Glass-Fiber body

    To many sports cars specialists, glass-fiber is a perfect material. It is lighter than steel and aluminium, easy to be shaped and rust-proof. Moreover, the most important is that it is cheap to be produced in small quantity - it needs only simple tooling and a pair of hands. There are a few drawbacks, though: 1) Higher tolerence in dimensions leads to bigger assembly gaps can be seen. This is usually percieved as lower visual quality compare with steel monocoque. 2) Image problem. Many people don't like "plastic cars".

    Glass-fiber has become a must for British sports car specialists because it is the only way to make small quantity of cars economically. In 1957, Lotus pioneered Glass-Fiber Monocoque chassis in Elite (see picture). The whole mechanical stressed structure was made of glass-fiber, which had the advantage of lightweight and rigidity like today's carbon-fiber monocoque. Engine, transmission and suspensions were bolted onto the glass-fiber body. As a result, the whole car weighed as light as 660 kg.

    However, this radical attempt caused too many problems to Colin Chapman. Since the connecting points between the glass-fiber body and suspensions / engine required very small tolerances, which was difficult for glass-fiber, Lotus actually scrapped many out-of-specification body. Others had to be corrected with intensive care. As a result, every Elite was built in loss. Since then, no any other car tried this idea again.

    Today, no matter Lotus, TVR, Marcos, GM's Corvette / Camaro / Firebird, Venturi and more, employ glass-fiber in non-stressed upper body. In other words, they just act as a beautiful enclosure and provide aerodynamic efficiency. The stressed chassises are usually backbone, tubular space-frame, aluminium space-frame or even monocoque. 
     

    Advantage:Lightweight. Cheap to be produced in small quantity. Rust-proof. 
    Disadvantage:Lower visual quality. Unable to act as stressed member.
    Who use it ?Lotus, TVR, Marcos, Corvette, Camaro, Firebird ... 
     

    Carbon-Fiber Monocoque

    Carbon Fiber is the most sophisticated material using in aircrafts, spaceships and racing cars because of its superior rigidity-to-weight ratio. In the early 80s, FIA established Group B racing category, which allowed the use of virtually any technology available as long as a minimum of 200 road cars are made. As a result, road cars featuring Carbon-Fiber body panels started to appear, such as Ferrari 288GTO and Porsche 959.

    There are several Carbon-fibers commonly used in motor industry. Kelvar, which was developed by Du Pont, offers the highest rigidity-to-weight ratio among them. Because of this, US army's helmets are made of Kelvar. Kelvar can also be found in the body panels of many exotic cars, although most of them simultaneously use other kinds of carbon-fiber in even larger amount.

    Production process

    Carbon-fiber panels are made by growing carbon-fiber sheets (something look like textile) in either side of an aluminium foil. The foil, which defines the shape of the panel, is sticked with several layers of carbon fiber sheets impregnated with resin, then cooked in a big oven for 3 hours at 120°C and 90 psi pressure. After that, the carbon fiber layers will be melted and form a uniformal, rigid body panel.

    Carbon-Fiber Panels VS Carbon-Fiber Monocoque Chassis

    Porsche 959, employed carbon-fiber in body panels only, is obviously ....
    .... inferior to McLaren F1's carbon-fiber monocoque. This structure not only supports the engine / drivetrain and suspensions, it also serves as a very rigid survival cell.
    Exotic car makers like to tell you their cars employ carbon-fiber in construction. This sounds very advanced, but you must ask one more question - where is the carbon-fiber used ? Body panels or Chassis ?

    Most so-called "supercars" use carbon-fiber in body panels only, such as Porsche 959, Ferrari 288GTO, Ferrari F40 and even lately, the Porsche 911 GT1. Since body panels do nothing to provide mechanical strength, the use of carbon fiber over aluminium can barely save weight. The stress member remains to be the chassis, which is usually in heavier and weaker steel tubular frame.

    What really sophisticated is carbon-fiber monocoque chassis, which had only ever appeared in McLaren F1, Bugatti EB110SS (not EB110GT) and Ferrari F50. It provides superior rigidity yet optimise weight. No other chassis could be better.

    Carbon Fiber Monocoque made its debut in 1981 with McLaren's MP4/1 Formula One racing car, designed by John Barnard. No wonder McLaren F1 is the first road car to feature it.

 
Car
Body
Chassis
Ferrari 288GTO (1985)carbon fiber panelssteel tubular space frame
Porsche 959 (1987)carbon fiber panelssteel monocoque
Ferrari F40 (1988)carbon fiber panels + doorssteel tubular space frame
McLaren F1 (1993)carbon fiber panelscarbon fiber monocoque
Ferrari F50 (1996)carbon fiber panels + doorscarbon fiber monocoque
Lamborghini Diablo SV (1998)mostly aluminium panels, with carbon fiber bonnet + engine lidsteel tubular space frame
Lamborghini Diablo GT (1999)mostly carbon fiber panels + aluminium doorssteel tubular space frame
 

    Engine act as stressed member - Ferrari F50

     
    Unlike McLaren F1, Ferrari F50's rear suspensions are directly bonded to the engine / gearbox assembly. This means the engine becomes the stressed member which supports the load from rear axle. Then, the whole engine / gearbox / rear suspensions structure is bonded into the carbon fiber chassis through light alloy. This is a first for a road car.  

    Advantage: lighter still.  

    Disadvantage: engine's vibration directly transfers to the body and cockpit. 

    In 1963, a revolutionary chassis structure appeared in Formula One, that is, the championship-winning Lotus 25. Once again, that was innovated by Colin Chapman. Chapman used the engine / gearbox as mounting points for rear suspensions in order to reduce the width of his car as well as to reduce weight. In particular, reduced width led to lower aerodynamic drag. Of course, the engine / chassis must be made stiffer to cope with the additional stressed from rear axle. Today, F1 cars still use this basic structure.

    Characteristics of carbon-fiber monocoque:

    Advantage:The lightest and stiffest chassis. 
    Disadvantage:By far the most expensive.
    Who use it ?McLaren F1, Bugatti EB110SS, Ferrari F50.
     

    Aluminium Space Frame

    Audi ASF

    Audi A8 is the first mass production car featuring Aluminium Space Frame chassis. Developed in conjunction with US aluminium maker Alcoa, ASF is intended to replace conventional steel monocoque mainly for the benefit of lightness. Audi claimed A8's ASF is 40% lighter yet 40% stiffer than contemporary steel monocoque. This enable the 4WD-equipped A8 to be lighter than BMW 740i.

    ASF consists of extruded aluminum sections, vacuum die cast components and aluminum sheets of different thicknesses. They all are made of high-strength aluminium alloy. At the highly stressed corners and joints, extruded sections are connected by complex aluminum die casting (nodes). Besides, new fastening methods were developed to join the body parts together. It's quite complex and production cost is far higher than steel monocoque.

    The Audi A2 employed the second generation of ASF technology, which involves larger but fewer frames, hence fewer nodes and requires fewer welding. Laser welding is also extensively used in the bonding. All these helped reducing the production cost to the extent that the cheap A2 can afford it. 
     

    Advantage:Lighter than steel monocoque. As space efficient as it. 
    Disadvantage:Still expensive for mass production
    Who use it ?Audi

    Lotus Elise

    Elise's revolutionary chassis is made of extruded aluminium sections joined by glue and rivets. New technology can make the extruded parts curvy, as seen in the side members. This allow large part to be made in single piece, thus save bonding and weight.
      
    To Lotus and other low-volume sports car makers, Audi's ASF technology is actually infeasible because it requires big pressing machines. But there is an alternative: extruding. Extrusion dies are very cheap, yet they can make extruded aluminium in any thickness. The question is: how to bond the extruded parts together to form a rigid chassis ?

    Renault Sport Spider bonds them by spot welding, while Lotus Elise uses glue and rivet to do so. Comparing their specification and you will know how superior the Elise is:

 
 
Renault Sport Spider
Lotus Elise
Weight of chassis
80 kg
65 kg
Torsional stiffness
10,000 Nm/degree
11,000 Nm/degree
Thickness of extrusion
3 mm
1.5 mm
 
    Lotus's technology was originated by its supplier, Hydro Aluminium of Denmark. Hydro discovered that aluminium extrusion can be bonded by epoxy resin (glue) if it is adequately prepared by a special chemical in the bonding surface. Surprisingly, glue can bond the sections together strongly and reliably. Most important, the aluminium extruded sections can be made much thinner than traditional welding technique. Why ? because welded joints are weak, so the thickness of material should be increased throughout a member just to make a joint strong enough. Therefore Elise's chassis could be lighter yet stiffer. 
     
    Glue can be clearly seen during production.
     
    Unquestionably, Lotus Elise's aluminium chassis is a revolution. I expect to see more British specialty cars to go this way. 
     
    Advantage:Cheap for low-volume production. Offers the highest rigidity-to-weight ratio besides carbon fiber monocoque.
    Disadvantage:Not very space efficient; High door sill.
    Who use it ?Lotus Elise, forthcoming Lotus M250, Opel Speedster