Hans Sebastian Bretzner was born in Austria on the 6th of May, 1930.  He would later move to Germany and train in the field of automotive design.  At the age of 18, he began an ambitious motorcycle project that would last two years.  

 

With the help of Mr. Brenner, an engine builder for the famed BMW side car racer Walter Schneider, young Hans would design and build the impressive R51/75. 

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The resultant motorcycle was a successful exercise in design, engineering and  craftsmanship as can be seen by studying the photos below.

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The  R51/75 was crafted using a combination of surplus wartime parts, factory prototype parts and one-off handcrafted parts.  The engine housing is an R75, originally used in BMW Wehrmacht Seitenwagen machines.  However internally, the motor is nothing like the original sidecar lump.  The cam was specially designed and ground for this motor.  The crank is of special construction as well,  balanced and polished.  The valve train fully balanced and lightened.  The heads are ported and polished. 

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The frame was specially constructed to house the motor.  As a result the motor sites higher up and further back than  a conventional R51 motor of the time.  As a result, the drive shaft is shorter than stock.  The gearbox has a close ratio racing cluster. 

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A close look at the front brake reveals something special.  The wheels are full hub version, with a larger diameter brake plate to accomodate it.  However, the brake is simplex with only one brake cam and lever.  BMW never produced this version, with the production variant seen on later R68 and R51/3 being duplex.  My contacts at the BMW archive indicate it was most likely a prototype part.  

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The bike was not of "official" BMW production and as a result does not have a corresponding BMW serial number.  Rather it has a unique 7 digit number, the first four characters of which are Hans Bretzner's birthday and the last three the house number of his grandmothers home. 

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The original typenschild which still accompanies the bike lists the model as R51/75, which is fitting.  Year of construction is 1950.  Two short years after finishing the bike, Mr. Bretzner would emigrate to the United States, arriving on February 18, 1952 aboard the ship Constitution.  Hans was enroute to Detroit, Michigan where he'd taken a position as car designer with General Motors.  Three years later in 1955 he had the R51/75 shipped from Munich to Detroit, but not before the bike had been gone through at the BMW works in Munich.

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By 1959 Hans' career was in full swing and he relocated to Nagoya, Aichi Prefecture, Japan, taking a position as a lead designer with Mitsubishi.  Realizing that it was not practical to take the R51/75 with him overseas, Mr. Bretzner sold the bike to fellow Detroiter Billlie Hummel.  While with Mitsubishi, Hans lead the design of the "Debonair", a decidedly American style sedan, that to this day has a cult following. https://mit-cardesign-t.com/en/front/design-stories-en/ds-debonair-1st-en/

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Billie Hummel and his wife fully enjoyed the R51/75.  Or should I say Billie enjoyed it.  Ms. Hummel found the "Rennbrotchen" pillion saddle a bit lacking in comfort.  However, I find that the red pillion loaf is quite a standout on the bike, and perhaps some day I'll have one reconstructed.  (Sadly it is no longer with the bike.)   Whilst under the ownership of Billie Hummel, the now 10 year old bike was shown at a car and motorcycle show in Detroit.   Perhaps because of the uncomfortable pillion arrangement, the bike was sold in the early 1960's not long after Mr. Hummel purchased a "new" R69S.  The R69S certainly was a more accommodating stead for riding two up, however Billie commented that the R51/75 engine was smoother than his R69S and the brakes stopped better, despite the simplex configuration. (See below for the original advertisement penned by Billie Hummel when selling the R51/75)

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In the mid 1960's the bike moved out west, having been purchsed by Dutch Becker who owned the bike from 1965-1967.   During this time, the valves were redone by  famed engine builders Taylor and Ryan in Whittier, California.  The valves were reportedly racing parts from Alfa Romeo.  Dutch subsequently traded the bike to Tom Bell who owned a service station in Hollywood on Beverly, near La Brea.  

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It was  a chance meeting in  central California between Duane Ausherman and as Duane describes it "some hippie kid" in 1972 that lead to the next chapter.  Duane spotted Tom Bell on the bike and inquired about purchasing it.  At the time Duane was owner and operator of BMW of Marin.  A deal was struck and Ausherman became the next caretaker of the R51/75. Thus began a new chapter for the bike in the San Francisco Bay Area

 

The next owner in line after Duane  was bay area resident Rod Miller.  Under Rod's ownership the bike would see a sympathetic cosmetic restoration.  Body work was completed by famed mechanic Brian Hilton of Ozzie's BMW in Chico, CA.  Rod also sourced an original Lugauer racing style tank, which remains on the bike today.  Additionally, Rod reached out to Peter Zollner, then head of the BMW archive in Munich in an attempt to get some history on the bike.  (Many years later I filled Rod in on the history.  At the time BMW had no records or info on the bike.)

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From Rod, the bike passed to another San Francisan,  Bob Garreett.  By that time the final drive had developed some problems and Bob sought the help of legendary tool and die maker Joachim Groeger of Redwood City.  Joe, a German ex-pat, was known throughout the Bay Area as the go-to guy for work on vintage BMWs.  The esoteric R51/75 was in need of an expert and Groeger was the right guy.  Groeger gave Garrett an estimate to complete the final drive work and at the time Garrett was not interested in putting the money into the machine.  Groeger, recognizing that the bike was something exceptional, purchased the bike and completed the final drive repairs.  

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The bike remained in the back of Groegers shop for a number of years.  At some point, the transmission was pulled out by some well intentioned friends of Joe, in an attempt to work out some clutch problems.  Unfortunately the work wasn't comleted and the bike sat in a sorry state for a number of years.  

In 2008 I convinced Joe to sell me the bike.  I promised to sort the clutch issues and get the bike running again.  Years before the Amal Fischer carbs had been swapped for Italian Dell'Orto's.  But there was two right side carbs on the bike, and they were a bit tattered.  I was able to source a NOS set of 30mm Dell'Orto's and fitted them to the bike.  I also repainted the cylinders and rechromed the valve covers.  The pushrod tubes were quite rusty, so I fabricated new ones and had those chromed as well.  

 

 

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