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All Yamaha YZ250F is ultimate corner carver

September 1st, 2009
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Ask any motocross rider where races are won and lost and odds on he'll tell you its all in the corners. A light, nimble and supreme handling MX bike can pick its lines and stick to them, enabling its rider to hang on longer and go harder.

So it's no surprise to learn that cornering performance was the guiding theme throughout the development of the new model YZ250F.

The next generation motocross development plan was hatched with rider involvement paramount. Trackside customer research led Yamaha engineers to discover that what riders wanted more than anything else was sharp handling. Riders of all abilities from pros right down to swappers. And by handling, they meant better cornering performance. Because a better handling bike lets a rider get on the gas sooner, is less tiring to ride and is the key to winning motocross races.

The development team's goal was to make quick cornering easy for riders of all skill levels. Not a simple task and certainly not something that could be achieved with small tweaks to the chassis or engine. The goal could only be reached by applying a highly innovative and synergistic approach to the science of motorcycle cornering.

The engineers were well aware that the 250 is not just a smaller displacement version of the 450, but a unique machine in its own right. So the task was to design a stand alone 250, not a scaled down 450cc MX bike.

By making a specific 250cc machine, the chassis can be tailored to deliver the handling response suitable for the class. The engine character of the 250 can then be tuned to suit that chassis. And by starting from a clean sheet of paper, not only was the goal of enhanced cornering performance met, the machine's overall performance was dramatically improved.

The new YZ250F is compact, lightweight and requires little input for rider balance. Which means it's less tiring to ride. The credit for this goes to the all new Bilateral Beam aluminium frame which features high lateral and torsional rigidity and offers a stable feel and precise handling.

The new frame converts sideways shocks smoothly to longitudinal shocks that are more readily absorbable by suspension. This makes the YZ250F very stable when cornering, which means it's easier to dive into corners with more speed.

By quantifying rider feedback, the chassis' rigidity could be more precisely matched to the rider. Additionally, the new frame layout made it possible to change the shape of the fuel tank. By moving the tank towards the centre of the bike, mass centralisation was significantly advanced. Further improvements were made by moving the radiator rearward and mounting it lower.

Improvement to the initial damping quality of the front and rear suspension units have resulted in improved traction and give the rider a better feel for the terrain. This allows more confidence when entering a turn and lets the rider hold a line with less effort. The result is less rider fatigue during a long moto.

An engine built like a Swiss watch
After thorough research and testing, Yamaha engineers decided to retain Yamaha's five titanium valve engine layout and FCR carburettor fuel delivery system. This combination was found to produce the best power output for the displacement while maintaining compact engine dimensions and light weight.

But the engine has been thoroughly refined for 2010 to result in even more punch. A higher lift intake camshaft, revised exhaust port and carb intake funnel shapes, new valve springs and retainers, smaller oil tank and revised ignition map for the CDI are just some of the many detail improvements that have been made to the 250cc powerplant. The new frame also allowed a reshaping of the air cleaner box which improves intake efficiency.

The result is a more rider-friendly engine with more linear power delivery. Low- and mid-range power characteristics and response are improved, giving the bike a lighter feel when cornering.

The changes were made possible in part by a measurement device used in MotoGP. This device allowed the engineers to quantify the transitional power characteristics when the rider begins to open the throttle after corner entry (1/4 - 1/2 throttle opening). Based on this data the engineers tuned the engine's response characteristics to give the engine an ideal power feeling for the rider.

Another small but important change was to modify the clutch lever ratio so as to reduce the effort required at the lever. This makes it much easier for the rider to slip the clutch.

Dart styling hits the bullseye
The YZ250F's straight frame-work directly expresses the lightweight feel and look a 250 should have. The plastic parts have been made as small as possible while still retaining their protective functions.

The bike's lightness is further expressed by emphasising its linear imagery. The flatness of the tank, seat and rear fender also facilitates rider movement and the relationship between the handlebars, pegs and seat make for a natural and roomy riding position. The dart-like styling allows function to follow form, as the YZ250F's elegant design contributes to its winning potential.

YZ250F Main Features

? ENGINE

1) New engine withrefined character
Improvements have been made to the liquid-cooled 4-stroke, DOHC single-cylinder 5-valve engine with its inherent qualities of excellent engine response and power. The improvements include optimised intake/exhaust systems, lightened valve assemblies, revised carburettor settings and new ignition advance characteristics. In particular the measures to reduce power loss in the valve drive mechanism have given this engine a new character.

The new engine is characterised by improved continuity in the climb through the rpm range, which especially improves drivability in the mid- to low-speed ranges. The 77.0 × 53.6 mm bore × stroke spec and the 13.5:1 compression ratio are the same as on the engine of the existing model.

2) Aluminium retainer and lightweight valve spring adopted

To improve the continuity in the climb through the rpm range, especially in terms of power development in the low- to mid-speed ranges, the retainer that holds the intake/exhaust valves has been changed to an aluminium unit (steel on the existing model) to reduce inertial mass in the valve drive mechanisms. In addition, the spring rate of the valve springs has been reduced to balance the reduced inertial mass.

Adoption of this new valve spring reduces the amount of force necessary to open the valves, which minimises power loss and thus contributes to superior torque characteristics in the low- to mid-speed ranges. Combined with a high lift intake camshaft, these changes help bring out drivability that exceeds expectations for a high compression ratio engine.

3) New carburettor settings and funnel shape
The settings of the FCR-MX37 carburettor have been optimised to accommodate the changes in the intake/exhaust systems. Also, the spec of the cam for the acceleration pump and the shape of the air passage on the air cleaner side have been changed. Together, these factors contribute to improved power development in the low- to mid-speed ranges.

As for the air cleaner, the shape of its funnel has been optimised and the shape of the cleaner box modified to best fit the new frame.

4) Modified exhaust port shape
In order to achieve the target performance, the shape of the exhaust port has been optimised (from a round cross-section to a "D" cross-section) to contribute to the new engine character.

5) Rigid mount silencer
Aussie spec models come with lightweight silencers. The length of the outer tube has been shortened by 55mm and the tail pipe shortened by 40mm. Also, in response to the new frame, the silencer mount has been changed to a rigid type that reduces weight.

6) Transmission ratios changed
In accordance with the new engine character, the gear ratios for third and fourth gears have been changed to achieve excellent ratio balance (third was 1.450 and now 1.444 while fourth was 1.227 and now 1.222)

7) New design reduces clutch operational load
Clutch spring load has been changed. Also, the length of the push lever cam has been modified to change the lever ratio and reduce operational load.

8) New-design crankcase cover
The crankcase cover has been given an oil level gauge window. Also, the drain bolt position has been changed to improve serviceability. The oil level gauge boss on the current model has been eliminated.
Also, the oil tank has been made more compact.

CHASSIS

1) New-design aluminium Bilateral Beam frame
In order to promote centralisation of machine mass, - and in particular centralisation of fuel tank mass - that contributes to fulfilling the aim of being the best cornering 250cc MX bike, a specifically designed all-aluminium Bilateral Beam frame has been adopted. This new frame is made up of about 20 parts of forged, extruded and panel aluminium parts welded together into a single unit.

In addition, parts and components other than the frame are also designed and positioned with centralisation of mass in mind. The total effect makes this new model significantly more compact than the existing model and that can be felt by the rider as a heightened level of rider-machine communication.

In terms of rigidity balance, the lateral rigidity and torsional rigidity have been increased compared to the present model in order to give a greater feeling. At the same time, it helps the rider experience an outstanding sense of rider-machine communication.

Also, it helps achieve an excellent balance that contributes to a feeling of agile lightness, good front wheel grip, strong traction and light handling in tight turns.

2) New dimensions adopted
The wheelbase, caster and trail dimensions have been optimised to bring out the characters of the new engine and frame. Wheelbase is shorter, caster angle relaxed and trail longer. The headstock is lower and the handlebars are higher for optimum performance. The 4-step adjustable handlebar positioning is the same as on the 2009 model. It enables settings to fit rider preference, course layout or surface conditions.

3) Further developed front suspension
In order to achieve dependable cushioning performance and smooth operation when entering a turn at high speed, the valve that produces the damping force has been optimised and the spec of the inner rod has been changed. This is a key factor for achieving front-wheel grip when leaning the machine over at the entrance to a turn.

The shape of the outer tubes has also been changed to optimise rigidity balance with the new frame. The handlebar crown is the same type as on the present model.

4) Rear suspension spring position optimised
The new model adopts the same basic rear suspension spec as the existing model with its excellent cushioning feeling and reliability although the position of the spring of the suspension unit has been lowered 29mm. The swingarm is the same type as on the existing model.

5) New-design flat seat
A new flat seat has been specially developed for the 250cc model. The rigidity of the seat bottom has been optimised for a better feeling and fit. The seat surface material is the same as on the 2009 model. It is vinyl chloride leather with an optimised mix of compounds added to the smooth wooly nylon base fabric to provide the right balance of grip and ease in sliding. The surface texture is designed to resist clogging of the surface grain with sand or mud to ensure superior grip and reduce the amount of dirt build-up after a ride.

6) Newly designed radiator adopted
The thickness of the radiator core has been increased and the mount changed from a 2-point to a 3-point mount, thus achieving outstanding heat-dissipation performance and greater unit reliability at the same time. The coolant volume and the cooling capacity are the same as on the existing unit.

7) New-design fuel tank adopted
The lightweight plastic resin fuel tank is designed to achieve the best relationship with the new frame and new dimensions. The centre of the tank has been moved closer to the rider to contribute to better centralisation of machine mass and a greater sense of compactness, which in turn contributes to a heightened feeling of rider-machine unity and lighter handling.

8) New lightweight parts adopted
This model adopts newly designed lightweight parts including a new-design sprocket, a plastic resin engine protector (under-guard), more compact oil tank, optimised brake pedal shape and smaller brake hose guides.

Also, new-design lightweight exterior parts have been adopted. These include new shapes for the front fender, rear fender, front number plate, front fork protector, side covers and air scoop, etc.

9) New body styling with smaller-surface exterior parts
This model has been designed with styling that accentuates compactness and lightness. This is achieved by keeping the surface area of the exterior parts to a minimum, accentuating compactness and expressing a sense of dynamic motion.

Also, the exteriors have the new textured plastic parts. They keep the bike looking good even if there are the scratches and dents on the surface caused by rough rider movement and contact.

10) White colour option
To further heighten pride of ownership, a white version is offered as a second colour. In addition to the model's high potential as a competition machine, this colour provides an image of refined quality

2010 YZ250F key dimension change chart

Change made 2009 model 2010 model
Silencer outer pipe tube length 380mm 325mm
Silencer tail pipe length 40mm shorter than 2009
Radiator fixation points 2 3
Cylinder head fixing point number 1 point 2 points
Intake cam lift 7.6mm 7.7mm
Exhaust cam shaft flange diameter 27mm 29mm
Intake valve spring wire diameter 2.2mm 2.1mm
Exhaust valve spring wire diameter 2.5mm 2.4mm
Exhaust pipe joint with silencer diffuser pipe diameter 50.8mm 45.0mm / 50.8mm
Clutch spring screw washer number 2 1
Clutch push lever cam height 4.4mm 4.0mm
Frame head pipe position 12 mm lower than 2009
Caster angle 27.1 degrees 27.4 degrees
Trail 117.4mm 119.6mm
Wheelbase 1476mm 1473mm
Ground clearance 371mm 376mm
Handle holder height 39mm 44mm
Number of aluminium parts in frame About 20
Rear suspension 29 mm lower than 2009
Wet weight 102.8kg 102.0kg

2010 YZ250F SPECIFICATIONS

Dimension?Weight
Overall length

2166mm

Overall width

825mm

Overall height

1304mm

Seat height

990mm

Wheelbase

1473mm

Minimum ground clearance

376mm

Wet(with oil and a full fuel tank)

102.0kg

Engine
Engine type

Liquid cooled 4-stroke,DOHC,5-valve

Cylinder arrangement

Forward-inclined single cylinder

Displacement

250cm3

Bore & stroke

77.0 X 53.6mm

Compression ratio

13.5 : 1

Starting system type

Kick starter

Lubrication system

Dry sump

Radiator capacity (including all routes)

1.0 litres

Engine oil capacity

1.2 litres

Fuel
Air filter type

Wet element

Fuel tank capacity

6.4 litres

Carburettor Type/Fuel supply

FCR-MX37 X 1

Electrical
Ignition system type

C.D.I

Spark plug model

CR8E

Transmission
Primary reduction system

Gear

Primary reduction ratio

57/17 3.353

Secondary reduction system

Chain

Secondary reduction ratio

51/13 3.923

Clutch type

Wet, multiple-disc

Gear ratio
Transmission type

Constant mesh 5-speed

Operation

Left foot operation

Gear ratio-1st gear

30/14 2.143

Gear ratio-2nd gear

28/16 1.750

Gear ratio-3rd gear

26/18 1.444

Gear ratio-4th gear

22/18 1.222

Gear ratio-5th gear

25/24 1.042

Chassis
Frame type

Bilateral Beam

Caster angle

27.4°

Trail

119.6mm

Tyre size (Front)

80/100-21 51M

Tyre type (Front)

Bridgestone M403A

Tyre size (Rear)

100/90-19 57M

Tyre type (Rear)

Bridgestone M404

Brake
Brake type (Front)

Hydraulic single disc brake

Brake type (Rear)

Hydraulic single disc brake

Diameter of disc (front/rear)

250mm/245mm

Inside diameter of drum / Effective radius of disc (Front)

224mm

Inside diameter of drum / Effective radius of disc (Rear)

219.8mm

Suspension
Suspension type (Front)

Telescopic fork

Suspension type (Rear)

Swingarm (link,suspension)

Shock absorber assembly type (Front)

Coil spring/oil damper

Shock absorber assembly type (Rear)

Coil spring/gas-oil damper

Wheel travel (Front)

300mm

Wheel travel (Rear)

310mm

Editor's note: Please note there are some small but significant differences between the Australian model and US model. And because AUS and Europe have different tyres, there are some differences to dimensions (eg length, seat height and ground clearance). Please quote AUS spec and use AUS images wherever possible. Key differences are:

Australia

US

Europe

Black rims on both colour models Silver rims on blue model, black rims on white Black rims on both colour models
Rounded muffler end cap Pointed muffler end cap Rounded muffler end cap
Gearing 51/13 Gearing 49/13 Gearing 51/13
Rear wheel travel 310mm Rear wheel travel 307mm Rear wheel travel 310mm
Carb jetting (same as EUR spec) Carb jetting Carb jetting (same as AUS spec)
Front suspension spring rate 4.5N/mm Front suspension spring rate 4.4N/mm Front suspension spring rate 4.5N/mm
Bridgestone M403/M404 tryes Bridgestone M403/M404 tryes Pirelli Mid-Soft 32 tyres
Dimensions (small differences caused by tyres, gearing and suspension changes) Dimensions (small differences caused by tyres, gearing and suspension changes) Dimensions (small differences caused by tyres, gearing and suspension changes)

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