Lotus is renowned for its innovative chassis technology, and its flexible vehicle architecture uses leading developments in casting to enter new segments
At the 75th Geneva International Motor Show, Lotus revealed the latest example of its Versatile Vehicle Architecture. Constructed almost entirely from aluminium sub-components, the company claims that the under structure reveals the next stage in the development of the innovative technology.
The cornerstone of this technology, for creating families of cars, is the inclusion of numerous common aluminium casting components. The structure reveals the high-pressure die cast corner nodes that are essential to an architecture that also draws on Lotus’s extensive experience with bonding and extruded aluminium.
“I was pleased to be able to demonstrate how far we’ve come with VVA,” explains Steve Swift, head of vehicle engineering at Lotus. “The under structure proves the thinking and we’re excited about the possibilities the technology brings to OEMs worldwide. We expect that the demonstration of this technology through a real under structure will stimulate greater interest from OEMs or consortiums who want to produce exciting products using cost-efficient, proven architecture,” he adds.
The vehicle that Lotus displayed at Geneva was named APX (Aluminium Performance Cross) – a concept production car, which demonstrated VVA in one of the most unlikely Lotus-type vehicles. The company opted for this platform because it wanted to prove that VVA is a technology that can be spread across a number of different vehicles within a portfolio.
APX is fundamentally a seven-seater (in reality a 5 + 2 with the two rear seats being occasional) four-wheel drive crossover vehicle with a front mounted 300hp supercharged V6 petrol engine. Weighing in at just 1,570 kg and with a power-to-weight ratio of 191hp per tonne, the APX has sportscar like performance: 5.4 seconds to 100 km/h (5.0 seconds to 60mph) before reaching a top speed of 245 km/h (152mph). These performance figures are as good as the highest performing four-wheel drive crossover vehicles from other brands, with up to 195 hp per tonne.
Crucially though, whereas those vehicles need higher output engines to compensate for heavy weight, APX does not. Combined fuel consumption for APX is estimated to be 8.7 litres per 100 kilometres (or 32 mpg) – impressive on its own and more so when compared to its production rivals which often consume more than 13 litres per 100 km (22 mpg).
Simon Wood, Director of Lotus Engineering, explains the rationale behind building the APX: “The first production car from Lotus to use the VVA will be the new mid-engine ‘super sportscar,’ due to go into production in 2008.
Customers are eagerly awaiting this vehicle; it will be a class leader and phenomenally high performing car.”
He continues, “However, we wanted to demonstrate the true versatility of the VVA technology, and what better way than to build a type of car that no one would expect from Lotus – a four-wheel drive crossover vehicle? I am delighted with this vehicle and we believe that this technology and strategy is what the motor industry must follow to produce niche vehicles efficiently and quickly. There is already a great deal of interest in both APX and VVA technology from our client base and we will work hard to see how Lotus Engineering can help them with their strategic product solutions.”
Traditionally, OEMs who want to gain competitive advantage through exciting niche vehicles have to either design a new platform or share one already available. Engineering a tailor-made low-volume platform is an expensive, time-consuming solution, while sharing a mainstream chassis normally results in compromises in performance and design.
Lotus VVA has been developed to bridge a gap in the investment-volume curve to exploit the benefits of producing at medium volumes but for niche markets, thereby giving the best chance of business case success and favourable returns.
VVA offers a fast-to-market, cost-effective approach to differentiated niche products by spreading the development, investment and bill of materials burden across a range of niche vehicle variants, without the compromise that stems from conventional platform sharing. The philosophy is based on the commonality and versatility of key elements of the vehicle structure and body systems across a ‘family’ of niche vehicle variants, with a combined annual production rate of up to about 50,000 units.
Structural components common to each ‘family member’ are arranged in different configurations in each variant around the ingenious corner nodes. The great advantage of this technology is that it can be used by one OEM who wants to develop a range of niche products, or by a group of OEMs keen to share investment but still retain a high degree of end product separation.
“We believe it is a technology that provides solutions to a wide range of manufacturers,” Swift says. “The worldwide vehicle market continues to sub-segment at a rate greater than its overall growth, leading to lower volumes per vehicle variant. OEMs are looking for architectures that give them a superior investment return with a high degree of product separation. VVA gives them that opportunity.”
The under structure is aluminium riv-bonded, consisting of high-pressure die-castings, stampings and extrusions, and uses advanced assembly techniques, including adhesive bonding, self-piercing rivets and flow-drill screws for construction. The self-piercing rivets are used in a similar way to spot welding on a conventional steel shell, with the flow-drill screws used for single-sided access on closed sections. Both suffice to hold the structure together during the bonding cure cycle, and prevent adhesive joint peeling in the event of a crash.
The heat-cured high strength structural adhesive is the main joining medium, and, used in combination with the mechanical fasteners, produces an immensely strong, durable joint and a lightweight shell with exceptional torsional stiffness. “Basically the VVA concept is a platform whereby high investment components such as high pressure die castings are carried over across a wide range of vehicles produced in different sizes and configurations, so the actual investment is spread over a large range of vehicles,” David Jenkinson Manager, Body Assembly Systems Materials and Process Technologies, Lotus Engineering explains. “So essentially it allows them to use high-volume technologies economically on low-volume vehicles,” he adds.
On the VVA platform, Lotus used 12 castings. There are two castings in each corner of the vehicle to form the central corner nodes, two spring housing castings at the front of the vehicle and two damper castings at the rear. “Essentially, we use castings in the corners because this allows a high degree of part integration,” says Jenkinson. “We were able to integrate about 12 pressings in just two castings in the front corners of the vehicle. It is all about intelligent use of the castings and making sure that you achieve whole levels of part integration, which tends to make them more viable.”
The VVA concept is unique in terms of the approach of carrying over those high volume castings, particularly because of difficult configurations and the unique approach of the castings as the cornerstones to that philosophy. “These castings are joined using riv-bonding technology as well, so they have to be developed to give them the properties to be able to be joined using a combination of self-piercing rivets and heat-cured epoxy adhesives, which meant that the elongation of the castings had to be quite high. All the castings in the VVA concept are aluminium and the main challenge that Lotus faced was the size of the castings.
Jenkinson explains that the front outer casting on the VVA platform weighed 6.4kg and it had a wall thickness of just 3mm, so the challenge was to produce a thinwalled casting of such a large size and at the same time achieving the mechanical properties that were required. A casting of that size to produce a thin wall of 3mm is quite an achievement.
“A lot of thought goes into the design, in determining the critical features within the casting,” Jenkinson adds. “Pinpointing where our master location is going to be becomes the most important feature on the casting, so everything becomes relative to that. “When the castings bear two halves to the mould, we work with the team and make sure that we put as many of the key characteristics in the same half of the mould tool, in our design. It is that relationship between the design and the manufacturing process that becomes really important.”
VVA is relatively new concept for Lotus but many of the technologies developed for the project will be implemented in any future products. Thus casting technology forms a critical part of the process. “It is fundamental, the cornerstone of the platform,” Jenkinson says. What is also worth mentioning, he tells AMS, is that “These particular castings are also designed to absorb energy in a crash; they are designed to actually absorb energy rather than fracture or transmit those forces to other parts of the car. The Crash Energy Management is similar to what Audi did with its A10 model.”
Jenkinson explains that the other benefit in terms of casting is from a packaging point of view. “Where the package is tight and we have to get metal into a very tight area, casting has allowed to do that efficiently. So we can tell you the gauge of the castings and we can actually put materials where you want, which obviously you can’t do with an extrusion or with a conventional pressing. The new technology gives us quite a degree of package flexibility as well, which is also a big advantage.”
“The castings on the VVA are extremely complex 3D shapes, which are not something that you would be able to achieve with an extrusion or forging. Even with a fabricated pressing, you are looking at a minimum of 12 pressings to replicate that kind of shape. People are using castings but the key message is that to make them economically viable, they need to be used in the right application and enable a great deal of part integration. There are instances where castings are being used for incorrect applications, where they don’t enable large degrees of part integration and then they become less attractive from an economic point of view,” Jenkinson explains.
It is worth pointing out that the VVA platform at present makes use of sand castings. “The VVA platform uses prototype castings, which are produced using the sand route, thus we are not currently produce high-pressure diecastings because it is a concept demonstration at this stage,” he adds.
According to him, there has been considerable interest from OEMs for the technology since it was shown for the first time almost three years ago. “We had a number of OEMs coming up to us and saying ‘wow’, so there has been a great deal of interest there. Then we showed the APX vehicle itself, just over a year ago, again there was a vast amount of interest. He declined to say from whence the interest has originated.
“The beauty of the VVA is that the cost of the casting is spread across all of the different vehicles and you would have the castings linked together by a combination of screws and different pressings. The beauty of that is for an ultra low-volume supercar, for the sake of argument an Esprit type car, you would have to be making 500 or 1,000 a year or maybe higher. You can then go to various other different types of cars that use the same castings, but just join them together in different ways. The castings are common but the bits in the middle are not. That is the clever thing about it; the high investment of the castings is spread over a number of different vehicles.
Jenkinson adds that, “I think that castings, as well as taking on a number of different operations, also make matching components very simple too; along the VVA structure the castings do all the complicated bits and interface with a simple cell, a very straight cell with no bend or pivot in it. So casting also enables simplification of matching components as well.”