Monday, September 02, 2013

Lightening a /2 flywheel

The stock BMW flywheel is about 13 lbs of tool strength precision steel, drilled and balanced. It has the rear main seal surface on its flange.  Timing marks are stamped into its circumference. When lightening a flywheel, the goal is to reduce rotating inertia while retaining the timing marks and balance.

I start off by taking some measurements on the backside. This gives me a diameter where I can begin to cut into the depth of the flywheel. I want to be in control of how thick the finished flywheel will be.  To maintain strength and the rigidity to withstand the clutch spring without flexing, I want the flywheel to be no less than 1/4" at any given diameter.

 I am beginning to thin the flywheel.
Steel is removed from the face of the flywheel.  The flywheel remains in balance during the entire process and, unlike milling holes in the flywheel with a turn table on a mill, when lightening on the lathe, the flywheel does not need to be rebalanced after lightening. 
Using a caliper, the flywheel is shaped to follow the contour of the opposing side.
The left side of the flywheel is cut, leaving a thin ring about 1/4" wide which contains the timing marks. 

This ring can be undercut to minimize the amount of steel out near the "heavy" edge.  

The ring, about 1/4" wide and supported by enough metal to be strong and light, bears the original and unchanged factory timing marks. 
The final product weighs approx. 8.25 lbs, a weight savings of about 5 lbs.  Versus cutting holes in the flywheel on a mill, most of this weight loss on the lathe-cut flywheel is at its circumference, where weight matters most when the flywheel is spinning.

An 8 lb flywheel is not very aggressive. It is just enough of a weight savings to notice an improvement in engine response and faster shifting speeds but, because the flywheel is not excessively lightened, starting,  idle quality and drivability are not adversely affected. 

The most beneficial effect of lightening the flywheel is to allow the motor to spin down faster.  This is really helpful when upshifting. It's been my opinion that one of the things that slows /2's down is slow up shifts.  One must wait for the motor to "spin down" in order to let the clutch out into the next higher gear. For example, you're in 2nd and wound way out past the "II"  redline mark on the speedo. You begin to the pull the clutch in and apply upward pressure with your left toe. Click, there's third. The motor is still winding down. Taking its good old time. Meanwhile I'm ready to release the clutch. Even if I let the clutch out smoothly, the bike will lurch forward and jerk the whole driveline. So I'm left to wait until the motor spins down low enough where I can release the clutch and get back on the gas.  Anywhoosit.  Scottie's Workshop offers this service on /2 flywheels. The cost for the service is $150.  
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