HRC Pursues the Possibilities of
Automatic Transmissions for Motorcycles
In the late 1950s, Honda began working on an automatic transmission for motorcycles as one of the technologies to make motorcycles easier to handle. The mechanism was called HRD (an acronym for Honda R&D), and its prototype was the HMT (Hydraulic Mechanical Transmission, commonly known as the Badalini transmission), a continuously variable transmission developed by Badalini in Italy.
Badalini structure
HRD, Honda’s automatic transmission for motorcycles based on the Badalini concept.
The HRD was the result of Honda’s modifications to the Badalini design, and although it was a very complicated mechanism, was nonetheless more efficient in power transmission and more compact than the torque converter transmissions that were becoming popular in automobiles, especially in the United States at the time. Soichiro Honda saw the potential of automatic transmissions for motorcycles. His idea that the hassle of manual shifting requiring clutch operation should be eliminated for products that demanded convenience, such as commuter models, was reinforced by the success of the Super Cub C100, which used an automatic centrifugal clutch.
Honda acquired the patent rights to the HMT and launched the Juno M80 scooter in November 1961, which combined an OHV horizontally opposed two-cylinder engine with HRD as Honda’s first automatic transmission motorcycle. However, although the engine was 124 cc and 11PS, which was high output for the time, its transmission efficiency was much lower than initially expected due to the inability to achieve high-pressure hydraulic operation because of concerns about oil leaks.
Juno M80, Honda’s first motorcycle equipped with the HRD automatic transmission.
The following year, the Juno M85 with a larger 170 cc engine was released, but it did not solve the root problem. The high price, driven by the huge HMT patent fees, had also hampered sales. In addition, the previously strong demand for scooters began to show signs of a downturn, and within a little more than a year production was discontinued and HRD disappeared.
Six years later, in 1968, Honda adopted its independently developed Hondamatic (torque converter automatic transmission) in the N360 k-car in Japan. With its motorcycles, in 1977, Honda introduced the CB750 FOUR Aira with Hondamatic, and in 1978, the Hawk CB400T with Hondamatic, but the market response was poor as the engine output could not be fully utilized.
HRD Resurrected in Motocross, Evolves into HFT
Honda's renewed focus on automatic transmissions for motorcycles began in the mid-1980s, when Soichiro, who had retired as president in 1973, organized an off-road motorcycle event for the Scout Association of Japan, but the event was washed out with rain, and the riders, mostly beginners, got stuck on the muddy course. Seeing the boys covered in mud and having a hard time, Soichiro said, “If we make a Badalini off-road bike, they wouldn’t need to slip.” His reasoning was that an automatic transmission would prevent the rear tire from slipping, because the torque fluctuation due to gear shifting would be small, but he must have felt a certain sense of shame about his unfinished dream. Although HRD research still continued, Honda was unable to find a way to put it to practical use. The HRD’s smooth operation and quietness were excellent, and if transmission efficiency could be optimized, it would be an ideal automatic transmission mechanism.
Many of the engineers at the institute who were involved in HRD were indeed frustrated, but Soichiro’s comment had changed everything. In 1985, HRD was officially approved and restarted as a research project for motorcycle-use.
The research theme was “HRD development for off-road engines, ultimately to confirm and demonstrate its performance by participating in races.” Tsutomu Hayashi, who had designed automatic transmissions and ABS (antilock braking system) for automobiles, was appointed as LPL. Hayashi, an expert in hydraulic systems, was of the opinion that “easy-to-use automatic transmission are especially important for motorcycles.” Knowing this, Hiroyuki Yoshino, a Honda director, head of Asaka R&D, and later the fifth president, recommended Hayashi for the position, who began research by analyzing HRD which was being studied for automobiles at the time.
Hayashi, however, finished his work in a month, and without conducting any basic research, suddenly drew up the complete schematics of HRD, which stunned his subordinates. However, when Tadashi Kume, then president, saw the drawings, he immediately green-lit it, saying, “If anyone in this company has a chance of winning the Nobel Prize, it’s Hayashi.”
The team determined that a 2-stroke, single-cylinder 250 cc motocross engine would be the most suitable for HRD to study, in terms of both power output and size. A prototype engine was soon completed, and without bench testing, it was mounted on a CR250R motocrosser for field tests within the R&D Center.
The HFT prototype layout. Although the chassis did not work, it was close at this stage.
As Hayashi warned his team, “I’m not sure about the clutch mechanism, be careful as it might pop out at any time,” they held onto the bike, and with its front wheel up against a building wall, started the engine. Then, they fearfully connected the clutch. The engine only groaned and the bike did not move.
“We thought we may as well test it with the most difficult bike, a racer, which was the most unsuitable bike for an automatic, but when we tried, cautioning everyone to keep their distance, it did nothing. The rear wheel didn’t even budge. I had only been in Honda for a year, and I was astonished. However, what I felt was amazing about Hayashi was that he had created a near-complete HRD for motorcycles, which I think was an invention comparable to the original Badalini,” said Nakajima.
Yoshihiro Nakajima, who was assigned to durability simulation test immediately after joining Honda, re-created several HRDs until he realized that the cause was the oil pump frame instantly rupturing due to hydraulic pressure.
“We did a pressure test at some stage, and it turned out that the assembly was losing pressure, so I dealt with this by integrating it using electron beam welding. HRD’s internal pressure is around 400 kg/cm2, so various parts get deformed. How much oil is leaking from the gaps? Which is better: nine thick plunger pistons compared to more, thinner pistons? There were no books or papers on the subject, so I had to try it for myself, but it was very interesting from a technical point of view.”
Although development progressed little by little, it took two years to develop the mechanism to ensure racing bike performance, and another three years to determine the shift characteristics and feeling in tests that simulated racing conditions. The process of maturing the HRD to actual racing specifications was particularly time-consuming.
The team was frustrated as no matter how many times they tested it, the evaluations were bad, and by 1989, they were beginning to feel a sense of resignation that it might not work. However, when foreign riders who came to Japan for the Japan Supercross in November of that year test-rode the HRD-equipped bike, they generally gave it high marks, saying that it was “not tiring,” “fast in the turns,” and “fun.” Although each rider had only three laps with the HRD, their lap times were within 0.5 seconds of the best time set by their manual transmission bikes. The decision was made to introduce the automatic transmission to the market.
In 1990, HRD was renamed HFT (Human Fitting Transmission), and was entered into the All Japan Motocross Championship as the RC250MA for a full season of racing. While undergoing development, it nonetheless achieved 7th place overall. The automatic transmission bike demonstrated overwhelming speed with its smooth shifting and high traction, winning 8 out of 24 heats in 12 rounds, winning the series championship with the most victories in the season.
HFT-equipped RC250MA won the 1991 All Japan Motocross Championship.
40 Years in the Making, Finally a Product
Although HFT had achieved its objective of technical verification in races, behind the scenes, studies and development were also underway for commercialization in a wide range of motorcycles. The HFT was tested on a CR250R production motocrosser in 1991, and was also tested on an NSR250R on-road model. The GL1500 and 4-stroke CRF450R motocrosser were also being considered as candidates.
“The NSR250R’s HFT even had a learning capability, so that at higher revs, the shift timing would also be at a higher engine speed. We aimed for 'riding fun.' The basic idea was to use HFT to create an easy-riding motorcycle that fits human senses.” (Nakajima)
A 10 PS HFT prototype was also developed for use in 4-stroke scooters and other vehicles, and was small enough to fit in the palm of a hand. There was also a 100 PS HFT unit capable of 200 hours of continuous operation, and Honda was even considering installing it in an automobile. However, the higher the revs and output, or the smaller the engine, the more stringent the requirements for HFT.
“Unfortunately, it did not make it to production because it was difficult to control the cost of processing and manufacturing, which is at the sub-micron (0.1μ) level. When it came to mass production design, there were higher hurdles than proof of concept, and I think it took about three years even to put them on the Rubicon, an ATV (all-terrain vehicle) for North America.” (Nakajima)
After a long process of trial and error, HFT was finally able to bring the TRX500FA FourTrax Rubicon, an ATV for North America, to market in 2000, and the DN-01 tourer, released in 2008, eight years after the TRX500FA was launched. The Rubicon, used on ranches and rough terrain, was powered by a 500 cc single-cylinder engine, while the DN-01, a tourer model, had a 680 cc V-twin cylinder engine, both of which had low to mid-speed engine performance with mild power fluctuations, making them ideal for HFT in terms of size.
TRX500FA
DN-01 equipped with HFT
“I think the reason why everyone was working so hard was because they saw so much potential in HFT that they thought, ‘This is going to be the future of transmissions.’ I was the one who drew the schematics and rode in the tests, so I knew the good and the bad. Once you get used to HFT, it is easier and more fun than manual transmission bikes. Everyone wanted to make this world a reality. Everyone who rode HFTs had these feelings. In that sense, we had gained the know-how for mass-producing HFTs, and felt satisfaction in our work.” (Nakajima)
Incidentally, the TRX500FA FourTrax Rubicon was named after the Rubicon River of Roman legend.
“We were worried about whether sales would go well, but we had no choice but to cross the Rubicon after coming this far.” (Nakajima)
This episode is symbolic of the 40-year struggle with HFT technology that began with HRD. In the end, the transmission efficiency of HFT was improved to 80% of that of a manual transmission.
