BMW R 4

BMW R 4

The BMW R 4 , released in 1932, was a single-cylinder motorcycle from BMW with a displacement of 400 cc.

history

Since BMW did not have a successor to the R 52 in the 500 cm³ class, the BMW R 4 with the same performance as the R 52 was an inexpensive mid-range motorcycle between the tax-free and license-free R 2 and the two-cylinder models of the 750 cm³ -Class brought to the difficult market at the time of the Great Depression. Series production began in December 1931, and was offered by BMW as an off-road model with a retail price of 1,250 Reichsmarks. From 1933 until production was phased out in 1937, the R 4 only cost 1150 Reichsmarks.

technology

engine

The longitudinally installed engine with vertical cylinder, fully encapsulated valve train and overhead valves developed 12 HP (9 kW) at 4000 rpm.

The stroke of the engine is 84 millimeters. The BMW R 4 was the first motorcycle with an engine with this longest stroke range of all BMW motorcycles. Only the later BMW R 3 and BMW R 35 models had engines with an identical stroke length.

The battery ignition is controlled by a camshaft operated breaker located in a cylindrical housing on the front of the engine. The battery is located on the left behind the engine.

drive

The R 4 had a manual gearbox with a drive shaft on the right side of the unsprung rear wheel.

BMW referred to the power transmission from the gearbox to the rear wheel as a "cardan drive", the drive shaft as a "cardan shaft" and the gearbox on the rear wheel as a "cardan housing" - technically correct it was only a shaft drive of the rear wheel, as there were no cardan joints.

The gear housing was flanged directly to the motor housing. The input shaft was driven directly by the single-plate dry clutch in the crankshaft's flywheel. The output shaft drove the drive shaft via a hardy disk in direct extension.

The kick starter was arranged on the right side - a sidecar operation was not planned.

Frame and chassis

The frame consists of two pressed sheet steel halves with four riveted cross members and the steering head. There was no rear suspension ("rigid frame"), the front fork consists of two fork blades that were reinforced with riveted metal sheets. On the fork sat a drawn short swing arm , which was supported on a nine-layer leaf spring below the headlight. The inner-shoe drum brakes on the front and rear wheels were designed as half-hub brakes according to the state of the art at the time, with a diameter of 180 mm and operated at the front via a cable and at the rear via brake rods.

Versions

The BMW R 4 was produced in 5 series.

The Series 1 from 1931 had a kick starter operated parallel to the vehicle's longitudinal axis.

In the Series 2 from 1933, the kick starter was arranged at right angles to the vehicle's longitudinal axis. The increased construction costs due to the bevel gear redirection made it possible to step without having to get down. The transmission was changed from three to four gears.

In the manuals, terms were Germanized over the course of the production period: “Type” became “Prototype”, “Kickstarter” became “Foot control lever”.

Technical specifications

Parameter	BMW R 4, Series 1
drilling	78 mm
Hub	84 mm
Displacement	398 cc
power	12  PS (9  kW ) at 4000 min -1
Empty weight	137 kg
total weight	337 kg
Tank capacity	10 L

See also

• List of BMW motorcycles

literature

• Udo Stünkel: BMW motorcycles typology: All series models from 1923 . Delius Klasing, Bielefeld 2008, ISBN 978-3-7688-2451-4 .

Web links

Commons : BMW R 4  - collection of images, videos and audio files

• BMW R 4. In: BMW History. BMW AG, accessed on December 19, 2015 (dossier from the BMW Group Archives). 
• Spare parts list for the BMW R 2/200 ccm, R 3/300 ccm, R 4/400 ccm motorcycles. In: BMW history. BMW AG, May 1942, accessed on December 29, 2015 (spare parts list for all series with pictures, 99 pages). 

Individual evidence

1. ↑ Fred Jakobs: BMW R 4. (PDF) The first off-road sports car from BMW. In: BMW history. BMW AG, 2006, pp. 40–43 , accessed on December 29, 2015 (Mobile Tradition live, 03.2006). 
2. ↑ ^{a b} manual for BMW bikes, type R 2 and R 4. In: BMW history. BMW AG, September 1933, accessed on December 29, 2015 (manual with pictures, 71 pages). 
3. ↑ Manual for the BMW wheels, type R 2, R 3 and R 4. In: BMW History. BMW AG, February 1939, accessed on December 29, 2015 (manual with pictures, 57 pages).