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2009 Yamaha YZ250F

Monday, February 22, 2010 11:43 PM



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.

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