The KTM 250 SX-F has dominated the MX2 World Championship for several years, and has just added a second AMA Supercross Lites title to its impressive track record. Who could steal the show from him? Successful marriage between a powerful and torquey engine and a light and manoeuvrable chassis, the 250 SX-F flies from podium to podium.
The 250 SX-F’s engine is considered to be one of the most powerful in the MX2 class paddock. Capable of reaching 14,000 rpm without flinching and with phenomenal torque present throughout the rev range, this engine has plenty of response. Its compact architecture allows better mass centralization and exceptional road holding. Let us recall in passing that it weighs only 26.1 kg, including electric starter.
The centerpiece of the compact DOHC engine, the new avant-garde cylinder head features two camshafts responsible for actuating the titanium valves using ultra-light rockers coated with a DLC coating. The new side mounting brackets (now made in aluminum and no longer in steel), have appeared to better secure the engine to the frame and save a few hundred grams..
The crankshaft of the 250 EXC-F offers greater rigidity. The big end bearing has two bush shells force-fitted directly into the crankpin. This design is the prerequisite for achieving extremely high speeds and the unrivaled power of the 250cc engine. This design increases engine life and allows longer crankshaft service intervals; a benefit for the user in terms of time, effort and costs.
CYLINDER AND PISTON
The shortened cylinder, with a bore of 78 mm, accommodates a proven bridge-box piston, manufactured by CP. The geometry of the piston head, perfectly suited to the new high compression combustion chamber (14: 1), is characterized by a rigid structure and low weight. The reduced oscillating masses give the engine its playful and muscular character.
In order to balance the mass forces, the engine of the 250 SX-F uses a multi-function balance shaft which drives the water pump and the timing chain at the same time..
ENGINE MANAGEMENT SYSTEM
Keihin’s electronic fuel injection engine management system features a 44mm throttle body. Thanks to the unique position of the injector, the engine response is instantaneous. And for added convenience, you have independent adjustment systems for cold start and idle. The ultra-light control unit manages a unique traction control and start-up control system to benefit from a healthy boost in the event of difficulty and optimum traction thanks to the different engine maps adapted to the gear engaged. A more powerful engine mode, selectable by means of the switch located on the handlebars, even makes it possible to reinforce the aggressive character of your machine.
Another strong point of the four-stroke: a new sophisticated and multifunctional engine mapping, which can be activated directly from the handlebars, and which fulfills a triple function:
– Activation of traction control
– Switching on the start control·
– Selection of an AC power curve (Standard or Advanced)
Traction control is a feature found on no other competitive model to date. It guarantees more efficient traction and better control of the motorcycle during acceleration phases. This is all the more evident in rainy weather. At the push of a button, you’re done. This map selection switch also enables start control to be engaged. But a successful start and it is the holeshot assured. The principle of operation is simple: The start control reduces the maximum speed of the engine at the exact moment you take off so that you can maintain better control of your machine. Pressing both buttons when the engine is idling and the EFI warning light located on the upper triple clamp comes on to warn you that the car is just waiting for a sign from the driver to jump. In addition to the standard engine maps, it has another even more powerful engine mode to boost the bike when the need arises..
The 250 SX-F is fitted with an electric starter as standard, which is a decisive advantage in reducing lap times in competition. Activated by a simple button, this ultra-powerful starter signed Mitsuba is associated with a proven training device.
ENGINE CASINGS AND COVERS
The design of the molded crankcases has a double advantage: reducing the weight and allowing a very centralized shaft configuration in order to make the engine even more compact and to obtain a much more efficient mass distribution. The new engine covers have a solid surface structure to reduce wear caused by the rider’s boots.
KTM’s proven integrated cooling system circulates the coolant from the cylinder head through the frame triangle to the aluminum radiators supplied by WP. Its incredible efficiency is due to the better management of internal air flow and the progress made in ventilation thanks to CFD calculations (numerical simulation of fluid mechanics). Well thought out, the radiator guards integrated into the front part of the spoilers are particularly useful in the event of an impact.
The new compact CSS (Coil Spring Steel) clutch features a lightweight, wear-free steel clutch housing and seven clutch discs that offer very high heat resistance thanks to their steel backing discs. Brembo hydraulic clutch system provides sharp and easy modulation.
To enhance the reliability of the 5-speed gearbox, the surface of the gears has received a special treatment for 2nd, 3rd and 4th gears. Design "zero dirt" of the selector prevents clogging and blocking of the lever articulation and guarantees perfect reliability of the gearbox. All 4-stroke engines are fitted with a gear sensor which allows engine characteristics to be adapted to each gear.
BATTERY AND WIRING HARNESS
As in the past, all SX-F engines are fitted with the proven electric starter – a KTM exclusive and a major advantage in motocross racing. The starter is associated with an ultra-compact lithium ion battery advertised at 495 g, which represents a significant weight gain compared to before. To avoid being hampered by the electrical circuit when removing the rear part of the frame, the battery and the wiring harness are now grouped together in a compartment above the air filter housing. Mechanics and DIY enthusiasts will appreciate.
Respecting the sound emission levels recommended by the FIM is a real challenge for all manufacturers. This is the reason why KTM has spared no efforts to develop an exhaust system at the forefront of innovation in this field. All 4 strokes are fitted as standard with collectors connected to a small additional chamber called the resonance chamber. They are also fitted with a shorter silencer mounted as close as possible to the engine. This has a dual interest: better centralize the masses and enhance driving pleasure. The internal design has also reached a significant degree of sophistication to keep the sound level well below the thresholds recommended by the FIM (112 dB (A) at 2 meters maximum), while guaranteeing maximum sensations for the pilot..
The main objective of the KTM engineers during the development of the new generation SX was very significant weight reduction and improved handling. The result is clear: the current SX-F models are among the lightest and slimmest competition motorcycles on the market, although the 4-stroke is now equipped with a very beneficial electric starter, while competing models must still be kicked off. This is a sign that a strong current is flowing between the motorcycle and its rider.
The high-tech chromium-molybdenum steel frame, a model of lightness, flexibility and maneuverability, offers excellent stability and proves devilishly precise in the turns. Thanks to its high torsional rigidity and low longitudinal rigidity, this frame allows better absorption of the energy generated by the front and rear suspensions when jumping or crossing a bump. Driving is more comfortable and less taxing. The mounting brackets that ensure the connection between the cylinder head and the frame are still just as light. Mounted laterally, they also help to limit the level of vibration and to refine the trajectory of the motorcycle in the bends. Engineers can be proud of their work because the frame is now a kilogram lighter than the lightest competitor frames available on the market today. Everything has also been done to facilitate maintenance and free up space for the tank and other components. The ultra-light rear part of the frame, made of stable aluminum profiles, weighs less than 900 grams on the scale.
Sophisticated footrests "zero dirt" prevent clogging of the gear selector even when driving through deep ruts.
AIR FILTER BOX
The air filter housing reinforces the protection of the air filter against contamination and guarantees optimum air flow. The cage in which the extra-large Twin Air air filter is housed is completely rigid, which helps to better hold the system in place. The design is so simple that one could replace the filter even with the face covered in mud. And this without the help of any tool. And in a matter of seconds.
In addition to significant weight savings, cast aluminum has ideal bending behavior. The one-piece molding process eliminates the imperfections inherent in welding and guarantees a more consistent build quality, but also more careful.
The ultra-light polyethylene tank has a capacity of 7 liters. Tank cap with internal thread ensures easy and reliable closing.
When it comes to the handlebars, KTM engineers have always known which direction to go. Profiled and made of high resistance aluminum, this NEKEN handlebar is no exception to the rule. It is equipped with rubber silent-blocks and can be adjusted to two positions. From MY 2017, the handlebars are fixed by means of two clamps (lower part) and a new trigger guard (upper part) which improves torsional rigidity. Replacing the handlebars is also easier if it should bend after a bad fall. right: an ODI vulcanized handle; on the left: the ODI lock-on handle as comfortable as it is practical, thanks to its mounting system without glue or wire.
Tailor-made to provide a comfortable support surface, this saddle is as resistant as it is comfortable. Pampered, the riders will benefit from a new seat cover on the AM 2017 intended to reinforce the grip and the feeling.
KTM’s proven integrated cooling system circulates the coolant from the cylinder head through the frame triangle to the aluminum radiators supplied by WP. Its incredible efficiency is due to the better management of internal air flow and the progress made in ventilation thanks to CFD calculations (numerical simulation of fluid mechanics). Well thought out, the radiator guards integrated into the front part of the spoilers are particularly useful in the event of an impact. Every detail counts for KTM.
When it comes to wheels, KTM engineers know a thing or two about them. Equipped with ultra-light CNC machined hubs and premium Excel rims, these wheels guarantee maximum stability despite a limited weight. They are fitted with the latest generation Dunlop Geomax MX 3S tires which are wider at the rear (110 / 90-19 instead of 100 / 90-19). The bike adopts an optimized secondary transmission (14/51 instead of 13/50).
Another interesting novelty on this bike is the WP AER 48 inverted fork with air spring and separate damping circuits, developed by WP in close collaboration with KTM. The damping functions are on the right, the air spring is on the left. The left fork leg has a dual chamber and air cartridge system to prevent pressure loss. Should the outer seal ever leak, the pressure exerted inside the inner cartridge will hold the motorcycle in position. The air spring is easily adjusted according to the weight of the pilot using a single valve. No special tools are required – the standard pump will do. The right fork leg incorporates a cartridge specially designed to adapt to the type of damping sought. Just turn the thumbwheel 30 stops to the desired position (compression up and rebound down) to choose the appropriate setting based on track conditions and rider preferences. For AM 2017, it is also important to point out that the outer tubes of the AER 48 fork have been redesigned to optimize the bending characteristics, provide more flexibility and limit friction forces. They also lost 265g, which is far from insignificant. More comfortable and more responsive than ever, these forks beg to be put to the test.
High quality CNC machined triple trees have been a trademark of KTM for years. Allowing precise steering, they significantly enhance the flexibility characteristics and, thus, the action of the fork. For the AM 2017, the design of the upper triple clamp has been revised and corrected according to the new divers. The handlebars are fitted with rubber silent-blocks by means of two clamps (lower part) and a new trigger guard (upper part) which improves torsional rigidity. Replacing the hanger is also easier, for example in the event of a fall. An hour meter, installed on the upper triple clamp, indicates when it’s time to perform maintenance or refuel.
The WP shock absorber has been developed taking into account the new architecture of the frame and the swingarm, just like the link system and the WP AER 48 front fork. With its various machined and black anodized components, this ultra-sophisticated shock absorber makes strong impression compared to competing products less well finished. It benefits from a new setting and a more flexible spring which go hand in hand with the new setting of the front axle.
More responsive, more efficient and more powerful than ever, Brembo brakes will tackle any situation. These small technological gems are associated with Wave discs from Galfer, renowned for their lightness. For AM 2017, the rear brake pedal has been lengthened by 10mm and the new brake pads are now less aggressive.
KTM factory riders collaborated on the design of the bike with R specialists&D KTM and the Kiska design agency, with a single watchword: to create a perfect harmony between the rider and the machine. The ergonomics and the contact points for the pilot have been particularly taken care of for maximum freedom of movement. The lowered saddle offers superior comfort, especially in the rear shell. The special shape of the front and rear mudguards is a perfect example of intelligent design: thanks to a well-thought-out structure and mounting, these mudguards guarantee maximum stability, while being lighter. Better yet: they stay clean longer.
Info and image credits: KTM
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250 SX-F 2016
Model marketed in
The technical aspect
2017 KTM 250 SX-F
- Tank: 7 liters
- Wheelbase: 1,485 mm
- Dry weight: 98 kg
- Train before
- secondary by chain
- Rear axle
(78 x 52.3 mm)