A new frame of reference
On Friday May 18, 1962, at Zandvoort, Formula One changed forever. In the blink of an eye, chassis design went from tubes to tub, courtesy of the radical Lotus 25. John Tipler recounts its story - from sketch on a napkin to ground-breaking debut
Think of Anthony Colin Bruce Chapman and automatically you conjure up groundbreaking cars: the ubiquitous Seven, the wedge-shaped 72, the ground-effect 79, the twin-chassis 88. And the car that rewrote the early-1960s F1 form book: the Lotus 25, its unique aluminium box-section chassis making it Formula One's first `monocoque'.
With a Coventry-Climax V8 amidships Jimmy Clark waltzed it to victory in the 1963 world championship. It was upgraded, modified, and re-designated the 33 for the '64 season, and Clark just missed out on the title — as he had done in '62 with the earliest 25. But he used a 33 to ease to his second championship, in '65. In all, 14 Lotus 25 and 33 chassis were built, winning 19 world championship GPs between them.
In an arena where tubular spaceframes had reigned supreme, the 25 came as a huge surprise. It landed in an unsuspecting Zandvoort paddock on May 18, 1962, and it became immediately apparent that Chapman had stolen a march on his rivals. Indeed, one or two of them were particularly dismayed, having just acquired, or ordered, the spaceframe Lotus 24, itself scarcely a month out of the box and, until Zandvoort, state of the Formula One art.
It's worth remembering that the 'new era' rear-engined revolution was scarcely four yea's old when the 25 appeared. Times were changing quiddy, and no-one effected change more dramatically than Chapman. His pencil-thin 21 of 1961, with its 1.5-litre Climax FPF, and which Innes Ireland drove to a win at Watkins Glen, set new standards for windcheating shape and laid-back driving position. In contrast, Phil Hill's world tide-winning Ferrari of '61, the Sharknose, was still running wire wheels at Zandvoort in '62. Porsche debuted its broad-shouldered spaceframe 1.5-litre F8 simultaneously with the 25, but while the Lotus soared, the Porsche hardly took off at all.
Legend has it that the 25 started as a drawing on a napkin. There might be some truth to this, for Chapman was always sketching ideas on fragments of paper; and senior Lotus personnel regularly frequented a local greasy spoon, the Maple Leaf Inn.
One lunchtime there in late summer 1961, Chapman asked what ideas his staff had come up with. There's no conclusive record of the resultant conservation, but according to Ron Hickman, then general manager of Lotus Developments: "John Standen [the components buyer] proposed using two Elan backbone chassis, each with the fuel tank inside, and linked together with bulkheads. To make the structure sufficiently aerodynamic, the box-sections would be tapered. The subsequent drawing was then refined by Chapman, who floated it as his own idea for the 25 at another lunchtime get-together."
Whatever the whys and wherefores, the background to the 25's chassis included all the chief ingredients of Lotus's construction philosophy. In his messianic quest for lightness, Chapman's cars were always pared to the minimum; only if something broke was it strengthened. Not all designers understood the importance of chassis layout and rigidity, but Chapman had always regarded suspension design, light weight and aerodynamic efficiency as crucial to successful lace-car construction. His 21 had the lowest frontal area of any car on the 1961 grid, and this was achieved by sitting the driver two inches lower than in the 18. Also, the 21 was the only car to attain the 450kg minimum weight limit stipulated for the '61 season. The seeds of the monocoque were here.
Conceived by October 1961, types 24 and 25 were built to comply with the FLNs new F1 regulations which came into force that year. They were powered by the new 1.5-litre Coventry-Climax 90-degree V8 FWMV, which had made its GP debut at the Nurburgring in August '61, in the tail of Jack Brabham's works Cooper T58.
The 24 was built on the first chassis made for Lotus by Arch Motors, and its Team Lotus debut was the Brussels GP on April 1, 1962. It was used in six other early-season races, Clark winning Snetterton's Lombank Trophy and the Aintree 200.
The 24 of team-mate Trevor Taylor also finished second in the Dutch GP. It was a very capable car, clearly, but the 25 had stolen the show, moved the goalposts.
Denis Jenkinson's Dutch GP report commented that, The most outstanding feature of the entry was Chapman's brand-new car.' He continued, 'This riveted monocoque structure, like an aircraft, makes for greater rigidity with less weight It was riding the bumps beautifully, but was naturally requiring a few minor adjustments, and Clark tried his old V8 Lotus  to make a comparison.'
The new car was good enough for Clark to qualify on the front row of the grid, alongside Graham Hill's BRM and the pole-sitting Lola ofJohn Surtees.
But was it the new Lotus?
There is some evidence that, despite the obvious sequencing of Lotus type numbers, the 25 was actually conceived before the 24. In the January 15, 1966 issue of Motor, Chapman told Philip Turner: "We designed the 25 first, and then, just in case it didn't do what we wanted, we took all the mechanical elements — engine, suspension, steering and so on — and designed a spaceframe to connect them." And that, of course, was the 24.
Chapman's response to the consternation in the Zandvoort paddock was that Lotus could hardly sell an unproven concept to its customers — and anyway, the 24 had a proven spaceframe chassis with more or less identical suspension and powertrain to the 25.
Team Lotus's chief mechanic at the time, Jim Endruweit, one of the few people involved in the construction of the 25 prototype, casts light on the furore: "When the 25 appeared, it really had been finished only that week. That's the truth of it Certainly there had been no testing. The 24 had done several races by then. Maybe they were running concurrently in Chapman's mind; the suspension design and geometry were pretty well identical.
"Colin wasn't one to miss a trick, and if people wanted cars, he'd say to them, 'Yes, I can do you this', which at the time meant a 24. Everybody was using spaceframes in 1962. Except us."
The 24 was equipped with the latest suspension geometry, but it lacked structural rigidity. Like the 21, a low frontal area was achieved by having the driver reclining, with much of the fuel located over his legs in the scuttle, and side tanks.
Endruweit: "On spaceframe cars, sidemounted tanks had to fit between the tubes, and they possessed a marked tendency to work loose, or the chassis tubes would chafe the aluminium. Fuel leaks were a way of life."
In the 24, the stresses around the pick-up points were alleviated by triangulations in the framework. These were spread more evenly in the 25's monocoque; the deficiencies in its structure were the cavities, meaning the cockpit and engine bay.
The 25 has been described as the first Formula One car to feature a monocoque chassis, but Chapman preferred to call it a 'twin-tube ladder-frame', or 'bathtub', since that's what the driver and engine sat in.
And in fact, he was not the first to utilise single-cell chassis construction: in 1910, Gabriel Voisin pioneered a monocoque airframe, while racing precedents to the 25 included Sir Alec Issigonis' single-seater Lightweight Special from 1938, the '49 Trimax 500cc F3 car, and the D -type Jaguar, which featured a central semimonocoque, as did the BRM P25. Lotus's own Elite (from '57) featured a glass-fibre monocoque. But until '62, the spaceframe was the preferred racing option.
The 25's chassis was based on two longitudinal aluminium torsion-boxes that ran either side of the cockpit. They were sufficiently wide and deep to form the lower flanks of the chassis, with the benefit of accommodating the fuel tanks — made-to-measure rubberised bags. The aft section of the structure formed the engine bay. The box-sections were linked by a series of welded steel bulkheads and a floor panel, as well as the engine itself. Glass-fibre body panels completed the enclosure.
The front and rear bulkheads carried the wishbone suspension. The outer skin was 16gauge 2L72 Alclad aluminium alloy sheet, and the inner skins, cockpit floor and seat back were 14gauge sheet. In the engine bay, the inner skin was in 20-gauge steel, rather than aluminium, because the engine was not a frilly-stressed member. Knowing that the new Coventry-Climax V8 could not accept chassis loads, Chapman incorporated the engine into the monocoque' package. It was a neat solution, since the cylinder heads of the semi-stressed V8 didn't project beyond the chassis.
The project was begun in November 1961, and six months later the type 25 was born. Typical mercurial Chapman.
The car possessed exceptional torsional rigidity, which in turn made the tyres work more efficiently. Its tub weighed much less than its multi-tubular counterpart, yet it was considerably stiffer. For instance, the aluminium tub weighed 701b complete with mounting brackets, while the 24's spaceframe weighed over 100lb once the brackets and aluminium tanks were added. The torsional rigidity of the 24's chassis was rated at 7001b/ft per degree, against nearer 1000lb/ft for the 25's monocoque, rising to 2400lb/ft per degree once the V8 was installed. These dramatic improvements ushered in anew era of handling and driving techniques.
"All Lotus cars understeered a bit," explains Trevor Taylor, works Lotus driver, 1961-63. 'This was fine on a fast circuit like Spa, but you can't cope with understeer on a tight circuit, so you had to make it oversteer. I grew up with spaceframe cars like the Lotus 18 and 24, where the chassis twisted slightly, and turn-in was much more gradual and, at first, I couldn't get on with the 25. It was such a stiff chassis that my first impression was, 'I'm never going to drive this, it'll scare me to death.' But we did lots and lots of testing, and eventually I got used to it".
The 25 had undoubtedly been wrapped around the exceptional talents of Clark — the inner dimensions of the cockpit fitting him exactly. This was his can And in a December 1962 Autosport, he explained that the 25 was apt to drift in corners more than the 18 and 24. He was quite happy when the back stepped out, as long as he had sufficient arm movement in the cockpit to grapple with it. He used the engine power to promote road-holding, because it gave more positive grip through a corner, and he employed liftoff oversteer to affect the car's handling, making the tail of the 25 come out in a corner.
The 25 may have been torsionally superior to the 24, but Endruweit recalls an incident at Zandvoort when they filled Jimmy's 25 up with fuel and he took off on a practice start: "He came back and told us how the inner sides of the tub had bulged as the fuel surged back through the fuel cells. I think it lifted his body up a bit as the sides flexed. We're talking a quarter of an inch or so, no big deal, but he certainly felt it"
Lotus's technical director Mike Costin's experience in the aircraft industry enabled him to advise on how the sheet metal for the tub could be assembled: "We worked out how we would jig it and rivet all the bits together. The jig was based on a flat plane, and the prototype was put together on 6in square planks of timber in Lotus's workshop. The front and back bulkheads were jigged onto that, with wooden formers for the panels to sit in."
While Chapman had done the original layout and exterior design for the 25, much of the detail work was carried out by chief draughtsman Alan Styman and Ian Jones. Styman: "The suspension parts for the 25 were virtually carried over from the 24, with new pick-ups to the tub. Our job was to interpret Chapman's layout and compile detail drawings for manufacture. Mainly, the first 25 chassis was made according to our drawings, but many of the parts were knocked up in-house and, if necessary, drawn later."
Endruweit: "It was designed as it went along. Nobody with any aircraft knowledge would have been flabbergasted by it. But we were under strict instructions that nobody else should come into the racing shop, except Colin, Mike Costin, the designer, and our own Team Lotus guys. But there weren't people prowling around trying to get pictures of it."
Or trying to copy it. In fact, rival teams were surprisingly slow on the uptake. Of the major players, only BRM came up with a (problematic) monocoque-and-spacefi-ame, the P61 in 1963, while Cooper, Brabharn and Ferrari clung to spaceframe technology for at least a couple more seasons. The most blatant copy of the 25 was produced in 1963 by BRP (British Racing Partnership),whose drivers were Ireland and (from 1964) Taylor.
"The BRP was a very similar car to the 25," says Team Lotus mechanic, Cedric Selzer, "but it wasn't as good by a long chalk. They made the mistake of not copying the suspension layout. When I asked hales about it, he said that if they had copied the suspension of the 24, which was identical to the 25, they could have had a good can".
Chapman was incensed at what he saw as a flagrant breach of copyright and began legal action against BRP, settling out of court.
The 25 had a revolutionary effect on race-car construction. Early in 1962, a state of virtual parity existed between the chassis designs of the top F1 teams running V8 engines. But when Team Lotus revealed its stressed-skin monocoque chassis in Holland, it was clear that Chapman had taken racing-car design to another level.
Our thanks to Cedric Selzer and Mervyn Garton Vehicle Movements (tel: 01604 858387) for their help with this article.
'Rivals were surprisingly slow on the uptake'
Technical specification - 1962
Type: Climax FWMV 90-deg V8
Bore x stroke: 63x60mm
Max power: 174bhp @ 8600 rpm
Carburation: 4 Weber 38DCNs
Ignition: Lucas electronic
Spark plugs: Lodge RL50
Gearbox: ZF 5DS10, five-speed
Clutch: AP, 7.25in, twin-plate
Halfshafts: Lotus, Metalastic UJs
Type: aluminium monocoque, steel cross-members
Track (f/r): 1321/1346mm
Suspension (f): top link, lower wishbone, inboard coil spring/damper
Suspension (r): top link, lower wishbone, twin radius arms, rollbar, inboard coil spring/damper
Steering: rack and pinion
Brakes: Girling, 10.5in discs
Wheels: Lotus, 15x5.5 (f)/15x8in (r)
Tyres: Dunlop, 5x15 (f)/7x15in (r)
Fuel tank: fireproof rubberised fabric