Z4 ///M Pages
In 1972, German automaker BMW decided to enhance its burgeoning motor sports position by creating its own in-house racing group known as BMW Motorsport. Soon, this group began designing; engineering and developing hot BMW's for the street, starting with a limited run of enhanced 5 Series cars between 1974 and 1980. It became a full-fledged development company in 1983 and 10 years later shortened its name to BMW M. When a new Motorsport group took over BMW racing activities in 1995, the M group was able to focus all its energies on M Cars, BMW Individual (custom-ordered cars) and BMW Driver Training.
Through the years, BMW's high-performance M cars have become legendary among enthusiasts, and their availability has spread across more model lines. Today's U.S.-market BMW M stable includes the long-running favorite M3, the awesome 500-hp V-10-powered M5 sedan and M6 Coupe, and the Z4 M two-seat roadster and coupe.
With the Z4 M Roadster and Z4 M Coupe, BMW brings the Z4’s advanced sports car technology and BMW M’s incredible 330-horsepower 6-cylinder engine together for the ultimate two-place performance vehicle. A wide range of new BMW and BMW M developments in suspension, steering, brakes, stability systems, safety features and luxury and convenience features are also a part of the Z4 M Roadster and Coupe’s genetic makeup.
The BMW M S54 6-cylinder engine: Awesomeness....
At the heart of the Z4 M Roadster and Coupe is BMW M's 3.2 liter 6-cylinder inline engine made famous in the M3 Sedan And Convertible. This award-winning engine has been characterized as “ferociously powerful“ by Car and Driver (May ‘03) and “an engine that likes to rev“ (Road & Track, December ‘03). The British magazine Engine Technology International has named it Best Engine of the Year for 3.0 to 4.0 liters for five years in a row. Now this remarkable unit gets to power lighter, even more agile BMWs than the M3s.
Statistically speaking, the results are dramatic: delivering 330 horsepower at a high-revving 7900 rpm, the internally-designated S54 engine propels the 3200-pound Z4 M Roadster to 60 mph in just 4.8 sec. Maximum speed is electronically limited to 155 mph; speculation about what the top speed might be without the limiter will intrigue enthusiasts, but BMW makes no claim in this regard.
In its broad concept, this engine shares its inline format with other BMW 6-cylinder engines. Virtually all its engineering details, however, are unique and targeted to the very highest level of performance. Unlike other Z4 engines, which employ lighter materials for their block and cylinder head, the S54 combines a cast-iron block with its aluminum head to achieve the engine's 3.2-liter displacement without lengthening the block. The block accommodates this engine's bore and stroke of 87.0 x 91.0 mm, both larger than the dimensions of other BMW 3-liter engines. However, playing larger roles than increased displacement in the 75-hp increase over the most powerful regular-production 6-cylinder engine is the special induction, combustion and exhaust engineering, together with its execution as a high-rpm engine. The starring role here is played by a cylinder head that could be termed “exotic.” Its key features include:
Double VANOS (Variable Nockenwellensteuerung) steplessly variable valve timing on the intake and exhaust camshafts. Though this system appears in all other current BMW engines, in the M unit it varies timing over a wider range and contributes in a major way to the engine's ultra-high power output.
VANOS pressure pump. In regular-production BMW engines the main oil pump supplies the pressure to operate VANOS; here the VANOS system has its own radial-piston hydraulic pump. Integral to the exhaust camshafts VANOS mechanism, the pump produces up to 120 bar (1740 lb./sq in.) of pressure to vary valve timing more quickly at the very high rpm this engine reaches. BMW M refers to its VANOS system as High-Pressure Double VANOS; it is also used in the M5 and M6’s 500-hp S85 V10 engine.
Unique valve mechanism. All BMW engines have an efficient valvetrain. Yet for an engine with the S54’s rpm potential, BMW M engineers needed less reciprocating mass than in the regular-production engines.
To achieve this, they created a unique actuating mechanism, using extra-light finger-type rocker arms. Pivoting on their own shafts (one on the intake side, one on the exhaust), these small arms provide the actuating surface between camshaft and valve. As the entire arm does not move the distance of valve lift, its effective reciprocating mass is less than its actual mass. All told, the valvetrains effective mass is 30% less than in the BMW 6-cylinder engines that have bucket-type hydraulic lifters; in turn, this allows lighter valve springs, which further reduce inertia. The system also has less friction.
As there is no hydraulic maintenance of valve clearance but it does have to be inspected and adjusted periodically. If adjustment is required, normally at Inspection I & II, it is done with shims (tiny metal discs of various thickness) without removing the camshafts. To see what is involved, see the S54 Valve Adjustment Page.
Whereas the “regular” 6-cylinder engines have a simplex (single) camshaft drive chain or chains, the S54 has a full duplex (double) chain driving its camshafts. As usual with BMW engines, the chain is hydraulically tensioned and needs no periodic adjustment or replacement.
Extra-high compression ratio. At 11.5:1, the M engine has the next-highest ratio in current BMW production, topped only by the M5‘s and M6's 12.0:1.
Machined surfaces. More “engineering finery” again: combustion chambers and intake ports are completely machined, for smoothness that facilitates airflow. The exhaust ports are partially machined. For durability, the valve seats are of especially hard steel. A 3-layer stainless-steel head gasket ensures effective sealing of the head to the block.
Head casting and sealing. Extreme strength in the cylinder head is achieved by making it a single aluminum casting. Though elaborate, this construction also saves a significant 29 lb. As this weight reduction is at the top of the engine, it helps lower the cars center of gravity.
The S54 won the International Engine of the Year award overall for 2001, and also claimed the "Best New Engine" that year as well as won the 3 - 4 liter category for six straight years in a row from 2001 through 2006. The S54 was also on the 'Ward's 10 Best Engines' list for 2001 through 2004
The DriveTrain
Z4 M Roadster and Coupe drivetrain: Getting the S54 power to the road
Like every M Car to date, the Z4 M Roadster and Coupe transmit power to the road via classic rear-wheel drive and include some premium and fascinating engineering into its drivetrain.
A transmission for sporting enthusiasts. The Z4 M Roadster and Coupe come with a 6-speed manual transmission. There is no automatic transmission offered. It is a ZF Type H gearbox, which features evolutionary improvements:
• Even more effective synchronization of shifts
• Lighter and more pleasing feel as shift lever is moved from gear to gear
• Firmer engagement of gear once selected
• Shorter “throws“ from neutral to each gear
• Use of lifetime transmission oil, which never needs to be changed
• “Clean bearings“ design for main transmission bearings; protects bearings from contamination, ensures adequate lubrication at all times and therefore enhances durability.
- As in other M models, the shift pattern on the gearshift knob is illuminated attractively at night.
Super-sized differential unit. A specific subframe allows equipping the Z4 M Roadster and Coupe with a heavy-duty differential with gears made of a special high-strength steel alloy to achieve quietness and durability. As at the front spoiler, rear diffusor and elsewhere along the underbody, airflow under the vehicle is carefully managed and targeted; here, a ribbed differential case and NACA air intakes for the differential unit help keep the oil cool.
M Variable Differential Lock. Together with the German division of GKN Viscodrive, BMW M engineers developed a special mechanical limited-slip that also appears in the M5 and M6. This M Variable Differential Lock specifically addresses low- and split-traction situations in a way that reinforces sporty handling while contributing to remarkable slippery-road abilities.
Any time a speed difference develops between the two rear (driven) wheels, a shear pump, driven solely by this difference, develops pressure in the silicon viscous fluid in which the lock operates. In turn, this pressure is directed to a multi-disc clutch that transfers driving torque to the wheel with the better road grip (“select high”). The greater the speed difference between the two wheels, the more aggressively the clutch engages. As soon as the difference between the two wheels’ speeds begins to diminish, the clutch starts to ease off.
This mechanism achieves its sophisticated action by entirely natural means. There is no external pump, no external source of lubrication or operating fluid. The very motion to be controlled - differences in speed between left and right wheels - generates its locking action. Viscous fluid is so-called because it develops internal force (via an increase in viscosity) whenever it is sheared; this is why the relatively small difference between one wheel speed and the other can generate the necessary action.
Dynamic Stability Control. This electronic traction and stability system, standard on all current BMWs, complements the M Variable Differential Lock.
DSC optimizes traction by electronic means, sensing wheel-speed differences and reducing engine torque and/or applying individual rear-wheel brakes. The crucial difference to the Z4 M Roadster driver between the M Variable Differential Lock and the DSC traction function is that the former in no way impedes power delivery, and is hence suitable for performance driving.
Yet in fact, even DSC’s traction function is calibrated to M-specific parameters. In combination with the fast-reacting engine, performance-oriented gearing and M Variable Differential Lock, its logic achieves the desired traction optimization in an M-compatible way…in other words, without undue interference with the differential lock’s ability to get power to the road.
The DSC stability-enhancing function is essentially unrelated to traction. Sensing differences in wheel speed in a critical cornering or avoidance maneuver, DSC detects any deviation from the normal cornering path (abnormal understeer or oversteer) and gently applies individual wheel brakes to help the driver keep the vehicle on the intended path.
As in other recently introduced BMW models, the Z4 M Roadster’s and Coupe's DSC system incorprates:
• Brake Drying. Acting on input from the windshield wipers’ rain sensor, the brake pads are periodically brought up to the rotors - just enough to eliminate any film of water between the pads and rotors, but not enough to cause any brake applications. This improves brake readiness in wet driving conditions.
• Start-off Assistant. Keeps the vehicle from rolling backward when stopped facing uphill. For 2 seconds after the brake pedal is released, the driver can then move off from rest without the car rolling back.
• Modulated ABS function. Thanks to new “analogized” control of the DSC brake valves, the anti-lockup function (ABS) is smoother than before. Instead of simply being fully applied and released, the application and release of these valves is now modulated.
Some DSC braking functions found in regular-production BMW models, such as Comfort Stop, are not included in the Z4 M Roadster because of its ultra-sporty driving character.
The Chassis
The Z4 M Roadster and Coupe chassis
Starting with the Z4’s already excellent chassis engineering - Motor Trend commented in its May ‘05 issue that the Z4‘s “sport suspension and solid chassis make this car a thrill to drive on canyon roads“ - BMW M has endowed the Z4 M Roadster and Coupe with typical M modifications that further enhance handling and tailor the cars' road capabilities to M’s higher performance level.
Front suspension. While retaining the Z4’s basic concept - strut-type with forged-aluminum lower arms - the M engineers created an essentially new version with:
• A wider front track (by 0.5 in.)
• Newly configured lower arms, still of forged aluminum; more complex shape with open sections for lightness, similar to those of M3s
• New steering knuckles, bolted to the struts (saves weight, sharpens steering response)
Such changes, subtle yet deep-reaching, are - as good as the basic BMW model is - key to achieving the “beyond exceptional” reflexes of an M model.
Rear suspension. Though modifications to the Z4’s Central Link (multi-link) rear suspension are less extensive than at the front, they are significant. Among others, the following major M elements are shared with M3s:
• Rear subframe - larger and stronger; accommodates differential with M Variable Differential Lock
• Wheel bearings
• Central (longitudinal) Links
• Arrangement of anti-roll bar.
As at the front, these modifications sharpen handling capabilities.
Ride height and suspension calibration. Like the Z4’s sport suspension, that of the Z4 M Roadster and Coupe are set lower compared to the standard Z4’s setup. Unlike the Z4, however, the M versions get an all-around new calibration of springs, shock absorbers, anti-roll bars and bushings to BMW M parameters; this includes specific auxiliary springs at the front and rear.
A new steering system. Alone among BMWs, Z4s have electrically assisted power steering. With this system’s vehicle-speed-sensitive power assist, it is notable for its combination of reduced driver effort in parking and at low vehicle speeds, and good road feel at higher driving speeds.
BMW M has somewhat different priorities: a greater emphasis on absolutely accurate road feedback and ultra-spontaneous response to the driver’s commands at the steering wheel. In this context, higher parking and low-speed efforts are acceptable. Thus M gave the Roadster and Coupe a hydraulically-assisted rack and pinion with a power-steering pump calibrated with assist characteristics optimized for the Z4 M Roadster and Coupe. This system reduces parking efforts somewhat, but not to the degree that vehicle-speed-sensitive assist does.
BMW M’s most advanced brake system. The Z4 M Roadster and Coupe come standard with BMW M’s most elaborate brake concept: compound, cross-drilled 4-wheel ventilated discs.
The brake rotors are 2-piece, with an aluminum “hat” and a cast-iron outer portion that is the actual friction surface. The hat and outer portion are connected by steel pins on which the aluminum and cast-iron portions, because of their differing expansion rates, can move relative to each other. This is an elaborate, costly construction that cuts unsprung weight and eliminates rotor deformation under hard-braking, high-heat conditions. In practical terms, this means virtually no tendency of the brakes to vibrate when hot, and also virtually no likelihood of the rotors cracking under the extreme heat conditions that might be encountered in driving on a race track.
At the front, the discs are of 345-mm diameter x 28-mm thickness (13.7 x 1.10 in.); at the rear, the discs measure 328 x 20 mm (12.9 x 0.79 in.). Cross-drilling, visible through the wheels, enhances heat dissipation beyond that provided by the rotors’ internal ventilation, further increasing the brakes’ resistance to fade, improving braking effectiveness in wet weather and reducing unsprung weight. Given the Roadster’s and Coupe's light weight, this system achieves phenomenal braking.
Wheels and tires: ultimate style and grip. M Double Spoke wheels, with a very open configuration to help get plenty of cooling air to the brakes, complement the Z4 M’s body design with a boldly sporty, yet elegant look. Sized 18 x 8.0 front and 18 x 9.0 rear, they are connected to the road by suitably wide, low-profile, high-speed-rated tires sized 225/45ZR-18 front and 255/40ZR-18 rear .
As with other BMW M models, the Roadster's and Coupe’s tires are not run-flats; run-flat tires meeting M requirements are not yet available. Also, the M exhaust system’s bulk precludes a spare tire. Thus like other BMW M models, the Coupe comes standard with the M Mobility System for use in case of a flat tire. A Tire Pressure Monitor alerts the driver to a significant loss of tire pressure.
M Mobility consists of a container of rapid sealant, a small compressor, and a hose to connect the compressor to the damaged tire. All this is carried in a container in the trunk; the system can seal punctures up to approximately 1/4 inch across. Omitting the spare tire saves about 45 lb. of weight.
The Roadster’s and Coupe's wheels incorporate the so-called Extended Hump rim shape, which helps keep the tire on the rim in case of deflation.
As in other M models, instrumentation is special, both optically and functionally. The instrument cluster reflect the M’s performance capabilities with special features:
• 9000-rpm tachometer
• Variable tachometer warning segment that lowers redline according to engine temperature: lowest with a cold engine, then increasing in increments to an 8000-rpm limit with a fully warmed-up engine. Although this feature appears in some regular-production BMW models, the BMW M graphics (with an arc of orange and red LEDs) are different.
• Oil-temperature gauge at bottom of tachometer dial
• 180-mph/300 km/h speedometer
• Distinctive graphics throughout, including red-on-gray dial pointers.
Through the years, BMW's high-performance M cars have become legendary among enthusiasts, and their availability has spread across more model lines. Today's U.S.-market BMW M stable includes the long-running favorite M3, the awesome 500-hp V-10-powered M5 sedan and M6 Coupe, and the Z4 M two-seat roadster and coupe.
With the Z4 M Roadster and Z4 M Coupe, BMW brings the Z4’s advanced sports car technology and BMW M’s incredible 330-horsepower 6-cylinder engine together for the ultimate two-place performance vehicle. A wide range of new BMW and BMW M developments in suspension, steering, brakes, stability systems, safety features and luxury and convenience features are also a part of the Z4 M Roadster and Coupe’s genetic makeup.
The BMW M S54 6-cylinder engine: Awesomeness....
At the heart of the Z4 M Roadster and Coupe is BMW M's 3.2 liter 6-cylinder inline engine made famous in the M3 Sedan And Convertible. This award-winning engine has been characterized as “ferociously powerful“ by Car and Driver (May ‘03) and “an engine that likes to rev“ (Road & Track, December ‘03). The British magazine Engine Technology International has named it Best Engine of the Year for 3.0 to 4.0 liters for five years in a row. Now this remarkable unit gets to power lighter, even more agile BMWs than the M3s.
Statistically speaking, the results are dramatic: delivering 330 horsepower at a high-revving 7900 rpm, the internally-designated S54 engine propels the 3200-pound Z4 M Roadster to 60 mph in just 4.8 sec. Maximum speed is electronically limited to 155 mph; speculation about what the top speed might be without the limiter will intrigue enthusiasts, but BMW makes no claim in this regard.
In its broad concept, this engine shares its inline format with other BMW 6-cylinder engines. Virtually all its engineering details, however, are unique and targeted to the very highest level of performance. Unlike other Z4 engines, which employ lighter materials for their block and cylinder head, the S54 combines a cast-iron block with its aluminum head to achieve the engine's 3.2-liter displacement without lengthening the block. The block accommodates this engine's bore and stroke of 87.0 x 91.0 mm, both larger than the dimensions of other BMW 3-liter engines. However, playing larger roles than increased displacement in the 75-hp increase over the most powerful regular-production 6-cylinder engine is the special induction, combustion and exhaust engineering, together with its execution as a high-rpm engine. The starring role here is played by a cylinder head that could be termed “exotic.” Its key features include:
Double VANOS (Variable Nockenwellensteuerung) steplessly variable valve timing on the intake and exhaust camshafts. Though this system appears in all other current BMW engines, in the M unit it varies timing over a wider range and contributes in a major way to the engine's ultra-high power output.
VANOS pressure pump. In regular-production BMW engines the main oil pump supplies the pressure to operate VANOS; here the VANOS system has its own radial-piston hydraulic pump. Integral to the exhaust camshafts VANOS mechanism, the pump produces up to 120 bar (1740 lb./sq in.) of pressure to vary valve timing more quickly at the very high rpm this engine reaches. BMW M refers to its VANOS system as High-Pressure Double VANOS; it is also used in the M5 and M6’s 500-hp S85 V10 engine.
Unique valve mechanism. All BMW engines have an efficient valvetrain. Yet for an engine with the S54’s rpm potential, BMW M engineers needed less reciprocating mass than in the regular-production engines.
To achieve this, they created a unique actuating mechanism, using extra-light finger-type rocker arms. Pivoting on their own shafts (one on the intake side, one on the exhaust), these small arms provide the actuating surface between camshaft and valve. As the entire arm does not move the distance of valve lift, its effective reciprocating mass is less than its actual mass. All told, the valvetrains effective mass is 30% less than in the BMW 6-cylinder engines that have bucket-type hydraulic lifters; in turn, this allows lighter valve springs, which further reduce inertia. The system also has less friction.
As there is no hydraulic maintenance of valve clearance but it does have to be inspected and adjusted periodically. If adjustment is required, normally at Inspection I & II, it is done with shims (tiny metal discs of various thickness) without removing the camshafts. To see what is involved, see the S54 Valve Adjustment Page.
Whereas the “regular” 6-cylinder engines have a simplex (single) camshaft drive chain or chains, the S54 has a full duplex (double) chain driving its camshafts. As usual with BMW engines, the chain is hydraulically tensioned and needs no periodic adjustment or replacement.
Extra-high compression ratio. At 11.5:1, the M engine has the next-highest ratio in current BMW production, topped only by the M5‘s and M6's 12.0:1.
Machined surfaces. More “engineering finery” again: combustion chambers and intake ports are completely machined, for smoothness that facilitates airflow. The exhaust ports are partially machined. For durability, the valve seats are of especially hard steel. A 3-layer stainless-steel head gasket ensures effective sealing of the head to the block.
Head casting and sealing. Extreme strength in the cylinder head is achieved by making it a single aluminum casting. Though elaborate, this construction also saves a significant 29 lb. As this weight reduction is at the top of the engine, it helps lower the cars center of gravity.
The S54 won the International Engine of the Year award overall for 2001, and also claimed the "Best New Engine" that year as well as won the 3 - 4 liter category for six straight years in a row from 2001 through 2006. The S54 was also on the 'Ward's 10 Best Engines' list for 2001 through 2004
The DriveTrain
Z4 M Roadster and Coupe drivetrain: Getting the S54 power to the road
Like every M Car to date, the Z4 M Roadster and Coupe transmit power to the road via classic rear-wheel drive and include some premium and fascinating engineering into its drivetrain.
A transmission for sporting enthusiasts. The Z4 M Roadster and Coupe come with a 6-speed manual transmission. There is no automatic transmission offered. It is a ZF Type H gearbox, which features evolutionary improvements:
• Even more effective synchronization of shifts
• Lighter and more pleasing feel as shift lever is moved from gear to gear
• Firmer engagement of gear once selected
• Shorter “throws“ from neutral to each gear
• Use of lifetime transmission oil, which never needs to be changed
• “Clean bearings“ design for main transmission bearings; protects bearings from contamination, ensures adequate lubrication at all times and therefore enhances durability.
- As in other M models, the shift pattern on the gearshift knob is illuminated attractively at night.
Super-sized differential unit. A specific subframe allows equipping the Z4 M Roadster and Coupe with a heavy-duty differential with gears made of a special high-strength steel alloy to achieve quietness and durability. As at the front spoiler, rear diffusor and elsewhere along the underbody, airflow under the vehicle is carefully managed and targeted; here, a ribbed differential case and NACA air intakes for the differential unit help keep the oil cool.
M Variable Differential Lock. Together with the German division of GKN Viscodrive, BMW M engineers developed a special mechanical limited-slip that also appears in the M5 and M6. This M Variable Differential Lock specifically addresses low- and split-traction situations in a way that reinforces sporty handling while contributing to remarkable slippery-road abilities.
Any time a speed difference develops between the two rear (driven) wheels, a shear pump, driven solely by this difference, develops pressure in the silicon viscous fluid in which the lock operates. In turn, this pressure is directed to a multi-disc clutch that transfers driving torque to the wheel with the better road grip (“select high”). The greater the speed difference between the two wheels, the more aggressively the clutch engages. As soon as the difference between the two wheels’ speeds begins to diminish, the clutch starts to ease off.
This mechanism achieves its sophisticated action by entirely natural means. There is no external pump, no external source of lubrication or operating fluid. The very motion to be controlled - differences in speed between left and right wheels - generates its locking action. Viscous fluid is so-called because it develops internal force (via an increase in viscosity) whenever it is sheared; this is why the relatively small difference between one wheel speed and the other can generate the necessary action.
Dynamic Stability Control. This electronic traction and stability system, standard on all current BMWs, complements the M Variable Differential Lock.
DSC optimizes traction by electronic means, sensing wheel-speed differences and reducing engine torque and/or applying individual rear-wheel brakes. The crucial difference to the Z4 M Roadster driver between the M Variable Differential Lock and the DSC traction function is that the former in no way impedes power delivery, and is hence suitable for performance driving.
Yet in fact, even DSC’s traction function is calibrated to M-specific parameters. In combination with the fast-reacting engine, performance-oriented gearing and M Variable Differential Lock, its logic achieves the desired traction optimization in an M-compatible way…in other words, without undue interference with the differential lock’s ability to get power to the road.
The DSC stability-enhancing function is essentially unrelated to traction. Sensing differences in wheel speed in a critical cornering or avoidance maneuver, DSC detects any deviation from the normal cornering path (abnormal understeer or oversteer) and gently applies individual wheel brakes to help the driver keep the vehicle on the intended path.
As in other recently introduced BMW models, the Z4 M Roadster’s and Coupe's DSC system incorprates:
• Brake Drying. Acting on input from the windshield wipers’ rain sensor, the brake pads are periodically brought up to the rotors - just enough to eliminate any film of water between the pads and rotors, but not enough to cause any brake applications. This improves brake readiness in wet driving conditions.
• Start-off Assistant. Keeps the vehicle from rolling backward when stopped facing uphill. For 2 seconds after the brake pedal is released, the driver can then move off from rest without the car rolling back.
• Modulated ABS function. Thanks to new “analogized” control of the DSC brake valves, the anti-lockup function (ABS) is smoother than before. Instead of simply being fully applied and released, the application and release of these valves is now modulated.
Some DSC braking functions found in regular-production BMW models, such as Comfort Stop, are not included in the Z4 M Roadster because of its ultra-sporty driving character.
The Chassis
The Z4 M Roadster and Coupe chassis
Starting with the Z4’s already excellent chassis engineering - Motor Trend commented in its May ‘05 issue that the Z4‘s “sport suspension and solid chassis make this car a thrill to drive on canyon roads“ - BMW M has endowed the Z4 M Roadster and Coupe with typical M modifications that further enhance handling and tailor the cars' road capabilities to M’s higher performance level.
Front suspension. While retaining the Z4’s basic concept - strut-type with forged-aluminum lower arms - the M engineers created an essentially new version with:
• A wider front track (by 0.5 in.)
• Newly configured lower arms, still of forged aluminum; more complex shape with open sections for lightness, similar to those of M3s
• New steering knuckles, bolted to the struts (saves weight, sharpens steering response)
Such changes, subtle yet deep-reaching, are - as good as the basic BMW model is - key to achieving the “beyond exceptional” reflexes of an M model.
Rear suspension. Though modifications to the Z4’s Central Link (multi-link) rear suspension are less extensive than at the front, they are significant. Among others, the following major M elements are shared with M3s:
• Rear subframe - larger and stronger; accommodates differential with M Variable Differential Lock
• Wheel bearings
• Central (longitudinal) Links
• Arrangement of anti-roll bar.
As at the front, these modifications sharpen handling capabilities.
Ride height and suspension calibration. Like the Z4’s sport suspension, that of the Z4 M Roadster and Coupe are set lower compared to the standard Z4’s setup. Unlike the Z4, however, the M versions get an all-around new calibration of springs, shock absorbers, anti-roll bars and bushings to BMW M parameters; this includes specific auxiliary springs at the front and rear.
A new steering system. Alone among BMWs, Z4s have electrically assisted power steering. With this system’s vehicle-speed-sensitive power assist, it is notable for its combination of reduced driver effort in parking and at low vehicle speeds, and good road feel at higher driving speeds.
BMW M has somewhat different priorities: a greater emphasis on absolutely accurate road feedback and ultra-spontaneous response to the driver’s commands at the steering wheel. In this context, higher parking and low-speed efforts are acceptable. Thus M gave the Roadster and Coupe a hydraulically-assisted rack and pinion with a power-steering pump calibrated with assist characteristics optimized for the Z4 M Roadster and Coupe. This system reduces parking efforts somewhat, but not to the degree that vehicle-speed-sensitive assist does.
BMW M’s most advanced brake system. The Z4 M Roadster and Coupe come standard with BMW M’s most elaborate brake concept: compound, cross-drilled 4-wheel ventilated discs.
The brake rotors are 2-piece, with an aluminum “hat” and a cast-iron outer portion that is the actual friction surface. The hat and outer portion are connected by steel pins on which the aluminum and cast-iron portions, because of their differing expansion rates, can move relative to each other. This is an elaborate, costly construction that cuts unsprung weight and eliminates rotor deformation under hard-braking, high-heat conditions. In practical terms, this means virtually no tendency of the brakes to vibrate when hot, and also virtually no likelihood of the rotors cracking under the extreme heat conditions that might be encountered in driving on a race track.
At the front, the discs are of 345-mm diameter x 28-mm thickness (13.7 x 1.10 in.); at the rear, the discs measure 328 x 20 mm (12.9 x 0.79 in.). Cross-drilling, visible through the wheels, enhances heat dissipation beyond that provided by the rotors’ internal ventilation, further increasing the brakes’ resistance to fade, improving braking effectiveness in wet weather and reducing unsprung weight. Given the Roadster’s and Coupe's light weight, this system achieves phenomenal braking.
Wheels and tires: ultimate style and grip. M Double Spoke wheels, with a very open configuration to help get plenty of cooling air to the brakes, complement the Z4 M’s body design with a boldly sporty, yet elegant look. Sized 18 x 8.0 front and 18 x 9.0 rear, they are connected to the road by suitably wide, low-profile, high-speed-rated tires sized 225/45ZR-18 front and 255/40ZR-18 rear .
As with other BMW M models, the Roadster's and Coupe’s tires are not run-flats; run-flat tires meeting M requirements are not yet available. Also, the M exhaust system’s bulk precludes a spare tire. Thus like other BMW M models, the Coupe comes standard with the M Mobility System for use in case of a flat tire. A Tire Pressure Monitor alerts the driver to a significant loss of tire pressure.
M Mobility consists of a container of rapid sealant, a small compressor, and a hose to connect the compressor to the damaged tire. All this is carried in a container in the trunk; the system can seal punctures up to approximately 1/4 inch across. Omitting the spare tire saves about 45 lb. of weight.
The Roadster’s and Coupe's wheels incorporate the so-called Extended Hump rim shape, which helps keep the tire on the rim in case of deflation.
As in other M models, instrumentation is special, both optically and functionally. The instrument cluster reflect the M’s performance capabilities with special features:
• 9000-rpm tachometer
• Variable tachometer warning segment that lowers redline according to engine temperature: lowest with a cold engine, then increasing in increments to an 8000-rpm limit with a fully warmed-up engine. Although this feature appears in some regular-production BMW models, the BMW M graphics (with an arc of orange and red LEDs) are different.
• Oil-temperature gauge at bottom of tachometer dial
• 180-mph/300 km/h speedometer
• Distinctive graphics throughout, including red-on-gray dial pointers.
