The BMW 3/20 PS was the first BMW automobile designed entirely by BMW. It was manufactured from 1932 to 1934, replacing the 3/15 model that was initially an Austin 7 manufactured under licence from the Austin Motor Company.
The engine used in the 3/20 was based on the Austin Seven engine used in the 3/15, but its crankshaft ran in plain bearings instead of roller bearings and had an stroke, generating a displacement of .Norbye, p.30 The new engine design also had a water pump and an overhead valve cylinder head. These design changes caused the engine to generate 20 hp, but did not change the taxable horsepower rating of 3 PS, thus giving the model designation 3/20.
The 3/20 was larger than the 3/15, with an wheelbase and a body lower than the 3/15.Unixnerd: BMW's Early History The 3/20 used a backbone frame unrelated to the 3/15's Austin Seven "A" frame.Noakes, p.24Robson, p.36 It inherited the 3/15 DA-4's independent front suspension and added a swing axle independent rear suspension, using a transverse leaf spring similar to that used at the front. The standard bodies for the car were built by Daimler-Benz in Sindelfingen.
Journalist and engineer Josef Ganz, who had criticized the 3/15 DA-4's suspension system in the magazine Motor-Kritik,Article ‘Die Schwingachse des Kleinen,’ Motor-Kritik magazine, issue 3, early February 1931 was contracted in July 1931 as a consultant in the design of the 3/20 AM-1.
Four versions of the 3/20 were built: AM 1, AM 2, AM 3 and AM 4, where AM denoted Automobil München.
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