Light Weight Flywheels Explained

[m sprank]

Originally posted in 2013. Re-posting for easier searching by members.

 

 

 

I get asked about LWFW's so often it made sense to post some info in one place. Hope this helps.

 

Any and all light weight flywheels will "chatter". It is the nature of a LWFW, along with the change in inertia.

 

Even the OEM SMFW can have some chatter/judder.

 

Enthusiasts considering a lightweight flywheel are often warned of "lightweight flywheel noise", or "chatter" more correctly called "gear rattle" or "gear lash noise". In making modifications to any car, there may at times be certain compromises in other areas of the car's behavior. Some may be completely acceptable, others may not. In a luxury-minded enthusiasts car like a LGT, owners are concerned about new or excessive sounds or rattles.

 

The original dual-mass flywheel does one thing very well - exhibits inertia, the resistance to changing it's state of motion. It is more difficult to change the speed of the dual-mass flywheel, whether accelerating it or slowing it down... that means besides inhibiting engine acceleration to some degree, it also inhibits the deceleration that occurs between every power pulse. The power pulse transition is smoother, therefore no rattle.

 

A single-mass flywheel is the same sort of design as a solid aluminum lightweight flywheel: a rigid disk of metal, either aluminum or steel. In the OE applications, these flywheels are equipped with a sprung-hub clutch. The sprung-hub clutch "splits the difference" between the these situations. The friction material (the disk itself) is not "hard coupled" to the hub, but has a springy nature to the assembly. It has a suspension, the springs you see around the hub. What this accomplishes is letting the power pulse couple the transmission's internal parts softly, and then de-couple softly. ("Softly" being a relative term here, it is just a few millimeters of travel, nothing that can be felt.) The net result is the difference between banging two pieces together and gently placing them together.

 

The lightened flywheel has less mass to damp the power pulse variations. The tolerances of the gears,splines and general internals are low. They are "loose". The dual mass units and heavier singles, dampen the engine pulses/harmonics through the shafts. The lighter units, dont do such a good job of it.

 

Subaru switched over to the SMFW for LGT's in 07. So using a 06-14 WRX set up is the same as a 07-09 LGT set up. 05-06 were the years that got the DMFW. Most likely to save coin, Subaru decided the change was acceptable.

 

Some users have complained of light "chatter" or "judder" when using an aftermarket clutch and the OEM SMFW. Seems to be more temp related and factored by the disc choice and pressure plate strength.

 

Misfires, CEL's etc are caused by a FW that is too light. Most LGT LWFW's are in the 14lb-15lb range and are not light enough to cause this. Going to 12lbs will deff throw a CEL. Generally the CEL is crank angle related and can lead to misfires.

[Mr-C]

Thanks!!

 

Guess I missed this earlier this week when I was doing some research.

 

I’d vote this being a sticky!

[GearJamr]

Having just replaced my clutch and flywheel to a SMFW, I have not noticed any change in "chatter" with the clutch. There wasn't any before (unless it was my error - not enough throttle) and there isn't any now. Pedal effort is pretty equal from before to after. I would think it is most likely due to incompatible components than the flywheel itself.