FWD to RWD Overview
front wheel drive ZT sports saloon and estate cars were intended, from the
outset, to be part of a suite of products which would extend to include extreme
sports performance derivatives. The fundamental strengths of the 75 platform –
from which the front wheel drive ZTs are derived – are such that the task of
creating executive sports saloons was relatively straight-forward; however it
would clearly be unacceptable to transmit very high power through the front
wheels only. The difficult decision was taken to use the basic body structure
of ZT, but to completely re-engineer the car under the skin to create a
rear-wheel drive performance platform. The new RWD platform – designed from the
outset to meet the demanding requirements of MG Sport and Racing’s forthcoming
ZT 385 – also plays host to the lower powered ZT 260, conceived as more
affordable, but no less entertaining practical British muscle car.
The FWD ZT’s KV6
engine was tuned to create a classic sports power curve; it’s a gem of an engine
that revs sweetly and enthusiastically straight up to it’s red line in a
typically European manner. To set aside the new family of RWD ZT product, the
team deliberately chose to start with an engine with quite different power and
torque characteristics, matching this to an accomplished sports chassis to
create a quite unique product with a very different driving experience indeed to
their FWD cousins.
project involving a completely different driveline transplant would have been
very labour intensive. However, MG Rover Group had the advantage that the
platform was one of the first to have been fully digitised during its design
process. This meant that the installation of alternative power units and
transmissions could be achieved rapidly in the virtual environment. In
addition, the decision to retain key elements of the base car’s underfloor
geometry, trim and electronics meant that the new platform could be manufactured
on many of the same state of the art facilities that produce Rover 75 and ZT,
vehicles renowned for their quality and reliability.
A number of
potential powerunits were considered before the final decision was taken to
employ the proven, robust Ford V8 used to power the infamous Mustang.
Delivering effortless performance, the 4.6-litre powerplant is well proven with
over a million units from this engine family produced each year. MG Rover
worked alongside Roush, an established American motorsport and engineering
company with a long, successful history of Ford powertrain integration projects
(indeed they’re one of the Dream Team businesses working with Ford to deliver
the GT40 programme). Roush used their in-depth knowledge of the engine to
develop a calibration which delivered the sharp throttle response demanded by
European customers, whilst also achieving the latest European emissions
legislation, all within the constraints of the tightly packaged ZT installation.
transmission has been adapted from its Mustang application for installation into
the ZT 260, whilst the two-piece prop shaft, with constant velocity joints,
(similar to that of the BMW M5) is supplied by GKN in Germany. Dana’s proven
Hydratrak® fluid coupling limited slip differential (similar to the unit fitted
to Holden’s HSV) was selected for it’s progressive characteristics and wear-free
torque transmission system. Thus the new drivetrain was built up from
extremely well proven, ‘bullet-proof’ components.
The vehicle has
been tested in a variety of extreme conditions. Prototypes have been
tested through the freezing depths of Swedish and North American winters;
they’ve been baked in the searing heat of the central Australian desert.
ZT260s have met the demands of our most exacting drivers on roads as varied as
the infamous Nürburgring road circuit to Europe’s highest speed test bowl at
Nardo in Southern Italy.
Behind all the
work lay the traditional MG credo – ‘Safety Fast’. The X12 team believes
that in the ZT 260 and ZT-T 260 it has produced a range of unique appeal and
character: responsive, agile performance machines that are fun to drive, yet
effortless mile-eaters – practical British muscle cars.
At the heart of
the new ZT 260 is the same robust 4.6-litre V8 engine that serves the American
Ford Mustang to such good effect. From the top down, it has a single
overhead camshaft on each bank, which operates two large diameter valves per
cylinder. The engine features sequential multi-point fuel injection,
aluminium cylinder heads and a thinwall cast iron block. The main bearings
are cross-bolted, giving an extremely stiff and solid structure (the reserves of
strength in this unit are such that it can be supercharged without requiring
internal modification and the same block can even be safely used for a 6.7-litre
version). To suit the ZT 260 installation, the normal pressed steel sump
is replaced by a special cast-alloy item, which suppresses noise transmission
and adds further stiffness to the bottom end.
mounting system is unique, employing twin mass-carrying Hydramounts and a pair
of rear mounts to provide a virtual-centre for excellent gearbox control.
exhaust and intake systems have been developed to comply with stringent European
drive-by noise legislation without sacrificing the engine’s torque or power.
In order to meet European emission standards, a bespoke MG catalyst system was
created, using a single high-specification catalyst canister for each cylinder
bank. These are mounted close to their respective exhaust manifolds for fast
light up and efficiency. The exhaust system is a low-back-pressure design
and is manufactured from heavy grade stainless steel throughout. The
design is twinned for almost the complete length of the car; balancing is
achieved by employing a short single pipe over the rear axle before splitting
again to feed large silencer cans either side of the rear floor.
The result is a
260Ps (191kW) power output and perhaps more importantly, a thumping maximum
torque figure of 410Nm (302 lb ft) for real ‘push-in-the-back’ performance in
requires a robust transmission. The proven Tremec TR3650 five-speed manual
gearbox is based on the unit used in the Mustang GT, but with unique ratios
specifically chosen to suit the ZT 260 and European driving conditions.
Fifth gear gives a 27% overdrive for effortless cruising at 29.7mph (47.7kph)
per 1000rpm engine speed and with so much torque available, fourth gear in the
260 is actually higher than the ZT 190’s fifth. To match the ZT’s
gearchange pattern and ergonomics, MG developed a unique directly mounted remote
specifically for the ZT 260, MG has developed its own concentric slave cylinder
hydraulic clutch actuation system and a unique clutch plate. As part of
the process of integrating the new drivetrain with the inherently refined ZT
platform, special attention was paid to NVH control and in particular to the
task of balancing good cruising refinement with a rewardingly sporting note when
driven hard. There is an acoustic shroud around the bell housing and tuned
sound absorption treatment for the new transmission tunnel. The two-piece
propshaft, with a centre steady bearing and constant velocity joints throughout,
is a high performance type with stiff, large diameter tubular construction,
similar to that used on the BMW M5.
To get the power
down to the road efficiently the final drive unit incorporates Dana’s Hydratrak®
limited slip differential. This uses a cartridge type fluid coupling that
apportions torque according to speed differences between the two rear wheels.
If one wheel begins to lose traction and spin, the coupling progressively and
smoothly increases torque transfer to the slower-rotating wheel. The
result is enhanced traction, stability and cornering feel.
generation 40mm tubular driveshafts are used to transmit drive to the rear
wheels, offering an exceptional combination of stiffness and lightweight.
260 Body Engineering
There are few
production precedents for the kind of driveline transplant involved in the
creation of the MG ZT 260 models. Even Triumph’s evolution from 1300/1500
to Toledo/Dolomite in the 1970s ‘only’ involved a change from front to rear
drive, as the engine was already longitudinally mounted. For ZT 260, the
move also included the switch from a transverse to an in-line powerunit
installation, an altogether more complex and demanding change.
In order to
package the large capacity V8 engine and gearbox, extra clearance was needed in
the central bulkhead and tunnel areas. New pressings and sub assemblies
replace the affected sections of the FWD bodyshell, including a deeper centre
tunnel extending to the rear of the front seat mounts.
Whilst much of
the front subframe structure is carried over from the original hydro-formed
component fitted to the FWD ZT, substantial modifications are required to
accommodate the new powertrain, it’s mounting system and the new rack location.
In altering the
bulkhead configuration, some of the space occupied by the original ZT heating
and ventilating system had to be given up. A new, more compact system, has
been developed around a unique, electronically-controlled water-valve heat
modulation system. The ergonomically sound and easy to use temperature and
distribution control pack of the FWD cars was deliberately retained to drive the
At the rear of
the car, some bodyshell changes were made to accept the new rear suspension and
final drive subframe installation. To optimise weight distribution the
main vehicle battery is now located in the boot well below the rear floor, which
brings the added benefit of lower battery operating temperatures.
The FWD ZT’s is
highly acclaimed for it’s excellent body control, agility and responsiveness.
The X12 team faced the challenge of creating a rear-wheel drive chassis that
builds on these core values. At the front, the MacPherson strut layout of
the FWD ZT is retained, though almost all components employed in the ZT 260
application are unique to this car. The existing forged alloy lower arm is
modified and mounted with an all new upright. The car is supported on
Bilstein monotube struts (which offer excellent camber stiffness and a large
piston area designed to guarantee fine tuning resolution). Unique linear
rate springs from competition specialists Eibach complete the package.
the longitudinal engine configuration, both the steering rack and the uprated
anti-roll bar have been re-engineered to sit ahead of the road wheel centreline
(rather than behind it). The steering system boasts a unique steering
ratio, whilst the rack itself is driven by a higher capacity power steering
pump. The ZT 260’s steering feel and feedback match the high standards of
the original design.
Some of the most
fundamental work in the X12 project was that involved in creating the new rear
axle installation, which employs a unique 6-mount sub-frame to accommodate the
brand new multi-link rear suspension and final drive unit. New Eibach
compound rate rear coil springs react against upper mountings within the
subframe, rather than against the standard ZT spring mountings in the underbody.
The lower spring pans are integral within the cast alloy transverse arms that
locate the rear ends of the hub carriers; location of the front ends is via a
light-alloy trailing arm, with upper and lower steel transverse radius arms.
monotube dampers are outrigged behind the hub carriers to maximise damper
control through the extra leverage provided. The rear anti-roll bar is
located behind the subframe and mounts to the rear transverse arms. This
intricate rear suspension provides adjustability of camber and toe setting and
was developed using advanced modelling techniques to minimise bump steer
effects. It plays a key role in delivering the enjoyable handling, ride
and refinement of the ZT 260.
mounting strategy is employed to locate the differential within the rear frame.
In conjunction with tuned external damping, the system offers excellent control
of final drive torque reaction within the frame.
is available in abundance from the uprated ZT 260 braking system. It has
‘H’-split hydraulics and the usual advanced Bosch 5.7 four-channel anti-lock
braking, with electronic brake force distribution.
The front brakes
are similar to the generously sized (325mm diameter) and proven ventilated discs
fitted as standard to the ZT 190, although the sliding pin front callipers are
now mounted behind the axle centre-line. To match the ZT260’s performance
potential and revised mass characteristics, the Chassis team worked with brakes
specialist AP-Racing to develop new 332mm ventilated discs with an opposed
piston alloy calliper design for the rear.
diameter, 7.5J rim alloy wheels and 225/45 ZR18 tyres are specified for all ZT
260 models, with the multi-spoke ‘Straights’ design on the 260 and an attractive
new ‘Apex’ 10-blade design for the highly-specified 260 SE. Original
equipment tyres are Contisport, chosen through extensive testing as the optimum
all-round tyre for stability with responsiveness.
Making the ZT 260
The kind of
lateral thinking that led to such an extraordinary low-volume product concept as
the ZT 260 was also applied to design the unusual production process on which it
is built. Perhaps the conventional approach to such a project would be to
ship the stock car to a specialist engineering company for labour-intensive
conversion. But that would have increased the selling price very
substantially, whilst placing at risk MG’s required standards of quality
control, value and durability and was hence clearly unacceptable.
After a careful
feasibility study and build trials, an ingenious strategy combining regular
production processes with dedicated facilities designed to accomplish unique RWD
assembly operations was developed. The bodyshell is built in the
conventional way in Longbridge’s highly automated West Works plant. The
only major variation is the new rear floor pressing required to accommodate the
rear differential. A special off-line facility has been installed to
complete the transformation into a RWD bodyshell.
With this work
completed, the shell re-joins the normal body conveyor to go through extensive
main line corrosion protection and paint processes to ensure it can benefit from
MG’s comprehensive cosmetic and structural corrosion warranties.
A dedicated ZT
260 parts reception and preparation area has been set up at Longbridge to handle
reception and ‘dress’ of the unique powerunit. Both front and rear
subframes are then sub-assembled with their relevant suspension, brakes and
powertrain components before being conveyed up to the assembly line. Their
arrival is synchronised with that of the corresponding ZT 260 bodyshell to
ensure no disruption the normal high volume manufacturing flow.
is completed on the main ZT assembly track, where the ZT 260 is subjected to all
the regular stringent quality control procedures of its FWD cousins.