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Reproduced from F1 – McLaren’s Road Car An Autocar & Motor Book free with the 2 March 1994
issue © Haymarket Magazines Ltd 1994 _________________________________ SUSPENSION It had been decided that the F1 would be a refined road car. A
harsh, noisy ride was out of the question but so too was the compromised
wheel control that results from road car rubber-bushed suspension. Steve
Randle, the car’s dynamicist, was therefore charged with creating a
stable suspension which did not incur the NVH (noise, vibration, harshness)
penalty of a rose-jointed race set-up. Suspension design does not begin, though, when the chassis engineer
starts to sketch wishbones and spring/damper units. In racing circles the
first requirement is to arrange the car’s principal masses correctly, a
discipline which Gordon Murray imposed on the F1’s design from day one. Instant steering response needs a low polar
moment of inertia in yaw, which means a wheel at each corner and the main masses — engine, fuel, occupants —
close to the centre of
gravity. In most road cars this is compromised
by packaging limits, but Having achieved the right distribution in plan
view the same must be done in side elevation. Starting with undesirable
weight transfer under cornering and then correcting it with anti-roll bars is
a compromise Murray could not accept, so the distances between the suspension
roll centre and body mass centroid had to be the same front and rear. Since
the roll centres must be low to avoid jacking effects, this meant the engine
had to be as low as possible in the body. Dry sump engine lubrication also
reduced engine height by valuable inches. Only when these basics were correct could design of
the suspension itself begin. Adaptive damping and ride height control were
ruled out on weight grounds. Progressive rate springing was omitted too but
for different reasons. First, the only way to achieve a stepless increase in
spring rate is either by using complex pushrod linkages or costly
taper-ground springs. Second, too much progression can suddenly increase
weight transfer when a wheel hits a mid-corner bump, making handling
unpredictable. What small amount of wheel rate progression there is in the F1
is an inherent feature of the suspension linkages themselves, supplemented by
carefully optimised bump rubbers. Wheel travel front and rear was set at a generous
90mm (3.5in) in bump and 80mm (3.1in) in rebound and the target unladen
bounce frequencies at 86 cycles per minute (1.43Hz) at the front, 108cpm
(1.80Hz) at the rear. With the finalised car slightly over target weight, the
actual ride frequencies have fallen slightly to 84.5 and 105cpm. Although
these frequencies are higher than those of everyday
road cars, they are still
low for a sports
car of this performance potential. It was the wheel rates and wheel travel which determined the downforce
generated by the underbody. Too much downforce would simply have squashed the
car on to its bump stops, making the handling dangerously unpredictable at
high speeds. Describing the suspension as double wishbone
sells it ludicrously short. Its cleverness lies in how longitudinal wheel
compliance has been engineered in without loss of wheel control. It is this
compliance which allows the wheel to move backwards when it hits a bump,
endowing the F1 with its remarkable ride. Different methods of achieving the required
compliance are used front and rear in the F1 because the suspension pickup
points, the forces acting on the wheels and the required geometrical
constraints are different at either end of the car. At the front wheels the priority was to prevent castor
wind-off under braking, which compromises stability. Here, where braking and
cornering forces are reacted through the tyre contact patch, a solution was
adopted which McLaren calls Ground Plane Shear Centre. Subframes on either
side carry the wishbones on rigid plane bearings but are mounted to the body
by four compliant bushes, each 25 times stiffer radially than axially. These
are aligned at tangents to circles which have the middle of the tyre contact
patch as their centre. The castor control of this arrangement is
outstanding. Castor wind-off has been measured at 1.02 degrees per g of
braking deceleration, whereas the NSX, 928 S and XJ6 measured 2.91, 3.60 and
4.30 deg/g. Toe change under braking and camber change under lateral force
are also very small. At the rear, where cornering and braking forces
are again reacted through the contact patch but tractive forces through the
wheel hub, a different configuration is used called Inclined Shear Axis.
Complicated by the lower wishbone mounting on the gearbox, which is itself
compliantly attached to the body, the suspension and engine mounts were
designed as an integrated system. Wheel control is again exceptional, the priority this time being
to control toe changes under braking and traction. Measured values are
0.04 deg/g toe-in under braking, 0.08 deg/g toe-out under traction, both of
which are negligible. Equivalent figures for the 928 S were 0.30 and 0.35 deg/g, both toe-in. Otherwise the steering and suspension broadly
conforms with road car practice. The castor angle and king pin inclination,
for example, are both relatively low at 46 and 8 degrees respectively.
However, the ground level offset (the distance between the centre-line of the
tyre and where the steering axis meets the ground) is 25mm compared with the
sub-10mm values typical today. Aside from longitudinal wheel compliance, one of
the critical determinants of a car’s ride quality and its ability to
maintain consistent tyre contact on bumpy roads is the ratio of its sprung to
unsprung masses. In a light car it is
therefore essential to have light suspension — easier said than done in a vehicle which needs tyres and
a braking system commensurate with a top speed of over 230mph. Everywhere that unsprung weight could be saved,
it was. The tyres — 235/45ZR17
front and 315/45ZR17 rear, developed specially for the car by Goodyear and
Michelin — were kept as small as possible consistent with the tractive.
braking and cornering grip demanded of them, and then subject to strict
weight targets. Likewise the 17´9in and
l7´11.5in cast magnesium
wheels, which are finished in a tough protective paint. Items such as the steering knuckles are specially
manufactured because readily available alternatives were simply not light
enough. The top wishbone/bell crank, which converts vertical motion of the
front wheels into horizontal motion at the transversely disposed
spring/damper units, is cast in aluminium alloy, while the lower front
wishbone and both rear wishbones are (like the front subframe) machined from
solid aluminium alloy
on CNC machines. Although it may sound like an indulgence, manufacturing the
wishbones this way was cheaper than forging them. Despite this concerted effort to keep down the
unsprung mass the final figures are, inevitably, still relatively high for an
1100kg car: 92lb (42kg) per corner at the front and 121lb (55kg) per corner
at the rear, equivalent to sprung to unsprung mass ratios of 5.5:1 and 5.8:1. The
equivalent ratios for a representative hatchback (Peugeot 306 1.8 XT) are
9.8:1 and 7.3:1. Brake system development for the F1 was entrusted
to the Italian company Brembo, well known for its motor racing expertise. But
of course the design brief from Gordon Murray was explicit. In order to
maximise brake pedal feel, he insisted that the brakes be unservoed. This
ruled out anti-lock, which in any case would have added unwelcome weight and
complication. To achieve acceptable pedal effort demanded long
moment arms at the wheels, so the ventilated discs are of large diameter
— 332mm at the front and 305mm at the rear. Cross-drilling of the
rotors provides improved pedal feel and helps clean the pad faces. Even with the large discs and carefully contrived
brake cooling, though, developing a friction material capable of hauling the
car down from 200mph-plus speeds
without fade, while still providing
sufficient bite when cold, proved a considerable design challenge. Front and rear brake
calipers are all four-pot, opposed piston types as favoured in racing
circles, not the floating calipers more typically used on modern road cars.
Naturally, they are constructed of aluminium alloy to save weight. Because of
their racing origins the rear calipers have no handbrake facility, so a
mechanically actuated, fist-type caliper is added. Gordon Murray’s
insistence on maximum brake feel dictated the use of calipers machined from
solid rather than bolted together from two halves. Again this is standard
practice in the senior race formulae, and for precisely the same reason: it
maximises caliper stiffness and so minimises lost motion. Pedal travel is
only a little over an inch. Although the
F1’s pop-up rear spoiler was not intended to be an air brake — it
is there to prevent forward migration of the aerodynamic centre of pressure
when the car pitches under braking, increasing braking stability and allowing
greater braking force to be applied at the back wheels — it actually
raises the car’s drag coefficient from 0.32 to 0.39. Activation of the
spoiler is controlled by brake line pressure, with a threshold speed of 40mph. When the spoiler is
raised, air pressure is developed at its base which is exploited to force
cooling air to the rear brakes. Ducts at either end of the spoiler, which are
uncovered when it deploys, convey the airflow down to the rear discs. ENGINEAlthough it is the numbing 627bhp peak power of
the F1’s engine (codenamed S70/2 within BMW) which garners headlines,
in many ways that represents the least of the challenges which faced the
design team. The fact that the 550bhp originally demanded by It was in other respects that BMW’s
considerable experience in designing road and race engines was to prove
invaluable. Firstly, It is natural to regard any powerplant capable of
delivering 627bhp and 500lb ft of torque (about 50 per cent more than a
modern Formula One engine, incidentally) as a thoroughbred race unit, but
that’s not so. It is instructive to compare the S70/2 with one of BMW
Motorsport’s less exotic creations, the six-cylinder engine fitted to
the M3. In most key areas — specific
output, specific torque, peak power revs, bore/stroke ratio and compression
ratio — the two
units are matched to within 5 per cent Only in its length and weight does the
F1 unit set itself significantly apart. This is what you would expect of an engine which,
in addition to being road-tractable, must be moderately stressed for a long
service life and practicable maintenance schedules. In the course of its
development the F1 engine was put through the same punishing 500-hoor bench
test as all BMW road-going powerplants, and its nominal service interval is
5000 miles. Emissions performance has not been compromised
either. As in the M3 engine, secondary air injection is used to reduce
pollutant levels during the critical warm-up phase. Until the four catalytic
converters reach light-off —
relatively quickly since they are closer-coupled in the F1 than in the M3 — air is injected into the
exhaust manifold to burn off excess hydrocarbons produced by cold start
over-fuelling. It is a
reflection of its short development time that the F1 engine uses, in the
main, only tried and trusted technology from BMW’s mainstream units.
The variable valve timing, for example, is closely based on the VANOS system
used in the M3. This simple, hydraulically-actuated phasing mechanism retards
the inlet cam relative to the exhaust cam at low revs, reducing valve overlap
and ensuring good idle behaviour and low-speed torque. Higher up the rev
range, under the control of the engine management computer, the valve overlap
is increased by 42 degrees (25 degrees in the M3) to improve engine breathing
and maximise power output. Despite their common valvetrain technology,
though, the F1 and M3 engines are tuned for significantly different torque
characteristics. Whereas the M3’s torque curve has its maximum at
3600rpm and is virtually a plateau from 3500rpm to almost 6000rpm, the F1’s displays
instead the inexorable climb of a traditional sporting engine, peaking at
5600rpm, only 1600rpmBelow peak power output. The F1 unit delivers a beefy
398lb ft at 1500rpm even so— 69 per cent greater than the M3’s
peak output and quite sufficient to ensure vivid performance in a car
weighing around 1200kg including driver. In fact, ensuring that the F1 was not
over-willing on small throttle openings posed one of the principal
development difficulties. Making the engine fuss-free in traffic was not
enough; it also had to be sufficiently controllable not to bury the car under
the lorry in front at the merest twitch of the loud pedal. Careful design of
the throttle linkage and TAG’s expertise in engine management were
relied upon to achieve this. Although considerable attention was paid to the
induction system (length, diameter and surface finish of the inlet tracts,
and the volume of the plenum chamber) variable geometry was resisted by BMW
as an unnecessary complication. A familiar problem in high-speed racing engines
is mixture preparation. At the high inlet air speeds encountered at high revs
there is insufficient time for the fuel to atomise fully if the injector is
placed close to the inlet valve, as it is normally is in road engines with
multi-point injection. Although the F1 engine runs at nothing like the
13,000rpm-plus of state-of-the-art racing engines like the Ford HB,
BMW’s engineers found that mixture preparation from a single injector
was not ideal across the whole rev band, so two Lucas injectors are used per
cylinder. The first, positioned close to the inlet valve, operates at low
engine speeds while the second, positioned further up the inlet tract, takes
over at high revs. A soft transition between the two, controlled by the
engine management computer, covers up the switch-over. Mixture preparation is
further improved in the lower injector by air assistance. A narrow jet of air,
drawn into the inlet tract by the partial vacuum created on the induction
stroke, ‘shears’ the fuel spray and breaks it up into smaller
droplets. As you would anticipate in an engine of this
sophistication, the closed-loop fuel injection is sequential. Fully mapped,
contactless ignition is likewise no less than you would expect, each cylinder
having its own miniature ignition coil, just as in the M5. Engine load is
sensed by hot wire. Combustion conditions are sufficiently remote from knock
limits that no knock sensor is necessary. The materials usage in this engine, like the core
technology, is also relatively conservative, drawing again on BMW’s
production engines. No titanium valves or conrods here. Both the head and
block are cast in aluminium, with a Nicasil coating to the cylinder bores
providing the necessary wear resistance. The lightweight pistons are of
forged aluminium, the con rods and the crank of forged and twisted steel, and
the exhaust valves are sodium cooled. Significantly, most of these features
can be found in the M5 powerplant. One notable exception is the exhaust system, a
bulky and potentially heavy item constructed, from the block to the silencer,
of Inconel, a particularly durable, heat resistant grade of stainless steel
which allows the use of a thinner pipe gauge (0.8mm). Further weight saving
is achieved by making the large 65-litre silencer of titanium and having it
double up as a crush member for rear impacts. A race engine feature which A
second race car feature, found on very few road cars, is dry sump lubrication. Although
more complex and costly than a conventional wet sump, it shaves vital inches
from the height of the oil pan and so allows the engine to be mounted lower. |
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