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Old 11-30-2012, 11:59 PM   #11
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Join Date: May 2008
Location: San Francisco, CA
Posts: 171
My Ride: 2000 323i
Good question! I've always been really bothered as to why BMW would suddenly throw in this unnecessarily complex piece when the old system was working just fine. But BMW is notorious for making things way more complicated than they ever needed to be.

Anywho, so the benefits are generally three-fold: increased fuel economy, less strain on the gear box, and to stop gearbox chatter. Fuel economy is increased by increasing the weight of the flywheel. A heavier flywheel produces greater rotational inertia. Greater rotational inertia means energy is, for lack of a better term, stored when the engine is not producing power. In reality, it all depends on how you drive the car since nothing is free, but it does boost fuel economy numbers in standardized testing. The last two are effectively the same thing. It helps reduce strain on the gearbox by increased using its increased mass to reduce gearbox chatter. Gearbox chatter is not just obnoxious, its damaging to an engine. That clicking is gears hammering on one another repeatedly. Sure, its not a lot of force, but it does do damage over time. On a vehicle that will see 200k to 300k miles, that likely could result in premature transmission failure. Gearbox chatter also interferes with shifter engagement, because the gears in the transmission are constantly rotating by just small amounts. All this is dealt with by allowing the heavier flywheel to absorb the driveline shock, which smooths out clutch engagement blah blah blah, I'm sure I'm loosing you guys.

Apparently theres a big movement among automotive manufacturers to get away from single mass flywheels and sprung hub clutches -I suspect to boost fuel economy numbers for the MPG target requirements set by the government.

Originally Posted by zhp43867 View Post
Very interesting!

Can anyone break down the reasons manufacturers use a single versus a dual mass flywheel?

Last edited by alexxander.foster; 12-01-2012 at 12:02 AM.
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