Lotus chassis engineering: Greatest Hits

Part of the exploration of Lotus involves looking into some of its engineering milestones. Lotus has such a wealth of engineering achievements and innovations to its name that it’s hard to pick a place to start, but for this article I’m going to concentrate on the area of chassis design. From when Colin Chapman first applied his ideas to lighten and stiffen Austin 7 frames to Richard Rackham’s ultramodern extruded aluminium Elise, Lotus have been at the forefront of chassis design. This article looks at some highlights.

Mark 6 spaceframe

The Mark 6 was one of the first cars to successfully adopt and popularize the spaceframe structure.

Lotus Mk 6 - picture from lotusvi.blogspot.com

The 1952 Lotus 6, as mentioned in the previous post (https://jtautomotive.wordpress.com/2011/10/21/colin-chapman-and-the-beginnings-of-lotus-part-2/), was Lotus’ first production car (or production insofar as over 100 were made and sold in kit form).

It was also the first Lotus not to be based on the Austin 7’s architecture. Chapman and his small group of helpers instead designed their own spaceframe chassis. This was significant because at that time the majority of cars were designed around a twin-tube or ‘ladder’ chassis. The ladder frame is so called because it consists of two large longitudinal members connected by several cross members, resembling a ladder laid flat.

Although twin-tube chassis were easy to make and durable, they had the major drawback of lacking torsional stiffness. This lack of stiffness is a problem as it can lead to flex in the chassis when cornering, causing changes in suspension pick up points’ positions and therefore inconsistent handling.

A spaceframe chassis is far more efficient and better suited to high performance cars. It involves many smaller diameter tubes arranged primarily in triangles, as a triangle is inherently rigid. The Lotus 6 was one of the first cars to be designed on spaceframe principles. Together with the Mercedes-Benz 300SL, another spaceframe car which appeared around the same time, its success paved the way for many more racing and sports cars constructed in this way.

Lotus 6 spaceframe chassis - illustration by author

Elite glass-fibre monocoque

The Elite was the first ever moulded monocoque GT car. 

Lotus Elite - picture from telegraph.co.uk: http://www.telegraph.co.uk/motoring/6251747/Buying-a-classic-Lotus-Elite.html

The 1957 Elite was an ambitious and forward thinking car. Rather than having a separate frame for the chassis, it featured a unitary glassfibre moulded monocoque body/chassis– the first GT car ever to be constructed in this way.

Lotus Elite construction - picture from http://www.erareplicas.com/misc/stress/deslogic.htm

The Elite was built in three main sections: the floor pan, including a small metal subframe for the front suspension and a flat undertray for low drag; a centre section running the length of the car incorporating the engine bay, transmission tunnel and boot; and the famously pretty body. The body itself was very effective aerodynamically, achieving a drag coefficient of just 0.29.

Although very clever, the Elite was unfortunately also very expensive and Lotus allegedly lost money on every one it sold. It may not have been a commercial success in its time, but the Elite was immensely successful in GT racing and has gone on to be regarded as an all time classic in terms of engineering, styling and performance.

Elan ‘backbone’ chassis

The 1962 Elan’s chassis layout would pave the way for the next three decades of Lotus road cars.

Lotus Elan - picture from telegraph.co.uk: http://www.telegraph.co.uk/motoring/classiccars/6670554/Classic-Lotus-Elan-buying-guide.html

For their next road car, Lotus moved away from the glassfibre moulded monocoque. The 1962 Elan instead featured another elegant and innovative (but more fiscally efficient) chassis design – the ‘backbone’ layout. Not truly a monocoque, as it is separate to the bodywork, the backbone chassis nonetheless shares structural similarities.  The central part of the chassis is compressed into a single steel section, forked at the front and rear for suspension mountings. The engine and gearbox sit in the vee of the front fork and the transmission runs along the backbone itself.  By utilising a central large box section, the torsional stiffness of this layout is much higher than that of a twin tube ladder chassis of a similar weight.

The backbone chassis would form the architecture of numerous future Lotus road cars, including the Europa, Eclat/Excel and Esprit. It was less expensive and easier to manufacture than a multi-tubular or spaceframe chassis. Another, unexpected, benefit in terms of ease of production was that it was possible to wheel the chassis with the running gear installed around the factory as the car was built up.

The Elan's backbone chassis - picture from http://lotuselan.wordpress.com

Type 25 – the first monocoque Grand Prix car

The 1962 Type 25 Formu1a 1 car brought the monocoque hull into modern grand prix racing – a revolutionary layout which remains in use in F1 today.

Lotus 25 at the 2011 Goodwood Revival - picture by author

An overview of Lotus chassis engineering achievements would not be complete without a look at some of their accomplishments in Formula 1. Colin Chapman and Lotus moved the game on dramatically at several points during F1 history, particularly in the areas of chassis design and aerodynamics.

The first F1 Lotus raced in 1958. Less than two years later they had won their first grand prix. In the early ’60s, GP cars were designed with very advanced spaceframe chassis. In 1962 however, the Lotus 25 rendered them all obsolete as it brought the first monocoque hull into modern grand prix racing. Rather than a complex arrangement of triangulated tubes, the 25 was based around an aluminium monocoque ‘bathtub.’

Its architecture is in effect a twin tube chassis. However, its side members are enlarged to form thin-walled box sections and joined with the floor and bulkheads to form an integrated structure.

Stiffer and lighter than a space frame, the essential principle of the 25’s construction still applies to modern F1 cars today.

Lotus 25 monocoque chassis - illustration by author based on diagram in the brilliant book 'Formula 1' by Zettergren/Mudsam

Type 49 – engine gets stressed

The 1967 Type 49 was the first F1 car to use the engine itself as a fully stressed chassis member – again an arrangement which remains the standard in F1 today.

Lotus 49 at Goodwood Revival 2011 - photograph by author

The first car to be armed with the celebrated Cosworth DFV engine, the 1967 Type 49 was significant because it was the first modern F1 car to use its engine as a load-bearing chassis member. The engine was secured to the bulkhead behind the driver’s seat and its block and cylinder head became mounting points for the rear suspension.

Previous racing cars may also have used the engine as a stressed or semi-stressed member, but generally utilised some kind of extra support. For example the 1964 F1 Ferrari involved the engine as a load-bearing member but had ‘wheelbarrow arms’ underneath to take some of the load.  The Type 49 was the first car where conventional chassis structure ended after the driver’s seat and had nothing but engine and fresh air beyond.

With Jim Clark at the wheel, the 49 won the first grand prix it entered and the world championship the following year with Graham Hill. Later in its four season life it became the first GP car to carry strut-mounted wings and also the first to appear in sponsors’ colours.

This overhead shot of Jim Clark in the 49 shows the suspension mountings and how the engine itself is bolted to the bulkhead behind the driver's seat

Type 72 – radiators get reshuffled, brakes go inboard

The 1970 Type 72 was another great leap forward for Formula 1 design.

Jochen Rindt in an early 72 - picture from http://f1insight.blogspot.com/2010/01/four-iconic-f1-cars-featured-at.html

By the end of the ’60s other teams had closed the gap to the 49. The opposition were using the DFV engine as well so the next Lotus would have to be another step forward in packaging, aerodynamics and handling to re-establish an advantage.

Chapman briefed designer Maurice Philippe towards the end of 1969 with a series of requirements: the radiator, previously located at the front of the car, was split into two and positioned either side of the driver (a layout which remains the standard today). As well as allowing a lower nose for a smaller frontal area, this also eliminated the weight of long cooling pipes and meant the driver didn’t get so hot in the cockpit. The iconic wedge shape reduced drag and created downforce whilst magnesium was used extensively to save weight. Torsion bar suspension and inboard brakes reduced the car’s unsprung masses and kept heat away from soft compound tyres.

Emerson Fittipaldi driving the 72 in iconic John Player Special livery - picture from http://f1insight.blogspot.com/2010/01/four-iconic-f1-cars-featured-at.html

The 72 was so far ahead of its time that it was raced for five seasons between 1970 and ’74 with comparatively minor development. This makes it one of, if not the, longest running Grand Prix cars ever. It won the constructors’ world championship three times and the drivers’ twice.

The 72's inboard brakes can be seen in this shot - picture from http://www.f1-info.cz/?gallery=f1foto/f1-historic-races/2008_hfo_hockenheimring

Twin chassis GP car

Never raced, the 1981 dual chassis Type 88 was a potentially revolutionary ground effect F1 car and Chapman’s last stand against the FIA.

1980 twin chassis Lotus 86 - picture from http://www.auto123.com/en/racing-news/formula-1/f1-the-stunning-twin-chassis-lotus-t88-of-1981?artid=115680

Banned from competition, the Lotus 86 (and later development 88) was an ingenious car which remains a curious footnote in F1 history. Following the outlawing of ground effect side skirts (another innovation pioneered by Lotus), the car attempted to regain downforce by effectively having two chassis, one inside the other. Drivers would be shaken violently in ground effect cars and to prevent this Lotus aimed to insulate the driver cell from the aerodynamic loadings. The inner chassis contained the driver’s cockpit and was independently sprung from the outer chassis, which was essentially one large ground effect surface. The 88 was constructed largely in carbon fibre – one of the first F1 cars to be built in this way (together with the McLaren MP4/1) – and powered by the Cosworth DFV engine.

Picture from http://www.projectm71.com/Lotushist.htm

Other teams lodged protests with the FIA on the grounds that the Lotus contravened the rules of moveable aerodynamic devices and the dual chassis car was banned from racing.  Colin Chapman remained resolute his design was legal but despite challenges and appeals the potentially revolutionary 86 never was allowed to compete.

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Elise epoxy-bonded extruded aluminium chassis

Like the backbone chassis it replaced, the aluminium Elise structure has formed the basis of virtually all Lotus road cars since.

The Elise's bonded aluminium chassis - picture from http://www.elises.co.uk/components/s1/chassis/index.html

Much has been written of the 1996 Lotus Elise’s chassis. Apart from its groundbreaking construction methods it is also notable for how effectively it ties in with the car’s styling.

Lead chassis engineer Richard Rackham and head of design Julian Thomson worked closely together on the Elise, a situation contrary to the friction which is often said to exist between automotive designers and engineers.

‘He’s a very exceptional engineer because he can appreciate the art of beautiful engineering,’ says Thomson of Rackham, ‘it’s very rare to find and I guess Colin Chapman and William Lyons were like that. Look at their work and they’re not necessarily designed aesthetically, but by their very purity they look fantastic.’

Work began on the Elise in 1994. The backbone chassis was no longer the lightweight solution it once had been; with the structure concentrated in the centre of the car it was becoming difficult to meet modern crash regulations without adding a lot of weight. The new car’s structure therefore needed to border the cockpit and to keep weight down would need to be made of either composites or aluminium.  For cost reasons, aluminium was the material of choice.

Rackham had previously worked on a project with Rover to design a vehicle using aluminium extrusions and felt there were advantages in this system. The process involves heating aluminium until soft and forcing it through a die to produce a long, constant-section member. Multiple parts can be made in this way and the process is quick and comparatively simple. However there was no previous work to base the design on and some lateral thinking was required. For instance, it is very expensive to bend extrusions so Rackham had to try and design out curves where possible.

Lotus Engineering worked closely with Danish company Hydro Aluminium with the aim of the project being an advertisement for the abilities of both companies. Not only the chassis but many other componenets were composed of extruded aluminium including suspension uprights, steering column bracket, doors, side impact protection and the famous pedal box (pictured).

Lotus Elise pedal box: four extruded sections with pedal arms cut to thickness depending on strength required. Picture from http://auto.howstuffworks.com/lotus-elise5.htm

The Elise was originally meant to take the pedal box from a Metro until Rackham hit upon the idea of a component composed of extruded sections which would fit the car’s overall styling, be cheap to produce and fit the Lotus ethos of elegant and functional engineering solutions.

Another first the Elise can claim is how its chassis is joined together. Rather than weld the members together Lotus opted to use bonded joints, a method used on aircraft for many years but never on a production car. The extrusions were thin in order to meet weight targets and so could have been damaged by welding. The extrusion method led to very precise joints and Lotus felt that bonding them as opposed to welding them was actually the lower risk solution. Having said that, they did build a back-up chassis with extrusions twice the thickness for welding just in case. Happily the bonding method was a resounding success and it was never needed.

The Elise also features a clever moulded glassfibre crash structure, metal matrix composite aluminium and ceramic brake discs and even a neat windscreen wiper design solution. Said wiper design is explained in the video below, an extract from a Discovery Channel documentary about the creation of the Elise.

The success of the Elise arguably saved Lotus Cars and was a valuable advertisement for Lotus Engineering, whilst its chassis became the basis for many other Lotus road and track cars including the 340R, Exige and Europa.

The 2009 Evora is the first Lotus since the Elise to utilise a new platform – like its ancestor it employs a bonded aluminium tub combining extrusions and folded sheet aluminium.

James Taylor