ALU-ISE
AutoCar
July 3, 1996
As we ready ourselves for next weekís drive of Lotusís exciting Elise, Keith Howard whets the appetite with an in-depth look at the state-of-the-art engineering below its skin
Whenever a new Lotus is launched, the cry goes up that it's a car Colin Chapman would have approved of. Pundits said it of the ill-fated Elan; they will say it again of the new Elise, which Autocar will be fully road testing next Week.
The difference is that this time it's true. The mid-engined Elise is closer to Chapman's original minimalist ideals than any car the Norfolk company has built in the past 20 Years. It's small it's light, it has a low centre of gravity, it's devoid of unnecessary frills. It is also, unlike the Elan, driven from the correct end .
But this absolute is not a retro-car in the manner of the Mazda MX-5. Returning to its roots in a modern context has required Lotus to develop and apply some radical technologies, nowhere more so than in the Elise's state-of-the-art chassis. It's a measure of the enthusiasm and skills at Hethel that this, the most technologically daring Lotus road car since the original Elite, was created in only 25 months and at the same time as the new V8 was bleeding resources elsewhere.
CHASSIS
Making a small, lightweight car in Chapmanís hey day was relatively easy. Take the original 60s Elan: you fibbed about the torsional stiffness of the backbone chassis. fitted such soft suspension that chassis flex caused no obvious ride problems and simply ignored the unwelcome feel that in a side impact there was nothing more substantial within the GRP doors than a flimsy steel window frame.
Its an unthinkable recipe for a modern Hethel product. Lotus's ride and handling team clearly established the importance of chassis rigidity during development of the fatter-day, front-drive Elan whose backbone steel frame eventfully achieved a torsional stiffness of 5800lb ft per degree. A better figure still was demanded of the Elise, which as well as being a great deal lighter than its overweight predecessor - a consequence of the Elan being designed by committee, says Elise program manager Tony Shute - would also have to equal or better its crashworthiness.
The solution lay in abandoning Lotusís traditional steel backbone ñ a consistent structure feature across 30 years of Lotus history since the original Elan - and replacing it with a chassis built from aluminium. More than that: one of a sophistication never before seen.
In the established jargon of aluminium automotive structure , the Elise chassis, because its constructed primarily by joining extruded sections, classifies as an aluminium spaceframe. So does the chassis of the Renault Sport Spider but a glance will tell you they are chalk and cheese. Whereas the Spider spaceframe is welded up from large, unsubtle box sections the Elise's is adhesively bonded into an elegant high-aided tub structure. Richard Rackham, the chassisís creator, prefers to call it a monocoque, but that too is a term with multiple connotations.
You don't need be an engineering aesthete to appreciate the superiority of the Elise solution: just look at the two cars' all up weights, The Sport Spider is knocking on he door of a tonne (930kg without windscreen, hood or heater), while the the Elise tips the scales at comfortably under 700 kg.
Its bare chassis weighs just 68kg yet has a torsional stiffness of 11.000Nm per degree 7400 lb ft per degree ) - a 27 per cent improvement over the Elan's. With the roll bar and body fitted, the overall figure rises to around ll,000 Nm per degree, matching may unitary steel-bodied saloons.
Transverse torque boxes front and rear are the key structural elements, together with the tall side rails which in concert with the extruded aluminium door beams, also provide a high level od side impact protection. A GRF crush box positioned ahead of the chassis, which doubles up a ducting for the radiator and cabin ventilation, provides defective energy dissipation ha a frontal impact and is simple End cheaply replaced in low-speed shunts which leave the chassis undamaged.
Anodising of the chassis components prior to assembly :all of which is done by Hydro Aluminium in Denmark provides both a suitable surface pre-treatment for the heat-cured Ciba epoxy adhesive and an attractive finish for the large areas of chassis exposed within the cabin. To protect the adhesive joints from peel failure in an impact, thread-forming rivets - the rivet equivalent of self-tapping screws - are used as a supplemental jointing method.
The complete elimination in welding also provide a high level of side impact the finished chassis. Suspension hard points are held to a tolerance of 0.5mm, compared with 2mm for Lotus's traditional steel backbone, There is some steel in the structure, but not much. Bonded steel inserts accept suspension loads, the roll hoop is steel to comply with FIA regulations (Lotus would happily have used aluminium here also, and so too is the engine subframe. The design team was concerned that an aluminium alternative would conduct heat from the catalytic converter to some of the chassis's adhesive bonds and weaken them and, in any case, so many load-bearing steel washers were necessary that, in Tony Shute's words_ "it made sense to weld them all together."
SUSPENSION AND STEERING
To say the Elise has double wishbone suspension at all four corners may be enough to establish its bona fides as an - uncompromised sporting car, but it actually -very little about how Lotus - an acknowledged master of ride and handling development - has gone about creating a chassis which promises a formidable combination: all the predictability and -friendliness of the Elan, enlivened with entertainment value of rear-wheel drive. To understand how Lotus has achieved that, you have to look at the details.
Avid watchers of Hethel suspension will have a good idea of what to expect: relatively soft springs and stiff dampers in the Lotus tradition, low roll centres, low levels of wheel camber change in bump, minimised castor and king pin inclination, and Hethel's "disturbance rejection" philosophy with regard to passive steering. Much of this was seen in the Elan, but as you'd expect with drive from the other end, the Elise is not quite Elan 1996.
A big surprise at the car's unveiling was the absence of anti-roll bars. The Elan had bars front and rear, but the Elise originally had none, A front bar has since been added, but not to reduce ultimate roll angle or dial in understeer: It's there to provide improved transient mild control and on-centre steering response, which was otherwise compromised by the monotube dampers, This is the first Lotus to use monotubes, chosen because, unlike twin-tube types, they can be	mounted upside down to reduce unsprung mass. Still more unsprung mass is saved by the use of extruded aluminium suspension uprights.
The Elan's most notable suspension feature was its front suspension rafts, designed to increase longitudinal wheel compliance and thereby improve ride quality. No equivalent is fitted to the Elise, whose wishbones are mounted directly to the chassis via interleaved elastomeric bushes These are over two-and-a-hall times stiffer than those in the Elan, So steer effects are reduced, Lotus's passive steer philosophy remains the same, though front and rear wheels both toe out in compression so as to reduce tyre slip angle if the outer wheels hit mid-corner bumps.
Steering is unassisted at 2.67 turns lock-to-lock for a turning circle of l0m (33ft); naturally, the steering gear is rack and pinion- To improve self-centring - which is reduced by the lower weight on the Elise's front wheels - castor has increased from 1 degree on the Elan to 4.25 degrees, rub radials (the distance between the centre of the tyre contact patch and where the steering axis rneets the ground) was ñ3mm on the Elan, a negative value chosen for optimum stability. In the Elise the offset l0.5mm positive, which promises improved steering feel. Anti-dive geometry is not used at the front, but both anti-dive and anti-squat are applied at the rear. The Elise is the first Lotus since versions of the Esprit to be supplied with Pirelli tyres as the factory fitment. 185/55R15 PZeros on 5.5xl5in rims at the front and 205/50916 PZeros on 7x16in rims at the rear: This is also the first Lotus in some time to use an "off-the-shelf" tyre rather than one developed specifically for the car. The differential in front and rear tyre widths goes some way to ensuring a table understeering handling balance, helped by -1.75 degrees static camber on the rear wheels and 45 per cent greater camber compensation in roll than at the front.
BRAKES
Cast iron has been the material of choice for brake rotors since the introduction of disc brakes during the '50s. The Elise heralds a new era, being the first production road car to use aluminium metal matrix composite (Al-MMC) discs instead, on all four wheels.
Aluminium is a better rotor material than cast iron for two reasons: its density is a third as much but its thermal conductivity three times greater These factors make it possible to construct a much lighter brake disc, with consequent savings in unsprung mass. The 282mm diameter discs on the Elise, which are the same front and rear, and weigh about half that of cast iron equivalents. Their large diameter and the low weight of the Gar also mean no brake servo is required, which saves more weight as well as improving pedal feel.
To provide the necessary abrasion resistance, aluminium discs have to be reinforced with a ceramic material, hence, Metal matrix composite. The Elise discs a 30 per cent by volume silicon carbide reinforcement, in particle form, and are manufactured by the US company Lanxide. The Al-MMC brake discs cannot withstand such high working temperatures as a cast iron disc, but their high thermal conductivity makes it much easier to move heat and dissipate it to the air via the wheel rim. A measure of just how effectively heat can be conducted away is maximum disc surface temperature measured at various test venues during the Eliseís development. Around Nurburgring it was a triffling 220deg C; at full pelt down the Stelvio pass the discs reached only 320deg C, With cast iron discs you might expect temperatures of the order of 700deg C. Really brutal treatment is needed to get the disc temperatures any higher: 30 maximum-effort. no pause stops from 60 to 20mph raise the temperature to around 450deg C.
Tony Shute, happily admits that the Elise brakes, which have achieved 1.29 deceleration in testing, are over-engineered, but, that will serve the car well when it takes to the track, hopefully in a one-make race series. He also anticipates remarkable disc life of up to 100,000 miles. Extraordinarily, the discs actually become thicker with use as a layer of the specially developed pad material, made by Allied Signal, is deposited on the disc surface.
AERODYNAMICS
The one area Lotus remains coy about is the Eliseís aerodynamics, CdA (drag coefficient times frontal area) is quoted as 0.59m2, but thatís all the hard data currently on offer. Guestimating a frontal area of 1.65 m2 , that translates into a Cd of 0.36 ñ nothing to write home about but then this is a short open car and hence a bad candidate for air-cheating. The pragmatic point is that drag is sufficiently low for the Elise to exceed its target top speed of 120mph. In standard form it cracks 122mph; with rear diffuser fitted (see below) it tops 126mph.
Lotus says nothing of front and rear lift, save that they have been carefully balanced for high-speed stability. But there are good reasons to suppose that, as road cars go, the Elise has low lift levels overall driving hot air from the nose-mounted radiator out through the bonnet contributes front downforce while the clean underside of the car bodes well for low rear lilt. too. Effectively the underside is flat all the way back to the engine bay, and this too can be cleaned up by fitting an optional rear diffuser panel which encloses the engine from below. This cuts drag as well as rear lift, increasing the top speed as noted above. Two small NACA ducts in the diffuser provide cooling air for the engine bay.
ENGINE AND TRANSMISSION
There's little to say here beyond, "Thank you, Rover". The long-stroke all-aluminium 1796cc in-line DOHC four - largest of the K-series powerplants - is lifted straight from the MGF, as are the five-sspeed transaxle and drive shafts. Rated outputs are 118bhp at 5500rpm and 122lb at 3000rpm. Early Elise literature talked of the engine management being remapped by Lotus, but in the event there was neither the time nor the budget for it.
Sadly, it looks as if there is no prospect of the 143bhp VVC variant ever gracing the Elise. Rover understandably intends to keep its clever variable valve timing system to itself.
AUTOCAR
3 JULY 1996