The Lotus 25 is the first World Champion car for the team founded by the Englishman Colin Chapman. The history of the car is very special, as Chapman develops the first drawings of this new chassis on a napkin while discussing this idea with Mike Costin, designer of the Lotus chassis.
The revolutionary feature of the new model is the choice to use, for the first time in Formula 1, the monocoque frame instead of the classic tubular frame.
With the adoption of the monocoque, the car results at the same time with a more resistant structure than the Formula 1 cars of this era, and three times more rigid than the previous Lotus 21, having, among other things, a lower weight.
The outcome also results in an extremely low and narrow car, so as to have a frontal area of 0.75 square meters compared to the average of 0.88 square meters of the other cars. To take advantage of every possible advantage, the driver's seat is also reclined, taking advantage of an idea already used on the Lotus 18, and ten years earlier on the NSU by Gustav Baumm, while as the engine the Lotus 25 has the Coventry Climax FWMV from 1.498 cm³ of displacement.
The birth of the Lotus 25 is not exactly easy, given that the Convetry Climax delays the start of the design of the 1498 cc engine until the last minute available, joining forces with the English teams in opposition to the new regulations.
In September 1960, however, it was clear that this provision would be implemented, and therefore the design of the new power unit began under the guidance of technical director Walter Hassan and Peter Windsor chief of drafters.
The choice falls on the eight-cylinder 90° V-shaped architecture and the general sizing takes its cue from the FWMC four cylinders of 742 cubic centimeters, while for the crankshaft the configuration with four 90° cranks is chosen, the so-called crossplane of American philosophy.
For the distribution it was decided to adopt a very simple solution, namely two single link chains, one for each bank, positioned at the front, two overhead camshafts and two valves per cylinder of large diameter.
The spark plug is placed slightly offset from the center of the combustion chamber, while an open deck structure in light alloy is used for the crankcase, with removable and wet cast iron cylinder liners.
In addition, there is also the presence of two gear pumps for oil recovery, mounted on the front side, each with its own suction duct in the crankcase. In the first year of use of the engine, fuel is supplied by four Weber double-barrel carburettors.
On August 1, 1961 the engine was mounted on a Cooper T58 chassis for a test session on the track. Performance is up to expectations, but major problems emerge with the cooling system, which goes under pressure due to poor sealing, due to thermal deformations, of the gaskets between the head and the cylinder liners.
Eventually Hassan will have to abandon the open deck structure for a closed one but with an original interpretation: the new cylinder barrel will have the upper anchoring flange blocked between the head and the edge, with the task of raising the previous support base on the monoblock.
In the first version, the unit releases a power of only 181 horsepower, but the constant development work gives rise to seven different versions over the next five years, bringing the latest evolution to a power of 213 horsepower.
Lotus 25 in Indycar
Model 25 is also made a version suitable for participation in the Indianapolis 500, called Lotus 29, characterized by a much longer wheelbase than the car from which it derives, as the engine to be housed is a Ford V8 with a power of 375 horsepower paired with four double barrel Weber carburettors. It is also equipped with six fuel tanks for a total capacity of 225 liters, positioned on the left side of the car to balance the centrifugal effect caused by the curves of the circuit all to the left.
The 1962 season
Many private individuals are interested in buying a model of the all-new Lotus 25, but Chapman categorically refuses to sell it, replacing the Lotus 24, with a tubular frame and not a monocoque. The 25 is therefore used only by Team Lotus, which has Jim Clark and Trevor Taylor at the wheel.
The racing debut takes place at the 1962 Dutch Grand Prix. The first victory was obtained at the Belgian Grand Prix at the hands of Jim Clark, while team mate Trevor Taylor completed the double in second position.
Two more victories for Clark in the USA and Great Britain followed later. Lotus remains in the fight for the world title until the last race of the championship in South Africa, where an engine failure causes the title to go to Graham Hill and the British team BRM.
The 1963 season
In 1963, however, Lotus proved even more competitive, and with the Lotus 25 Jim Clark won seven victories in the Grand Prix of Belgium, the Netherlands, France, Great Britain, Italy, Mexico and South Africa, obtaining a second place in Germany and a third place in the United States, winning a total of 54 world championship points that earned him the 1963 world drivers title, while the English team, thanks to the one point also won by Trevor Taylor, won the constructors' world championship.
The 1964 season
The following season Jim Clark took three more wins in Holland, Belgium, and Great Britain, while team mate Peter Arundell took two third places in Monaco and Holland. Despite the good performances, both Clak and Lotus give way to John Surtees' Ferrari, winning a total of 47 championship points in total. Once again the World Championship escaped Lotus at the last race due to an engine oil leak that crowned John Surtees world champion.
The 1965 season
In 1965, the Lotus 25 took three wins in the first four races of the season, more specifically in South Africa, Belgium and France. Although the car’s competitiveness is still very good, the car was replaced by the also winning Lotus 33 from the British Grand Prix, effectively contributing to the victory of both the drivers 'and manufacturers' titles.
The Lotus 25 conquers a total of twenty-five victories, also obtained in its use in the minor championships, seventeen pole positions and thirteen fastest laps, winning the Drivers and Constructors World Championship in 1963, and contributing significantly to the conquest of both titles in the 1965 season, before being replaced by the Lotus 33.