'Steel Origami': The BMW 3 Series

For years, convertibles have been made by, essentially, cutting the top off of a coupe, adding the requisite cloth top and mechanisms, and calling it good.

For years, convertibles have been made by, essentially, cutting the top off of a coupe, adding the requisite cloth top and mechanisms, and calling it good. Michael Brachvogel, deputy project director for the BMW 3 Series convertible, said, not entirely joking, that the first generation 3 Series convertible (1986) was “made in a garage with its top cut off.” BMW has certainly come a long way since then in many ways, as evidenced, at the very least, by the 2007 3 Series convertible, which, incidentally, is the fourth generation of the product, with the other two being 1993 and 2000 models. Not only was this version engineered to be a convertible (Brachvogel, incidentally, was also the director of the body-in-white activities for the 3 Series coupe), but like many of the open-topped models that are appearing of late, it is a hardtop convertible, featuring a power-operated three-piece steel roof. 

There is rear-wheel drive. The front suspension has a double-pivot McPherson strut in the front and a five-link arrangement in the rear. The vehicle has BMW’s latest Dynamic Stability Control feature that fundamentally assists when driving on slippery roads. Among the interesting attributes of this system is that it offers automatic brake drying (i.e., the pads are lightly applied periodically during wet weather so as to dry them); brake standby (i.e., when the driver suddenly lifts from the accelerator, the brake pads are preloaded to facilitate brake response); and brake fade compensation (i.e., the brake pressure is maximized in extreme driving situations so as to provide consistent pedal pressure regardless of how hot the brakes become). (The brakes, incidentally are 12.3-in. diameter front/11.8-in. rear for the 328i and 13.7-/13.2-in. for the 335i.)

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One of the things that Brachvogel said they tried to do with the vehicle is to provide a low shoulder line. He suggested that a purpose of a convertible is to have an open-air top, so why design the vehicle so that the driver and passengers are hunkered down below the beltline? He noted that it is “not dignified” if you can only see someone’s head, and that not only the head, but part of the chest must be visible, as well. Another objective was to have a short and flat rear end and a long hood. The long hood leads to a short A-pillar. There is no B-pillar. And, yes, there is the Hofmeister kink on the C-pillar. In keeping with the openness even with the top up, the rear and side windows are 30% larger than those found on the previous-generation convertible.
Weight savings is not only achieved through the use of things like high-strength steel supports, plastic front fenders, and aluminum roof rails , but also by lightweight engines. Both the 328i and 335i engines have DOHC in-line sixes that feature magnesium/aluminum composite construction. In the case of the 3.0-liter 230-hp engine in the 328i, the engine is 22 lb. lighter than the engine it replaces yet has more power (230 hp vs. 184 hp) and torque (200 lb-ft vs. 175 lb-ft).

According to Brachvogel, there was consideration between a soft and hard top. When they determined that they would be able to make the hardtop fit, only then did they go with the three-piece steel roof. He described it as “steel origami.” The roof opens in 22 seconds and closes in 23 seconds. He points out that even though the folded roof is packaged in the trunk, the trunk line is 30 mm lower than on the coupe. Compared with the coupe, there is not a great weight penalty for the convertible top. That is, a 328i coupe with a manual transmission has a curb weight of 3,351 lb; it’s 3,417 lb. for the automatic. For the 328i convertible, the comparative numbers are 3,792 lb. for the manual and 3,858 lb. for the automatic, or 441 lb. more. Apparently, that delta is one that BMW engineers have kept fairly consistent throughout all four generations of the vehicles.