Introduction

The Mark III Range Rover was unveiled by Land Rover at the 2002 Detroit Motor Show. This model was a complete redesign which was started by BMW under their ownership of the Land Rover brand, however by the time the car was released the ownership had transferred to Ford, who had finished the design of the car. Among many other innovations came the introduction of 4 wheel cross-linked independent air suspension, the purpose of which was to eliminate problems in the MK II’s air suspension setup while also improving both off road ability and on road handling. The current Range Rover is one of the world’s most popular and impressive SUV’s and is well known for having vastly superior off-road abilities compared to other vehicles in it’s class such as the Hummer H2 or ‘soft-roaders’ such as the BMW X5. The air suspension used in the current Range Rover is a setup which is not used by many other consumer vehicles and contains many interesting design aspects which have helped improve the cars performance. This air suspension system is the subject of this report. History

The evolution of the Range Rover [1]

The original Range Rover was launched in 1970 and immediately changed the 4x4 market forever. It introduced excellent off road performance without sacrificing luxury, comfort and on road performance. It was so successful that it remained in production until 1996 and remains the only vehicle to ever be displayed as a work of art in the world famous Louvre museum [3]. In 1994 a new model was launched, with the original model being renamed the ‘classic’ and staying in parallel production for a further 2 years. The new model introduced electronically controlled air suspension, a development of the classic model’s unique air spring setup. It was also aimed at the top end of the market and as such was a very luxuriously equipped vehicle. This vehicle continued the tradition of excellent off road performance however was criticised in some circles for excessive body roll under road driving conditions [2]. This vehicle was replaced by the current Range Rover in 2002. Suspension System Requirements When designing the suspension set-up for the current model there was one aim; to improve the on road performance without compromising the excellent off-road performance. The finished design represented a radically different approach to anything Land Rover, or any other off-road vehicle manufacturer, had ever undertaken. A fully independent suspension system was going to be used. These have typically been viewed as unsuitable for off-road use due to issues with ground clearance [2]. Beam axle setups had long since been viewed as optimal for extreme off-road use due to the axle articulation they

offer which reduces the likelihood of the vehicle components becoming grounded on rocks or other obstacles. The Range Rover design managed to combine the benefits of fully independent suspension and those of a beam axle such that this model offered better axle articulation than the heralded Range Rover Classic [2]. Syspension System Schematics Front:

Diagram adapted from [4]

Rear:

Diagram adapted from [4]

Suspension System

The suspension system consists of fully independent, electronically controlled, cross-linked air suspension. Being an independent system gives ride and handling benefits, benefits which had previously been restricted to road cars but could now be transferred to an off-road vehicle [5]. The main aim of this system is ‘to maintain the vehicle at the correct ride height, irrespective of load’ [4]. The ride height of the vehicle is adjustable thanks to the air springs, with off road settings increasing the ride height by 2 inches above standard, while conversely the vehicle can be lowered until it almost rests on the bump stops to allow easy access into the vehicle. Some suspension data for the vehicle is summarized in the following table [4].

Suspension Travel (mm) Bump Rebound Total

On road 115 155 270 Front Off road 175 95 270 On road 140 190 330 Rear Off road 190 140 330

The air suspension system operates between 9 and 13.7 bar pressure. The cross-linking aspect of the suspension system is the key to adapting the benefits of a fully independent setup to the off-road arena. The air springs on the 2 front and 2 rear wheels respectively are linked by electronically controlled valves. When actuated these allow air to move quickly between the left and right sides of the vehicle. This in essence simulates the behaviour of a beam axle, offering the benefits in axle articulation while also helping to smooth out bumps and offer a smooth ride over rough surfaces. Additionally this system helps to ensure that any wheel which is partially hanging has the maximum ground contact force possible. This is necessary to provide maximum traction of difficult terrain. This scenario is demonstrated by the front wheel on following image.

Maintaining traction on a partially unloaded wheel [6]

Front: The front suspension is built on a twin tube style MacPherson strut design. This design permits a reduction in unsprung weight over more traditional beam axle designs. The front struts did however have to be carefully designed to offer maximum ground clearance [4]. Other key aspects to this design are that it uses a small amount of packaging space, thus allowing the passenger compartment to be large as demanded by this type of luxury SUV. The high bonnet line of the Range Rover means that the typical high installation height of this type of system is not a problem. Furthermore the fact that this design uses air springs rather than traditional coil springs allows the installation height of the front suspension to be lowered, maximising space in both the cabin and engine bay. This type of front suspension is only successful on the new Range Rover due to its move towards a monocoque body rather than the separate chassis and body of the previous iteration. A monocoque offers increased suspension space and provides a strong top mounting, both of which are key features to the success of such a system [7]. A typical disadvantage of these types of suspension is that they can provide a harsher ride than say a double wishbone setup, however the use of air springs in this vehicle compensates for this and the vehicle has a reputation for having a level of harshness which is nowhere near that of the MK II model [2]. The vehicle also features a front anti-roll bar. This is constructed from 30mm diameter steel bar mounted between the 2 struts. These help to reduce body roll under road conditions, a common problem with SUV’s which have a high centre of gravity due to their need for ground clearance. The anti-roll bar is a vital component of the suspension system on this vehicle, as it allows the vehicle to meet one of its main objectives, namely to handle excellently on-road at typical motorway speeds. Rear: The rear suspension utilises a double wishbone design rather than the MacPherson strut used on the front. This type of design offers more control over the motion of the rear wheels with suspension travel, for example camber and caster angles are easily controlled. Careful design of the geometry allows for fine adjustments in the vehicles handling characteristics for example the level of understeer or oversteer. Possibly the best trait of this system however is that it offers increased strength which is vital as these vehicles are frequently used for towing [2]. Suspension travel is also increased over the front suspension. The upper wishbone is shorter than the lower, which is important as it means that the vehicle’s load space isn’t compromised. This design also helps reduce body roll when combined with the anti-roll bar [8]. The rear anti-roll bar is made from 23mm diameter solid steel. This is connected between the 2 lower wishbones and performs the same function as that on the front suspension, helping to create vehicle handling which has more in common with a normal saloon car than a typical SUV at road speeds.

The rear dampers were designed especially for this vehicle and are a mono-tube design. This is a less common design than traditional twin tube dampers, and provides a lightweight construction. This is useful as it counteracts the increased weight of the rear wishbone suspension over the front MacPherson struts. Also this design offers better heat dissipation [5] and more consistent performance than twin tubes [4]. The following diagram indicates the location of key suspension system components.

Adapted from [4]

Vehicle Feedback This vehicle was designed with the intention of taking the Range Rover back to the very top of its class. Its success or otherwise can be measured by the feedback received from customers and professional road testers. Some feedback received follows: “The ride is one of the best in class, killing bumps and massacring

road imperfections” – Top Gear Magazine [9] “It isn't as sporting as the best of the high performance 4x4s, but it

is pretty good and will blow them away when the going gets tricky” – Parkers [10]

“it's incredible the sort of angles this thing can climb up on normal

road tires” – Orichalon123 on ciao.co.uk [11] Conclusion The latest iteration of the Range Rover was released to widespread acclaim as it managed to successfully build on the strengths of the previous models, namely in luxury and off-road performance while adding a ‘limousine’ matching ride on-road [10]. The success of this model is to a large extent due to the innovation of the chassis and suspension designers at Land Rover and their introduction of the unique electronically controlled cross-linked air suspension system.

A Range Rover displaying it’s excellent off-road ability [10]

References [1] – Official Land Rover press release, Land Rover, 2002

[2] – www.rangerovers.net accessed 5/11/08

[3] – http://www.carseek.com/reviews/land-rover/range-rover/ accessed 4/11/08

[4] – Range Rover workshop manual, LRL0424ENG (2nd Edition), Land Rover – Technical Service, 2002

[5] – MECH5185 Course notes, D. Towers.

[6] – http://www.familycar.com/roadtests/landroverrangerover/Images/OffRoad3.jpg accessed 6/11/08

[7] – Setright, L.J.K., "MacPherson Strut: Legs to Support the Car", in Northey, Tom, ed. World of Automobiles (London: Orbis, 1974), Volume 11, p.1235

[8] – http://auto.howstuffworks.com/car-suspension4.htm accessed 6/11/08

[9] - http://www.topgear.com/uk/land-rover/range-rover accessed 6/11/08

[10] - http://www.parkers.co.uk/cars/reviews/land-rover/range-rover-2002.aspx?page=2 accessed 6/11/08

[11] - http://www.ciao.co.uk/Land_Rover_Range_V8_Vogue__Review_5788853 accessed 6/11/08

Front cover image adapted from official Land Rover press release image