One of the current BMW M3 and M4's most distinctive features is their carbon fiber driveshafts. Sadly, the two BMW performance cars will become a little more mundane in the future, as the company announced it will transition the cars to steel driveshafts. The reason has to do with emissions.
BMW will begin adding a particulate filter to the exhaust system at some point in the future to stay compliant with stiffening emissions regulations. Carbon fiber driveshafts have a larger diameter and therefore take up too much space under the car. The "M-specific high-performance" steel driveshaft being used in its place is narrower enough to work.
BMW says that "the superlative performance and handling qualities" won't change with the use of the new driveshaft. We suspect that discerning drivers may notice a slight difference. Not only will steel add to the car's overall weight (however little), it will also represent more mass for the engine to move. This could mean the engine will be slightly less eager to rev. Of course, BMW could compensate for this by shedding weight elsewhere, possibly with a lighter flywheel or half-shafts.
Either way, though, this change still makes those M3 and M4 models produced in November 2017 and thereafter less special than those made before.
According to BMW, "for production-related reasons, this pre-enabling (of the new driveshaft for future emissions equipment) needs to be carried out universally." Presumably, this includes the United States, but as the announcement was made by BMW's global headquarters, we reached out to BMW's American office to see if the changes will indeed affect US-spec M3s and M4s. Indeed, according to a BMW spokesperson, the U.S.-spec M3s and M4s will feature this change.