Section 60, sub-section 2A
© Copyright, 2012, R. Fleischer (except the photo)
Well below is a photo of a properly lightened flywheel. Lightened Airhead flywheels are sometimes installed on early heavy flywheel models (pre-1981). Although this article is specific to the /5 & later, the basics ALSO apply to BMW's before the /5 models. There have been instances, rarely, of lightened flywheels (Clutch Carrier) on the 1980/1981and later models.
From 1980/81, BMW changed to a totally different type of 'flywheel', which is then called a Clutch Carrier & is much lighter, & the clutch parts were totally different, & the transmission input shaft spline was shortened to match. The entire assembly was much lighter. This accounted for some of the lack of smoothness of the later models, as the engine had less stabilizing/smoothing mass at the crankshaft output. This was particularly noticeable at idle rpm, but also noticeable for more buzzy-ness (higher frequency vibrations) at higher rpm, while actually riding.
The later models with the lighter clutch assemblies generally required, AND ENABLED, slightly quicker down-shifting, as the rpm drop was faster when the throttle was chopped, not to mention the more sudden effect on the rear wheel upon a throttle chop. The faster shifting is generally regarded as a nicety. Shifting to a higher numerical gear number can also be nicer.
Lightening the flywheels on earlier BMW's will enhance shifting, by enabling faster shifting, because the flywheel has inertia, & with less weight, less inertia, the flywheel...with engine crank, etc., attached, will slow down at a faster rate during shifting. You will have smoother shifting IF you shift FASTER, before the rpm drops too much (assuming downshifting). The acceleration will be somewhat faster, as less mass is being speeded up. Engine braking is reduced with a lightened flywheel which can help handling during braking. When shifting upwards, and using the clutch (rather than just the throttle chop technique with pre-shift lever loading), the shifting can also be quicker.
There ARE downsides to lightening a flywheel, besides cost.
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