Virtual Prototyping support TACOM in Predicting Truck Stability

U.S. Army Tank-automotive & Armaments Command (TACOM) researchers are using computer simulation to investigate concerns about the yaw stability of trucks equipped with super single tires. Super singles are very large tires, typically about 50 inches in diameter, that can provide increased traction when they are used to replace dual tires in the drive position. Concerns have been expressed, however, that these tires might reduce yaw stability, increasing the chances of a rollover, because they have lower lateral stiffness than dual tires. Using mechanical simulation software, the researchers simulated a situation that would be expensive and dangerous to test in the real world: a truck with super single tires performing constant radius turns at increasing speeds until an inside tire lifted off the ground. The simulation quantified these concerns and helped the researchers provide guidance to TACOM engineers developing performance specifications for Army trucks.

TACOM is responsible for procuring and managing the US Army fleet of both wheeled and tracked vehicles. TACOM’s Research, Development and Engineering Center (TARDEC), provides engineering and scientific support to vehicle designers, developers and system managers to maximize the performance and capability of these vehicles and ensure the safety of their occupants. The vehicle users (Battle labs etc.) are responsible for writing operational specifications, such as: a truck needs to be capable of operating on terrain ranging from the desert to the Arctic Circle. TACOM engineers are responsible for converting these into performance specifications, such as the vehicle needs to be able to make sudden lane changes at 45 mph. As part of these responsibilities, TACOM engineers are continually seeking out the performance limits of old and new vehicle technology.

In the past, TACOM engineers relied nearly exclusively on physical testing both on the test track and on physical test bed simulators, to determine vehicle capabilities. More recently, an increasing amount of this work is being done using computer simulation. Computer simulation allows engineers to evaluate the performance of equipment under extreme conditions that could be dangerous, damaging and expensive to perform on the test track. Computer simulation also provides more data in most situations than physical testing. Physical testing results are usually limited to a small number of locations where sensors are placed while computer simulation provides data from throughout the problem domain. Finally, with computer simulation it’s relatively easy to change the vehicle design parameters being evaluated, to lengthen the wheelbase or reduce its weight for example. These types of changes are expensive and time-consuming to make during physical testing or after the vehicle is produced.

In a recent example of their increasing use of simulation, TACOM researchers investigated the effects on yaw stability of fitting a 6X6 military truck with super single tires. There are currently two versions of this truck in inventory. The older version is equipped with 11.00X20 bias ply tires, singles on the front axle and a dual tire configuration on the rear axle. The upgraded variant is essentially the same truck except that it has one 14.00R20 super single tire at each suspension station. This upgrade was made in order to improve the off-road mobility of the truck. TACOM researchers wanted to investigate possible tradeoffs in terms of yaw stability in order to provide guidance in upgrading trucks in the future as well as suggestions on the operational capabilities of the current inventory.

The researchers modeled the truck, it’s suspension system, and the tires using LMS DADS mechanical system simulation software. This advanced multi-body dynamics simulation software isemployed throughout the world to create virtual prototypes of vehicles and other mechanical systems with general constraints, gravitational forces, and forces due to contact. DADS has the ability to produce accurate 3D models, solve the non-linear equations of motion, output realistic 3D animation, and graph loads, velocities, accelerations, and positions. These capabilities along with it’s proven numerical stability 
and accuracy, make it a powerful design analysis tool for mechanical engineers. TACOM researchers have used DADS successfully for a number of years, using it to model and simulate over 100 military vehicles. Over this period, they have taken advantage of DADS open architecture to develop a considerable number of user-defined subroutines. These subroutines make it easier to re-use simulation models for complex subsystems, such as tires, braking systems and power trains.

TACOM researchers modeled the troop/cargo transport truck with a high level of detail for the critical suspension components and tires. The front axle supports the truck chassis through a pair of leaf springs and the rear two axles share two leaf springs with the chassis supported through a trunnion in the center of each rear leaf spring. Shock absorbers are used on the front axles only and their characteristics were obtained from the manufacturer. The leaf springs were modeled as vertical springs acting between the vehicle and the axle. Tire data sets for the tires were obtained from the tire manufacturers, including aligning moment versus normal force; vertical, lateral and longitudinal forces versus deflections versus tire normal forces; and lateral and longitudinal slip force versus slip angle versus tire normal forces.

In order to validate the accuracy of the virtual prototype, field tests were conducted with an instrumented truck. The test data was recorded as the truck traversed discrete bump and pothole obstacles at an army proving ground. These discrete obstacle scenarios were chosen because of the relative ease with which they can be replicated in simulation. The computer model was made to traverse equivalent virtual test courses and the displacements, accelerations and rates from the field data were compared with the results from the simulation. Once the channels were shifted to accommodate transients in vehicle speed in the physical tests, the results matched up quite well. Percentage differences between simulation and physical test measurements were between 5% and 12% for all comparisons.

The test that was employed to gauge the vehicle’s yaw stability was to control the vehicle’s steering to follow a circular path whose radius was 30.5 meters. The vehicle speed was slowly increased until the lateral acceleration exerted on the vehicle caused a tire to lift off the ground. The key parameter that was monitored in these simulations was the tendency of the vehicle to oversteer – to move into a sharper turn without intervention by the driver. This tendency is caused by the rear wheels slipping laterally more than the front wheels. This tendency is dangerous because the lateral acceleration that a vehicle undergoes is inversely proportional to the turning radius. Without intervention by the driver, an oversteered vehicle will continue on an ever-decreasing radial spiral until it turns over. Braking only exacerbates oversteering, because it transfers weight from the rear to the front wheels, increasing slippage of the rear wheels. An understeered vehicle, on the other hand, one that tends to move out of the turn, is desirable because it does not require immediate correction by the operator.

Handling diagrams were created for the vehicle hauling a 2270 Kg payload, a 4540 Kg payload and a 6810 Kg payload with the payload centered, shifted forward and shifted back. The results showed that the truck equipped with the dual tire configuration on the rear axles exhibited the desired understeer characteristics for all tested payload weights and locations. On the other hand, the truck equipped with the super single tires exhibited only slight understeer or neutral steer characteristics with higher payloads or when the payload shifted rearward. This can be attributed to the reduced lateral stiffness of these tires. Because this is a very steady state test, it seems reasonable to assume that under adverse operational scenarios, such as panic obstacle avoidance events that include brake application, the vehicle would very likely pass over to the problematic oversteer side of the diagram.

By using a virtual prototype created in DADS, TACOM researchers were able to consider several possible solutions to improve the yaw stability of the truck. They were able to simulate using different tires on the front and rear so that the rear tires would provide substantially greater lateral restoring force than those on the front. The disadvantage of this approach is that it requires doubling of spare parts inventories. The fact that a rolling tire generates greater tire to ground slip forces than a skidding tire in lock-up means that the implementation of anti-lock braking systems might help to relieve the problem. A more radical solution would be the implementation of active, speed sensitive rear axle steering to augment the front axle steering that would act to keep a heavy truck on the understeer side of the handling diagram at high speeds.

As this application demonstrates, computer simulation is being used to cost-effectively investigate vehicle design options. This is leading to improvements in procurement effectiveness and operational guidelines for existing equipment. Simulation reduces concerns about the safety of physically testing equipment to its extremes as well as the cost and time involved in building and testing custom configurations. Physical testingwill continue to be required, both to calibrate virtual models and to provide real world validation of simulation results. The net result of developing virtual prototype’s is that TACOM can fulfill its primary mission of developing safe and reliable mobility and armament systems that support Army readiness, faster and at reduced costs.