Though the BMW Isetta 300 may have been, by some accounts, the best selling one cylinder car ever produced, by the late 1950s, Europe was well on its way towards rebuilding, as were European families, and BMW needed to introduce a four seat model. Fritz Fend was upgrading the Messerschmidt to the four wheel Tiger 500 and BMW followed suit in a manner of speakeing. The 250/300 Isettas that BMW built had been based around a BMW motorcycle engine and again they used an existing powerplant, this time the 582cc horizontally opposed twin from the R67 bike.Though the front of the Isetta with its distinctive front opening door and foldaway steering wheel (along with the Isetta front suspension)was maintained, a new perimeter frame with box sections and tube crossmembers was designed. They also designed a new rear suspension.
The Isetta 300 uses a rather complicated drive system to get power from the rear mounted engine and transmission to the wheels. It involves a Hardy disc (I believe those are similar to the Rotoflex couplings used on Lotus Elans and Sumbeam Imps), a cardan joint made of rubber, a cardan shaft, and finally a second Hardy disc, which in turn drives a chain case with a duplex chain running in an oil bath led driving a rigid rear axle. No differential was needed because the rear track was so narrow. Actually the Isetta started out as a three wheeler but that design proved to be unstable. The Isetta 300’s suspension and drive setup was not suitable for the weight of a four passenger car so BMW came up with something that served them in good stead for over 40 years, using a conventional differential and a semi-trailing arm independent rear suspension. BMW used that same basic suspension design in all of their cars (except for the M1) from the 600 until the E30 BMW 3 Series cars of the 1990s.
Access to the front seat was as with other Isettas, through the front door. A second conventional passenger door was added for rear passengers. I suppose that makes this a two-door, though the only one that’s at all like it is the Stout Scarab, that has one door for the driver and a single common door for the passengers.
The BMW 600 car didn’t sell as well as BMW hoped, with fewer than 35,000 sold from 1957 to 1959. Consumers could purchase a VW Beetle, something a lot more like a real car than a motorcycle with four wheels with an oddball front door. So unlike the smaller Isettas made by BMW, the BMW 600 is fairly rare (BMW seems to have dropped the Isetta name for the bigger car). It did, however provide the basis for the BMW 700, which sold much better and created the foundation for the company’s later success with the 1600 models.