The Lotus Position
Lotus unveiled the Elise at last monthís Frankfurt Motor Show. Keith Reynolds describes its epoxy-bonded aluninium chassis.
ìElise features the latest thinking in styling, aerodynamics and ride, yet it is a true Lotus. Its ultralight design is intended to meet all worldwide requirements for safety, comfort and emission and noise control. The appeal of this model is not limited to its looks; throughout the project Lotus engineers have applied technologies of the future with simplicity and purity of design. The result is an advanced race bred high performance sport car with distinctive new styling which delivers the most fun on four wheelsî.
Romano Artoli, Chairman of Group Lotus and Bugatti Industries
The aluminum industry's ultimate goal of confronting steel head-on in the motor industry awaits the material's adoption for unitary car body manufacture - the construction method of choice where production Volumes exceed 100 000 to 150 000 units per annum. That the production processes exist for this was demonstrated last year when Ford in the US built 40 unitary bodied AIV; (aluminium intensive vehicle) prototypes in association with Alcan, but no major car manufacturer has yet made a firm commitment to this technology.
In other sectors of the motor industry, however, where production Volumes are lower, aluminium is making considerable inroads. Here so-called spaceframe construction, using assembled extruded sections, is a cost-effective proposition through which considerable weight savings are achievable. Earlier this year at the Geneva motor shove Renault announced the Renault Sport Spider, a two seat, mid-engined sports car based on a welded aluminium chassis and GRP (glassfibre reinforced plastic) bodywork. Now Lotus has unveiled an even more significant development in the form of its new Elise, seen for the first time at September's Frankfurt motor shot.
Aluminium and composites combine
In common with the Renault, the new Lotus is a lightweight two-seat sports car with an aluminium chassis and GRP body, but uniquely the chassis is adhesive bonded throughout - a new technology developed by Lotus and Ciba Geigy which brings significant practical benefits in respect of weight reduction, production processing and dimensional accuracy. Some thread-forming riveting is used to support the structure while the one part Ciba epoxy is heat-cured in an oven, and to prevent peel. The aluminium chassis was developed in conjunction with Hydro Aluminium Automotive Structures of Denmark. Hydro builds the chassis units and ships them to Lotus in Norwich.
As can be seen, 'spaceframe' is not an accurate description of the chassis structure - it does not comprise a triangulated lattice of tubes as the term suggests. Instead it is a tub structure, with front and rear torque boxes linked by high side rails, constructed from a combination of custom extruded sections (in 6063 alloy) and sheet (made from recycled material). The term spaceframe in this context is used to describe any structure which is not built, like todayís high-volume steel-bodied cars, from pressed sheet metal. Richard Rackham, Lotus engineer in charge of chassis design for the Elise, prefers the term monocoque.
Lotus has never used an aluminium chassis in any of its precious road cars, but the philosophy of achieving high performance, exceptional handling and good fuel economy through light weight is a Lotus tradition, imbued in the company by founder Colin Chapman. Aluminium was therefore a natural choice for the Elise chassis. But minimum weight - 65 kg for the bare structure - had to be combined with high torsional stiffness for reasons of both handling accuracy and ride quality.
The roofless construction of the Elise makes this requirement particularly difficult to achieve. When modern saloon cars are re-engineered as cabriolets it is typical for them to lose 50 percent or more of the saloon body's torsional rigidity, despite local reinforcement of the bodyshell. Consequently they rarely handle quite as crisply, nor do they ride as well, as the saloons from which they are derived.
Lotus's last convertible, the Elan, employed the companyís familiar cruciform backbone chassis, constructed of steel. Painstaking development eventually achieved a torsional stiffness of 7900 Nm (5800 Ib.ft per degree), rising to 8900 Nm (6600 Ib.ft per degree) with the GRP body fitted - a figure close to that of many modern unitary-bodied saloons. A measure of the advance embodied in the Elise chassis is that, despite its lightness, it has a torsional stiffness figure 27% superior to the Elan's (approaching 10 000 Nm/deg and increasing slightly With the rear subframe, GRP body and roll over bar attached), which means it matches or surpasses the majority of roofed cars.
This rigid platform, as well as being the ideal starting point for Lotus's ride and handling engineers, has also contributed considerably to the Elise's crashworthiness, particularly in side impacts where the high chassis side rails, extruded anti-intrusion beam in each door and absence of a central tunnel - a feature of all Lotus's backbone-chassis cars - all assist in minimizing intrusion and risk of occupant injury. Resistance to frontal impacts is assisted by a replaceable energy-absorbing structure molded in (PRIG; rear impacts are absorbed by cross-linked hollow aluminium crush members, which again are replaceable.
Repairing after impact damage
In the case of an impact severe enough to cause serious structural damage to the chassis, it can be replaced in its entirety like previous Lotus backbone chassis. Repair methods As which retain the chassis' high dimensional accuracy have been developed for localized damage to the front suspension pickup points, cohere steel inserts are bonded in to accept the Ad heel loads.
Dimensional accuracy, indeed, was one of the key attractions of a boned structure over a welded one. For a high performance car, suspension and steering ëhard points' need to be accurate to within half a millimeter. Achieving such accuracy with a welded structure is not possible without post-machining, because of the distortion introduced by the welding process. (This applies whether the welding is performed manually or by robot.) A bonded structure can also be made lighter, because thicker sections are not required in the vicinity of joints to compensate for local weakening of the metal caused by the welding process. This removes the need for heat treatment of the finished chassis.
Cost is always a potential issue with aluminium structures, due to higher cost of the raw material. But Lotus claims the Elise chassis is price-competitive with previous steel designs. Moreover, its inherent corrosion resistance means a significantly longer life. All the sheet and extruded chassis sections are anodized prior to assembly, which as well as providing the surface pre-treatment required for high-strength bonding also ensures that the chassis is and remains attractive to look at. In a car where the bare chassis is open to view in the passenger compartment, this was an important design consideration.
Use of extruded aluminium sections in the Elise has been extended to many components, including the foot pedals and the pedal box which supports them, and to the suspension uprights. The car also breaks new ground in being the first to use aluminium metal matrix composite brake discs, manufactured by Lanxide in the US, which are about 25 percent the weight of an equivalent cast iron item. Together with the extruded uprights, these help ensure that the 675 kg Elise has a lightweight suspension as well as a lightweight body.
Oliver Everett, Head of Sales and Marketing for Lotus Cars, sees a side appeal for Elise: ìWe plan an initial production volume of 700 units per year, at a base price in the UK of under £20 000. When we begin deliveries in mid-1996, there will be many enthusiasts who still no longer have to dream of driving their own Lotus - Elise is affordable and exclusive." Says Tony Shute, Elise program manager adds: "The results so far speak for themselves - we like to think Colin Chapman would have approved."
ENGINEERING WITH ALUMINIUM