In 2007, Bou won the championship title for the first time. The Honda factory machine continued to evolve along with Bou, supporting his winning streak, which has continued for an outstanding decade. Through interviews with Honda, WCT journalist Shuji Fujita reveals the secrets of the machine that became a driving force behind Bou’s decade-long winning streak.
Looking back, 2005 is a big turning point in the machine's history. In 2004, Takahisa Fujinami won the Trial World Championship title, becoming the first Japanese world champion, riding a 2-stroke Honda Montesa COTA315R. In the following year, Fujinami was not able to replicate his victory on his 4-storke Montesa COTA 4RT, losing out to Adam Raga(Gas Gas) on an old 2-stroke machine. In 2006, Raga won two consecutive world championship titles, but in 2007, Toni Bou brought the title back to Honda on a 4-stroke machine, which was to be the start of his decade^long winning streak. In 2006, Fédération Internationale de Motocyclisme (FIM) announced regulations necessitating that all bike engines be 4-stroke. Honda was first to compete with a 4-stroke machine, while other kept to their 2-stroke engines. I interviewed engineers at Honda about how did developed the 4-stroke machine to win, despite the adverse conditions.
The COTA 4RT was innovative in being the first to use FI (electronic fuel injection) for a trial bike, along with new 4-stroke engine.
The riders requested “a 4-stroke like an old 2-stroke,” which was unreasonable. At first, we tried carburetors, but the engine would easily stall due to violent postural changes through the trial run. We adapted FI so the bike has great pickup and traction. In 2005, we solved the no-battery problem and launched the 249.1cc RTL250F, and developed a factory machine based on it.
In 2006, it was a big shift that we increased the engine displacement to 298cc and changed everything. Combining the data logger and ECU (engine control unit), the device we developed for a factory machine, made it possible to gather more information and quick response to rider's requests while they rode. For example, the normal engine performs one arithmetic process per second, but we increased it to 4 times per second. Toni Bou won the championship title after he switched to 4-stroke since 2007, riding the machine developed by two champions, Dougie Lampkin (the rider who achieved 7 consecutive wins from 1997 to 2003, riding a 2-storke Honda factory machine in 2000-2003.), and Takahisa Fujinami. Bou commented that he was “lucky to ride on a good machine" and his first impression on the bike was that “It's light and easy to jump at high step.” It was a major change to increase the engine displacement to 313cc with twin plugs to meet Bou's expectations.
ECU integrated throttle. Factory machine version (left), and production version (right)
In 2004, the year before the RTL250F was launched, Tomoyuki Ogawa competed selectively in the Japan Grand Prix riding a RTL250F
The twin plug was also a trial machine-first, and it succeeded in improving the rider's performance.
We ran into a problem with irregular combustion as we increased engine displacement to meet the rider's requests. We adapted the idea of the twin plug from a researcher, and that fixed it. Phase ignition improved feel and combustion, even at low revs. Combustion changes according to the shape of the combustion chamber. Combustion changes according to the speed of combustion, use of the twin plug, and where and when ignition occurs. With the twin plug, one plug ignites at the center of the combustion chamber and the other at the edge. It is possible to adjust the combustion speed by ignition location. Ignition at the center causes fast combustion, but at the edge, combustion is slower. The combinations can be wide, such as twin ignition or single ignition. The ignition patterns are: single ignition (igniting the main or sub plug); and for for twin ignition, phase ignition, and simultaneous ignition. Since there are many choices, it was hard work to find the best feel for the rider, so we wrote it down in the map. If the rider wants to go slow while opening the throttle, the ECU senses it, and controls the best type of ignition according to the map.
The twin plug
Twin plug vs. single plug
It must have been hard to switch from 2- to 4-strokes.
It was, but there is a big difference between the length of the tracks in Japan, and the rest of the world. Riders such as Lampkin, Fujinami and Bou, along with other test riders, taught us the benefits of a 4-stroke engine. For example, they taught us the difference in how 4-strokes and 2-strokes jump up at the high steps. 4-strokes bumped up into the wall of a staircase and clambered up by traction, while 2-strokes clamber up a wall by momentum. Bou and 4-strokes are very compatible because he doesn't open the throttle that much, since he had been riding 2-strokes. With suspension, Bou had been riding with Lampkin's specifications at first. Later Bou started to use the same part which Fujinami used to use. I was surprised while watching the free practice, as Bou totally went to the high step which other riders don't do because it's too dangerous and could cause serious injury if the rider fell. His riding skills were in a different dimension. When I asked Bou about the bike he told me “I like the machine's stability. Engine and suspension are always functionally the same.” His physical abilities are also at a high level. He can jump high, even more so with a lighter machine. I was glad that I made it lightweight in 2006. Bou's riding skills are outstanding and he and his machines are very compatible. This machine is here because of Lampkin, Fujinami and Bou. Fujinami became a champion on a 2-stroke in 2004, then the following year he switched to 4-strokes. Our hearts were as one, to develop the machine to reproduce last year's glory. It was also a big factor that Lampkin won the championship title on a 4-stroke for the first time in 2005.
The 4-stroke Montesa COTA 4RT debuted in 2005
Bou riding the significantly changed COTA 4RT with great physical ability at Japan Grand Prix in 2007
Mr. Haga, what areas you had difficulties in?
We struggled the most with traction. 2-strokes and 4-strokse have very different characteristics when taking off on slippery surfaces. Rough throttle operation is no problem with a 2-stroke, but with 4-strokes, the rear tire slips right after the throttle is opened. Riders were not used to it either. We improved it with different FI settings. The clutch is very important to riders. They have high expectations, and their expectations vary a lot. Basically, the machine is easier to handle by half-clutching. On the other hand, riders want a direct clutch feel when they are jumping. It is very difficult to achieve these two opposing goals. They want a light clutch because it is easier to control, but they want it heavy for a more direct feel. With a 4-stroke machine, they control the bike by releasing the power through the clutch, so the clutch also have to cover that. There was no certain answer. If we increase the engine displacement to improve the rider's performance, we need to change the clutch again. It was a cat-and-mouse game. The clutch's drivability and throttle operation are necessary. We need them both, which made it very hard to settle. We could easily go around in circles and get lost. The throttle and clutch are joined in trials, because control of the clutch is at the absolute edge, which is unique to trials.
Engineers interviewed seemed so happy when they looked back on their strenuous episodes in the past
In 2013, Bou was on the podium in every race, powered by the new twin plug engine
Was a weight problem achieving the twin plug design?
Yes there was. Simultaneous ignition requires only one big ignition coil, but phase ignition requires two ignition coils which adds more weight. We didn't want more weight, and there was also a problem of where to locate the two ignition coils. It has to be housed neatly so it will not hit to rider's legs. When I heard we were going to make it a twin plug, I could feel my face twitching. Eventually, two ignition coils were put on the left side of the engine. In the first year there was an exhaust sound problem, but we solved it by dividing the silencer's internal structure into two parts, and succeeded to make the machine more comfortable to ride. About downsizing, I rewrote the drawings to save 1 gram. For example, when we put the engine hanger to the engine rather than the frame, it reduced the bike’s weight by 1 gram. I think one big reason for ten consecutive wins is Honda's long history in trials. Masaya Yamamoto (Japan champion 1982-1986. Now riding around the world) was riding an air-cooled 4-cycle machine. Takahisa Fujinami, who had been watching Takumi Narita (riding in the world championship since 1990, on an air-cooled 2-cycle bike.) became a rider. As they were switching from 2-strokes to 4-strokes I felt that this bike we now have is a masterpiece that had been handed down from the past as they challenged, and overcame many difficulties, seeking the perfect machine to win.
COTA 4RT was made lighter through the tireless efforts of the engineers
This year, Bou is reached his 10th consecutive outdoor title, or 20 consecutive titles, including indoor competition
Article: Shuji Fujita