Lotus Seven
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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.Template:Fact It is characterised by extremely high acceleration (0–60 mph in 3.1 seconds) and a mid-range top speed (up to 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. 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 1967–1968 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.
Replicas
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:
- 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)
- ESTfield from RaceTech (using Lada parts)
- Esther [1]
- Dala7 (a taller and wider design using Volvo parts) [2]
- Rotus, built in Florida with Japanese components
- Gregory
- Fraser Clubman from Fraser Cars Ltd
- Vindicator Sprint and the four seat Vindicator Family by Vindicator Cars [3]
- Irmscher 7[4]
Analysis of the Seven's performance
Top speed
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 110–135 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 (clam shell) 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 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. More recent acceleration times (for top level models) are world beating for production cars with 0–60 mph below 3.5 seconds. The high power-to-weight ratio is excellent with almost any engine.
Braking
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.
Handling
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 trunk and small gas tank assure that it remains fairly constant. It is, however, more front heavy than more modern high performance cars.
Suspension
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 1990's. 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 1980's 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 1990's as supplies of live axles dried up, manufacturers started to introduce various independent and semi-independent designs. Most popular is the Semi-independent DeDion, 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.
Unsprung weight
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 affect 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.)
Aerodynamics
In general, un-aerodynamic cars tend to be free of adverse aerodynamic effects on handling, but the front fenders, 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.
Steering
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
Nothing unusual.
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 [[5]] 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.
Mechanical details
Engines
After the English Ford flathead (L head or side valve) a BMC series A was used, then push rod overhead valve Fords of 1340 and 1500 cc with the intake and exhaust on the same side of the head. These were often Cosworth modified, making these cars Lotus Ford Cosworths, like the racing cars. The Cosworth 1,340 "Super Seven" had 85 hp. and the 1,500 "Super Seven 1500" had 105 hp. These were replaced by the cross flow, when it came out. The crossflows are most popular as 1600cc and 1700cc and are still the preferred choice for many owners, due to them having more grunt than modern k-series engines and being easily tuned.Template:Fact The acceleration finally caught up to the handling when the Cosworth/Ford twin cam 1,600, as in the Lotus Élan, 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 Duratec engine giving 260bhp.
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 either 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.
Weight
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.
Suspension
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.
Image:Standard Companion splines.JPG
Problem areas
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.
References
- Jeremy Coulter. The Lotus and Caterham Sevens. Croydon: Motor Racing Publications Ltd., 1986. ISBN 0947981063
- Lotus Super Seven Series II owner's manual, Lotus Components.
External links
Contains a comprehensive history of the 7 sports car, technical specifications, reviews and projects.
The modern day custodians of the legend.
Ultimate performance in a Lotus 7 skin.
The Lotus 7 club of Great Britain
The Lotus 7 club's Internet forums
The California Caterham Club
Canada-based Super Seven maker, using a Suzuki GSX1300R Hayabusa motorcycle engine
- http://dmoz.org/Recreation/Autos/Makes_and_Models/Lotus/Seven/
- Lotus Seven Club Sweden
- US Caterham Site
- http://www.davebean.com/title.htm Parts and information source.