The Yamaha YZ250 is a two-stroke motocross race bike made by
Yamaha Yamaha may refer to:
* Yamaha Corporation, a Japanese company with a wide range of products and services, established in 1887. The company is the largest shareholder of Yamaha Motor Company (below).
** Yamaha Music Foundation, an organization estab ...
. The model was launched in 1974 and has been regularly updated and is still in production with new releases every year.
Engine
The original YZ250 of 1974 used an air-cooled 250cc two-stroke engine of 70 mm bore and a 64 mm stroke, which was improved semi-annually. The air-cooled motor was replaced in 1982 with a 249 cc liquid-cooled
two-stroke
A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a Thermodynamic power cycle, power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being comple ...
reed-valved engine with a
YPVS {{Unreferenced, date=February 2018
The two-stroke power valve system is an improvement to a conventional two-stroke engine that gives a high power output over a wider RPM range.
Operation of a two-stroke engine
A '' stroke'' is the action of a p ...
exhaust valve for a wider spread of power. The bore on the water-cooled engine was reduced to 68 mm and the stroke was increased to 68 mm producing a displacement of 247 cc. In 1999, the bore was further reduced to 66.4 mm and the stroke lengthened to 72 mm producing a displacement of 249 cc. The longer stroke engine resulted in a lower redline, slightly less top end power and greatly improved torque at lower RPM. The engine produces a peak at 8,800 rpm and of torque at 7,500 rpm, with a 9,000 rpm redline.
Chassis
On its introduction in 1974, the YZ250 used a single backbone steel frame with a twin-shock swingarm. The following year, reflecting the factory racer Hakan Andersson's factory racer, the swingarm became a monoshock unit, initially made of steel, then aluminum from 1978. The
swingarm
A swingarm, or "swinging arm" (UK), originally known as a swing fork or pivoted fork, is a single or double sided mechanical device which attaches the rear wheel of a motorcycle to its body, allowing it to pivot vertically. The main component of ...
rear suspension system featured more than 12" of travel, while the telescopic inverted front forks had of travel.
In the 2005 model year, the YZ250 gained a new frame made from aluminum that reduced the dry weight to approximately . All YZ250s feature front and rear hydraulic disc brakes.
Timeline
Racing
* 5 AMA National Motocross Titles
* 9 AMA National Supercross Titles
[
* AMA National Supercross 2004 championship, by Chad Reed.
* 6-time AMA National Offroad Championships by Jason Raines]
See also
* YZ85
* YZ125
* YZ250F
References
YZ250
Off-road motorcycles
Motorcycles introduced in 1974
Two-stroke motorcycles
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