M3 chassis: The Best of 3 Series and BMW M Engineering

In developing the M3, BMW M engineers set for themselves the target of making the great-handling 3 Series Coupe and Convertible even more capable. Here's what they did:

Front suspension. Within the same basic concept, BMW M further developed the strut-type front suspension for even more tenacious roadholding:

• At 59.4 in., the M3's front track is 1.5 in. greater than that of 330Ci models.
• The lower suspension arms are unique to the M3.

To form an ultra-rigid basis for the suspension system, the M engineers added a thrust plate to handle the immense lateral thrust generated by BMW M3's tires in cornering. It is made of aluminum 3 mm thick, and attaches in the area between the left and right lower suspension arms. The thrust plate even incorporates an NACA air intake that helps cool the transmission.

The bearings, bushings and cushions on which the suspension arms pivot are all M3-specific. In contrast to the Ci models' arc-shaped lower arms, BMW M3s' unique arms have a "bat wing" shape for even greater strength. As on the regular models, they are made of forged aluminum to keep unsprung weight low and strength high.

Other M3-specific front suspension components include:

• Unique steering knuckles
• Modified wheel bearings
• Separate top mountings for the springs and shock absorbers; on standard 325/330Ci models they mount together.
• Subframe - from BMW 3 Series Convertible, even for BMW M3 Coupe
• Reinforcing braces to the strut towers, visible with the hood open.

Steering. Power assist is calibrated for extra-firm road feel, and steering return action is enhanced by increased caster.

Rear suspension. Here too, the track is increased (by 1.6 in.), and both pairs of lateral links (lower and upper) have steel balljoints instead of the 3 Series' rubber bushings at their outer ends. There are many other detail distinctions from the standard rear suspension as well.

For the larger, stronger BMW M3 differential, M engineers developed an entirely different subframe and a specific mounting system for it. To enhance rigidity here, where the suspension meets the vehicle structure, a V-brace was also added. (This is analogous to the front thrust plate.)

As the most prominent element of the multi-link rear suspension system, the massive Central Link is retained. At its front end, the link pivots on a large rubber bushing that is firmer in the M3 than in "regular" 325/330Ci models. The axle halfshafts are upsized for extra strength, and the wheel carriers are special to BMW M3 as well.

Springs and shock absorbers. BMW M3 coil springs are calibrated for an ideal blend of firmness and compliance. The twin tube gas pressure shock absorbers - with hollow piston rods to minimize inertia and mass - are likewise perfectly calibrated for sports-car response. Anti-roll (stabilizer) bars - 26 mm front, 21.5 mm rear - are sized front-to-rear for ideally responsive, yet not nervous, handling.

Brakes: even more powerful. Powerful brakes are always a BMW strength, and current 330Ci models achieve outstanding braking ability with front discs of 325-mm/12.8-in. diameter and rear discs of 320-mm/12.6-in. diameter. The M3 goes a step further with 328-mm/12.9-in. rear discs, and the rotors are thicker all around: 28 mm at the front, vs. 22; and 20 mm at the rear, vs. 19. A tandem booster, sized 10 in. / 9 in., provides extra vacuum assist over BMW 330s' single 10-in. booster. As always on BMW M Cars, all four discs are ventilated for high fade resistance. In Road & Track's December '03 comparison with two key competitors, BMW M3 Coupe achieved the shortest stopping distance from 60 and 80 mph: 112 ft. and 197 ft. respectively and the only "excellent" overall brake rating.

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