Lotus Super Seven
The Lotus Seven is a small, simple, lightweight two-seater open-top sports car from Lotus, which has been called "a motorcycle on four wheels". It is the most successful inexpensive exotic sports car. It is characterised by extremely high acceleration (0â€“60 mph in 3.1 seconds) and a mid-range top speed (up to 250 km/h, or 155 mph, depending on engine). The drag coefficient is around 0.7, mostly because of the flat windscreen and the wheel arches.
The original Lotus Seven was launched in 1957, after the Lotus Eleven was in (limited) production. (The name was left over, due to a model having been abandoned or re-names.) Based on the first series produced Lotus, the Lotus 6, it was powered by a 40 bhp Ford Side-valve 1172 cc 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. These had even simpler structure and a close look at such details as bushings of rubber, rather than brass or Delrin, as well as removable weather protection and optional heaters and engine fans, shows that they are mainly for use on roads. In 1970, Lotus radically changed the shape of the car to create the slightly more conventional sized Series 4 (S4), with more fiberglass replacing some of the aluminum. Having closer serious competition from all the moderately spartan British sports cars, this model was not accepted widely enough that either Lotus or Caterham continued it for long.
An S2 Lotus 7 driven by star Patrick McGoohan was featured in the opening credits of the 1967â€“1968 television series The Prisoner.
In 1973, Lotus decided to shed fully its British tax system-inspired partial kit car image and concentrate on its more up-market models. As part of this plan, it sold the rights to the Seven to Caterham Cars. Caterham had been a Seven dealer since the very early days, and at this time they were the sole suppliers. After a brief period producing the Series 4, including assembly of the last "kits" supplied by Lotus, they introduced their version of the Series 3 car, and have been making, and refining, this car ever since as the Caterham Super Seven. Note: the car pictured here is a replica, not a Lotus or Caterham Seven.
Since the design of the Lotus Seven is so simple, over 90 companies have offered replicas or Seven-type cars over the years such as:
- Caterham ( www.caterham.co.uk) now own the rights to the 'official' Super Seven
- Locost by Ron Champion
- Stalker V6 Clubman by Brunton Automotive
- Several models by Westfield Sportscars Ltd
- Raptor by Tornado Sports Cars
- Several models from Robin Hood Engineering Ltd
- Donkervoort from Netherlands with Audi-Turbo-Engines
- HKT from Germany also with Audi-Turbo-Engines
- Hauser from Switzerland with BMW engines
- Tiger Z100 from Z Cars Ltd
- MK Indy from MK Engineering (using Ford Sierra parts)
- Mitsuoka Zero 1 from Toyama, Japan
- ESTfield from RaceTech (using Lada parts)
- Esther ( www.esther.se/)
- Dala7 (a taller and wider design using Volvo parts) ( www.dala7.se/)
- Rotus, built in Florida with Japanese components
- Fraser Clubman from Fraser Cars Ltd
- Vindicator Sprint and the four seat Vindicator Family by Vindicator Cars ( www.vindicator.co.uk/)
- Irmscher 7 ( www.irmscher7.de/)
- Elfin ( www.elfin.com.au) , Australian manufacturer of the MS8 (V8 powered) and Type 3 Clubman
- Super Martin from France
Analysis of the Seven's performance
A Seven's top speed greatly depends upon the body configuration, engine power and gearing. In practice it is unlikely that even the most powerful current Seven's would exceed 150 mph easily, while more typical top speeds are in the range of 110â€“135 mph. The original models with open fendered non-envelope body layout, upright windscreen and bug headlights had difficulty exceeding 90 mph. In addition, the original (clamshell) style front fenders tend to lift the front end at higher speeds, increasing instability and resistance. Later cycle guards help alleviate this tendency, and recent low height wind deflectors that replace the windscreen improve top end speed.
Low speed acceleration
Nearly all Sevens, due to their extremely light weight (around 500 kg) have excellent acceleration, especially up to 70 mph, depending on power. For their time, the original late 1950s Sevens could beat most contemporary saloon carsâ€”and by the early 1960s, with improved Ford-Cosworth engines could take on most high performance sports cars with 0â€“60 mph time in the low 7 seconds. More recent acceleration times (for top level models) are world beating for production cars with 0â€“60 mph below 3.5 seconds. The high power-to-weight ratio is excellent with almost any engine.
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 disc 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 (similar to a Lamborghini Countach) 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 aluminum. The lower frame tubes are five inches from the road, while the wet sump, bell housing and one frame tube are lower.
The front/rear center of gravity is nearly equal and the lack of a boot and small gas 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) suspension configuration is fairly traditional, but for the purpose of reducing cost - 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.
A number or changes to the front suspension were tried in racing circles in the 1970-80s mostly involving using a full upper wishbone and a separate anti-rollbar and this approach was finally adopted by most manufacturers by the early 1990s. This approach has much greater allowance for adjustment in suspension settings, especially Camber.
For the rear suspension - Lotus originally used a live axle (or solid axle) rear suspension. 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 response.
In the early 1990s as supplies of live axles dried up, manufacturers started to introduce various independent and semi-independent designs. Most popular is the Semi-independent De Dion tube, but others have also tried rear wishbones to great effect.
Tires (tyres) and wheels
Tires that look small in comparison to those of most other cars are actually oversize for a car of this weight.
Track and wheelbase
The track is noticeably wider than that of the Austin Healey Sprite / MG Midget.The wheelbase is long relative to the length of the car and the distribution of mass.
Due to the solid axle with which most (but not all) models are fitted and the very low overall weight, this is not particularly good. Again, the effect on the handling is controlled at the expense of the ride. (Ride harshness is aggravated by the lack of springs or thick cushions in the seats, which are only pads on the floor panels.)
In general, un-aerodynamic cars 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, so unless these are changed, the good handling, like the good straight line performance, is limited in speed range. It can be argued that this is not usually important in a car intended for public roads.
Delivery of power to the wheels and brakes
These are not unusual, beyond a rather smooth application of torque.
Yaw and pitch angular inertia (polar moment)
It is very short, and the nose and fenders are of very light fiber glass. There are often no bumpers at all, for exactly this reason.
Physics favors a small car in handling, also, because angular inertia depends more on size than turning or rolling torque does.
Roll angular inertia
The body is much narrower than the track, and the fenders are very light. It is about three feet high, with nothing but the roll bar (if fitted) and windshield sticking up that high.
Position and support for the driver
The driver is upright and held in place between the transmission hump and side panel.
The rack and pinion steering provides a minimum of play and friction. The light weight assures light steering without power assist, even with very large tires. The ratio is quick.
Alignment of the wheels
Rigidity of the frame
Like racing cars of the time and the equally respected and more expensive Mercedes-Benz 300SL coup, 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 https://www.cnet.navy.mil/nascweb/sas/stress.htm construction, in which the flat aluminum body panels, and especially the floor, triangulate the largely rectangular steel tubular frame structure. This gives a ridgid 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 Fords of 1340cc and 1500 cc with the intake and exhaust on the same side of the head. These were often Cosworth modified; the Cosworth 1340cc "Super Seven" delivered 85 bhp and the 1500cc "Super Seven 1500" 105 bhp. These were later replaced by the Ford Kent engine, better known as the Ford crossflow, in 1600cc and 1700cc models designated SuperSprints; in their 1700cc guise, a crossflow delivers up to 135 bhp. The acceleration finally caught up to the handling when the Cosworth/Ford twin cam 1600, 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.
The latest models are the most powerful. The CSR 260 the fastest with a Cosworth tuned 2.3 L Duratec engine giving 260 hp (194 kW). The Rover K-Series engine has been replaced by the 1.6 L Ford Sigma engine with 115 hp (86 kW), 140 hp (104 kW), or 160 hp (119 kW). A 2.0 L Duratec with 210 hp (157 kW) will be used in the Superlight R400.
Frame and body
It had a front engine and rear drive like most cars of its time, with a tubular steel frame with stressed skin aluminum bodywork. The body panels were flat to avoid the expense of more elaborate aluminum bodywork. The nose and wheel arches were thin colored fiberglass. Often neither the body nor the arches were painted.It was a bit over three feet high with the top up. The track was relatively wide. The ground clearance was 5 inches to the main frame tubes, but components extended at least half an inch lower. Because the upper frame tubes were high for rigidity, there were no doors, but only removable hinged side curtains. Bumpers were not standard, as they would have degraded the very low angular inertia (polar moment) as well as the total weight. A heater could be installed.
Early Lotus Sevens weighed around 1100 lb (500 kg). Although the weight crept upward as production progressed, it remained remarkably low for a production car of over a litre displacement. Superlight production models weigh well under this and the latest high performance model reports a stunning, world class power / weight ratio in the region of 600 hp (447 kW) per metric ton or 3.7 lb per hp.
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 center 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.
Broken off splines from the Standard Companion estate car (station wagon) half axles of a Lotus Seven series II
The series II had problems with its Standard Companion estate car (station wagon) rear axle and differential, however Caterham has supplied stronger parts.
The anti-roll bar has broken in at least one old car, but without the serious consequences that might be expected for a member that takes braking loads.