Cameron Brown and John Sessions
Oregon State University, Corvallis, Oregeon, USA
Tropical forest conditions pose special problems for forest transportation because a number of adverse factors are present. First, most tropical forests have prolonged wet periods with high intensity rainfall. Second, in many areas of the tropics, good quality rock is scarce and expensive. Third, harvest removals under the selective management systems designed to sustain tropical forests are usually low and may limit investment in roads. The objective of this paper is to suggest the use of variable tire inflation pressure for log trucks as a means of reaching transportation management goals in tropical forests. A synthesis of concepts and applications of variable tire inflation pressure in North America is discussed. Parallels are drawn between benefits and costs in North America and tropical forest regions.
KEYWORDS: Variable tire inflation, central tire inflation, tropical forest transportation.
Forest management requires a reliable transportation system to provide access to the forest. Roads, however, can have major environmental impacts through their location, design standard, construction method, and maintenance level. Roads can also drive management prescriptions as initial harvest removals may be required to pay for road investments. High road costs are sometimes used to justify high harvest removals.
Tropical forest conditions pose special problems for forest transportation because a number of adverse factors are present. First, most tropical forests have prolonged wet periods with high intensity rainfall. Second, in many areas of the tropics, good quality rock is scarce and expensive. Third, harvest removals under the selective management systems designed to sustain tropical forests are usually low and may limit investment in roads.
The objective of this paper is to suggest the use of variable tire inflation pressure for log trucks as a means of reaching transportation management goals in tropical forests. A synthesis of concepts and applications of variable tire inflation pressure in North America is discussed. Parallels are drawn between benefits and costs in North America and tropical forest regions.
VARIABLE TIRE INFLATION
One way to overcome many of the transportation problems in tropical forests and obtain additional benefits seen in other areas of the world would be to apply the concept of Variable Tire Inflation to log trucks in tropical forests. Variable Tire Inflation (VTI) is the concept of matching truck tire pressures with specific hauling conditions, defined by speed, terrain, load, and road surface strength. The idea of varying tire inflations on vehicles has been around since at least WWII when the US military developed systems that allowed the driver to control tire pressure for the purpose of improving vehicle mobility. In the early 1980s, United States Forest Service engineers began exploring the implications of reduced tire pressures for use on their large system of aggregate surface roads . A number of tests, both structured and operational, have been completed since that time and each have shown that significant benefits can be obtained by matching tire inflation pressures to hauling conditions.
During a typical hauling cycle, both log hauling and road construction vehicles carry a load during only part of the cycle and use a variety of road surfaces and speeds. In order to match tire pressures for each of the hauling conditions, tire pressures must be altered during the haul cycle. These variations in tire pressure can be achieved in two ways:
Stationary "airing stations" or truck mounted Central Tire Inflation (CTI) systems. Airing stations are locations on the haul route that have facilities to allow a driver to manually increase or decrease tire inflation pressures to a specified level. This represents a simple and inexpensive approach to matching tire pressures with hauling conditions but it is limited in application because of the time and inconvenience involved in manually changing pressures. A minimum of one station would be required at the landing and additional stations could be added at points where the haul condition changes significantly, such as a logging road and highway intersection. Because truck drivers are limited to a single pressure configuration between air stations and their inconvenience generally precludes use of more than two stations in a hauling cycle, optimal pressures for each haul condition are not always available.
Conversely, truck mounted Central Tire Inflation (CTI) systems allow pressures to be altered at any time and as often as the driver feels that it is helpful. The driver is able to control the system from within the cab and generally has preset buttons designated for each portion of the haul cycle and additional settings used for severe conditions such as steep, slippery grades. An example of the pressures used on a commercial CTI operation are shown in Table 1 and Figure 1. CTI allows the tires to be inflated or deflated while in motion by control values that route air from the fixed-axle housings, through a seal to the rotating wheel, axle shaft or hub. Air is provided by the air brake compressor and a fail-safe priority value assures brake operation. While a retrofit CTI system can cost from US$7,000_15,000, the convenience provided by this system ensures optimal tire pressures for every haul condition are available and therefore maximizes the benefits associated with VTI. As consumer demand for CTI systems increase, manufactures of new vehicles should be able to factory install the option on new trucks at a reduced cost.
Table 1: Operational tire inflation pressures used by Weyerhaeuser for a CTI system 
Haul Cycle Phase
Commercial CTI systems are likely to be the only effective system of varying tire inflation pressures when there is a wide variety of road conditions and potential speeds occurring over a haul route. Alternatively, if the haul route is limited to low speed, non-paved roads, airing stations at the landing and the mill may be able to provide an acceptable range of pressures at a reduced initial investment. In cases where neither CTI or airing stations seem viable, it may be of benefit to use constant reduced pressures (i.e., off-highway, loaded condition) if the haul route is relatively uniform in terrain and speed.
Figure 1. Tire Inflation pressures for a drive axle during a typical log haul cycle
Tire deflection and vehicle speed have been suggested as the guide for determining safe and effective low inflation pressures for commercial vehicles . Tire deflection is the change in tire section height from the freestanding height to the loaded height (Figure 2). Deflection is usually expressed as a percentage that represents the change in height over the freestanding section height. Hodges  reported that deflection in the range of 20-22% was optimal for reducing road damage but requires vehicles to travel at speeds less than 35 mph. Conventional highway tire inflation pressures (100 pSI, 689 kPa) normally fall in the range of 10-12% deflection. The inflation pressure required to achieve a particular deflection will vary depending on the weight of the vehicle's load and the tire's location on the vehicle.
Figure 2. Tire deflection is the criteria used for setting inflation pressures
When a constant inflation pressure is used on an industrial vehicle, it is set to ensure vehicle stability and appropriate tire temperatures during the most demanding circumstance the vehicle will encounter. For log trucks, this condition typically occurs when the vehicle is fully loaded and traveling at highway speeds. When the vehicle is no longer carrying a load or operating on paved roads at high speeds, its tires are effectively overinflated. Overinflated tires contribute to a variety of problems including increased vehicle vibration, tire wear, occurrence of punctures, and damage to the road's surface . A system that allows the driver to optimize tire performance for a given speed, load, and road condition by adjusting the inflation pressures will benefit the road, vehicle, driver, and environment.
Tire Pressure and Footprint Size
A vehicle's impact on the road occurs at the contact patch or "footprint" of each tire. At lower inflation pressures, the footprint of a bias ply tire will increase in both length and width, while a radial tire's belt plies limit the contact width of the tire to its design width  and force the footprint to only increase in length (Figures 2 and 3). This is an important difference because a longer footprint serves as a more effective means of increasing traction than a wider footprint, and it ensures proper tire wear because only the tread of a radial tire ever contacts the ground. The Tire and Rim Association has only endorsed tubeless radial tires as appropriate for use with variable tire inflation systems .
Figure 3. A radial tire's footprint will change size with different inflation pressures
To propel a vehicle along a road, energy is transferred from the engine, through the drive train to the driven tires. On a paved surface, the transformation of this energy into movement is dependent on the amount of energy available at the wheel, and the frictional force developed between the tire and road surface . On unpaved road surfaces, only a percentage of the energy available at the wheel is transferred into movement because some is lost to tire slip and soil shear. The road surface must be able to withstand the shear stresses created along the length of the footprint in order to prevent 100% slip from occurring. Washboarding results from the slip cycles occurring at the drive wheels that excite the road/tire/suspension system into a particular frequency [4,20]. As the size and frequency of the slip cycles is reduced, the washboard's ridges will be spaced further apart until the ridges disappear. Rolling trailer tires do not slip and are therefore less prone to initiate washboarding, although they may magnify existing washboards if the tires become resonant . Wash-boarding is most likely to be induced by cyclic bearing stresses .
Rutting of the road surface is another type of road damage that occurs from vehicle traffic. The road section under the tire's footprint must be able to withstand the downward pressure applied by the tire's footprint or else lateral soil shear (rutting) will result . The amount of rutting will depend on the strength of the surfacing, depth of the surfacing, subgrade strength and the pressure exerted on the soil by the tire's footprint.
Rutting will occur if the road section (surface and subgrade) is not able to support the vertical pressure exerted by the tires.
Both types of damage are related to the size of the tire's footprint. For a given normal force, a tire with a small footprint (high pressure) spreads the normal force (tire's load) over a small area, while a tire with a large footprint (low pressure) spreads the normal force out over a larger area. Thus, to develop a given thrust, high pressure tires will create larger shear stresses on the road surface than low pressure tires . The high pressure tire will also place more vertical pressure on the surface and subgrade of a road to support the same load . This difference in shear stress and vertical loading can be especially important on roads with poor quality surfacing or weak subgrades. Tires with large footprints are more likely to produce shear forces within the limitations of the surface material and pressures within the limitation of the road section. The small footprint of tires at high pressures will therefore cause more washboarding and rutting than tires at low pressure.
Several studies have been published since 1983 that show significant reductions in road damage [3,9,11,15,19] and many others have quantified the reduction in road maintenance required when low inflation pressures are used. A structured test at the Nevada Automotive Test Center (NATC) reported a 76% reduction in road maintenance and a 70% reduction in surface material replacement between highway tire pressures (90 pSI, 620 kPa) and low tire pressures(25-53 pSI, 172-365 kPa) . A Weyerhaeuser operational study showed that the road maintenance required on a weak sandstone rock road was dramatically reduced with the use of lower inflation pressures . Grau  showed that in a controlled test with two identical trucks running on adjacent lanes, the number of passes before road maintenance was required was 1.5 to 21 times greater with low tire pressures. A US Forest Service operational study in Alaska showed that by reducing construction equipment tire inflations to 22% deflection (42-62 pSI, 290-427 kPa), rutting was reduced from 12-18 inches (30-46 cm) with daily blading to 0-4 inches (0-10 cm) with no blading requirements.
Depth of Surfacing
An additional benefit of a low inflation tire is the reduced depth of surfacing required to support the vehicle and load. The weight placed on the soil is spread out over a larger area and results in lower pressure on the ground. With less pressure to distribute to the subgrade, a reduction in the depth of the surfacing can be achieved (Figure 4). The surfacing must also be thick enough to ensure acceptable dynamic loadings reach the subgrade. A study completed two decades ago showed that wide single tires had a greatly reduced spring rate when compared with duals and produced significantly smaller dynamic loadings, allowing a 20% reduction in surfacing material . Low pressure tires can provide similar results because of their capacity to absorb shock and vibration better than high pressure tires, thus reducing dynamic loadings from surface roughness. Low pressures also reduce the impact from dynamic loadings by reducing the surface roughness, such as washboarding and potholes, that produce the dynamic loadings.
Figure 4. Low pressure tires require less surfacing to distribute loads to the subgrade
Reductions in aggregate thickness in the range of 20-30% with the use of low pressure tires have been reported by Steward . The reduction in surfacing aggregate can significantly reduce road building costs, especially when suitable material is not available on the job site, or can increase the life of a road for a given road strength. Figures 5 and 6 use equations developed by Army Corp of Engineers, Waterways Experiment Station [21,10] and methods from the USFS Aggregate Surface Design Guide to illustrate the impacts of lowered tire pressures on earth and aggregate surfaced roads. In Figure 5, for earth roads it is assumed that the compacted surface layer is constant at 6 inches (15 cm). This example for earth roads shows that 5 times as many trips can be made over the road if tire pressures are reduced from 100 pSI (690 kPa) to 50 pSI (345 kPa). The aggregate road example (Figure 6) shows that 25% less aggregate surfacing material is required when tire pressures are reduced from 100 pSI (690 kPa) to 50 pSI (345 kPa).
Allowable rut depth = 3 in
Surface Depth = 6 in
Surface CBR value = 14
Subgrade CBR Value = 8
Figure 5. Earth road durability as a function of tire pressure
Allowable rut depth = 2 in
Truck passes = 500
Surface CBR value = 30
Subgrade CBR Value = 10
Figure 6. Variations in surfage aggregate depth required as a function of tire pressure
Healing of Damaged Roads
A healing affect on roads with washboarding and/or rutting has also been reported in the variable tire inflation literature [2,4,12,16,25]. Both operational and structured studies have shown that if low pressure trucks are used on roads with existing washboarding and/or rutting, the road surface will improve or "heal" with repeated passes. This phenomenon has been compared to the use of a pneumatic roller/compactor to maintain a road  because if the vehicles are able to vary their path over the roads surface, lowered tire pressures have the same effect. Even if drivers are forced to remain in a single set of ruts, washboarding will shrink and rutting will not get worse. Lowered tire inflation cannot heal all types of degraded roads but it is safe to assume that if a pneumatic roller can improve the road, then so can vehicles with low tire inflation.
Summary of Benefits Related to Roads
_ Decreased road damage and longer road life.
_ Decreased road maintenance and associated maintenance costs.
_ Reduced use of surface aggregate in road construction and maintenance.
_ Ruts and washboarding can be healed.
Variable tire inflation has a similarly positive effect on vehicles as it did on roads. At very high inflation pressures, tires act like rigid wheels and have very little contact area with the ground. As discussed previously, lower inflation pressures increase footprint length and allow the tire to function like a pneumatic spring. These two concepts,increased contact with the ground and tires that provide shock absorption, allow vehicles travelling on rough roads to perform better and be less expensive to operate.
When operating off-road with tires that are at pressures intended for highway travel, vehicle speed is limited by the impacts and vibrations that threaten the drivers ability to control the vehicle . Decreased inflation pressures allow tires to act like pneumatic springs that absorb vibration and bumps, thus producing a smoother and more stable vehicle ride [2,3,8,12,19,24,25]. A structured test performed at the Nevada Automotive Test Center measured vibration levels that showed seven times more vertical energy was imparted to the vehicle with high inflation pressure in comparison to the low inflation pressure vehicle . This result also suggests that dynamic loadings on the road structure are reduced because the road is providing an equal and opposite reaction force.
This improvement in vehicle ride has allowed faster average speeds on rough roads and reduced haul times in several operational trials [2,12,19].
In order for a vehicle to move, it must be able to produce a thrust that is larger than the sum of the resisting forces (i.e., grade, friction, rolling, and air resistances). The net torque a vehicle is capable of producing at the soil/tire interface is a function of the amount of energy available from the engine, the vehicles drive train efficiency and the wheels radius. As the torque is transmitted to a wheel, some amount of slip will occur between the tire and the soil before stress builds up to support the thrust. As slip occurs, tire and soil deform and rolling resistance is created. As torque at the wheel overcomes rolling and other resistances, the vehicle moves . If the soil cannot provide enough resistance to support the thrust, 100% slip will occur and the vehicle will not move.
The amount of resistance a soil can provide is determined by its cohesive and frictional properties. Cohesion properties can provide a small amount of traction and the remaining resistance provided by the soil comes from frictional properties. While traction from cohesion will improve with a larger footprint area, traction generated by frictional properties depends strongly on footprint length to develop a given thrust. For equal thrust, a longer footprint creates less shear stress on the soil and therefore requires less slip (deformation of the soil) to build up enough resistance to begin moving or keep moving, when compared with a shorter footprint. The longer footprint also provides more opportunity for the soil to build up shear strength before it fails (100% slip) because of the additional bearing surface area along the axis of the shear stress as compared to a wider footprint. These two factors increase power efficiency and improve mobility.
Rolling resistance also affects vehicle mobility. As inflation pressures increase, rolling resistance from tire deformation decreases and the rolling resistance from tire sinkage increases. The two effects oppose each other and suggest that a properly inflated tire should minimize the power consumption from both effects . This ideal pressure will be less for softer road surfaces because tire sinkage will be larger on soft roads than on paved surfaces.
The larger footprint of a low pressure tire places more tire tread in contact with the ground, increases floatation by spreading the weight of the vehicle over a larger area, and reduces tire slip. Several studies using lowered tire pressures have shown that mobility is greatly increased in course grained (sandy) and muddy soils [2,12,19,25] while fine grained (silt and clay) soils showed smaller but significant improvements . One structured test showed no improvement in the grade a log truck could climb, but did report that the truck with lower tire pressures was able to climb further up the slope .
A US Forest Service study on the Siuslaw N.F. using CTI trucks found that loaded log trucks were capable of climbing grades of 18-21% unaided with reduced tire pressures . Weyerhaeuser provided the following evidence of gradeability:
"With the trucks tires inflated to 620 kPa (90 pSI), the driver tried to pull the load up the hill. The road was wet from a rain. He spun out at about 12% adverse in the middle of a turn before he got to the 17% grade. The driver backed the truck up to get out to the turn and lowered the tire pressure to the severe condition setting (205 kPa, 30 pSI). He then started the truck and drove it out unassisted." .
A traction evaluation conducted by FERIC on a flat, gravel surface showed that drawbar pull could be increased 39% by lowering the tire inflation pressure from 90 pSI (620 kPa) to 30 pSI (207 kPa) . Both FERIC's and Weyerhaeuser's operational studies found that CTI trucks experienced large traction gains in slippery, muddy conditions.
Handling and cornering were improved, and the need to chain tires or assist vehicles was greatly reduced [2,12]. A reduction in the slipping and hopping of drive tires typical of high pressure trucks on steep gravel roads was also reported [2,12,20]. This improved mobility permitted the vehicles to travel faster when off-highway and on steep grades, or in muddy or rough road conditions . The higher speeds possible on rough roads also provides more momentum for use in difficult traction conditions, leading to increased mobility.
Improved traction can also help to slow the truck down. No studies examined this relationship in detail but the Weyerhaeuser operational study reported:
"The traction improvements have also increased the effectiveness of braking on steep favorable grades. Low pressure tires are less likely to skid on a wet slippery road. This has been especially noticeable on the short logger, the pup truck, and the lowboy truck. These trucks haul heavier loads than normal trucks and have more trouble braking on steep grades." 
Some of the largest concerns about variable tire inflation have been related to tire wear and damage. Either of these concerns can shorten tire life. In the logging industry, very few tires come out of service due to casing fatigue, most are retired because of belt separations, sidewall damage, or too many section repairs . The use of low inflation pressures on rough roads reduces the amount of damage sustained by a tire because they are better able to absorb and roll over objects that might otherwise cut or penetrate a high inflation tire . Lowered tire pressures have also been shown to reduce the number of rocks that get caught between duals because of the reduced space between the tires [11,25].
The Weyerhaeuser study showed that tire tread wear rates did not differ significantly between low and high pressure tires . FERIC reported that tread wear on the original drive tires was reduced by 47% and retreads achieved a 25% reduction in wear in comparison to the average of the control fleet . The US Forest Service trial at the Nevada Automotive Test Center found a 15% reduction in tread wear with the low pressure tires . This indicates a wide range in tread wear can occur with differing haul conditions, tires, and driving techniques. It does appear safe to conclude that low pressure tires can have similar or better wear rates than high pressure tires on rough roads.
Each of the studies mentioned above also assessed the structure of the low inflation pressure tires at the completion of the study. FERIC found that,
"the tread faces were in exceptionally good condition, having few cuts and almost no chunking of the tread blocks. No sidewall damage was noted and the tires had significantly fewer rock penetrations than was typical of drive tires in logging service in the same area. Toyo Tire's analysis of the low pressure tires found no adverse effects of low pressure operation."
and Weyerhaeuser found:
"a reduction in tread chipping and rock retention occurred. Goodyear did a laboratory test on each tire as it was taken out of service for recapping or failure and these examinations showed that low pressure tires had fewer belt separations than the high pressure tires. If this trend continues, it could indicate longer tire life with low pressure in off-highway applications."
The studies have shown that when low inflation pressures were used with appropriate speeds and loads, tire life is increased because of reduced damage to the tread and sidewall, and potential increases in tread wear. Reduced damage and fewer rocks caught between duals results in fewer failures and can lead to improved productivity for vehicles using low inflation pressures on non-paved roads. Additional benefits can be realized if CTI systems are used on vehicles because tire pressures can constantly be monitored and leaking tires can be identified. Slow leaking tires can be continuously inflated by the CTI system until the truck can reach a suitable repair shop, eliminating costly road side assistance.
Tire inflation pressures affect fuel consumption because tire temperatures and rolling resistances affect the efficiency of the system. A Goodyear Tire and Rubber Company study showed that a 10 pSI (69 kPa) reduction in truck tire pressure will cause a one percent loss in fuel economy on paved roads . On softer roads, this relationship is less well defined because rolling resistances are not always the lowest with high tire pressures . Low pressure tires penetrate into the soil less than high pressure tires and therefore have less rolling resistance from the soil. This is countered by the increase in rolling resistance occurring from tire deformation at low pressures. Slip can also affect fuel economy because it is a measure of how efficiently the power provided to the wheel is being use to propel the vehicle. We know that there is less slip on non-paved roads with lower tire inflations  and therefore a more efficient transfer of energy that suggests improved fuel economy for low pressure tires. The net affect on fuel economy from the combination of rolling resistance and slip is not well known but it appears to be insignificant.
Vehicle Repair and Maintenance
The softer vehicle ride provided by low inflation tires on rough roads also serves to reduce vehicle repair and maintenance. The vehicle is subjected to less vibration and reduced impacts because of the additional suspension provided by the low pressure tires.
The Nevada Automotive Test Center study found that low tire pressures transmitted 85% less shock and vibration to the suspension of a truck and that damage to truck components was reduced by as much as 87% and repair costs by 83% . FERIC's year long operational study found that a truck using CTI technology required 30% fewer repairs than the standard fleet average and that 91% less time was spent repairing cracks in frames and other components . Both FERIC and Weyerhaeuser detected less vibration related repairs, ranging from 20-75% [2,12]. The large variation in results is likely due to different road and driving conditions in each of the study areas, with the roughest roads yielding the highest savings. Decreases in truck repair and maintenance have also been reported in several US Forest Service trials but were not quantified [3,25].
A common and costly repair to logging trucks operating on steep aggregate roads is a broken rear differential. They are most often broken by the slipping and hopping of the drive tires on steep grades. Hopping occurs as the tires shears the soil under the contact patch and then falls down into the depression it created [4,20]. This process continues until the digging effect provides enough contact area to support the forward movement of the vehicle. During tire slip, the tire rotates faster and less torque is transmitted to the tire. As the tire grabs onto the larger contact area, a large torque demand is suddenly placed on the drivetrain (rear differential) . It is the repeated incidences of these torque spikes that breaks drivetrains. A US Forest Service study found that the peak rear-axle torque of a loaded log truck climbing a 20% grade was reduced by 14% and average torque was unchanged when low tire pressures were used . Simonson states that lower tire pressures translate more engine power to its intended purpose, rather than wasting it in road damage (slip) and additional gear train wear (torque spikes) .
We can probably assume that the largest savings in repair and maintenance costs will be realized on trucks using low inflation pressures on the most severe road conditions (steep and rough). However, the evidence presented above shows that any vehicle operating on non-paved roads will experience reduced repair and maintenance costs, leading to less down time and increased productivity.
Summary of Benefits Related to the Vehicle
_ Higher average speeds on rough roads, reduced haul times,
_ Decreased damage from shocks and vibration,
_ Reduced drivetrain stresses, fewer failures,
_ Reduced repair and maintenance costs,
_ Reduced down time, more productive hauling,
_ Longer vehicle service life.
_ Improved traction and braking,
_ Reduced need to provide assist-vehicles on steep adverse grades,
_ Fewer stuck vehicles that require rescue,
_ More landing locations becoming available, higher vehicle capabilities,
_ Longer haul seasons.
_ Fewer flats, more productive hauling.
_ Potentially better tread wear, more miles (km's) between retreads,
_ Less tire damage, fewer cuts and penetrations,
_ Longer tire life, more retreads possible,
_ Truck spends less time in the tire shop.
Almost every driver involved with the use of variable tire inflation systems has commented on the improvement in vehicle ride and reduced fatigue after a day of driving [2,3,8,12,19,24,25]. Variable tire inflation systems that allow lowered tire pressures to be used on rough roads can help make the drivers working life more comfortable and productive while possibly even reducing work related back problems.
Driver safety can also be improved with variable tire inflation systems because pressures can be altered to provide optimum tire performance on any road condition.
Optimal pressures improve driver safety by allowing better vehicle control, better braking capabilities, fewer failed climbs and related back-downs, fewer tire failures, and better monitoring of tire pressures with CTI systems.
Summary of Benefits Related to the Driver
_ Reduced driver fatigue,
_ More productive drivers,
_ Fewer back problems, less sick time, fewer compensation claims,
_ Safer working environment.
Non-paved roads that are situated close to streams or have runoff reaching streams have been identified as potentially significant sources of sediment. Their exposed soils provide a source of sediment and the road itself can provide a means of transporting it, especially in steep terrain. Although road location and design can minimize the amount of sediment that reaches a water course from these roads, some level of sedimentation will likely always be occurring depending on the road surface, amount of precipitation, and road traffic.
Improved water quality through a reduction in sediment production has proven to be a potentially large benefit associated with the use of variable tire pressures on non-paved roads. Rutting caused by heavy vehicle traffic on sloped roads causes road surface runoff to concentrate into channels and develop larger erosive powers [7,13]. The reduced rut depth associated with low tire pressures decreases the concentration of flow over the road surface, reduces erosive power, and reduces sediment production from roads. Low tire pressures can also reduce the amount of sediment that is readily available to be transported off roads through smoother, better compacted road surfaces, less soil disturbance (i.e., damage and repair cycles,) and reduced surfacing aggregate breakdown (production of fines) [6,13].
Several studies have been completed by the US Forest Service to assess the difference in sediment production between low and high pressure tires. A study completed in 1991 showed a decrease in sediment yield of up to 55% while hauling in wet conditions with low tire inflation pressures . A three year study was completed in 1994 that found reductions in sediment of 54% (year 1), 84%(year 2), and 63% (year 3) when low pressure trucks were used in place of trucks with normal highway pressures . The variation in sediment production is correlated with the amount of rainfall and truck traffic that the road sections were subjected to during the tests. Low tire pressures provide greater reductions in sediment with higher rainfall and truck traffic. Higher rainfall produces more opportunities for erosion and sediment transport, while increased truck traffic provides more opportunity for rutting, channelization, and soil disturbance (damage and grading) which can translate into sediment production. The largest reductions in sediment associated with the use of variable tire pressure can be expected from roads that experience large amounts of rain and heavy vehicle traffic.
Additional reductions in the amount of sediment that reaches water courses can be achieved through the design of road systems that are less likely to result in sediment reaching the stream. The most sensitive road locations are those immediately adjacent to streams, because eroded soil can quickly enter the stream, and flooding may even wash entire road sections away . Landslides can be one of the largest contributors of sediment into a water course and thus road location must avoid areas that are prone to failure. The improved traction and mobility of vehicles using low pressure tires on non-paved roads allows road designers greater flexibility in road design. This provides more flexibility to avoid potentially hazardous areas such as headwalls and areas adjacent to streams. Steeper grades allow roads to reach ridgetops faster and provide shorter sections of midslope roads that have a higher potential for failure than ridgetop roads in unstable terrain . Ridgetop roads also reduce the likelihood of sediment reaching a water course because any sediment leaving the road surface must pass through more soil or vegetation before it reaches water.
Summary of Benefits to the Environment
_ Reduced sediment production from non-paved roads,
_ Improved flexibility in road location: less sediment reaches streams, fewer landslides.
IMPLEMENTATION AND POTENTIAL PROBLEMS
Studies have shown numerous benefits to the landowner, truck operator, and environment are associated with the use of variable tire inflation. This would suggest that the implementation of the idea would be an easy task because of the apparent win-win situation for all parties involved. One of the problems that may be hindering the widespread use of this technology in the Pacific Northwest of the USA is that only a portion of the economic benefits of installing and operating a variable tire inflation system accrue to the parties who pay for its installation and operation. Contract truckers are especially isolated from many of the economic benefits because even if a landowner provides an economic incentive for using VTI vehicles, the purchaser of the timber sale may not pass on the savings to the contract truckers. The ideal situation is when a single firm is responsible for all aspects of the operation (forest land, roads, equipment, and people) because all the benefits of VTI will ultimately accrue to that firm. An example of this situation would be a large company operation, such as Weyerhaeuser, that owns their own logging and truck equipment and operates on their own land. In this case, each of the benefits to roads, vehicles, people and the environment would be realized by the firm that must bear the additional owning and operating cost.
Although this organization provides the most direct link between costs and benefits, it need not be present for VTI systems to be implemented.
Implementation can also be complicated by different types of road users. If only a portion of the vehicle traffic on a non-paved road is using low tire pressures, then the benefits to the road and environment will not be as great. The low pressure traffic will provide some "healing" of the road damage caused by the high pressure traffic but the net effect will be a reduction in benefits realized.
One situation where traffic would need to be restricted to VTI vehicles would be if the road was constructed with a reduced depth of surfacing aggregate for use with VTI vehicles [10,26]. Normal highway pressure vehicles would quickly destroy roads designed with a thinner surfacing layer and thus should not be allowed on the road.
Thus, designing a new road system or a portion of a system for reduced tire pressures will result in a long term commitment to the use of the technology . A tradeoff must be made between building less expense roads and restricting traffic over a portion of the road system. It seems reasonable that dead end spurs and temporary roads would be ideal candidates for reduced surfacing thickness because they would not hinder hauling in other parts of the transportation network.
In situations where commercial CTI systems are found to be too expensive, stationary airing stations located at landings and where high speed haul will commence can provide the desired changes in tire pressure. This method of obtaining variable tire pressures is particularly helpful in situations where there are few variations in the road conditions and investment capital is limited. Potential problems with this scenario are the exposure of airing equipment to vandalism and theft, a source of power is required, and it is slower and more labor intensive than CTI systems.
APPLICATIONS IN TROPICAL FORESTS
Many of the conditions that favor variable tire inflation are present in the forested regions of the tropics. Low volume removals, high rock costs, and large amounts of rainfall combine to make hauling conditions difficult and erosion potential large. Sediment production from some forest roads surfaces in Malaysia have been estimated to be as high as 25 m3 per 100 m of road . Study results from the Pacific Northwest have shown that lowering tire pressures on non-paved roads can result in longer lasting roads that are functional over a wider range of moisture conditions, less required maintenance, and reduced sediment production. New roads that are to be built with aggregate will require a smaller volume of the expensive material. Increased vehicle mobility will allow more areas to be reached and temporary roads will not have to be built to as high as standard. All of the benefits seen in the Pacific Northwest should be realized in tropical forests and many will be magnified because of the poor road conditions and high levels of rainfall. In conclusion, variable tire inflation systems that allow low tire pressures to be used can produce significant improvements in the hauling economics and environmental impacts occurring in tropical forests.
Technology transfer and lack of investment capital may be barriers to the use of high technology systems such as CTI in the tropics but simpler solutions such as airing stations or constant reduced pressures can be used to realize the majority of the benefits.
Barriers to the implementation of new ideas and technology will always exist until we choose to find a way around them.
Significant benefits to the road, vehicle, driver and environment are all possible when tire inflation pressures are set to match the hauling condition; defined by speed, load, terrain, and road surface strength. Low tire pressures can be used on non-paved, low speed roads in order to minimize road and vehicle damage, maximize vehicle traction and mobility, allow reduced depths of surfacing on new roads, minimize sediment production, and provide a safer, more comfortable working environment for the driver. As vehicle speed increases, higher tire pressures should be used to maintain stability and fuel economy on paved roads. In order to match the tire inflation pressures with changing haul conditions, commercially developed "Central Tire Inflation" systems are available to allow the driver to change tire pressures from inside the cab, or stationary "airing stations" can be set up to manually alter tire pressures.
The benefits of variable tire pressure vehicles can be lumped into two categories that are of great interest to forest land managers: economics and environment. Economic gains can be seen when trucks are able to use low tire pressures on non-paved roads because of decreased road maintenance costs, decreased vehicle repair and maintenance costs, increased tire life, potentially lower road construction costs, and increased hauling productivity. Productivity gains are realized because of less "down-time" for the road system, vehicle, and driver that result from fewer breakdowns, less required maintenance, reduced need for vehicle assistance in the field, and improved ability to haul in adverse conditions. Environmental benefits are realized by the decrease in sediment production and an increased flexibility in road location that allow designers to minimize road location on potentially unstable terrain.
The studies have shown that the largest benefits from variable tire inflation will be seen when vehicles are operating on non-paved roads that experience high levels of rainfall, steep road grades, rough terrain, and poor surfacing material or no surfacing material. In summary, variable tire inflation will benefit any vehicle using non-paved roads, and will provide the greatest improvements over highway pressures when hauling conditions are at their worst.
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