Driving report Yamaha factory crosser YZM 400 F
It’s a pony
While the competition in the Cyclo-cross World Championship galloped powerful battle horses, Yamaha spurred lively ponies – with success. MOTORRAD had the opportunity for a hot ride.
“At the end of the season, everything will be in the press,” says Yamaha chief engineer Shiohara as a matter of course.
The concept is astonishing: before the season it was unthinkable that a small four-stroke engine could beat the large-volume competition in the 500cc World Championship. The works drivers Andrea Bartolini and Stefan Johansson succeeded in doing this many times in 1997. The technology of the mysterious YZM is apparently rather conventional. The Japanese sports department did not look into the cards; individual parts from the inside were taboo for the press. After all, the often questioned displacement of really only 397 cm³ was confirmed. The riddle about the control valve on the left of the cylinder head has also been solved: It guarantees constant pressure conditions in the engine housing and thus a secure circulation of the 0.8 liter oil supply.
That the YZM is uncompromisingly trimmed for maximum performance becomes clear as soon as you kick off. Only an energetic kick brings the extremely highly compressed five-valve engine to life, an automatic decompression system is missing. The first few meters then show how it shouldn’t be done: The speed is too low, the YZM acknowledges the sudden opening of the throttle with a refusal to work, as the mixture supply through the huge intake ducts collapses. After just one or two rounds, however, every YZM novice is carried away by the enthusiastic maneuverability. The roaring exhaust sound on over 10,000 tours acts like a drug. The usable band is actually quite wide, although it only begins at speeds at which larger four-stroke engines already make big jaws.
It’s impressive how spontaneously the works crosser reacts to every twist of the throttle – on the other hand, even a Husaberg seems phlegmatic. The characteristics are not comparable to any current four-stroke crosser, reminiscent of a toxic 250 two-stroke. The traction on slippery courses tends to be better, the performance can be brought to the ground more easily. The many good starts of the two works drivers in the past season prove that there is enough power even at the top. No wonder with almost 60 hp and a live weight of just 102 kilograms. The YZM is correspondingly light-footed when accelerating, the front end constantly striving towards the sky. The switch foot is not overused. The three upper gears of the four-speed gearbox are sufficient, the first serves as the »paddock gear«. In the Supercross two gears were enough for the American works driver Doug Henry to beat the two-stroke 250s with the YZM 400 F.
The agile, light engine also determines the driving behavior of the YZM. It doesn’t dive as deeply into the waves as other four-stroke engines. The light engine and compact chassis put less strain on the suspension, which can therefore be adjusted to be surprisingly soft. When braking before bends, the fork and handlebars stay calm. Despite the razor-sharp handling, the little four-stroke doesn’t know how to hit the handlebars, the straight-line stability is reassuring. The YZM can only get a bit stubborn if, for example, it is accelerated out of the curve at high speed. Then the stern wedges violently at the edges. Curves and jumps are the YZM’s favorite profession, which is where it turns out to be a real fun bike. The gyroscopic torques are low because of the small centrifugal masses in the engine, which is why the lightweight folds into tight turns by itself. When exiting a curve, it is also much easier to choose the line than with any other four-stroke engine.
A consolation for technology freaks: The disused factory crossers are put in the shredder, 1998 will Yamaha but keep going with new, improved machines. And other four-stroke engines will follow the YZM concept. Because the secret of success can be copied: It’s not a trick- it’s a pony.
High-tech in series?
The concept is the same, the execution inevitably completely different: Like the factory racer, the series YZ 400 F, also in a red / white design, has a displacement of almost 400 cm³, but with a 92 millimeter bore and 60.1 millimeter stroke. This is intended to broaden the speed range even further. The high compression of 12.5: 1 suggests that the four-stroke engine that can be bought is as good as the factory unit. The top performance is given with a promising 55 HP, the speed should be limited to 11,000 rpm. That would guarantee a lot of fun. The small four-stroke engine has dry sump lubrication with the main frame as a reservoir so that it lasts over the long term. In addition to top performance, lightweight construction was given top priority in the development. Magnesium covers on the left and right of the engine and on the head are evidence of this. The balancer shaft is not a contradiction in terms, because it was only through this that an extremely light, yet stable crankshaft became possible. The goal of the diet: The handling of the dry, 107 kilogram 400 F should come close to that of the factory machine, despite compromises in favor of everyday suitability. From January 1998 the first of the 12,600 marks four-stroke crossers are due to arrive in Germany, the talk is of 200 units for the spring. It is not yet clear whether there will also be an enduro version. From France, however, it is reported that an approved version in a blue design will follow from April, which will be used by off-road legend Stephan Peterhansel in enduro sports – and in rallies? – should be used.
Yamaha YZM 400 F.
Water-cooled single-cylinder four-stroke engine, milled aluminum housing, magnesium cover, two overhead camshafts driven by chain and intermediate gears, five titanium valves operated by bucket tappets, steel connecting rods, wet sump lubrication with oil pump, oil volume 0.75 liters, crankshaft chamber sealed off, Keihin FCR flat slide carburetor , Ø 45 mm, optional versions with kick or electric starter, oil bath clutch, four-speed gearboxBore x stroke 95 x 56 mmHub volume 397 cm³Nominal output min. 57 HP at 10,000 rpm, max torque 45 Nm at 9,000 rpm, max. Speed 12,000 rpm, compression 12.5: 1 chassis, single-loop frame made of tubular steel with split beams, bolted rear frame made of carbon fiber with integrated intake box, Öhlins telescopic fork or Kayaba upside-down fork with adjustable rebound and compression damping, swing arm made of welded aluminum sheets, Öhlins or Kayaba central spring strut, articulated via a lever system, with adjustable spring base, rebound and compression damping, magnesium hubs and aluminum rims Dimensions and weights Weight without petrol Kick starter version: 102 kg E- Start version: 105 kg tank capacity 8.7 liters
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