The Lotus Seven is a small, simple, lightweight two-seater open-top sports car produced by the British manufacturer Lotus Cars (initially called Lotus Engineering) between 1957 and 1972.
It was designed by Lotus founder Colin Chapman and has been considered the embodiment of the Lotus philosophy of performance through low weight and simplicity. The original model was highly successful with more than 2,500 cars sold, due to its attraction as a road legal car that could be used for clubman racing.
After Lotus ended production of the Seven, Caterham bought the rights and today Caterham make both kits and fully assembled cars based on the original design.
The Lotus Seven design has spawned a host of imitations on the kit car market, generally called Sevens or sevenesque roadsters.
The Lotus Seven was launched in 1957 to replace the Mark VI as the 'entry level' Lotus model, The Seven name was left over from a model that was abandoned by Lotus, which would have been a Riley-engined single-seater that Lotus intended to enter into the Formula Two in 1952 or 1953. However, the car was completed around Chapman's chassis as a sports car by its backers and christened the Clairmonte Special.
Externally similar to Chapman's earlier Lotus Mark VI, but with a different tubular frame similar to the Lotus Eleven, the Seven was powered by a 40 bhp (30 kW; 41 PS) FordSide-valve 1,172 cc inline-four engine. It was used both on the road and for club racing (750 motor club in the UK).
The Lotus Seven S2 followed in 1960 and was supplemented by the Lotus Super Seven S2 from 1961. The Super Seven initially used the larger Cosworth modified 1,340cc Ford Classic engine and later examples were fitted with 1,498cc or 1,599cc engines. The Seven S3 was released in 1968. In 1970, Lotus radically changed the shape of the car to create the slightly more conventional sized Series 4 (S4), with a squarer fibreglass shell replacing most of the aluminium bodywork. It also offered some luxuries as standard, such as an internal heater matrix.
Between 1970 and 1975, following a representation agreement, Lotus Argentina SA obtained the licence to manufacture the Lotus Seven in Argentina. This production reached approximately 51 units. These vehicles were not replicas, but built under licence and original brand Lotus.
Under the Purchase Tax system of the time cars supplied as a kit (known as "completely knocked down" or CKD) did not attract the tax surcharge that would apply if sold in assembled form. Tax rules specified assembly instructions could not be included, but as the rules said nothing about the inclusion of disassembly instructions, they were included instead and all the enthusiast had to do was to follow them in reverse. However, once the UK joined the EEC on 1 January 1973, the VAT system was adopted instead so the tax advantage of the kit-built Lotus Seven came to an end.
In 1973, Lotus decided to shed fully its "British tax system"-inspired kit car image and concentrate on limited series motor racing cars. As part of this plan, it sold the rights to the Seven to its only remaining agents Caterham Cars in England and Steel Brothers Limited in New Zealand.
Caterham ran out of the Lotus Series 4 kits in the early 1970s. When this occurred and in accordance with their agreement with Lotus, Caterham introduced its own brand version of the Series 3. They have been manufacturing the car ever since as the Caterham Seven. Steel Brothers Limited in Christchurch, New Zealand, assembled Lotus Seven Series 4s until March 1979 when the last of the 95 kits provided by Lotus was used up. Steel Brothers had a much wider range of factory options than the UK models with carpet, centre console glove-box, radio, window-washer and hard top. Sold largely to competition enthusiasts, the NZ cars also had engine modifications, close-ratio gears and adjustable suspension as factory options. As such they were very successful in local racing. With officially licensed production stopping in 1979, the last Lotus badged Seven, a Series 4, was therefore produced in New Zealand.
Steel Brothers Limited attempted to make a wider, modernised version of the Series 4, the Lotus Super 907, using the twin cam Lotus 907 engine. In the spring of 1978 it was announced that this was to be sold in the United States - but the American importer had no funds and the project came to naught. The single finished Super 907 was moved from the New Zealand to the United States in 2010 to undergo a full restoration.
Analysis of the Seven's performance
A car with a tuned Ford 1172 cc engine and close ratio gearbox was tested by the British magazine The Motor in 1958. It was found to have a top speed of 80.4 mph (129.4 km/h), could accelerate from 0-60 mph (97 km/h) in 6.2 seconds and had a fuel consumption of 31.0 miles per imperial gallon (9.1 L/100 km; 25.8 mpg‑US). The test car cost £1,157 including taxes of £386. They commented that car could be bought in component form and then it would have cost £399 for the parts from Lotus, £100 for the Ford engine and gearbox and £27 for the BMC rear axle.
A Seven's top speed greatly depends upon the body configuration, engine power and gearing. Early models with low-powered engines had difficulty exceeding 90 mph (140 km/h), although a race-prepared Seven was clocked at 127 mph (204 km/h) whilst driven by Brausch Niemann through a speed-trap at the 1962 Natal Grand Prix. In addition, clamshell style wings tend to create drag and generate lift at higher speeds. Cycle wings help alleviate this tendency, and low height Brookland aeroscreens or the lighter Perspex variants that can replace the windscreen help improve top end speed. Sevens do suffer from front end lift at high speed — the nose creates more lift than downforce at speeds over around 70 mph (110 km/h), although retro fitted "winglets" may counter this.
Low speed acceleration
Nearly all Sevens, due to their extremely light weight (around 10cwt / 500 kg) have excellent acceleration, especially up to 70 mph (110 km/h), depending on power. For their time, the original late 1950s Sevens could beat most contemporary saloon cars—and by the early 1960s, with improved Ford-Cosworth engines could take on most high performance sports cars with 0–60 mph time in the low 7 seconds.
The choice of brakes varies considerably between models, and over the evolution of the car. The less powerful early models had drum brakes all round, while more powerful and later models had rear drums only (especially on live axle cars from the early 1960s) or discs all round. With the popularity of semi-independent (De Dion) or fully independent rear suspension, most manufacturers have opted for discs all round, as is current common practice in the automotive industry.
Physics favours small cars in braking and Sevens have excellent stopping distances, but one of the effects of light weight and powerful (non ABS) brakes is the tendency to lock up, especially at the front under strong braking. The cooling surface-to-weight ratio improves with reduced scale, while the light weight makes vacuum assistance unnecessary.
The highest part of the car is about three feet (900 mm) from the road and it has a cloth top and side curtains with plastic back and side windows. The supports for the top and the windshield frame are aluminium. The lower chassis tubes are five inches (127 mm) from the road, while the wet sump, bell housing and one chassis tube are lower, meaning the centre of gravity is very low.
The front/rear weight distribution is nearly equal and the lack of a boot and small petrol tank assure that it remains fairly constant. It is, however, more front-heavy than more modern high performance cars.
In the original Seven, the front lower A-arm (or "wishbone") of the double wishbonesuspension is traditional, but for the purpose of reducing weight, the upper suspension integrated an anti-roll (anti-sway) bar into a horizontal suspension arm. This approach formed a pseudo-wishbone which was semi-independent in nature. This approach worked well with early crossply tyres, but with later radials, the configuration seriously affected its adjustability.
For the rear suspension, Lotus originally used a live axle (or solid axle). This approach was very cost effective, since most production saloon cars up to the 1980s used these components. A mixture of Ford and Austin components were used. One disadvantage of live axles is higher unsprung weight, affecting handling and ride on rough surfaces.
In general, cars with non-optimised aerodynamics tend to be free of adverse aerodynamic effects on handling, but the front wheel arches, of all but the Series I, cause lift at high speeds. Like the good straight line performance, the car's nimble handling is limited in speed range, and this is not usually important in a car intended for public roads.
While the car's frontal area is small, the Lotus Seven has the highest drag coefficient () of any known production car - ranging from 0.65 to 0.75, depending on bodywork.
Additionally, the clamshell front wings develop lift. This is accentuated by the slight natural lift caused by rotating wheels. Consequently, Sevens have exhibited understeer at high speeds.
The rack and pinion steering provides a minimum of play and friction. The light weight assures light steering without power assistance, even with very large tyres. The ratio is quick.
Rigidity of the frame
Like racing cars of the time and the equally respected and more expensive Mercedes-Benz 300SL coupe, it had a multi-tube space frame with high sides to allow a stiffer frame (longer lever arm). However, the Series II and other road versions had simpler frames than the more race oriented Series I.
It is a stressed skin construction, in which the flat aluminium body panels, and especially the floor, stiffen and effectively triangulate the largely rectangular steel tubular frame structure. This gives a rigid frame with few tubes and very little body weight that does not contribute to the frame stiffness. The flat panels avoid difficulties in shaping aluminum sheet into smooth compound curves. On the down side, it does not allow attractive curves or streamlining.
After the English Ford flathead (L head or side valve) with 49 hp (37 kW), both BMC series A and FWA s.o.h.c. Coventry Climax engines were used, then push rod overhead valve (OHV) Fords of 1,340 cc and 1,500 cc with the intake and exhaust on the same side of the head. These were often Cosworth modified; the Cosworth 1,340 cc "Super Seven" delivered 85 bhp (63 kW; 86 PS) and the 1,500 cc "Super Seven 1500" 105 bhp (78 kW; 106 PS). These were later replaced with the Series III Seven in 1968 by the Ford Kent engine, better known as the Ford crossflow, in 1,600 cc and 1,700 cc models designated SuperSprints; in their 1,700 cc guise, a crossflow delivers up to 135 bhp (101 kW; 137 PS). The acceleration finally caught up to the handling when the Ford Kent engine Lotus Twin Cam 1,558cc, as in the Lotus Elan, was used.
Frame and body
The Lotus Seven was designed with racing in mind, and lightness was of primary concern to Chapman. A front-mounted engine driving the rear wheels (a similar layout to most cars of the day) and a very lightweight steel spaceframe was covered with unstressed aluminium panel bodywork. The body panels were mainly flat to avoid the expense of more elaborate curved bodywork, and the simple cloth lined plastic doors were hinged from the windscreen. The nose-cone and wheel arches were originally aluminium parts, but these were replaced in the later S2 and S3 models with painted or self-coloured fibreglass.
Early Lotus Sevens weighed around 1,100 lb (10cwt/500 kg). Although the weight crept upward as production progressed, it remained remarkably low for a production car of over a litre displacement.
Broken off splines from the Standard Companion estate car (station wagon) half axles of a Lotus Seven series II
The front was by "A" arms and coil springs with an anti-roll bar serving as the front half of the top A arm. The rear had trailing arms, a triangular centre locating member and solid rear axle.
The geometry and high (relative to total) unsprung weight gave it some bump steer, which owners sometimes treated by moving the supports forward and lengthening the trailing arms.
A model that was sold in the US had independent rear suspension and a Coventry Climax engine.
An S2 Lotus Seven (registration plate "KAR 120C"), driven by creator/director & actor Patrick McGoohan, was featured in the opening titles of the 1967–1968 television series The Prisoner - although the car in the final episode, "Fall Out", was another; driven by Caterham Car's MD Graham Nearn.
A Lotus Seven appears as one of the steal-able cars in Grand Theft Auto: London 1969, under the name "Locust". Ironically enough there is a replica of the Lotus Seven made under the same name and another version made under the trade name of the Locost.
In the second game of Sonic Drift series, Tails uses a car called the "Whirlwind S7", which is a reference to the Lotus S7.
In the game Rage Racer the Age Pegase (top notch handling) is a reference to the Lotus S7
How to assemble and improve any Lotus Seven or Locost sports car kit.
Because of the Seven's relatively simple design, over 160 companies have offered replicas or Seven-type cars over the years. Such cars are often referred to as "sevenesque" or simply a "seven" or "se7en". Sometimes they are also called clubmans or "locost". Some examples are:
Rotus, originally built with components from Japan in Hagerstown, Maryland. Since founder Chris Custer owned a Toyota dealership, the first cars used a Japanese spec 2litre twin cam Toyota engines and five speed gearbox. Many other engines were used over the years including Mazda rotary up to Rover/Buick V8s.