New two-stroke crossers


New two-stroke crossers

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Something is happening in the two-stroke off-road scene. Honda also puts the 125 cc in an aluminum frame, and KTM shows the courage to come up with completely new solutions with the PDS spring system.

For many years off-road fans had to learn to be happy about the little things in crosser life. As little as possible, and certainly no more than necessary, seemed the watchword of the designers and accountants in the mine guild when it came to further development. New ideas and trend-setting concepts stayed in the drawers – at least until the beginning of last year.
Because under World Championship driver Rupert Walkner, of all things, industry mini KTM kissed the gently slumbering crowd of technicians awake last year. The magic word: Progressive Damping System, or PDS for short. Instead of the previously usual deflections, in the KTM monoshock manufactured by the Swedish suspension specialist Ohlins, the shock absorber itself took on the progressive increase in the spring and damping curve. A GP race victory by Mr. Walkner confirmed the functionality of the concept in the same season.
After the takeover of the Dutch suspension manufacturer White Power by KTM, the Dutch are also working on their own PDS concept and have been equipping the 250 factory crossers with it since the beginning of the year. Reason enough for the courageous Alpine Republicans to be the first series producer to add the new concept to their in-house 1998 Moto Cross and Enduro two-stroke models.
However, the Crosser cannot expect too much weight savings from the new suspension system. The redirection saved is almost completely absorbed by the corpulent PDS strut (for comparison: the piston rod is 18 instead of 14 millimeters thick, the housing x instead of x millimeters, and the spring weighs x kilograms more) together with the reinforced frame in the area of ​​the upper strut mount. In the end, just under a pound is missing on the scales. Ease of assembly, lower maintenance requirements and, above all, the possible off-center installation, which can straighten the path from the air filter to the carburetor, are the real arguments that should convince. For now, however, only KTM’s two-stroke fan base is allowed to be avant-garde. The four-stroke line will be converted to the PDS system in 1999 at the earliest.
Another delicacy is also only available for the two-stroke league. In 1998, the long-awaited, completely new in-house eight-liter two-stroke engine made its debut in a modern design, i.e. with a right-hand kick starter, left-hand chain and hydraulic clutch actuation. The 125 is also available as a 200 for the American and Australian scene. The market launch of the conventionally working Extreme front fork from White Power is currently reserved for the 250, 300 and the new 380 cm³ models.
And what else is going on in the dusty element? After Honda caused a lot of turmoil in the stud guild with the CR 250 in the light metal bridge frame at the beginning of this season, the Reds are now also putting their 125 cc in the stiff aluminum corset. The quarter-liter machine remains largely untouched for this year, just like the 500.
The biggest two-stroke car from Kawasaki, the KX 500, has endured the same existence for years. New stickers, a grass-green seat cover, that’s it. The KX 250 is different: In addition to many detailed improvements, a power-jet carburettor shines above all, which enriches the mixture in the lower and middle speed range to promote performance, while at the same time improving the revving by leaning the mixture at high speeds. The KX 125 also benefits from the new carburetor technology.
Suzuki leaves the RM-Crosser essentially the same as before, with the exception of changes to the details and design. Only with the ignition do the men in yellow break new ground and use a system that mixes the mixture more effectively with several staggered ignitions instead of a single strong spark should ignite.
Yamaha will remain true to the tradition of incorporating very easy-to-dose engines into their blue plastic cover for the coming model year. With rather unspectacular changes to the engine and transmission, the technicians preserved this advantage above all in the 250. The new YZ 250 also shares larger brake discs, reinforced rear swing arm and reinforced frame with its little sister. The test ride on the currently very successfully used in the half-liter World Cup YZ 400 F, however, Moto Cross fans have to hold back for a while. The highly acclaimed four-stroke engine should be in the shop windows in December of this year at the earliest. How far Yamaha had to move away from the feather-light and extremely strong factory machine in the series production of the small 400 series remains to be seen despite the first photos.

PDS system

With the PDS system, KTM supplier White Power is entering new technical territory – at least when it comes to large-scale production. In contrast to a conventional monoshock system, in which a lever arm compresses the shock absorber increasingly faster in the course of the spring travel and thus allows the spring and damping forces of the monoshock to increase progressively, this task must essentially be taken over by the PDS shock absorber itself. White Power uses two elements for this: two damper pistons with different maximum flow rates and a conical needle, similar to a jet needle in the carburettor. Unlike Ohlins, the Dutch do without a progressively designed nib. If the full spring travel is available, the displaced oil bypasses the first piston during compression and flows through a hole in the piston rod only through the second piston – the pressure damping is weak, the compression is easy. As the compression travel increases, the conical needle closes an increasingly larger one Cross section of the piston rod. Some of the oil must make its way over the second piston. The pressure damping increases, the compression movement is opposed more and more resistance. At the end of the spring travel, the needle closes the piston rod completely. All oil must flow through both pistons. The pressure damping reaches its maximum value.

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