Kawasaki Ninja 250R Final Drive Ratio Discussion
From the beginning the Ninja 250s had a custom final drive ratio. The 2005 has a ratio of 15:41 and the 2008 has a ratio of 14:38 (currently 15:38). Both had a stock ratio of 14:45. The taller final drive allows the engine to operate at just shy of 1000 RPMs per each 10 MPH of indicated road speed in top gear (interestingly, this would translate to running the Escort at highway speeds in second gear). The main benefits are reduced engine "buzz" at highway speeds and assumed reduced engine wear. Increased fuel economy was a secondary consideration with this modification.
The decision to alter the final drive was based on:
- 1) the Ninja 250 Riders Club Site fuel FAQs,
- 2) the 100,000 mile Ninja 250 posts on the Ninja 250 Riders Club,
- 3) the number of impulses per mile as compared to most 4 cylinder motorcycles/cars,
- 4) the RPM of the theoretical piston/cylinder wall critical velocity (9247 RPM for the 250),
- 5) the rather flat roads in my area of the country,
- 6) the power to weight ratio of me on the Ninjette as compared to the power to weight ratio of a typical economy car, and
- 7) the horsepower and torque curve of the Ninja 250 engine.
I felt I would be getting pretty close to optimal with the 15:41 final drive on the 2005 (JT Sprockets; JTF 516.15T and JTR 486.41T respectively) which provides for a 2.73 gear ratio.
The 2008 provided the opportunity to take a slightly different approach as the new generation Ninjette has a stiffer suspension and a tighter chain slack specification. This enables one to run a smaller rear sprocket with out increasing the risk of "chain slap" on the rear swing arm. The downside of course is that one will have to pay more attention to the chain slack on the 2008s and newer. To reduce both the unsprung weight of the 2008 and the overall cost of the modification I opted to replace only the rear sprocket with a (JT Sprockets JTR 486.38T) which provides for a 2.71 gear ratio.
As the torque curve of the "J" model Ninja 250 engine has increased mid-range torque over the older models at the first chain and sprocket replacement a 15:38 sprocket combination (JT Sprockets; JTF 1539.15T and JTR 486.38T respectively) was used to achieve a 2.53 final drive ratio. The 15:38 final drive offers the following advantages over the stock ratio 1) cruising engine speed is kept under 7000 RPM (the engine spins at about 6400 RPM at an indicated 70 MPH in top gear) virtually eliminating any perceptible engine "buzz" at highway speeds and 2) freeway entry can be accomplished with one gear change (engine "red line" in second gear occurs at approximately 65 MPH). Additionally the JT Sprockets JTF 1539 front sprocket is lighter than the stock unit.
Front Sprocket JTF 516.**T (Diagram courtesy of JT Sprockets)
Front Sprocket JTF 1539.**T (Diagram courtesy of JT Sprockets)
Rear Sprocket JTR 486.**T (Diagram courtesy of JT Sprockets)
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The Yeager's Kawasaki Ninja 250 Site
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