The problem, near as anyone can tell, is two-fold. First, there is extreme sensitivity to wheel balancing. Second, the front control arm bushings, aka thrust arm bushings, are not stiff enough. BMW used different control arm bushings for every model 6 Series, E28 5 Series and E32 7 Series. All are too soft-except for the E32 750iL bushings. These can be made to fit the E24 and E28, but they have to be machined professionally. Aftermarket companies offer E32 750iL bushings already machined and ready to install. Among them are BMP Design (www.bmpd.com) and Bavarian Autosport (www.bavauto.com). If your post-06/82 production 6 Series has a chronic front suspension vibration and other possible causes have been eliminated, use these control arm bushings. They are also a good performance upgrade for modified cars.
During this period, the now-dreaded Michelin TRX tire and wheel combination replaced the familiar 195/70-14 tires and 6x14-in. alloy wheels. The TRX promised revolutionary handling improvements but in order to deliver them required a specially shaped wheel bead area. In order to prevent TRX tires from being mounted on wheels without the specially shaped bead area, Michelin built it in a metric size, in this case, 225/55VR-390. In truth, the TRX did offer improved steering response when the technology was new, but it was quickly eclipsed by "regular" tire technology, after which the Michelin TRX became little more than a monumental pain in the ass. Drivers couldn't mount normal tires on TRX wheels. Instead, they could only buy Michelin TRX tires, which were very expensive and decidedly yestertech in terms of performance. Michelin TRX snow tires worked better for their intended purpose but were unspeakably expensive. Still, BMW would buy into TRX for the next five, long years.
Meanwhile, BMW owners sought aftermarket wheels and factory alternatives. Today, TRX wheels are routinely junked in favor of the same options-aftermarket and factory alternatives and normal-sized tires. Typical replacement fitments include 195/70-14, 225/60-14, 215/60-15, 225/50-16 and 235/45-17. The 17-in. size often requires body modifications for increased clearance, especially on lowered cars-most drivers considering this fitment will also have a completely tuned suspension with sport shock absorbers and shorter, stiffer aftermarket coil springs. Replacement Michelin TRXs are now considered "vintage tires" and are sold by specialty vendors such as www.cokertire.com at sky-high prices.
By 1983, 633CSi pricing was nudging the $40,000 mark in the U.S. The German-spec 635CSis could be personally imported to the U.S., so-called "gray market" cars so quick and fast they could suck a U.S.-spec 633CSi into the air intake and cough it out the exhaust while costing less money in the process. This was becoming a not insignificant sales threat. Moreover, the U.S.-spec 533i had nearly identical performance numbers at a savings of over $10,000. BMW of North America was confronted with the low power situation; something had to be done. BMW AG, typical of the era, could not understand why Americans wanted more power. After all, they reasoned, we could only go 55 mph-a speed considered laughably slow in Europe, where serious drivers had access to cars that could and did triple the U.S. speed limit. The notion that there could be wholesale disregard of speed limits here was inconceivable to the Germans, when in fact that was and still is precisely the norm in the U.S.