Memorable MC: 1973 Yamaha Monoshock

In 2010, the idea of an off-road machine having two rear shocks, rather than one, is as radical as the concept of a monoshock was in 1971. This is the story of how Yamaha was the first company to race a Monoshock in Grand Prix motocross. It’s a lovely tale and absolutely true too.

The 1973 Yamaha Monoshock a novel design with a fascinating history.

Some of it came directly to me from Roger DeCoster, one of the great riders in the history of world motocross. Although we weren’t exactly going on vacation together, I knew Roger quite well and liked him tremendously as much for his intelligence and integrity as his magnificent riding ability. Roger was closely involved with the story of the monoshock and wrote to me in the Autumn of 1978.

Fortunately, I still have the letter.

I was also heavily involved with GP riders in Belgium and had extremely warm relations with Yamaha at its European race headquarters in Amsterdam. Finally, I knew and worked with British GP star John Banks who was riding what was, at the start of the 1973 season, considered to be the best handling bike in the 500cc class – the Cheney BSA. This is how the jigsaw fits together from all four sides:

In the late 1960s and early 1970s, one of the great characters of Belgian motocross was Lucien Tilkiens, an accomplished designer and manufacturer of industrial steam cleaning machinery and water softening devices and also a lecturer in mechanical engineering at the University of Liege. Mr. Tilkiens had been involved in motocross for many years and was very popular with the riders – always willing to lend a hand with machine troubles and give assistance in fabricating special parts.

In 1971 Mr. Tilkiens’ son, Guy, was racing a 380cc CZ and managed to crash the bike heavily – almost in front of his father. Dad saw the accident in detail and instead of berating his son for falling off, he came to the conclusion that the

The monoshock design allowed the rear wheel to stay planted in the center without side-swaying and caused less fatigue on the rider.

rear wheel had kicked so viciously because the force being applied to it could not be adequately dissipated in the frame. Mr. Tilkiens felt that the near vertical mounting of the rear shock, which was normal practice at the time, prevented the frame from handling extreme loading. Mr.

Tilkiens felt that if the force were transmitted along the spine of the frame to the steering head, the chassis would be much better able to deal with it, particularly if the impact was intercepted by a damper en route.

Having the mechanical ability to translate his ideas into metal, it was not long before a frame was built housing a long damper interposed between the swinging arm and steering head. This became the Monocross which Yamaha later put into production.

Guy Tilkiens reported favorably on the chassis and all through the 1972 season, work progressed on the new design. Having all the right contacts, Mr Tilkiens was able to get truly expert input and took his bike to the Mol circuit where Suzuki factory riders Roger DeCoster and Sylvain Geboers tested it.

Their response was enthusiastic and DeCoster, who had engineers from the Suzuki factory staying with him at the time, arranged for Tilkiens to have a new Suzuki motocross machine into which a monoshock unit could be built. Then there would be a direct comparison between a known quantity with which DeCoster was familiar, and the new system.

The single shock Suzuki was built and once again the reports were favorable. Mr Tilkiens became enthusiastic about selling the idea to the factory. What happens next depends on whether you are standing on the Yamaha or the Suzuki side of the fence.

This is what Roger told me: “Suzuki sent Mr. Tamaki over to assist on the tests with us at the circuit of Mol where Sylvain lives. The result was that we both liked it and told Mr. Tamaki our feelings about it. He insisted that there was no reason that the Tilkiens system would be better just because of the monoshock and the direction in which the unit was working.

These were Mr. Tilkien’s main arguments for accepting the single shock system.”

In the early 70’s, competitors were typically their own pit crew, so riders had to know the ins and outs of their machines.

“Later, thinking about it myself, I realized that Mr Tamaki was right and that the better feeling of the monoshock was not caused by the system or direction, etc. but because of the 50% increased rear wheel travel compared to our conventional machine and also because of the very good craftsmanship of Mr. Tilkiens and his ability to adjust the damping characteristics of the unit.”

Yamaha saw the story differently.

At the time, Yamaha works rider, Katsuhiko Sao was on a racing trip to Europe as part of his reward for winning the all-Japan Moto Cross championship. With him was Toshinori Suzuki, a free thinking development engineer with the brief of keeping in touch with the latest developments in motocross. Mr. Suzuki was unhappy with the suspension media then in use and so had more than a little interest when he saw Guy Tilkiens’ Suzuki, apparently without any rear suspension.

He studied the bike at the meeting and then, with characteristic efficiency, things began to move very quickly.

The next time that Toshinori visited Yamaha’s European headquarters in Amsterdam, he mentioned that the Tilkiens’ Suzuki looked very similar to a “secret” Suzuki which Yamaha knew was being tested in Japan. This comment might have gone unnoticed had it not been for Yamaha Europe’s Managing Director, Mr. Nagoka, who had issued a brief to his staff that an active search must be made for new methods of suspension since frames and suspension had changed very little since the early 1950s.

A meeting was called on a Thursday in July and by the next day, Lucien Tilkiens had been identified as the builder and designer of the frame. This was decades before e-mail, or even fax machines, so secretaries sat at Telex machines feverishly bashing out terse teleprinter messages between Japan and Europe as the plan was developed.

Yamaha’s 250 engine with its efficient reed-valve layout was key to the Monoshock’s success in 1973.

The result was that three senior executives headed for Belgium, bright and early the next morning. This was no fact-finding mission but one with real power. The team was composed of Messrs. Nackamura, Kuratomo and Tanaka and, after some considerable difficulty, they located the large country house from where Lucien Tilkiens operated his steam-cleaning machinery business.

Mr. Kuratomo, being fluent in French, conducted the negotiations and after several hours of interesting discussion, it was realized that Mr Tilkiens would be amenable to an offer from Yamaha.

Suzuki had been considering the system for two months and after such a long period without any positive move, Tilkiens was, not unnaturally, becoming rather impatient. His problem was that although he was a trained engineer and university lecturer, he could not prove mathematically what was happening on the race track. Suzuki’s engineers at Hammamatsu were far less interested in the subjective opinions of the riders than in empirical evidence of what the men in the saddle claimed to be happening and these facts were not forthcoming from the mathematical data which was submitted to them.

That Mr. Tilkiens could not prove his case in theory turned out to be not too surprising. Yamaha eventually protected the Monocross system with a battery of 30 patents covering the chassis design, the damper unit and the method of attachment to the frame but still no-one managed to prove in mathematical terms the effect the system had on handling.

Small wonder then, that Lucien Tilkiens had such an uphill struggle.

Yamaha was very proud of its Monoshock machine and its top rider Hakan Andersson in 1973.

Both Yamaha and Tilkiens exhibited considerable integrity and although an initial letter of agreement was drawn up between the two parties, it was agreed that the Belgian should contact Suzuki again and allow a further week for a reply. No reply came from Suzuki but the next day, a Sunday, an equally high-powered delegation from Honda Belgium arrived at Tilkiens’ house and began some hard negotiating. However, by this time Lucien had an agreement that if the Monocross did not go to Suzuki, then Yamaha would have it and the Honda team were politely told that they were too late.

Suzuki showed no further interest within the allotted time and by the following week Tilkiens had begun work on a Yamaha Monoshock machine and a team was being groomed in Japan to carry out parallel development.

The facts after this stage are well known but still the monoshock’s importance to motocross has a faint cloud hanging over it. 1973 was Yamaha’s season, at least in the 250 class – about that there can be no doubt. The 250 Yamaha engine, with its very efficient reed-valve layout, was far and away the best powerplant of the year and their number one rider, Hakan Andersson, was at his peak – and probably the best rider in the competition.

These factors, far more than the monoshock, resulted in success.

In an interview with American motocross collector, Terry Clark, who now owns the actual 1973 World Championship winning bike, Hakan said: “The first time I saw the monoshock was in February of 1973. It was at a very remote track in the woods, near a small village in northern Belgium named As. The Japanese chose this place because the Monoshock project was top secret and they were very careful not to be seen.

Hakan Andersson and the Yamaha Monoshock won the Wuustwezel round and eventually the championship in 1973.

“I was very skeptical about the bike when I first saw it. After testing it, my first impression was that it felt very strange. It was very harsh in the rear and the seat kept hitting me every time I went over the bumps.

The rear shock had too much compression damping and there was no rebound damping at all. I was very surprised that my lap times were about equal to my best lap times on the two shock bike.

“The Japanese really wanted me to start the Grand Prix season on the Monoshock bike but I felt it wasn’t ready yet. They put no pressure on me and left the final decision up to me as to what bike to use. For the first two GPs I used the standard bike.

“It took about two months of testing nearly every day to get the compression and rebound damping and also find the right spring rate, preload and gas pressure before I felt the bike was up to its potential. The Yamaha engineers, and my friend and mechanic Eije Skarin, did a great job to sort out all the problems in the beginning.

“At the third round of the GPs in Belgium, I felt the bike was ready. We picked Belgium for the monoshock debut because the track at Wuustwezel was similar to the one we tested on for so many months. This was a calculated and well thought out decision.

The bike was working very well that day and I was riding very well too. I had just recovered from the ‘flu that I had at the Italian Grand Prix and I was feeling very good and confident.

“When the day was all over I had won overall. The bike performed excellent. Yamaha was very happy and we decided to use the monoshock for the rest of the season.”

At the end of the season, the combination of Andersson’s brilliant riding, the excellent reed-valve motor and the long

Hakan Andersson and his championship winning Yamaha Monoshock motorcycle.

travel suspension gave Yamaha the 250 title. How much the single shock played in the success is very debatable.

What was the most tangible result of the monoshock system was that by the end of the 1973 season, every bike in the GP circuit was obsolete. 1974 would require the application of a whole new concept in chassis design and the key element would be suspension travel – not the monoshock.

John Banks, who campaigned a conventional twin shock Cheney throughout the whole season, remembers how quickly the change came about.

“We came back from Carlsbad, where the American GP was held in June, and the bike was great. I had just got a second equal and it was as competitive as anything in the world. It handled real good and it was quick enough down the straight, then, before we knew it, it was all over. The bike was just rubbish – scrap!

You just had to have long travel rear suspension or it wasn’t worth starting. It was fantastic how quick things changed.”

Our thanks to Terry Good – – for this assistance with this article.

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