YZ250F: radically new engine layout

YZ250F: radically new engine layout

“The goals were very clear for us” says our development engineer. “Better power and optimal handling. So we adopted the reversed-engine layout that was so successful on the 450. At the same time, we didn’t want the power output to be too aggressive. Easy control and overall rideability was our main focus.”

2014_YZ250F_Off road_Throttle response from 236-539614 (gc_single_col)Throttle response
2014_YZ250F_Off road_Power tuner to re-adjust the engine from 236-539590 (gc_single_col)Power tuner to re-adjust the engine
2014_YZ250F_Off road_Reversed engine head from 236-539596 (gc_single_col)Reversed engine head
2014_YZ250F_Off road_Straight intake design from 236-539608 (gc_single_col)Straight intake design
Fuel injection
The new engine also features a fuel injection system. With the following merits:
It gives a great throttle response and doesn’t need to be set to temperature or altitude as a carburetor does.

Power tuner
It also allows to connect the accessory Yamaha ‘Power Tuner”. The connector is already on the bike. Easy to use and without the need to bring a PC to the track, the Power Tuner lets you adjust ignition timing and fuel injection amount and check maintenance items like engine hours.
This way you can set the power delivery character of the bike to the different surface conditions, or to your personal taste! It also allows you to re-adjust the engine if you mount accessory parts like an Akrapovic muffler, or GYTR performance parts.
 
Engine features
The engine uses the same ‘reversed engine head’ philosophy as the current YZ450F: by positioning the intake in front and inclining the cylinder to the back, the intake tract is more straight. This gives a boost in performance as well as a unique style for the bike. And it has great merits for mass centralization too, as we will see later.
“Due to the good mid range power, the bike is nice and friendly to ride, not only for the racer but also for the average rider. The new engine has an easier to use power band, and revs fully on till its top end when you accelerate out of corners” says test rider Erik Eggens.

Important new performance features, apart from the straight air intake design and snake type muffler, are:
  • A more compact combustion chamber design with DOHC, 4-valve head and narrower valve angle
  • A new two-ring piston ring design (with only 1 compression ring and 1 oil ring) which is lighter and has less friction, for less hp loss.
  • By offsetting the cylinder from the centre of the crankshaft, friction losses are further reduced. This is because at the moment of greatest combustion force, the conrod position is more straight under the piston and the piston is not pushed into the cylinder wall so strongly. This system was launched on the 2010 YZ450F and is now incorporated in the YZ250F as well.
  •  


2014_YZ250F_Off road_Fuel tank sits low in the frame from 236-539578 (gc_single_col)Fuel tank sits low in the frame
2014_YZ250F_Off road_Exhaust pipe from 236-539566 (gc_single_col)Exhaust pipe
2014_YZ250F_Off road_Frame construction from 236-539572 (gc_single_col)Frame construction
2014_YZ250F_Off road_Handle crown from 236-539584 (gc_single_col)Handle crown
Mass centralization
The development team tried to make the bike smaller, lighter and easier to use.
The reversed engine layout also helps, with concentrating all heavy masses around the center of gravity:
- The air cleaner, which is a relatively light component, is in a high and forward position;
- The fuel tank is moved rearward and sits in a low position in the frame where the fuel level doesn’t have a big influence on the handling;
- The muffler is moved forward;
- The exhaust pipe, which needs a certain length for good low and mid range torque, winds around the cylinder head like a snake, again centralizing the weight.

In addition to this, many smaller components like the fuel pump are also moved toward the center of the machine. All those details improve the handling.
And thanks to the new frame and the continued search for lighter components, with the shapes of parts throughout the machine shaved down as much as possible to reduce weight, the handling is improved even more.
 
Chassis features
Though it may look similar, actually the main frame is an all new construction. This is not a simple aluminum frame as some other manufacturers make. It is made up of 10 different types of forged, cast and extruded aluminum parts that are welded together into one unit with outstanding strength, rigidity balance and shock absorbing capacity that contribute to superior cornering performance. Because on a motocross bike it is very important to have the right amount of flex versus rigidity, some parts have a very complex shape. For example the tank rails are constructed by hydro-forming, which allows the thickness of the upper, lower, right and left portions to be different, can you imagine…
An increased handle crown rigidity, combined with a larger front axle (from 20 to 22mm) helps to aim for and trace lines through turns at will.
Refined front and rear suspension settings with new inner tubes and revised spring specs give better stability and better ground contact.
The new, slimline body design with minimal plastic body parts and a thinner radiator design saves weight and makes you feel more at one with the bike than ever. 
 


2014_YZ250F_Off road_Riding from 236-539602 (gc_single_col)Riding
2014_YZ250F_Off road_Compact from 236-539554 (gc_single_col)Compact
2014_YZ250F_Off road_Air cleaner from 236-539548 (gc_single_col)Air cleaner
2014_YZ250F_Off road_Design is beautiful and functional from 236-539560 (gc_single_col)Design is beautiful and functional
2014_YZ250F_Off road_YZ250F 2014 from 236-539911 (gc_single_col)YZ250F 2014
Riding
Erik explains the benefits of the new layout:
“The handling of the bike is very very stable. This is typical strong point of Yamaha. Not only on the straight, with obstacles like ruts, square bumps and jumps, but also inside a rut it remains very stable and you feel confident that you have control over the front wheel.”
“Also on entering the corners, where other bikes are nervous and don’t find enough grip, you can let the bike turn in and the front will follow its track. Turn after turn after turn. And because of the more compact and lighter body, the bike can be leaned into a turn more quickly. Because the bike is so sensitive, you can 'feel' the ground very well, you get a very direct feedback for the surface and for how the grip conditions are at any point of the track."
 
Practical details
- The air cleaner box is easier to reach;
- The transmission and gear shifting mechanisms have been redesigned for smoother and quicker shifting;
- The graphics on the bike are not stickers but a new ‘in-mold’ design for greater durability;
- The fuel tank can be lifted up and rotated 180º to rest on rear frame, without disassembly of hoses: for easy maintenance access;
- The handlebar is rubber mounted for less fatigue when riding and, with less vibrations coming through, a better ‘feel’ for the front wheel;
- The muffler body is bolted instead of riveted, for easier maintenance;
- The fuel tank capacity is increased to 7.5 litres.

Design
The silhouette of the machine is like a lean athlete. Typically Yamaha, the design is both beautiful as well functional. The bodywork is minimal, with a slimline design and the clean line on top of the seat allows you to shift your weight around on the bike easily without any barriers – not even a fuel cap!

Technical specifications

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