Upgraded brakes

All Z3s had ventilated front and solid rear disc brakes; the Z4 3.0i gets ventilated rear discs, and both models have larger-diameter rear discs than those of the predecessor.

Equipment is as follows:

• 2.5i - Ventilated front discs of 286-mm/11.3-in. diameter; solid rear discs of 280-mm/11.0-in. diameter.
• 3.0i - Ventilated front discs of 300-mm/11.8-in. diameter; ventilated rear discs of 294-mm/11.6-in. diameter.

Also new here are two refinements to the handbrake mechanism. One is a self-adjusting actuating cable, reducing the need for periodic adjustment; the other is a newly designed cable linkage that ensures equal handbrake force on both rear wheels.

Run-flat tires, standard on both models

Every Z4 comes standard with BMW's Run-Flat Combination (RFC) wheel-tire system, an important step forward in personal safety, convenience and in-vehicle space utilization.

The system consists of self-supporting tires, special wheel rims and a Flat Tire Monitor. The tires have special sidewalls that include specific inserts and highly heat-resistant rubber compounds. These features allow a deflated tire to maintain its essential shape and guidance characteristics for a considerable distance.

While maintaining essentially the handling and safety standards of high-performance tires, the RFC system offers these advantages:

• No roadside tire changes. It will be seldom, if ever, necessary to stop and change a damaged tire. Thus when confronted with a flat (a condition revealed to the driver by the Flat Tire Monitor), the driver can continue on for up to 90 miles at speeds up to 50 mph until reaching a safe and/or convenient place to have the tire repaired or replaced.
• Tire stays on rim, thanks to specially developed Extended Hump wheel rims.
• Stability systems remain functional. All Dynamic Stability Control functions remain fully in effect, even with a deflated tire.
• Increased trunk space. A spare wheel and tire take up considerable space. With RFC, no spare is needed, so the trunk can be larger.
• Weight savings. The weight of a spare tire is eliminated.

A key element of this system is the Flat Tire Monitor (FTM). Whenever tire pressure drops by 30% or more, the tire's rolling radius changes significantly; this wheel will turn at a different speed from the others. Via the DSC wheel-speed sensors, FTM recognizes any major deviation in wheel speed (and therefore tire pressure). Within a short time, the system triggers a visual and audible warning to the driver.

Anytime the tire pressures are adjusted, the technician or user should initialize FTM via its control button at the forward end of the console, according to the procedure described in the owner's manual. This button can also be used to de-activate FTM, as may be desirable under certain conditions.

Wheels and tires: four combinations in 16-, 17- and 18-inch sizes

Each Z4 model comes standard with beefy standard wheels and tires, and offers an upgrade as part of its optional Sport Package. All wheel designs are new; all tires are performance-type.

2.5i standard. Ultra-open Cross Spoke wheels are dimensioned 16 x 7.0 and carry 225/50R-16 V-rated tires. These wheels are especially light, saving about 2 lb. each and thus enhancing the low unsprung weight already promoted by the aluminum suspension components.

2.5i Sport Package. 17 x 8.0 Turbine wheels wear 225/45R-17 W-rated tires for this upgraded equipment.

3.0i standard. Wheel and tire sizes and the tires' speed rating are equivalent to those of the 2.5i Sport Package; the wheel design is Twin Spoke.

3.0i Sport Package. In this ultimate Z4 wheel and tire combination, handsome Ellipsoid wheels are sized 18 x 8.0 front / 18 x 8.5 rear and carry 225/40R-18 front / 255/35R-18 rear W-rated tires.