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-   -   Light Weight Flywheels Explained (http://legacygt.com/forums/showthread.php?t=203149)

m sprank 03-08-2013 08:59 PM

Light Weight Flywheels Explained
 
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.

All Competetion Clutch kits for the LGT currently use and include a LWFW. They will chatter.

06+ WRX (which shares compatability with the 05-09 LGT) uses the same kits. 02-05 WRX use a different kit (which can use the OEM FW). So, for 05-09 LGT owners all current Comp Clutch offerings include a LWFW and must be used with the provided LWFW (the pressure plate will not bolt up to an OEM FW).

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+ 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.

Comp Clutch LWFW's are 14.5 lbs.

V8Bait 03-08-2013 09:07 PM

Wow, I did not know you can cause a misfire with too light of a flywheel. Could you also get a misfire if you were to use a LFW and a lightweight crank pulley? I know this is not a good idea in the first place since it will be hard with a DD starting from a stand still.

m sprank 03-09-2013 01:57 PM

Adding a LWCP and a LWFW can cause misfires. It also puts a lot of vibration on the bearings (causing undue/excessive wear). We do not recommend using a LWCP and a LWFW on DD cars.

RallyMachine 03-10-2013 12:49 AM

Quote:

Originally Posted by m sprank (Post 4331680)
Adding a LWCP and a LWFW can cause misfires. It also puts a lot of vibration on the bearings (causing undue/excessive wear). We do not recommend using a LWCP and a LWFW on DD cars.

Engine bearings or transmission bearings?

m sprank 03-11-2013 08:44 PM

Engine. Crank and rod bearings take the "punishment". Nothing to absorb/dampen any of the vibration on either end of the crank.

Max Capacity 03-12-2013 09:31 AM

Quote:

Originally Posted by m sprank (Post 4331680)
Adding a LWCP and a LWFW can cause misfires. It also puts a lot of vibration on the bearings (causing undue/excessive wear). We do not recommend using a LWCP and a LWFW on DD cars.

Yep, that's been a general rule for a while.

Don't use both.

acumenhokie 03-14-2013 08:16 AM

I would assume this rule holds true for the 5th gen? I'm running an ACT streetlite flywheel and was looking at getting a lightweight pulley. I definitely have a good bit of gear rattle when coasting already.

m sprank 03-14-2013 12:08 PM

Holds true for any engine. Take weight off both ends and eliminate any ability to dampen/absorb vibration. The vibration must go somewhere. Crank and rod bearings take an extra "beating".

mattg 03-14-2013 12:15 PM

I have a LWCP along with a 06 WRX FW. Is that a setup you would endorse?

GTTuner 03-14-2013 12:26 PM

Quote:

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.

Mike, excellently explained. I miss my DM setup. I always quoted on here that heavy flywheels win drag races, everyone just thought I was farting out my mouth. Correct me if I am wrong but the reason a stock WRX clutch has so much smoother driveability is due to the dual sprung disk. IIRC the ACT I installed recently was not double sprung, as most high performance disks are not. I believe this is why some still experience chatter even with an OEM WRX flywheel.
I am so disenchanted with my experience's with different clutches in this car I am tempted to go back to a competly OE set up and detune my car.

Great post Mike, Thank you.

mattg 03-14-2013 04:12 PM

I like my Act HDSS & WRX fly so much better than the OEM setup, even with the heavier pedal and decel noise.

GTTuner 03-14-2013 04:34 PM

Quote:

Originally Posted by mattg (Post 4340748)
I like my Act HDSS & WRX fly so much better than the OEM setup, even with the heavier pedal and decel noise.

It strange how inconsistant the ACT clutches are. I have the same set up. I got it because I liked how it felt in a friends car. In my car the pedal effort is unacceptable, so much more than the same clutch in other cars. As far as gear noise and NVH I have virtrually nothing noticable over my DM set up. :confused::confused:

m sprank 03-14-2013 08:32 PM

OEM SMFW and a LWCP are fine.

Dual sprung disc is normally "smoother" than single. But, most aftermarket performance pressure plates are single spring.

I too have had ACT experiences across the board. Far more consistent results with ClutchMasters and Competition Clutch.

El Gabito 03-15-2013 07:05 AM

So for the guys with dmfw - what can we replace with? 06+ WRX or only 06-07?


Sent from mobile

m sprank 03-15-2013 04:25 PM

06+ WRX OEM SMFW. Same part number used in 07-09 LGT's.

MPN: 12342AA090

http://www.infamousperformance.net/s...le-Mass/Detail


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