Rotohak Reborn

By Bob Pelikan

It was July ’93 when Rotohak first appeared in the BMWMOA News.  It had just been completed that spring, was painted red, and looked like a K100 with a sidecar on the side.  What you couldn’t tell from the picture was that the K100 engine was an empty shell, a fake.  Hidden in the trunk of the sidecar was a normally aspirated Mazda RX7 engine which powered both the sidecar wheel and rear motorcycle wheel through a VW automatic transmission.  It went to the national in Oshkosh that July on a trailer because I had not yet sorted out a steering problem, making it uncontrollable at speeds over 40 MPH.

The steering problem turned out to be a tight bushing at the lower end of the front hub and once that was remedied, I spent the next six years cruising the west, including a trip to the national in Durango in July 19xx.  It turned out to be an excellent road machine, very stable at high speed, comfortable to ride on long trips.  My longest one day ride was the 1150 mile return home to San Francisco from the Durango rally.

I have built a number of mechanical contrivances in my lifetime starting back in high school and none came together with so few problems or ended up working as well as Rotohak.  I’d have to say that the project exceed my expectations. 

By 1998, I wasn’t riding it much so I sold it.  I had immediate seller’s remorse.  What was I thinking?  I told Bill, the chap in Chicago that bought it, that should he ever decide to sell I wanted the first right of refusal to buy it back.

In the summer of 2001, I got a call from Chicago Bill saying he needed the garage space, and was willing to sell it back to me at the same price he’d paid.  Three weeks later it showed up on a truck, a little tired looking from having spent three winters in Chicago but running as sweet as ever.

After riding it for a few weeks, I began planning a ground up restoration, mainly to make sure all the pieces I’d hand made 10 years ago were still holding together.  In the process, I thought it would be cool to have a bigger, more powerful engine and a modern body on the motorcycle. 

I found a sequential twin turbo intercooled Mazda Cosmo engine at a local Japanese engine importer and began a year long project of fitting the engine into the chassis and of fitting the sidecar body around the larger engine.  The Mazda Cosmo is a muscle car sold in Japan but not imported into the US and therefore not supported here by the Mazda dealers.  The motor is a rotary, similar to that of an RX7 but has larger ports and makes a whopping 325 HP. 

The first hurtle was to was to squeeze the engine into my sidecar frame.  The turbos make the engine 6” wider than its normally aspirated cousin it was replacing so the frame and left rear suspension had to be modified.  The second hurtle was the bundle of 45 wires coming out of the computer which had been cut off at the plug.  Since the Cosmo is not supported by Mazda in the US, I had no local source of documentation for the engine.  As luck would have it, one of my sons was living in Japan at the time, teaching English.  He went into the local Mazda dealer, bought a shop manual, and shipped it off to dad.  Of course, it was in Japanese, but with the help of a Japanese speaking friend of a friend, I was able to get the wiring color codes and other pertinent information translated into English.  Shortly thereafter it was running.

Coincidentally with the engine project, I was gathering R1150RT body parts via the News classifieds, the Internet, and a local BMW shop dumpster.  The trick to fitting 2002 R1150RT body parts to a 1985 K100 is to graft front and rear RT sub frames to the center section of the K frame so everything lines up and the body parts bolt right on.  I had to retain the K center section because it was attached to the sidecar.  Eventually, it all came together and actually looked like a 2002 R1150RT complete with valve covers where they belonged.

In the cavernous opening in the center of the R fairing, I fit a 16 gallon marine gas tank inside the hole and hooked it to the RT gas cap.  An external high pressure fuel pump mounted behind the tank supplies the injectors.

The last major task was to modify the sidecar body to clear the turbos.  After cutting away the unwanted fiberglass in that area, I used Styrofoam and plaster to form a fender like shape needed to clear the turbos.  I then covered the Styrofoam with successive layers of fiberglass matt and cloth, removed the Styrofoam and, with a little Bondo, had a descent looking body appendage that looked like it belonged.

The finishing touches involved polishing the aluminum engine parts, having Mike Corbin’s wizards do new seats for the bike and a new interior for the sidecar, and having a painter friend put a nice two part yellow paint job on the plastic. 

The first time I took it out on the highway, I babied it up to about 50.  With all the gauges in the normal range and hearing no unusual noises, I thought; I wonder if these turbos work.  I slowly cranked open the throttle.  When the second turbo kicked in and the front tire gently lifted off the road, a big grin crept across my face and I quietly muttered under my helmet, “I finally have enough horse power”.

At the end of the day, I realize that Rotohak, as fun as it is to work on and to ride is just an object.  The real joy of my life is my family, wife Jerilyn who brightens every day and my three sons who make me proud to be their dad.  It was Jerilyn after all who inspired the Rotohak project in the first place by telling me she was tired of sitting on the back of my BMW.