Bentley Mulsanne on production line

Building the new Bentley Mulsanne

Mixing hand-built and cutting-edge technology in a luxury car factory is no mean feat, says E&T.

Bentley has always been a carmaker with an iconic brand. It set numerous speed and endurance records in the 1920s, with its famous 3-litre, 85bhp engine providing speeds of 80mph and more. It successfully competed at Indianapolis, the Isle of Man and Brooklands - and became inextricably linked with the history of the famous 24-hour race at Le Mans. In the hands of the legendary Bentley Boys, Bentley achieved Le Mans victories in 1924, 1927, 1928, 1929, and 1930 - taking the first four places in 1929.

However, it was another 73 years before Bentley could go back and win again at Le Mans, which it did in 2003 after becoming part of the VW Group.

The VW Group has secured the car maker's future and invested in new technology, so although Bentleys are still hand built, they now have a little help from modern high-tech equipment. Part of that is a new Body in White facility. The £25m facility enables Bentley's skilled craftsmen and women to work alongside state-of-the-art robotic technology to produce the bodyshells for the new Mulsanne. Given the complex shape of the Mulsanne, new production techniques such as superforming had to be combined with traditional hand-brazing and polishing. The new facility allows these techniques to be undertaken side by side before the bodies go on to paint and final assembly.

There are not many car makers that would welcome their customers in to see their actual car being built, but Bentley has a state-of-the-art Body Assembly Hall at Crewe where owners can watch their car being assembled from a gallery above the hall floor.

Offering this facility was not without its challenges as owners had to be protected from the welding flashes below, which involved Bentley carrying out their own research on what tint was required on the glass, explains Andrew Robertson, production planning manager at Bentley Motors.

In a usual car factory you will see armies of robots aligned down the production line, each programmed to carry out a single weld but not at Bentley. Its approach is to use just two robots and move the chassis around so that both robots have access. This is a simple solution that makes better use of the space and reduces the investment in robots.

Bentley has also abandoned standard 'hot weld' technology on the body which can cause distortion of panels at the weld points. It has instead adopted plasma-brazing 'cold weld' technology, which leaves a pristine finish for painting.

Elsewhere in the hall, suspended computer-controlled guns are used to assemble body sections mounted on jigs. These hand-held tools allow accurate positioning of the 6,500 spot-welds required to optimise torsional rigidity on each monocoque steel bodyshell. 'No other manufacturer goes in for spot-welding on this scale,' says Robertson.

'Everything here is done by hand, from attaching the 240 individual copper and other metal studs used in the floor pan to hold the wiring looms and carpeting in place, to applying the sealant, which will later be baked in the oven,' he explains. Between 540 and 560 components are used to construct each body.

A computer-controlled Hemming press is an important addition, doing in 12 minutes what would once have taken a day. Loaded and operated by a four-man team, it presses outer skins onto hinged components - doors, boots and bonnets - at up to 100t. The skins are glued rather than welded, to be set in an oven no hotter than a hair drier. Two hundred and ten closures can be completed each week by the team.

When it comes to testing the product twice a year, a complete body is built in the Body Assembly Hall only to be taken apart again. This may be drastic, but on an everyday basis, one in five components is chisel-checked for structural efficiency.

It takes about three weeks to turn those 540-odd components into a complete, inspected body shell ready to go on to the next stage. Working a single eight-hour shift, the teams currently send 35 bodies on their way to the paint shop each week. And as each body goes on its way, it is already earmarked for one particular customer.

The metal shells that will eventually become Bentley's new flagship are created using a combination of high-strength steels and aluminium. 'The Mulsanne's hewn from solid appearance without visible panel seams is impossible in volume car manufacture,' says Robertson. 'The seamless sweep of the Mulsanne's roof, rear wing and boot can only be achieved by crafting a hand-brazed seam midway down the D-pillar, a task still best performed with 'hand and eye'.'

In complete contrast to this is the use of superforming, which is essential to create the highly-complex 3D curves of the front wings. This advanced technique, primarily used by the aerospace industry, heats a single sheet of aluminium to 500C and forces it onto a single surface tool.

In pre-production, completed bodies are being torn apart with specialist cutting gear to analyse the breaking point of a seam or weld as well as minutely measuring the accuracy of finished components and bodyshells - 588 functions and relationships are measured to ensure complete precision.

Bentley is also ultrasonic-testing individual welds to analyse their strength and consistency. 'During these early, pre-production stages, we are exhaustively testing the build process to achieve absolute precision and quality for our customers,' says Gary Picken, senior production manager.

Assembly

Once the car reaches the assembly line stage it moves slowly through the factory, allowing workers building the car to carry out their individual tasks. They can also raise and lower the cars to suite their individual access needs.

All the parts arriving at the work stations on the production line are laid down in a car manifest to make sure that everything is matched to that particular vehicle. Each car is individually tailored to the owner's requirements, meaning that there are considerable variations in the vehicles on the line.

Wood and leather

When it comes to fitting out the opulent cabin in the new Bentley Mulsanne, the company has responded to customer demand, even to the extent of the smell of the leather hides. This has resulted in the reintroduction of a complex, traditional tanning process to create a rich, mature leather aroma, evoking that of vintage Bentleys. Customers can select from a broad palette of hides, including 24 'standard' colours as well as a wide selection of rich 'heritage' colours from previous models. These colours may be specified in single-tone or duo-tone combinations in four different styles.

It takes three hours to inspect and cut the complete set of leathers needed for each Mulsanne. 'The hide is inspected by eye to make sure there are no imperfections,' says Robertson. 'There are 380 cut parts that go into each vehicle - 30 per cent more than previously. A total of 15 hides are used per car. The cutting machine scans the hide to optimise the best use of leather and it only cuts from perfect areas. After cutting, the parts are inspected again to make sure there are no imperfections. They are then all hand stitched.'

When it comes to wood Bentley is an expert, having the largest wood shop of its kind in the luxury car industry. It takes approximately five weeks to turn a rough 'root ball' (individually selected by Bentley experts) into a full set of mirror-matched, fine-polished leaves.

Mirror-matching of the veneers is a process that takes place throughout the whole of the car. Customers can select from an extensive range of premium quality veneers, unbleached for a natural appearance, including Walnut, Piano Black, Oak, Bird's Eye Maple, Vavona, Olive Ash and Sapelli Pommele, harvested worldwide from sustainable sources.

Each piece of veneer is stored in a special humidity-controlled chamber and then applied onto a solid walnut base, which has been machined in a series of five-axis CNC machines to provide the final shape. They are then sent on for sanding prior to the application of lacquer. The lacquer process entails as many as five coats of polyurethane polyester material up to 1mm in thickness. Once the polyester material has been cured over a three-day period, the part is then flatted and polished to a mirror-like finish.

Engine management

Bentley's new 6 litre, twin-turbocharged engine blends state-of-the-art technology with a classic V8 configuration.

The very latest technologies have been employed, including two new control systems: cam-phasing and variable displacement, delivering even greater refinement as well as a 15 per cent improvement in emissions and fuel consumption.

The pioneering application of these new technologies in combination allows the Mulsanne's engine management system to adjust the V8's breathing for improved engine idle quality and torque delivery. However, by closing the valves of four of the eight cylinders, fuel economy is maximised when cruising.

All the major building blocks of the Mulsanne's engine are new, including the lightweight components such as pistons, connecting rods and forged crankshaft, which reduce reciprocating mass and internal friction for improved engine response.

These significant changes are complemented by the introduction of a new eight-speed automatic transmission offering virtually imperceptible gearshifts.

A state-of-the-art balancing machine ensures all the spinning components are rigorously tested (and, if necessary, rebalanced using tiny washers). The level of accuracy achieved by this process ensures there is not a hint of vibration on idle. Alongside the use of the balancing machine, Bentley's engine builders still fit each of the pistons into the V8 block by hand as well as the 16 individual ball-bearing,s which are inserted into the valve train.

Once assembled, every Mulsanne V8 is tested over an 80-minute cycle by the team. The 'hot testing' process tests the engine from idle through to maximum revs and the load on the engine is varied to simulate real-world driving conditions.

A host of new technologies and parts may have been introduced, but a signed engine maker's plate is still attached to every Crewe-built V8.

Quality control

While quality checks are fundamental to every stage of the Mulsanne's build, it is at the penultimate sign-off stage - 'Checkpoint 7' - that the complete car is given the most rigorous examination possible. 'Every Mulsanne must pass through 503 individual checks, which lasts a full two hours and features all of the functional measures you'd expect of a modern automotive plant, but unlike mass-produced cars, at Bentley they review every single Mulsanne,' explains Robertson.

After testing on the shake rig and rolling road, every Mulsanne completes a rigorous on-road evaluation. Each car is driven on a wide range of carefully selected roads offering variations in camber and surface quality. Road types range from narrow country lanes to fast dual carriageways, allowing the Mulsanne to be comprehensively evaluated.

Once the car returns to the Pyms Lane factory, it enters a special test facility that simulates monsoon-like weather conditions to ensure that it is completely watertight before heading for the very final sign-off stage - 'Checkpoint 8'. 'Here, the car progresses through a long line of ultra-bright overhead and side lights (nicknamed the 'shipping lane'), which enables the quality team to inspect every surface in minute detail,' concludes Robertson.

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