GP’s Classic Steel 105: 75 YZ250B

For this week’s GP’s Classic Steel we are going to take a look back at the bike that started the monoshock revolution, the 1975 Yamaha YZ250B. 

For this week’s GP’s Classic Steel we are going to take a look back at the bike that started the monoshock revolution, the 1975 Yamaha YZ250B. 

 

 

In 1974, Yamaha unveiled the first production machine to offer true long travel suspension, the YZ250B. Quite literally a works replica, YZ was pricy, exotic and nearly impossible to acquire, but for those worthy of its ownership, it offered performance potential far beyond anything else available at the time.

 

In the late sixties and early seventies, the state of motocross suspension was pretty grim. Even at the Factory level, most bikes offered at best 3-4 inches of mediocre travel. Fork technology was poor and shock performance was generally considered even worse. With so little travel, one good hit was often enough to snap a frame and saving your bike was just as important as going fast. 

 

The thing that set the new YZ apart from anything else on the track was its revolutionary rear suspension system. The original “monoshock” design was the handiwork of Belgian engineer Lucien Lilkens, who had seen the evil handling of family friend Sylvain Geboers’ Factory CZ and thought he could come up with a better design. The first prototype (pictured above) featured a modified CZ chassis married to the shock off a Citroën car.  Terry Good Photo

 

In these nascent days of motocross suspension, the quest for more power was quickly outpacing the technology to effectively use it. The new two-stroke works machines from CZ, Husqvarna and Suzuki were far lighter and quicker than their four-stroke predecessors, but also harder to manage. The explosive deliveries of the early piston-port two-strokes, combined with the flex-prone frames and poor suspension of the day, made getting power to the ground a challenge for even the most talented riders.  

 

By 1971, Tilkens had gone through several prototypes (using his son Guy as a test rider) and moved on to a Suzuki chassis built from parts sourced from Geboers. Both Geboers and teammate Roger DeCoster were incredibly impressed with the prototype and convinced Factory Suzuki to look into purchasing the design.  While the bike offered a clear performance advantage, Suzuki’s head engineer, Mr. Tamaki could not work out a mathematically sound reason for the lap-time advantage (completely overlooking the two extra inches of suspension travel) and decided to pass on the design. After Suzuki passed, both Honda and Yamaha contacted Tilkens about selling them his design. Eventually Lucien settled on Yamaha and gave birth to the modern suspension revolution.

 

In the late sixties, one of these riders was Sylvain Geboers. Geboers, along with teammate Roger DeCoster, were not particularly happy with the performance of their Factory CZ’s. The big 400 Chay-Zed was proving a handful on the track, with most fingers being pointed to the Czechoslovakian bike’s hard-hitting and difficult to manage motor. 

 

Prior to the release of the first YZ250A, Yamaha’s production motocross bikes had been based on their successful line of DT enduro bikes. Stripped of all the DT-1’s street running gear, the DT2-MX offered a decent base for the racer on a budget, but far less performance than the best machines from Europe.

 

The original YZ250 was the culmination of nearly a decade of development on three continents. The Jones family, who were hired to develop the all-new DT-1 250 Enduro into a race machine in the late sixties, laid much of the groundwork for the original YZ. Gary (pictured), his brother DeWayne and father Don took the groundbreaking dual-sport and transformed it into a machine capable of wining America’s first AMA 250 National Motocross title in 1972.  Motocross Action Photo

 

At the time, one of the people involved in Sylvain’s racing program was a friend of the family by the name of Lucien Tilkens. Tilkens, an engineer by trade, had been a motocross fan his whole life and an avid inventor. Early on in Sylvian’s career, he had helped with the setup of his bikes and now he had some ideas about what might be the real problem with the works CZ’s.

 

Pure Unobtanium: While the Americans were stuck racing a modified DT-1, the Europeans got this beauty. The top-secret Factory Yamaha YZ637 was introduced at the third round of the ’73 Grand Prix season and offered less chassis flex and twice the travel of any other bike on the track at the time.

 

Tilkens’ hypothesized that the real problem was not the motor, but actually the chassis. From watching Geboers’ and DeCoster’s machines, he deduced that it was actually as excess of twisting and flexing in the rear suspension that was causing the problem. Lucien believed that having the shocks placed vertically was overstressing the frame and a better design would be to have the shock laid down in line with the steering head. This would allow the chassis to better dissipate the forces from the rear suspension With this in mind, he set about designing a rear suspension that could better handle the rigorous demands of motocross.

 

The secret to the original Tilkens’ monoshock design was this laid down shock placement. By placing the shock in this position and mounting it to a large triangulated swingarm, Tilkens was able to double the available travel and increase chassis rigidity significantly. The downsides of this new design were the weight, which went up significantly, and shock fading, which was a problem due to lack of airflow to the damper.

 

By 1968, Tilkiens had his first attempt fabricated and ready for testing. In order to combat flex in the rear suspension, he fabricated a large triangulated swingarm that braced the rear against the twisting forces of the motor. Then he cobbled together a rear suspension system by modifying the shock from a Citroën car and ran it through the backbone of the frame. The basic frame was pilfered from a CZ360, but after the many modifications, it was nearly unrecognizable at a Czechoslovakian product. 

 

The arrival of the YZ637 on the circuit in 1973 changed the sport forever. Håkan Andersson dominated the remainder of the ’73 GP season on his monoshock equipped YZ and served notice that long-travel suspension was here to stay.

 

While both Geboers and DeCoster were intrigued in Tilkens’ experiment, neither one was too keen to risk their motocross careers testing some Frankenstein bike. Luckily, however, Lucien had someone available who was a little more willing to ride the experimental machine, Tilkens’ son Guy. 

 

In late 1973 (America had to wait till ‘74 to get the bike), Yamaha introduced their first truly serious motocross racer, the YZ250A. The YZ was offered in incredibly few numbers (less than 1000 in the US), and required a resume to be submitted before purchase. The bike was 7/8ths a works bike and offered blistering performance and a price tag to match (twice the cost of the garden variety MX250).

 

In 1968, Guy Tilkens was both an aspiring engineer and an amateur motocross racer. While not a professional, Guy was a capable rider at the local level and very interested in motorcycle design. This proved the perfect combination to help work out the bugs of the new design. Guy was capable of both providing feedback of a racer and help with the engineering problems presented by an all-new suspension design. 

 

While based loosely on the mill found in the MX250, the YZ’s 246cc reed-valve power plant offered a much different flavor on the track. Power was extremely explosive, with very little low-end and a very light flywheel. Just getting the bike going from a standstill could be difficult for a novice and keeping the YZ in the powerband required above average skill.

 

By 1971, Tilkens had moved on to a completely new chassis of his own design. The new frame maintained the basic layout of the original prototype, but replaced the Citroën damper with a gas-charged unit designed by Tilkens.  The original CZ running gear was also ditched in favor of the Suzuki parts obtained from Sylvain Geboers. 

 

In 1974, Pierre Karsmakers would take a monoshock equipped Factory YZ to the inaugural AMA 250 Supercross Title. Dave Friedman and Dom Emde Collection photo

 

By this point, both DeCoster and Geboers had moved on to Factory Suzuki and were very interested in the progress Tilkens was making with his engineering exercise. Reportedly both of them had tried the Tilkens’ prototype and turned faster times on it than on their Factory built racers. At first, it was not clear exactly why this was and the Suzuki engineers struggled to understand the advantages of Tilkens’ innovative design. Apparently the most obvious advantage, increased suspension travel, did not register with them at the time. The extra few inches of movement was actually an accidental side effect of the laid down shock and was not the main focus of Tilkens’ design. Because he had come at it with the goal of strengthening the chassis, not increasing travel, this benefit was largely overlooked.

 

In 1975, Yamaha used the success of Karsmakers and his YZ to sell the potential of the new monoshock equipped MX250 and MX400. The MX series was intended to be the bikes real people would by, while the YZ’s were looked at mainly as a marketing and development exercise.

 

While much of the YZ was unique, its DT roots were still evident in some of the details. This upper triple clamp is sourced straight from the DT line and still has mounting holes for the speedometer and tach.

 

When Tilkens approached Suzuki about licensing his design, they hesitated because they could not work out a mathematical reason for the lap time advantage. In spite of DeCoster and Geboers’ insistence that they should buy it, Suzuki’s head engineer, Mr. Tamaki, chose to pass on the design. While initially a setback, Tilkens did not have to wait long for other suitors. Both Honda and Yamaha had heard the reports of Tilkens’ amazing suspension and made overtures to the Belgian inventor about selling the design to them. In the end, Tilkens went with Yamaha and set in motion events that would take motocross into the modern era.

Say WHAT ?!?: The YZ’s pathetic excuse of a silencer looked to have been plucked right off a mini bike. Obnoxious and virtually useless, this unit was inadequate even by mid-seventies standards.

 

Yamaha was quick to integrate the new suspension to their race program and debuted the new monoshock at the third round of the 1973 World Championships, on Håkan Andersson’s works YZ637. This was the first usage of long-travel suspension in professional competition and the start of a suspension revolution. The YZ offered fifty percent more travel than any other bike on the track and Håkan dominated the day and the rest of the championship. 

 

Handcrafted out of fiberglass, the side plates on the YZ were light, but fragile. Rivets built into the plate held a sealing ring for the filter and constant attention needed to be given to their condition to assure proper sealing of the air filter.

 

In 1974, Yamaha would release their first true production “works replica”, the YZ250A. Prior to the introduction of the YZ, Yamaha’s productions racers were based off of their DT Enduro line and not nearly as serious as the premium machines from Europe. The new YZ250A changed all that and proclaimed loud and clear that Yamaha was serious about the motocross market. Available in very limited numbers and aimed at experts only (riders had to submit a resume to buy one), the exotic YZ cost fifty percent more than the Honda CR250M Elsinore and was worth every penny. 

 

Experts only: In the right hands, the YZ was capable of achieving amazing speed, but for most riders, the MX250 was a better choice.

 

 While the sturdy chromoly steel frame, powerful reed-valve motor and Thermal Phase shocks on the YZ250A were cutting edge for the time, they were still not as trick at the works mono’s found on Andersson and Karsmaker’s Factory bikes. For 1975, Yamaha looked to rectify this trickness deficit with an all-new YZ250. The new bike would once again offer a huge performance increase over the garden variety MX250 Yamaha, and perhaps most importantly, incorporate the radical new monoshock for the first time. 

 

In America, the YZ250B kept the yellow and black color scheme of the of the ’72-’74 US Factory bikes. In the rest of the world, however, the YZ could be found running the white and red color combo of Håkan Andersson’s ’73 works bike.

 

The new YZ250B maintained the basic motor layout of the “A” model, but did away with much of peakiness of the previous year with major porting changes aimed at beefing up the midrange. This had been a major complaint with the pro oriented ’74 and even expert riders appreciated the broader power curve of the new “B” model. The transmission and much of the running gear was maintained from ’74 and the new bike had to soldier on with the mediocre forks from the year before.

 

The “Monocross” rear suspension on the ’75 YZ250B offered 6.5” of rear wheel travel. This was quite a bit more than any 250 available at the time. While the travel was plentiful, the action was anything but plush. Even experts found the rear action too stiff and it was prone to kicking if the throttle was backed off in the rough.

 

 By far the biggest change on the new model was the addition of the monoshock, which necessitated an all-new frame to accommodate the large and long DeCarbon shock used for damping. The shock itself ran the length of the chassis and sat nestled under the seat and tank. This layed-down position was the key to the mono’s suspension advantage and gave the bike roughly twice the travel of most bikes available at the time. The downside of the new design was weight, as the beefed up frame, oversized shock and assorted hardware bloated the weight nearly 20 pounds over the ’74 model. 

 

Let’s just say people were a lot less uptight in the seventies…

 

 On the track, the new YZ was a huge step forward in performance in spite of its prodigious weight gain. The revamped power plant was much less demanding and easier to go fast on. Peak horsepower was down almost two horsepower, but the midrange was much improved. Low end was still virtually non-existent and with its extremely light flywheel and hard hit, the bike continued to be aimed at experts only. 

 

Hard landings were taken in stride by the new Monocross rear suspension. If you were aggressive and fast, it worked well, but if you backed the pace down in the least, the rear became busy and harsh.

 

The factory expansion chamber on the new YZ was very trick for the time. The up-pipe snaked out of the center port exhaust and crossed over the motor from left to right.  In order to accommodate this unorthodox design, Yamaha actually manufactured the pipe in two sections that mated just past the head pipe.

 

The new monoshock was too stiff for many and prone to kicking if you backed off the throttle (Yamaha-Hop), but for those fast enough to make it work, it was a revelation. Travel was a massive for the time 6.3 inches and obstacles could be attacked as never before. The forks were largely a carryover from the year before and by far the weakest link in the package. Based on the units found on their DT-MX line, they were far from state of the art and incapable of handling the speeds the motor and shock were capable of generating. For most riders, an upgrade of the forks internals or a compete swap to a set of Betor units was mandatory. 

 

 

Much like the shock and motor, it took a skilled hand to make the most of the YZ’s handling. If ridden aggressively, the bike turned and tracked well, but if the rider backed out of the throttle, the Y-Zed liked to stand up and push the front end.

 

Handling was once again aimed at expert users and the bike demanded aggression to work properly. If ridden hard, the YZ handled well, but if you backed off, the rear end would kick and send way to much force to the woefully undersized forks.  Turning also demanded aggression and the bike tended to stand up in turns if the rider was not 100% committed.  Braking was a carryover from the year before and it continued to be excellent. The front could be locked with a firm grip and the rear was very powerful, if a bit touchy. Overall, the bike was a huge technological achievement, but not perfect.

 

 

The strap on tank was taken right from the Yamaha Factory racers and added a bit of “works” cachet to the new machine. While the relativity low and comfortable, the alloy tank was wider than most of the competition due to the clearance needs of the shock, which ran the length of the bike below the tank and seat.

 

By far the weakest link in the YZ25B package was its forks. Not nearly as trick as the Monocross rear, the front sliders on the YZ were underspung and underdamped for the speeds the bike was capable of attaining. Perhaps worst of all, they were undersized and prone to an extreme flex that hurt the bikes handling.

 

 

When originally envisioned, the YZ line was designed to be a very limited production machine for serious racers only (less than 1000 were sold in the US in ’74 and ’75). The bike was very costly for Yamaha to produce and was only supposed to serve as a marketing platform to sell more MX250’s.  Yamaha had the less costly MX line for normal folks and the YZ’s were for the pros only. By the end of 1975, however, Yamaha was beginning to rethink this two-pronged strategy. 

 

 

In 1975, for about two thirds the cost of a YZ250B, you could have yourself a nice monoshock equipped MX250. The MX cost $1200, to the YZ’s $1800 and offered an easier-to-ride motor and softer suspension.

 

 

For 1976, Yamaha chose to reintegrate the YZ and MX line into one machine that was higher performance than the MX, but less costly to produce than the YZ. The result was a new YZ250 that took parts of both machines and sold it at the same $1300 of the competition. A new 246cc motor offered a wider and easier to manage powerband and a new set of 34mm Kayaba air-forks smoothed out the track. In the rear, revamped calibration for the monoshock took much of the bite out if the YZ’s pro-only ride. It was a new, more mainstream YZ, which kept the look and name of the “works replica” but made it more accessible to the common man. 

 

 

One and done: In 1975, the Yamaha introduced the world to the joys of long travel suspension. The YZ250B would be the first shot in a suspension war that would dominate the latter part of the decade. In 1976, Suzuki would introduce their own version of a long travel rear that leapfrogged the YZ and took the lead at a then astonishing eight inches of travel. By 1979, all the bikes would be punching a foot of movement front and rear. For the YZ, 1975 would be the last year of the true “works replicas”, because in 1976, the MX and YZ line would be merged into a less costly and more rider friendly version. The new bike would maintain the YZ name, but lose much of its predecessor’s exotic nature. Such is the cost of progress.

 

 

After the introduction of the monoshock YZ250B, Suzuki would be quick to come out with their own version of a long-travel rear suspension with their new RM250A. The new RM would actually offer more (and better) suspension travel than the YZ, and do it without the added complexity of the monoshock design. While the original Monocross rear suspension was not perfect and its performance was quickly eclipsed by more conventional competition, its significance cannot be overstated. Lucien Tilkens’ vision set the stage for an explosion in motocross technology that would propel us into the modern era.  The Monoshock YZ was the first machine to show the promise of long-travel suspension and served as the catalyst of a suspension revolution. 

 

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