Lightweight single mass flywheels

[hooblah]

Hey guys I've got a B6 S4 and I'm contemplating putting on a lightweight single mass flywheel to replace the dual mass, mainly for performance reasons. The thing is I keep hearing that it causes the gears to rattle with a high load at low rpm. Most of the time I'm cruising below 2k in the highest gear possible and this worries me.
So onto my question.. Why does this happen? 20 years ago everyone had single mass flywheels and you never heard of gears rattling.

 

[desertstorm]

This is more so with Diesels than Petrol cars. As you say DMF's didn't really exist 20 years ago and they were first bought in as a way of isolating the diesel engine from the transmission and thus improving the Noise, Vibration , Harshness (NVH) of the engine.
It has another effect in that by moving the springs from the clutch friction plate this is lightened thus the synchro on the gearbox doesn't have as much rotating mass to deal with so gear changes are improved.
The same way it works on a diesel it also works on petrols but to a lesser extent. By going to a lightweight flywheel you will potentially get more vibration and noise as the heavier flywheel absorbs and smooths vibrations.

 

[hooblah]

Thanks but that doesn't quite answer the question.
The question was why do the gears rattle in the first place? Old cars with single mass flywheels didn't do it, so why do newer ones?

 

[desertstorm]

The difference is in the description of the flywheel. Old Single Mass Flywheels were heavy, Having a heavy mass absorbs the vibrations and smooths the output of the engine. You can use a SMF as long as it is heavy enough. If you use a lighter weight flywheel it doesn't absorb the harmonic vibrations from the engine which is what causes all the noise in the gearbox. These vibrations can causes issues and in extreme cases things will brake.
http://flat6innovations.com/index.php/broken-crank
Manufacturers don't want to spend money if they don't have too. They would rather fit a nice heavy SMF from new as this would be cheaper.
However the DMF gives better vibration damping and better gear changes due to reduced weight of the clutch disc the synchros in the gearbox don't need to work as hard.