Bentley claims that the engineering challenges posed by the new State Limousine built for the Queen often bore no relation to those of any conventional car, even though the vehicle is based on the latest Arnage.

Built by a consortium of British motor industry manufacturers and suppliers to celebrate the Queen#;s Golden Jubilee, the imposing limousine will be presented as a gift to Her Majesty in time for its first official duties on June 4.

Designed for a minimum lifespan of 25 years and 125,000 miles, it is expected to be the Queen#;s principal transport at state and ceremonial occasions.

The major partners joining Bentley Motors in the consortium are Mayflower Vehicle Systems (bodywork), Leoni Wiring Systems (electrics), TWR Group (trim packaging), Radshape (brightware), Ricardo PLC (powertrain) Intier (interior trim substrates) and MSX International (structural analysis and validation).

Bentley#;s responsibilities extended to the design, styling, chassis and construction of the car and also devised, commissioned and oversaw the project from start to finish.

The ‘glasshouse#; design of the rear cabin was an area that posed questions few design teams have ever faced. With such a large rear cabin and so much glass, the car will potentially be subject to huge solar loadings which could swiftly result in intolerable interior cabin temperatures. Normally this issue would be at least partially addressed by fitting heavily tinted glass but the Queen was keen that visibility both into and out of the car was compromised as little as possible.

The solution was the fitment of laminated glass in all windows with a reflective coating sandwiched between the two layers. This allowed a tint of just 15% and will be barely noticeable inside or out. The tint on the roof panels is 40%.

Uprated air-conditioning is also fitted to ensure the vast rear cabin stays cool in the hottest weather, and without the benefit of airflow over the car while on ceremonial duty. The average speed at which the car will travel at processional occasions is around 9mph, though this may fall as low as 4mph. As a result of exhaustive computer modelling and testing at the Motor Industry Research Association (MIRA) in the UK, a system has been devised that produces a large and slow moving mass of cool air, distributed silently about the car.

As soon as the styling of the car had been decided, the designers set to work seeing how it could be realised using both the latest CAD techniques and by dissecting a standard Bentley Arnage. One weighty problem was retaining sufficient rigidity within such a heavy car without resorting to unattractively thick pillars. As the first Royal state limousine to use monocoque construction rather than the body-on-chassis method of the Phantom series of Rolls-Royces, the pillars are stressed members required to do a lot more than merely support the roof.

Once this problem was resolved by detailed computer analysis of the performance of certain high-grade steels, attention turned to the execution of the rest of the design.

Just some basic statistics give an idea of the size of the challenge involved. At 6220mm/ 249 inches, it is over 800mm/32 inches longer even than a standard Bentley Arnage, while its 1770mm/70 inches height dwarfs that of the 1515mm/60 inches Arnage. Its wheelbase is 3844mm/154 inches, not only 728mm/29 inches more than an Arnage, but also upwards of 1.3metres/ 4.3 feet longer than that of an average sized family saloon. Most daunting of all for the engineers was a car with a likely kerb weight of 3390 kg/ 7474 lbs some 1400kg/ 3086 lbs. – a family saloon – heavier than an Arnage.

The State Limousine is powered by a modified version of Bentley#;s new 400bhp, twin-turbo 6.75-litre V8 engine that made its debut in the recently launched Arnage R and, with 616lb ft/ 835 Nm of torque, gives the big limousine an impressive turn of speed right up to its electronically limited top speed of 120mph (193km/h). Modifications have been made to the air boxes to allow them to fit under the bonnet, while a larger alternator is fitted to cope with the added demands of the electrical system.

The engine can run on liquid petroleum gas (LPG) to not only extend the range of the car but also dramatically reduce emissions.

The engine drives a standard four speed GM 4L80-E gearbox, as used in the Arnage, which directs power through up rated driveshafts to the rear wheels.

The limousine#;s suspension is by double wishbones at each corner tuned to provide the right balance of low speed ride quality for processional events and high speed stability.

The rear doors, hinged at the back rather than the front to facilitate easier access, required extensive work before a hinging system could be found that would cope with their weight, package effectively and allow an extraordinary 81 degree opening.

It would be understandable to presume a car such as the Bentley State Limousine is brimming with every electronic gadget available. In fact the Queen has quite specifically requested that the car should not be filled with such items.

Nevertheless, the highly specialised role the car will play has necessitated the fitment of a large number of components vital to the execution of the Head of State#;s duties.

Inside there is a glass division between the front and rear compartment that can be lowered by its passengers from a console mounted between the seats. An intercom is also fitted.

Both rear seats are height adjustable so passengers are seen at the same level and squarely within the rear side window. Both rear windows can also be operated from either rear seat.

Not surprisingly, details of the security equipment fitted to the car are not being made public.