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 to it, and today 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. Many of those cars are still produced to this day. Along with manufacturer supplied kits, there are entirely self-built sevenesque cars referred to as Locosts.
The Lotus Seven was launched in 1957, after the Lotus Eleven was in limited production. 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.
Based on Chapman's first series-produced Lotus Mark VI, the Seven was powered by a 40 bhp (30 kW; 41 PS) Ford Side-valve 1,172 cc inline-four engine. It was mainly for lower budget club racing on short tracks (750 motor club).
The Lotus Seven Series 2 (S2) followed in 1960, and the Series 3 (S3) 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. The S4 model was not widely welcomed, and Lotus sold few cars. What is believed to be the oldest of these Series 4's survives to this day, and can be seen most weekends around Sussex and Kent. A familiar face at Lotus events up and down the country, this was the second Series 4 ever built, and has proved critics of the S4 wrong; it's looks ageing better than many predicted, and its performance and reliability unsurpassed by any of the original Sevens.
The British tax system of the time (Purchase Tax) meant the car could be supplied as a kit (known as "completely knocked down" or CKD) without attracting the tax surcharge that would apply if sold in assembled form. Tax rules specified assembly instructions could not be included, but in a typical Chapman-inspired piece of lateral thinking, there was no rule covering the inclusion of disassembly instructions. Hence all the enthusiast had to do was to follow these in reverse.
Having joined the EEC on 1 January 1973, the UK had to abolish Purchase Tax and adopt VAT instead. VAT does not allow for concessions such as "CKD", so the tax advantage of the kit-built Lotus Seven came to an end. (Note that VAT does allow for variable rating and even zero-rating" of certain goods and services; but the Government still opted not to indulge the kit-builder).
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. After a brief period producing the Series 4, including assembly of the last "kits" supplied by Lotus, Caterham introduced their version of the Series 3, and have been manufacturing and refining this car ever since as the Caterham Seven.
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 16.2 seconds and had a fuel consumption of 31.0 miles per imperial gallon (9.11 L/100 km; 25.8 mpg-US). The test car cost £1157 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) 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 guards help alleviate this tendency, and low height Brookland aeroscreens that replace the windscreen help improve top end speed.
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 vary 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 drums at the rear only (especially on live axle cars from the early 1960s) or discs all round. With the popularity of semi-independent (DeDion) 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 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 wishbone suspension 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, Austin (Rover) components were used. The disadvantage of live axles is higher unsprung weight since the springs (and shock absorber) have to carry the weight of the axle and differential, affecting handling and ride on rough surfaces.
In general, cars with non-optimized 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. It can be argued that 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. The introduction of the Series IV Seven improved the car's Cd. 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, 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 "sexy" curves or streamlining.
After the English Ford flathead (L head or side valve) with 49 hp (37 kW), a BMC series A was 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 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 Cosworth/Ford Twin Cam 1,600, as in the Lotus Elan, was used. There was also a model, sold in the US with a Coventry Climax engine and independent rear suspension.
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.
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.
The series II had problems with its Standard Companion estate car (station wagon) rear axle and differential. This was later solved on the Series III with a Ford Cortina rear end.
The tubular spaceframe chassis suffered from rust, especially from the inside which would lead to its sudden and unexpected collapse.