You won’t find them on any U.S.-specification models, but Honda Motor Corporation did create market-specific transmissions primarily for the domestic market. So, what’s a “rotary-gearbox”? It was designed for stoplight-to-stoplight driving conditions that exist in Japan (and now a lot of other large cities, worldwide).
With the rotary gearbox, you start out in NEUTRAL, just like any other motorcycle, with the selection of 1st gear being one click DOWN from neutral. Once you launch in 1st gear, selecting 2nd gear requires another click DOWN (rather than a click up through neutral to get to 2nd gear in a “return shift” transmission). Need 3rd gear? Click DOWN again. And 4th gear, too? YEP! So, what’s next? NEUTRAL!
The rotary gearbox shift pattern is 1 down, 2 down, 3 down, 4 down for gears, then down for neutral and then start over again! A normal “return-shift” transmission uses 1 down for 1st, up for 2nd, up for 3rd and up for 4th (and or 5th/6th gears).
On Honda’s “little bikes,” (various 50-65-90cc models-but not all) the transmissions shift down for “upshifts” and up for “downshifts,” however they are all still return-shift transmissions. The big problem with this transmission design is that riders inadvertently could select 1st at high speeds, if they forgot what gear they were in and hit a neutral after 4th, then thinking that they just missed a shift, hit the shift lever again… into 1st gear! Obviously this causes great potential harm to the machine and rider in this circumstance. There may not have been a “law” against rotary-gearboxes, however they didn’t seem to be welcomed in the U.S. at any time.
Apparently Honda did try to send some rotary-gearbox models to North America, but they didn’t come to the U.S., but to Canada, instead. An avid Canadian collector has told me about contacting a mechanic who worked at the Honda dealers in the 1959-60 era and was tasked with doing “conversions” on bikes at the dealerships, removing the rotary gearbox components and replacing them with return-shift versions.
When Honda sent two “test unit” RC70 Scramblers to Alan D’alo in Los Angeles in 1959 ( even before American Honda had even setup shop in LA), the bikes came with basic 250cc Dream engines, complete with rotary-gearboxes. Local racers were still wrestling with the “left side shift” gear levers on these early Hondas, because many of the European racers came with “right side shift” transmission selectors. Having to remember which side of the engine to shift gears with and then the “rotary” gearbox function was more than most riders wanted to deal with, back then.
The “fix” for the RC70 riders was to commandeer engine assemblies from the 1959 CE71s, once they were released for sale through the fledgling AHMC distributorship. The CE71 was Honda’s U.S.-spec “Dream Super Sport” model, which featured a return-shift transmission, a larger 24mm carburetor (vs. 22mm carburetors that were stock on Dream engines) and a horsepower upgrade because of higher compression pistons and more aggressive camshaft timing rates. A CE71-powered RC70 Scrambler combination won several local SoCal dirt races in the hands of plastic-fender guru, Preston Petty. Petty had also trimmed off the factory-installed leading-link suspension front end and machined up a new steering head which could hold AJS telescopic front forks and a wider, smaller front wheel in place of the skinny 21” stocker.
The 1960-61 pre-production Honda 250 Scrambler prototypes also came with single-carb Dream engines, but with return shift transmissions. When the 1961 CB72 Hawk sport bikes were released, the first thing that happened to the “next generation” 250 Scrambler was again a motor transplant, but this time from a CB72, which had dual carbs, even higher compression pistons and a 10,000 rpm redline.
In fairness to Honda, they were not the only ones who offered “rotary gearboxes” to the public. Early Yamahas, the Bridgestone 175s and Lilac motorcycles also had this feature for a few years, among other now-extinct manufactured models. Honda continued to offer the rotary-gearbox option on domestic 250-305cc Dreams and CYP77 Police bikes into the mid-1960s, but no other “large” models were so equipped after that.
So, how do you make a transmission into a rotary gearbox design? First of all, the transmission gears are really just the same as the return-shift types. All the changes come in the shift selector parts and particularly the shift drum and shift forks. The shift drum has only one continuous shift fork slot, all the way around the circumference of the drum assembly. The shift forks have to overlap each other in order to reach both gears and still maintain contact with the shift drum. There are other parts which have to be used for the installation. See photos for listing of the parts and images of specific fork/drums.
A quick search for the term “motorcycle rotary gearbox” actually turned up new, small-bore Hondas, marketed in Vietnam (and probably elsewhere) still offer that transmission today!
Bill “MrHonda” Silver