The Long Read: Lotus and the “25” – Part 4

Part 4: The Lotus 25 – The Second Chassis Revolution

In Part 1 we covered motor racing in 1961, Part 2 gave the status of chassis design “thinking out of the box” – the torsion box and Part 3 covered the surprising and extensive Lotus catalogue in 1962. Here in Part 4 we can bring this rambling tale together!

With the 1.5-litre F1 engines offering approximately 100 bhp less that the 2.5-litre engines of 1960, accordingly driving styles needed to be smoother to conserve speed. Dunlop racing tyres were improving rapidly and mid-engined configurations (whether 4-cylinder, V6, V8 or flat-8) allowed inclined seating positions which lowered the centre of gravity and threw the emphasis on unsprung weight and minimised frontal areas. The net result was that cornering speeds increased, cars were faster and lap times improved.

The next piece of the puzzle was Colin Chapman’s design, which by all undisputed accounts, was outlined by John Standen, the Purchasing Manager of Lotus. The author was not around at the time… so an acknowledgement of recommended reading is required!

How and why was the Lotus 25 conceived?

As mentioned in Part 2, the chassis of the future Elan was designed by Colin Chapman during a weekend in 1960. This however does not mean that it was a hurried job of secondary importance. More importantly, it led to the unexpected technical discovery of surprisingly good torsional rigidity.

However, the connecting up of the dots began with a lunchtime “brain storming” in a local café nicknamed “The Trough” in November 1960. The participants were discussing the problem of fitting aluminium fuel tanks to space frames (ref: the photos of the Lotus 24 provided by Paul Matty Sports Cars and featured in Part 3).

It was John Standen (Lotus’ Purchasing manager) who first suggested joining the front and rear of the car with “two fuel tanks”. Standen was alluding to Chapman’s torsion box application (he had priced this up for the Lotus Elan) and contemporary aeronautical practice of fitting rubber fuel tanks within torsion boxes. Standen presented it as follows to John Tipler:

“For years I’d been brainwashed by Mike Costin on the merits of aircraft structures, and I had the idea that something like this would be more of a rigid structure. So I said to them, “Why don’t you take a pair of fuel tanks, join them together with bulkheads at front and rear, with the front suspension hung off the front bulkhead and, the engine mounted behind a firewall towards the back, and hang the rear suspension off the transmission?” At the time it was laughingly dismissed by all those around the table – there was Mike Costin, Colin Chapman, Nobby Clarke, Fred Bushell and Ron Hickman – and then I sketched it out on a paper napkin. They still pooh-poohed it, so I thought no more about it, until some days later, when Colin came into my office, flung a roll of drawings onto the desk and said, “Don’t laugh, but what do you think of these?”. He knew I had been annoyed that they had scoffed at my ideas back in the café. He’d done three of four drawings for the new car, which were based on what I had drawn on the napkin.”

This concurred with another interview with Ron Hickman (designer of the Elan) by Tipler:

“Jimmy Clark was afraid of one thing, and that was being trapped in a burning car. Bent spaceframes made that more of a possibility and with a punctured fuel tank it would only take one spark to create an inferno… Standen proposed using two Elan backbone chassis, each with a fuel tank inside and linked together with bulkheads. To make the structure aerodynamic, the box-sections would be tapered.”

The “Four-Wheel fuel tank”

The idea was conceptually a revolution, but Lotus could not simply introduce it at the start of the 1962 season for reasons of technical and financial viability. The Lotus 24 was already packed with untested components (see Part 3) and in any case, even in May 1962, the new package was late and untested for the season.

But before we look at the innovations, consider how Motor Sport described the Lotus 25 in June 1962:

“Casting away all thoughts of small-diameter tubes and space-frames, the new Lotus 25 has a chassis formed by two long boxes of riveted construction, made from 1.6mm L72 aluminium sheets. These are spaced just wide enough for the driver to sit between and are joined by a similar box behind the driving seat, the instrument panel, the undertray and a square-tube framework at each end. These boxes carry petrol so that the chassis and petrol tanks are one and the same thing, the boxes being approximately 10in. deep by 6in. wide. The front suspension is bolted to the front and the rear suspension on the back, while the V8 Coventry-Climax engine sits in the cradle behind the driver, readily accessible and not covered by frame tubes or such-like. The “monocoque” chassis also forms the lower half of the body, the top half, with windscreen being merely a cover over all the mechanism and the driver. Front and rear suspensions are identical with the previous Grand Prix Lotus as is the ZF gearbox with its rubber universal-joint drive shafts. This riveted “monocoque” structure, like an aircraft, makes for greater rigidity with less weight and provides improved engine mounting, accessibility and servicing. The V8 Coventry-Climax engine has an improved carburettor layout with the four double-choke Webers mounted transversely and in a row, providing improved inlet passages and simplification of the throttle linkage.”

Many observers described the chassis as a “monocoque”. Colin Chapman described it as a “tub” and the name has stuck. The distinction is important as this design is not comparable with previous “monocoque” structures both by the combination of the torsion box and aircraft quality execution.

There were other important details which were often misunderstood:

• The L72 Alclad aluminium tub was rivetted to the steel bulkheads using De Havilland aircraft techniques. It is very difficult to attach aluminium to steel any other way and indeed welding aluminium is difficult as although aluminium welds will be strong, the aluminium immediately around the welds is annealed as the material qualities are weakened unless “overengineering” is used. This is why in certain cases, steel is lighter for a given strength than aluminium.
• The photo shows the tub composed of D-section aluminium torsion boxes (dark grey fuel containers). These boxes, linked by a stressed undertray, contain rubberised fuel cells (capacity 118-litres) made in period by an aircraft supplier, FPT (now replaced with modern FP3 tanks). They were specially clipped in place and accessed by an oval hole. These were far more reliable, cheaper, and safer than aluminium tanks rattling against a tubular frame (and held in place with bungees!).
• In the foreground, the steel front bulkhead provides the suspension attachment points and the “pedal box” (the master cylinders are visible) and component subframe (for the triangular 3 litre dry sump tank, the combined oil/water Serck radiator and the “bomb” Lucas fuel pump – a huge device to feed the fuel injection). Again, this is very different to a space frame mounting.
• The fuel filler is unreassuringly perched in front of the instrument hoop (steel) which braces the “fuselage”. Filling was a slow process. In addition, the car needed a special swirl tank to counteract the lack of baffles in the rubber tanks.
• In period, the roll over hoop was more triangular and used as a bridge to strengthen the tub. Today this has been made to suit FIA standards.
• The red seat (with a “sausage” cushion) conceals a small fuel tank, a firewall and the roll over protection. This is the point where the full height “fuselage” is combined with the half height “trouser legs” to support the Coventry Climax FWMV engine. The engine is a “semi-stressed” component with eight mountings that have a major importance to the torsional stiffness.
• The design shows brilliant use of the different moduli and stressing of each material. The high loading on the bulkheads could only be taken by steel (this was the solution used well into the 70s) at the time. The loadings on the aluminium tub were different in nature to those on the bulkheads. The upper bodywork of the Lotus 25 was made of unstressed fibreglass / Lexan.
• The connections of the tub to the bulkhead were made with intricate rivetted flanges that required practical aeronautical experience. As late as 1965, Ferrari were still fitting steel tubes inside their “tub” to try to match the torsional rigidity of the 25 (which had by then evolved into the 33).

The chassis cannot go without analysis as the reader who has managed to get this far can well imagine! The weights and torsional strengths of the respective chassis were stated as follows in period. The figures for the Type 26 Elan seem to be overstated and are not defined enough (with engine / fibreglass body etc) to be taken seriously.

Type N°	Year	Weight (kg) (sources)	Torsional strength	Description
24	1962	45 (Tipler)	700 lbft / degree	F1 single seater chassis made of steel tubes with brackets and fuel tanks
25	1962	32 (Tipler quoting Len Terry)	1,000 lbft / degree without the engine and 2,400 lbft / degree with the FWMV8 engine	Aluminium twin torsion box tub with steel bulkheads and brackets
26 Elan S2	1962	38 (Road & Track, Motor)	4,000 lbft / degree (without the fibreglass body)	Steel single torsion box leading to four suspension uprights.
33	1965	(Tipler quoting Len Terry)	2,400 lbft / degree with the FWMV8 engine	Aluminium twin torsion box tub with steel bulkheads and brackets
79	1978	(Ludvigsen quotingMartin Ogilvie)	3,000 lbft / degree	F1 car with ground effect (at the beginning of the season…)
Elise	1995	68 (Lotus Cars)	10,000 lbft / degree	Road car. Aluminium rivetted and epoxy bonded chassis and steel rear subframe

Anorak note: In the 1990s, the first cars to use aluminium extrusions were Renault (Spider) and the Lotus Elise using the same supplier and entering the market at the same time (1994) with aluminium supplied in both cases by a Danish firm, Hydro Aluminium. Renault used a welded aluminium chassis which was much heavier than the Elise which used aircraft derived epoxy, and an aluminium alloy used by Lotus that could not be welded. It is significant that the Elise also uses a steel rear subframe. This echoes the steel bulkhead of the 25…

The engine

The regulations required dual circuit braking systems, a circuit breaker, a starter motor and to carry all the oil needed for racing which for a 300 km distance will have been borderline.

The rear suspension comprised outboard coilover damper units with reversed lower wishbones, upper links and twin radius rods attached to the rear bulkhead. The drive shafts are not used to locate the suspension. As with Lotus Elan 26Rs, the original system of hollow tube drive-shafts and Metalastic doughnuts have been replaced by sliding drive-shafts. The rear roll bar is not adjustable (very rare in 1962) but was available in different sizes. The rear bulkhead connects the rear of the engine and transmission to the “trousers” – the half height part of the tub.

In period the Lotus 25 used a ZF 4DS10 transmission which was originally conceived as a 4-speed, front wheel drive transmission for a Hanomag van of all things!

This was modified by Lotus by turning it upside down and then adding a gear to the gearset (by machining the gears and rewelding them together) and we can presume that the truck did not originally have the magnesium casing. These modifications of original equipment were a regular tour de force of Lotus and indeed ZF adopted the modifications in 1964 and it became the 5DS10. Although 25R4 now uses an easier to maintain Hewland gearbox, history does not relate whether a Hanomag van was ever treated to a Coventry Climax FWMV8…

The front and rear brakes comprised 263mm discs operated by the two large reservoir Girling master cylinders visible to the aluminium two pot callipers (probably AR type) operated as a dual circuit.

Dunlop tyres advanced prodigiously at this time with the advent of synthetic rubber with a D12 green spot for wet weather and scrubbed D9 for dry use. Essentially, tyres were becoming lighter, wider and with higher hysteresis which required more sensitive damping.

The airjet screen and the yellow stripe

This screen was first fitted in 1963 at Zandvoort and the yellow stripe followed from the Silverstone GP that year. The venturi created between the Lexan screen and fibreglass body forced air over the driver allowing a lower screen and better management of rain and insects. It is surprising that modern F1 teams have not tinkered with this concept for more downforce. In period the roll cage height was often below the driver’s head…

The screen height varied according to the track. This was a trend probably started by Malcolm Sayer on the Jaguar D-type (only accepted by the late Norman Dewis who was faster than “pros” along Mulsanne). The screen was cut down at helmet level to improve visibility for Jim Clark. Similarly beautiful mirrors are being remanufactured by Peter Denty Racing.

Did the Lotus 25 win the 1962 World Championship?

In the hands of Jim Clark, in 9 events, the Lotus 25 gained 6 pole positions, 3 victories, a 4^th place, and a 9^th place. But it retired 4 times in his hands and 5 times in the hands of Trevor Taylor. The 25 was quite clearly the fastest car of the season.

However, as mentioned already, racing reliability is an essential. Graham Hill’s BRM finished every race and won the final three races of the season. With this, Hill sealed the title at the last race in South Africa when, for want of a single locking washer, Jim Clark’s Coventry Climax ran out of oil whilst in the lead.

That BRM won the title had a great deal to do with the management of BRM by Tony Rudd from 1960 onwards. Several years later in 1969, Tony Rudd joined Lotus where he was involved with road car development.

Once again, make sure to tune in next week as we wrap up the topic…