High Speed Off-Road Vehicles

Suspensions, Tracks, Wheels and Dynamics

Bruce Maclaurin

• Publisher: Wiley
• Year: 2018
• ISBN:
  9781119258810
• Edition: 1
• Topic: Technologie et ingénierie
• Subtopic: Génie mécanique

A concise reference that provides an overview of the design of high speed off-road vehicles

High Speed Off-Road Vehicles is an excellent, in-depth review of vehicle performance in off-road conditions with a focus on key elements of the running gear systems of vehicles. In particular, elements such as suspension systems, wheels, tyres, and tracks are addressed in-depth. It is a well-written text that provides a pragmatic discussion of off-road vehicles from both a historical and analytical perspective. Some of the unique topics addressed in this book include link and flexible tracks, ride performance of tracked vehicles, and active and semi-active suspension systems for both armoured and unarmoured vehicles.

The book provides spreadsheet-based analytic approaches to model these topic areas giving insight into steering, handling, and overall performance of both tracked and wheeled systems. The author further extends these analyses to soft soil scenarios and thoroughly addresses rollover situations. The text also provides some insight into more advanced articulated systems.

High Speed Off-Road Vehicles: Suspensions, Tracks, Wheels and Dynamics provides valuable coverage of:

• Tracked and wheeled vehicles
• Suspension component design and characteristics, vehicle ride performance, link track component design and characteristics, flexible track, and testing of active suspension test vehicles
• General vehicle configurations for combat and logistic vehicles, suspension performance modelling and measurement, steering performance, and the effects of limited slip differentials on the soft soil traction and steering behavior of vehicles

Written from a very practical perspective, and based on the author's extensive experience, High Speed Off-Road Vehicles provides an excellent introduction to off-road vehicles and will be a helpful reference text for those practicing design and analysis of such systems.

1
Tracked Vehicle Running Gear and Suspension Systems

The running gear systems used on high speed, mainly military, tracked vehicles provide four essential functions:

• the transmission of drive to a relatively large number of road wheels;
• the distribution of the weight of the vehicle over a relatively large area;
• a large suspension displacement to allow high speeds over rough terrains; and
• a particular requirement of military armoured vehicles, the running gear system should occupy the minimum space in the overall vehicle envelope in order to maximise internal hull volume (as will be shown in Section 8.4, this is a particular attribute of tracked vehicles compared to wheeled vehicles of similar soft‐soil performance).

In addition, the running gear must be of minimum weight, reliable, easy to maintain, and compared to some other vehicle components, relatively cheap to produce.

1.1 General Arrangement

Figure 1.1 shows the running gear of the Warrior Infantry Fighting Vehicle (IFV) and is typical of modern practice. Trailing suspension arms carry rubber‐tyre road wheels and operate transverse torsion bars which run across the floor of the vehicle. Rotary vane hydraulic dampers are incorporated into the pivots of the front, second and rear road wheel stations. Link tracks run under the road wheels and around hull‐mounted drive sprockets and return idlers. Track pretension is adjusted by means of oil‐filled rams reacting against the idlers, which are carried on short pivoting arms. The drive sprockets are front‐mounted but could be at the rear of the vehicle, depending on the position of the power pack. Small diameter rollers support the top run of the track. The track link pivots are rubber‐bushed and the links are fitted with replaceable rubber road pads to minimise road damage and reduce noise and vibration.

Figure 1.1 Warrior running gear layout. Source: Courtesy of Ministry of Defence.

Figure 1.2 shows the arrangement on the Leopard 2 Main Battle Tank (MBT). Rotary friction dampers are built into the front three and rear two axle arm pivots. The vehicle is fitted with rubber‐bushed double‐pin tracks (see Chapter 2).

Figure 1.2 Leopard 2 running gear layout. Source: Courtesy of ATZ.

1.2 Transverse Torsion Bars

Modern high‐strength spring steels, used with suitable presetting, shot peening and corrosion prevention techniques, allow nominal shear stresses of up to 1250 mPa to be used with a reasonable fatigue life [1.1, p. 226]. Suspension torsion bars are only loaded in one direction and so can be ‘preset’. To preset a torsion bar, it is wound up to induce partial yielding in the outer layers of the bar. On release, the outer layers take on negative shear stresses and torques opposed by positive stresses and torques in the inner layers of the bar (Figure 1.3).

Figure 1.3 The principle of presetting a torsion bar.

The relationship between the various variables that affect the maximum shear stress in the bar can be explored by setting up a suitable spreadsheet. The vehicle will be considered as a notional MBT with a sprung mass of 600 kN and an effective torsion bar length of 2.13 m. The variables that can be considered are the axle arm length (initially taken as 450 mm), the number of road wheels (initially taken as 12) and the stiffness of the bar. The latter can deduced from the ratio of wheel loads at full bump and at static F_B/F_S, initially taken as 3:1, and the required static to bump suspension displacement Δ_SB, taken as 350 mm. This gives a heave natural frequency of about 1.2 Hz, which is typical for an MBT. The shear modulus C is set at 76 mPa [1.1, p. 226]. The diameter of the bar is left open.

This gives a maximum shear stress q_max of 1326 mPa, which can be considered too high for a good fatigue life. Increasing the arm length to 500 mm increases maximum torque on the bar, but also reduces maximum wind‐up angle; q_max reduces to 1258 mPa. This may be acceptable depending on the duty cycle. Measurements show that the front wheels nearly always have the most severe duty, largely because of the pitching motion of ...