1973 Yamaha YZ637


1973 250 World Champion

The bike featured here is the very bike Hakan Andersson won the 1973 250 World Championship with. It was Yamaha's first World Championship and just as important, this was the first long travel motocross bike in history. This is the bike that started it all. Looking back, it is easy to come to the conclusion that this is one of the most historical motocross bikes of all time. The long travel suspension that we enjoy today started right here. It made its debut at the third round of the 1973 250cc grand prix season at Wuustwezel Belgium. It had over fifty percent more travel than any of the other bikes that day. All the work that Torsten Hallman did in 1972 and all the work that Hakan, Hakan's mechanic Eije Skarin (pronounced Aya) and the Japanese engineers did in months of rigorous testing, was about to pay off. Hakan won the overall that day and then went on to absolutely dominate the season. With three grand prix's to go at Hyvinka Finland, Hakan clinched the World Championship. It wasn't until the seventh round at the French grand prix that Adolf Weil's Maico came out with the forward mounted shocks. In this race during the first moto, Hakan got a last place start and came blazing through the pack to nearly win. The FIM officials were sure something was not right and ordered that Hakan immediately take a drug test. Drugs were not the issue here. Hakan was riding fantastic and also he was on a bike that was changing motocross history. Things would never be the same.

Designated the YZ637, this would be Yamaha's last works YZ. The following year Yamaha would change the internal code of its works racers from YZ to OW. For 1973, the Japanese factories were focusing on the chassis but nobody thought of increasing the suspension travel. Prior to 1973, normal suspension travel for a grand prix motocross bike was about seven inches in the front and a little over four in the rear. Improvements in suspension and handling primarily came from reducing unsprung weight, shock damping and spring rate adjustments. How the suspension revolution got underway was actually by complete accident.

A Belgian man by the name of Lucian Tilkens had an idea that by transferring the rear suspension load from the rear wheel to the steering head instead of the rear chassis would make the bike handle better in the rough. This idea came to him by watching his son Guy Tilkens racing an amateur motocross race on Sylvain Geboers 1969 works 380 CZ. Convinced this was the way to go, Mr Tilkens took the CZ and converted it to what would be the very first monoshock bike made for motocross. The design was somewhat similar to the Vincent road bikes but instead of using a very short shock, the shock went all the way from the cantilever swingarm to the steering head. The shock that Mr. Tilkens used for this project was from a Citroen automobile and was gas pressurized. 

Once the prototype was complete, family friend and factory Suzuki rider Sylvain Geboers was asked to test the odd looking CZ at a very rough sand track in northern Belgium. The test results were shocking. The CZ tracked better than Sylvains works Suzuki. Convinced Tilkens was on to something, Sylvain told Roger DeCoster about it and they both decided to supply Tilkens with a spare works Suzuki frame to build a Suzuki monoshock bike.

Once Tilkens completed the bike Suzuki engineers were sent to investigate and a secret test session was scheduled. When the engineers arrived and testing began, the results once again proved the monoshock was better but Mr. Tilkens could not mathematically prove to the Japanese that his theory of transferring the rear suspension load to the steering head was better. The special Suzuki was sent to Japan for further evaluation and even though both DeCoster and Geboers tried to convince Suzuki to buy the patent, Suzuki declined. Nobody noticed the increased rear suspension travel.

Mr. Tilkens then went to Torsten Hallman at Yamaha. Torsten had been hired by Yamaha in the spring of 1971 to develop the factory YZ bikes into grand prix winners and for 1972, Torsten had already made significant improvements to the original 1971 YZ637 works bike. It was Torsten's responsibility to ensure that the Yamaha works team had the most competitive equipment possible so naturally he was interested in Mr. Tilkens system. Torsten remembers the story well....

Torsten Hallman's comments:

In about June of 1972 we received an expermential bike with a special rear suspension converted from one of our 72 works bikes. It was made by a man from Belgium named Lucien Tilkens. It was the first single shock rear suspension motocross bike. The monoshock. This system had already been to Suzuki and Honda. Both manufacturers had previously tested the system and turned it down. We tested the new bike in northern Belgium near Mol for months. The tracks near Mol are all sand and very rough. The test sessions were top secret and none of the other Yamaha team riders even knew about it, I was the only one. Yamaha had flown over from Japan twenty-five engineers to help with the evaluation. I could not understand why so many engineers. I later learned many years later that the engineers were from different departments. If the testing went well, they planned on using the design on many different products like street bikes, mopeds, snowmobiles etc. We tested the bike for thousands of laps over  several months and all kinds of data was recorded. The original test bike was very heavy but still the lap times were good and it seemed to be better than the standard bike. Yamaha was going to leave the decision up to me on whether to buy it or not. It was a tremendous amount of pressure with all the engineers and attention the project got, to be the one to make the decision. After many laps, meetings and evaluation, I decided they should buy it. Yamaha took my advice and bought the patent from Mr. Tilkens.

Now that Yamaha owned the patent on the monoshock, they built a much lighter test bike than the test bike built by Mr. Tilkens, which was 10kg. heavier than our standard bike. We then arranged a top secret test session with our team riders. We even had lookouts to make sure nobody was near. Hakan Andersson was the star of our team, and having just taken second place in the 1972 world championship behind Joel Robert, he was riding very good. The test session began in northern Belgium on a very rough track. The advantage for the monoshock bike should have been very clear. It had longer wheel travel and was much more rigid than the standard bike. After Hakan's initial ride on the bike, he came in and said he didn't like it. He said he preferred the twinshock bike. I could not believe it! The lap times were good and I thought for sure the monoshock was better. Hakan was a very conservative guy and didn't like change. He decided that the twinshock bike was better.

This put me in a very bad position. Here I was, after Yamaha spent all this money to send all those engineers over for months, buy the design and go through this whole ordeal, and there number one rider won't ride it! Yamaha put all their money on Hakan and he didn't want to touch it. He didn't even test it much at first. I think his mind was made up before he even sat on it. Now the Japanese were very confused. They just spent all this time and money on this new suspension system, and they were getting mixed reviews. I was now asking myself "what have I done?" I went to Hakan in private and explained to him the situation that it did not look good for him not to be using it. I practically begged him to use it. He agreed to test it further.

 At first the bike was too stiff to suit his riding style. We did a lot of work on the shock to correct this and after much testing, we got the shock working pretty good. This was the result of many test sessions in which Hakan was involved. In the meantime the 1973 grand prix season started, and Hakan rode the first two rounds on the two shock bike. The shock on the new bike was still the major issue. After countless laps of testing, the bike got better and better and the lap times were showing the result. By the third round of the 250 grand prix' at Wuustwezel Belgium, it was decided to debut the bike in its first grand prix.

The day of the first GP at Wuustwezel, I was very nervous because of all the pressure on me regarding the decision to purchase the monoshock. The Japanese were not shy about reminding me that it was on my recommendation that they spent all this money on a system that had mixed reviews. This would also be the first time the public and the press would see the factory Yamaha team with this new suspension system. The bike received a lot of attention that day and everybody had their eyes on Hakan. The whole world would soon find out how this system stacked up against all the other factory bikes. The stakes were very high. When it was all over, Hakan had won the overall! I was so relieved. I knew it was a good system, but with all the pressure and controversy, you just never know. It felt so good to have this pressure off of me. The team was also very excited and it was decided that this bike would be used in the remaining GP's. Hakan went on to dominate the season and was crowned world champion with three GP's remaining. This was the start of long travel suspension.

Roger DeCoster's comments:

At the end of 1971 Sylvain Geboers and I had been working with our mutual friend Lucien Tilkens on this monoshock project with a 1971 works Suzuki chassis. For some time Tilkens had been talking to us about this idea he had about transferring the force of the rear shock horizontally instead of vertically to make the rear suspension better. He built a prototype of his idea on one of Sylvain’s old works CZ’s and Sylvain had tested the bike and told me about it and said it worked better.

Sylvain and I decided to give Tilkens an old works Suzuki frame that was broken and let him make a monoshock Suzuki for us to test. We were both afraid the Japanese would find out about this but we were also afraid to tell them about it until we had tested it and were sure it worked. We gave him the frame and he converted it to a monoshock. Then we put it together with some works Suzuki parts and had a bike.

Sylvain and I took the bike to Mol to test it. Mol was a very rough sand track and there were pine trees on each side of a very rough narrow straight. The straight was so rough and close to the trees that we always backed it off there because if the bike swapped you could easily hit one of the trees. When we ran the initial test laps on the monoshock, we discovered immediately that the bike was so much better on the rough track. Going down the rough narrow straight with the pine trees on each side, we were going wide open down the straight not backing off at all and the bike was tracking straight. Sylvain and I were both convinced this was way better and we knew we needed to convince Suzuki to go for this. Tilkens had a patent on the monoshock and wanted to sell it, so we arranged a meeting between Mr. Tilkens and the Suzuki chassis designer Mr. Tanaki.

Mr. Tilkens was trying to sell the idea to Suzuki based on the direction change of the shock force. When Mr. Tanaki arrived we went to the test track to show him how good it was. On the track the bike once again performed fantastic but Mr. Tilkens could not convince Mr. Tanaki that it was because of his idea of changing the direction of the force. He kept shaking his head and saying, “That is not the reason, there is no way, and theoretically it doesn’t make sense.” Mr. Tanaki also argued that the initial direction of the force is going to be the end result of the force and it didn’t matter which direction the shock is angled. Hearing this Sylvain and I were telling Mr. Tanaki “But it works better and we can go faster.” Mr. Tanaki was a real nice guy and he was standing there scratching his head and trying to figure things out but mathematically could not be convinced so Suzuki declined to buy the idea.

What we all over looked was that the bike had over 50% more travel than the conventional bike. Nobody noticed it. It was amazing that we all were staring at this bike and nobody understood why it was so much better. It was kind of hard to tell because the bike looked different without the conventional rear shocks but still it’s funny to look back on that. To this day I’m still not sure if Mr. Tilkens ever realized that it was because of the increased travel that it worked so much better.

Roger DeCoster seen here testing the secret monoshock Suzuki couldn't convince Suzuki to purchase the Tilkens monoshock system. 

Terry Good comments:

I was a junior in high school when all this was happening. Like many others I used to dream of being in Europe at these races. I actually got to meet Hakan at the 1974 Trans-am at Honda Hills in Ohio. I remember being so nervous just to ask him for his autograph. It was a special time for me and many others that lived through this era. I never thought I would one day own this bike.

In 1997 I went to Belgium with Jim Pomeroy for the 50th anniversary of the 500 Belgium Grand Prix. We stayed at Joel Robert's house and the night before the Grand Prix there was a party at the Citadel in Namur. At about one O'clock in the morning Joel and I were just starting to leave, and there was Hakan sitting at a table by himself. Joel stopped to talk, and introduced me to him. We then sat down at the table and talked for a while. One of the first things I asked him was if he still had the 1973 Championship bike. When he told me he did, I just sat there in shock for awhile. I asked him if he would sell it and the answer was no. He told me that over the years at least a hundred people had tried to buy the bike and he wanted to keep it. At that point I was just glad that it still existed and I hoped that one day I could see it. Weeks later back in the states, I could not get that bike out of my mind. I dug out all of those same magazines that I used to take to school and went through them over and over. I couldn't take it anymore. I phoned up Jim Pomeroy and asked him to call Hakan and ask him about selling the bike. The answer was still no. I then asked Jim to call him back and ask if it would be alright if I called him. The answer was yes to that. I phoned Hakan and introduced myself again. He actually remembered me from Belgium and believe it or not from Ohio in 1974. I explained to him my passion and goal of preserving motocross history and someday building a museum for all these special bikes. Finally after Pomeroy calling him and Joel Robert even calling him and putting in a good word, he decided to sell me the bike. When the crate arrived, inside along with the bike was the jersey he was wearing at the Honda Hills Trans am from 1974. It was and still is a dream come true.

After the purchase, Hakan and I became good friends and he invited me to Sweden to watch the 500cc Swedish Grand Prix. Hakan is still involved with motocross and infact it is his club that puts on the race. I took him up on the offer and there I met his long time mechanic Eije Skarin. Over dinner one night Eije told me the history of this bike. They used a total of four bikes that year and this one was used in a total of three Grand Prix's. Finland, Russia and Sweden. Hakan also used it in some post season races in Sweden. I also learned that Eije was very instrumental in the development of the bike, working along side the Japanese for hours on end to develop it into a World Championship winner.

 Hakan Andersson laying waste to the competition on a bike that would forever change motocross.  French Grand-prix 1973.


1973 Yamaha YZ637 Photos



 

Hakan Andersson's comments:

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. 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 GP's 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, 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 GP's 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 was feeling very good and confident. When the day was all over I had won the overall. The bike preformed excellent. Yamaha was very happy and we decided to use the monoshock for the rest of the season. The next GP was at Payerne Switzerland and I won both motos there. We were really starting to gain momentum. In between grand prix's we tested all the time, mostly in Belgium but we tested at other places as well. We would also test at a track that would be similar to the track at the upcoming grand prix, and set the bike up for that type of track. We basically had a different shock for each grand prix. It was in Yugoslavia that we had our only mechanical failure. The frame broke near the swingarm pivot. My mechanic Eije welded the frame in between motos (neither one of us thought it would last but it did) and I went on to win the second moto.

We had two Japanese engineers that traveled all over Europe with us and after the races and during testing they were always in contact with the factory in Japan. Every little detail was reported and parts were being changed, updated and flown over all the time. The bike was changing every week and it was improving every week too. Yamaha was very serious about winning. I had so many parts that there was no way I could ever use them in ten lifetimes. It was crazy. The other factories were seeing our advantage and were now trying to play catch up. In France we got our first counter attack. Adolph Weil showed up with the first bike with forward mounted shocks on his factory Maico. That day in the first moto, I started dead last and worked all the way up to second at the last lap. One more lap and I would have won for sure. The F.I.M. officials did not believe it was possible to come from behind like that with out being under the influence, and they forced me to take a drug test. That was the first and only drug test I ever took. After that race we had so much momentum, we had the best bike and I was riding at my best. It was then that I thought I had a real good chance of being world champion. Going into Finland, with four grand prix's to go I had enough points to possibly clinch the world championship that day. When the day was over, I had won both motos and was crowned World Champion. My life long dream had come true.

When I look back now, I realize that it was Yamaha and myself that started the whole suspension revolution more than thirty years ago. And that how important it was for the whole motorcycle industry. In my opinion this was the single most important advancement in motocross. All the bikes today now use a single rear shock suspension similar to the one I used, only improved.

Note:

On May 2nd 2001 Eije Skarin died after a long battle with Brain cancer. He was Hakan's best friend and mechanic from the beginning. Eije was a major contributor to the development of the monoshock, often working late hours behind the scenes. Eije was a kind and soft spoken man. His brilliance and hard work played a major role in this Championship. He is missed by all those who new him.

Check out our Interviews page for more on Hakan Andersson.


Detail Photos



Details


    • The works motors for 1973 used an all new crankcase design made from sand cast magnesium and featured a new style clutch actuating arm that went through the top of the cases. This gave a much more positive clutch feel. Prior to this they used the DT style actuator that had a worm gear attached to a small case. This system resulted in too much flex and the clutch action suffered. All the gears are hand machined and drilled out as is the clutch basket. Even the primary gear attached to the crankshaft is machined and drilled for less weight. Titanium is used inside the motor where ever possible. The cylinder is a special works model with a porting arrangement designed more for torque. The cylinder was then modified by Hakan's long time mechanic Eije for even more torque. Eije modified the shape of the rear transfer port to distribute the fuel mix more evenly. This was done by cutting the cylinder fins and accessing the port from outside the cylinder. Once the modification was finished, a plug was then welded in place. The result was a much broader power band. This modification was done very late in the year, possibly after the GP season ended. Magnesium cylinders were tested but discarded because they didn't dissipate the heat well. There was also at least a half a dozen different reed cage and intake manifold set ups available that altered the torque curve. A 36mm mikuni was used exclusively. Magnesium carburetors were tested but there was a problem with the aluminum slide sticking when the motor got hot. Notice the shift lever, it is cut and re-welded near the end for the perfect length for Hakan. Every single fastener is either titanium or aluminum. The motor is very light.

 

    • There were several different ignitions available, each offered a different torque curve. Hakan settled on the one for more bottom end power. With the new shock location a new pipe design was used that starts on the left, crosses over the top of the motor and exits on the right. This was done to maintain the proper tuning length. It was made in two parts so it could be easily removed. Different pipes were used depending on the track. The pipes themselves are a work of art. They are hydro-formed out of very thin gauge steel. No silencer was used and the bike is very loud. Yamaha went all out to save weight on this bike, even the fitting on the end of the spring where the pipe attaches to the cylinder is titanium!

 

    • The monoshock location restricted space for the air box, so a duel filter system was incorporated to maintain the proper air volume. The air box itself is made of very thin layered fiber glass, as are the air filter covers. The entire assembly weighs just ounces. Also because of the monoshock location, the black box and ignition coil that were previously mounted under the tank, were relocated as you can see in the photo below. The black box was placed in such an unconventional location for the time, that an American magazine actually published a photo of it claiming that it was a lead weight ballast! 1973 was the first year that the works Yamahas used the Timken style bearings at the steering head. This provided a much more positive front end. The top triple clamp is sand cast magnesium while the bottom is billet aluminum, and all the related fasteners are titanium. This was also the first year for Yamaha to knurl their fork legs where they make contact with the triple clamps. As you can see, the chassis was a large part of Yamaha's focus for 1973.

 

    • Hakan's bike displays a certain mystique that was unique to the early Japanese works bikes. They were very cobby and very functional looking. It was this combination that had the fans going to the races to see the bikes as well as the race itself. They were constantly changing from week to week as motocross bikes were now entering a hyper-evolution phase in design. More often than not, it was the bikes themselves that stole the show.

 

    • The front forks are hand made except for the sliders which are cast aluminum. The stantions are 35mm and made from a thicker and higher quality steel than on any of the production bikes. Several damping rods were available that offered different travel and damping characteristics. Obviously different spring rates were offered too. As the shock was developed, changes had to be made in the front forks to compensate. With the added travel the bike now had, the front to rear balance now became more crucial. Keep in mind, since rear suspension travel increased over fifty percent nearly overnight, development was now in uncharted territory. The front hub is sand cast magnesium and has a titanium spacer in between the wheel bearings. The rear hub is sand cast magnesium and all related hardware is machined titanium.

 

    • The works swingarm is handmade and extremely cobby in appearance. There are welds all over the place. It even has a formed bend in it to avoid being hit by the chain. It looks as if it was made from a works twin shock swingarm. Because of the triangular design, there was now new found rigidity at the rear never seen before on the two shock bikes. The rear sprocket is billet aluminum. In the photo below, stripped of all its bodywork, you can imagine how unique this design was compared to any other bike from that time. The frames were sent to Sweden blank and Eije would have to weld all the brackets on to mount all the hardware. This bike was truly hand built.

 

    • The heart of the whole bike is the monoshock itself. It is a handmade unit with an aluminum body and a reservoir body design that is unique only to the 1973 bike. The shock uses springs, oil and nitrogen. In the reservoir, there is a rubber membrane that separates the oil from the nitrogen. The shock worked its best with a relatively low amount of nitrogen, too much nitrogen would cause the rear to kick too much. The low use of nitrogen, according to Hakan, was one of the secrets to get the shock to work so good. On the production YZ's the pressure was over 300 PSI and on the works bikes it was less than half that. New internal valving was used to compensate for this. This system was never passed on to the production bikes. If you look at Bob Hannah's race reports in the "Tech Sheets" section, you can see that his shocks were set up very similar in 1976. Bill Buchka (Bob Hannah's mechanic in 1976) told me recently that the development that went on with this bike was basically carried thru until 1976. Below you can see the method used to get a variable spring rate, one spring inside the other. The inside spring is for the small bumps and the outside spring is for the bigger ones. Thousands and thousands of test and race laps went into getting the shock to this stage. There were tons of different valves, tuning jets and spring rates available. The only problem that occurred with the shock itself, was that the aluminum body would distort occasionally and cause the shock to bind. This was remedied by the use of a steel body. The basic design was used on the works Yamahas thru 1976. Notice the extra tubing needed for the top shock mount. This made for a very solid mount and there were no failures with this design.

 

    • The swingarm was lengthened at the shock mount for even more travel. The bike was going under a constant evolution throughout the season, it changed every week. The rear brake pedal arm was cut and angled forward for better leverage, resulting in more braking power.

 

    • This bike still wears the original grips used by Hakan in 1973. Note how they are cut where his thumb went. Even the brown duct tape that kept the dirt out of the handlebars remains. The throttle assembly is sand cast magnesium. When Hakan and Eije crated the bike for shipment to Chicago, they were careful to leave the handlebars in the exact location as they were when it was last ridden. This was done by removing the top triple clamp and leaving the bars attached. The bike sits as it did after that very historical year!

 


Historical Photos


 

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