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Old 12-02-2010, 07:18 AM   #112
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Joined: Sep 2008
Location: Quesnel, BC
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Originally Posted by Dan Căta View Post
I agree that a flywheel takes power from the engine but I was thinking that since the most power would be directed to the rear wheel, the flywheel would not consume that much power.

And, why didn't they design a smaller diameter flywheel and a more powerfull starter engine ? That would have been a big plus in flywheel bottleneck ;)

The flywheel doesn't really *take* power, think of it as a power storage device. Newton wrote some laws... his second law states: The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector. So, the more mass an abject has the more force is necessary to get it moving, so a lighter flywheel will require less input energy to get it accelerate faster. The issue is, say you made your flywheel, clutch, wheels etc weigh in at 1kg - acceleration would be fantastic, but every external force would easily stop that assembly from spinning. You wouldn't be able to ride your bike as you would stall it at every intersection when you release the clutch. As the flywheel assembly is on the other side of a gearbox that acts as a power multiplier it as a two way effect. This is why a taking 1kg off the flywheel has more "acceleration" affect than taking 1kg off the wheel. But, say you add 10kg to the flywheel, acceleration rate goes down but you ability to sustain a given speed goes up... with a heavier flywheel you can go off roading and crawl along at idle in 1st, hit a big rock and you bike is less likely to stall as all that energy is stored in that larger mass that will want to keep moving. So same principal starting at the intersection - some flywheel mass is necessary so you don't stall the engine if you take off at an engine speed slightly above idle.

So, for a BMW S1000RR, a lighter flywheel is going to help as you arn't loading it up for a transcontinental off-road journey. Put that same flywheel on a R1200GSA and you'll be cursing at it as you will have to rev your engine to 4000 rpm just to start moving - the GS is more of a tractor than you think!
David Marshall - Quesnel, BC, Canada -
2006 BMW F650 "YZ" Dakar | 2013 BMW G650 "YZ" Sertao
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