axle, tridem and quad axle configurations that may include at least one liftable axle to allow multi axle trailers to negotiate tight corners. The use of lift axles is known to damage roads and bridges since the load that was carried by the lift axle must be distributed to the other axles when the lift axle is raised. Therefore, many provincial governments, including Ontario, have instituted freight policies that will eliminate the use of lift axles within the next 10 to 20 years. Therefore, the only way to maintain current vehicle loading is to replace straight lift axles with steerable lift axles.
Steerable lift axles cannot be raised by the driver in the cab and are capable of steering as much as 20 degrees when the vehicle is turning. Fitting continuous flush mount side guards (as shown in Figure 28) may pose serious technical challenges to the vehicle manufacturers due to the need to allow self steer axles to steer outside the lateral envelope of the trailer.
Trailers with multiple self steer axles may be required to carry gapped side guards; or complicated panels that move with the self steer axles will have to be designed and integrated onto the trailers.
4.8.5 Collection of snow, ice, mud and debris
Another possible side effect of side guards that should be considered, particularly in Canada, is their tendency to collect snow, ice, mud and other debris. During winter months, Canadian truckers must be aware that their vehicles could be trapping hundreds, if not thousands, of extra pounds of snow and ice. This increases GVW and increases braking distances since the driver may not be aware their vehicle has taken on unwanted weight. It is still not clear if side guards will prevent snow from entering under the vehicle or if snow will accumulate on the panels themselves, particularly on the backsides which will have structural elements that may naturally trap foreign objects and snow/ice. A literature search did not reveal any test data or results on this topic.
A representative from one of the fleet operators involved in the Transport Canada belly fairing project  indicated that (anecdotally via teleconference with NRC-CSTT) when comparing trailers with and without belly fairings, the trailers with the fairings tended to collect less snow and ice than the baseline trailers which did not have any side guards or fairings. The brackets for the fairings tended to collect some snow and ice, however, the trailer as a whole retained less snow and ice due to the snow plow effect of the side fairing. The fairings on these particular trailers were mounted with less than 12 inches of ground clearance which is considered relatively low. It is likely that guards with more ground clearance would allow more snow to enter underneath the trailer.
Without any quantitative test data to corroborate these observations, it may be necessary to investigate this phenomenon in greater detail to determine if side guards help, or hinder vehicle weights during inclement weather.
4.8.6 Break angle and Interference around Snow and Ice Reduced ground clearance is important for optimum safety performance of a side guard and drag reduction of a fairing, but this can cause manoeuvrability problems in some circumstances.
Operators of trailers already fitted with side guards have also mentioned damage issues due to striking snow banks or going around curves in the yards or anywhere the guards have potential contact with the ground.
Another logistical challenge of operating belly fairings and side guards is interference with objects as the trailers break over an angle. Rail crossings, humps in the yard are only a few such areas where any device attached to the side of a trailer is at risk. Newer, more flexible materials have alleviated this issue somewhat. Regardless of construction, operators must be aware of reduced clearance over such a long span of trailer. The low rider belly fairings, which sit only eight inches above the ground, are particularly at risk for damage when crossing an obstacle.
4.9 Canadian Statistics Tables 21, 22 and 23 are drawn from the Statistics Canada annual Canadian vehicle Survey of
2007. These data are useful when attempting to predict the overall environmental and safety impacts to the fleet at large.
There are many different ways to distinguish between classes of heavy vehicles. Since this study relied heavily on annual statistical data, it made sense to group the vehicles based on the same groupings used by Statistics Canada. Therefore, some of the groupings may not be intuitive to those familiar with heavy trucks. Three different weight classes of vehicles were defined for straight trucks and for tractor trailer combinations for this portion of the study. The basic classifications are shown in Table 20.
The following definitions may be found in the Statistics Canada data:
Vehicle-kilometres: is the distance traveled by vehicles on roads.