During the 1960s, as the Japanese manufacturers entered the World GP one after another, the performance of the race machines began to improve dramatically. In the ensuing race to develop more competitive machines, the trend was toward higher and higher power output.
When Yamaha first began competing in the World GP in 1961, its 250cc RD48 had a 2-stroke air-cooled in-line 2-cylinder engine with a maximum power output of 35 PS. That was far from enough power to compete against the rival manufacturers. So Yamaha went back to Japan and returned two years later with the RD56 that had a max. output of 45 PS. With this new machine they could now compete on even footing with the other manufacturers. After that the power development continued, first to 48 PS and then 55 PS. With these machines Yamaha dominated the competition, winning the championship two years in a row.
That dominance didn't last long, however. Soon, Honda came out with a 6-cylinder machine with awesome power, forcing Yamaha to fight back desperately with the liquid-cooled V4 engine RD05 and 05A that pumped out over 70 PS.
It was an era in World GP when the only regulation limitations were on engine displacement. That meant that the way to get more power was to increase rpm. That created the need for smaller, lighter pistons. For that, a twin cylinder engine naturally had the advantage over a single-cylinder and a 4-cylinder had the advantage over a twin. This simple fact led to a rapid increase in the number of cylinders, to the point where some manufacturers were building 50cc machines with three cylinders, 125cc machines with five cylinders and 250cc machines with up to eight cylinders.
The big problem with these machines was that the more cylinders and higher rpm meant a narrower power band. To remedy that problem the manufacturers increased the number of gears in the transmission, to the point that where 50cc machines with 14 gears and 125cc machines with 12 gears were not uncommon.
The RA31 racer that Yamaha began developing in 1966 as the successor to the RA97 was no exception. The calculation was that reducing the displacement of the RD05's V4 engine to 125cc and giving it an 11-speed transmission would create a machine that would reach a max. output of 39 PS / 16,000 rpm.
It happened that one of the engineers in charge of the performance testing began to have big doubts about that development plan, however, when he heard one of the test riders saying that they would get confused about what gear they were in when riding with the 11-speed transmission. It made the engineers ask themselves how many gears were actually necessary considering the engine character. Then they began to think about how many gears would actually be the ideal number from the rider's standpoint if the engine performance were not a factor.
To answer these questions the engineers went to the Yatabe test course with the latest telemeter equipment and the theory-oriented rider Akiyasu Motohashi. What they found was that each gear change meant a loss of drive force to the rear wheel for about 0.2 seconds. Considering the fact that World GP riders raced for a competitive edge of just one or two tenths of a second per lap and the fact that they changed gears anywhere from 20 to 30 times in each lap, having to make fewer gear changes would definitely be a competitive advantage.
That realization led to the conclusion that they should try to create an engine with a broader power band that would enable fewer gear shifts, even if it meant sacrificing a bit in terms of max. power output. As a result of that decision the Yamaha engineers built the RA31 with a 9-speed transmission and developed it to the point that it produced 44 PS / 16,800 rpm. This machine won ten of the twelve rounds in the 1967 World GP to claim the championship.
One of the engineers involved at the time describes the lesson they learned in the following way. "The 9th gear was used essentially as an overdrive gear for long downhill straights, so in effect it was actually an 8-speed transmission. In other words, it was our 8-speed against the other makers' 12-speeds. Do you know what that difference meant? The rider is concentrating to race consistency at lap times that vary at no more than 0.1 sec. per lap. In a race that state of concentration continues for 45 minutes. In the heat of the race where tension is so high that mistakes can happen at any time, eliminating the need for even one gear shift puts the rider in a safer situation and increases the chances of winning. I believe this was when we first learned how important it is to build rider-friendly machines."