Crankshaft Harmonics - Flywheel Mass
10-23-2006, 06:55 PM
#1
Crankshaft Harmonics - Flywheel Mass How much, if any, does the mass of the flywheel play into the damping of crankshaft harmonics. I guess I am looking at aluminum vs. steel and wondering if the mass has an affect, if the material stiffness has an affect (aluminum being 1/3 as stiff as steel) or whether there is no interaction or detrimental affects of going to a lightweight flywheel on a normal street driven car (non special purpose) rather than a heavier steel/cast iron flywheel. All driving aspects aside, I am looking at if there are any long term issues with the lighter flywheel.
10-23-2006, 10:26 PM
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If the engine was unloaded from the drivetrain, the flywheel mass would have
a larger effect on harmonics. I don't know how to quantify the differences,
but in the above case the mass plays a role in reducing harmonics.
With the driveline engaged, the difference in flywheel mass has more to do
with accelerating, than it does with harmonics. In this case, it's the crank
arm furthest from the load which produces the most deflection.
Other Points of Interest: Drag Racing
I know there are a few threads on here pertaining to launching a car with
reduced flywheel mass. You might want to check those out as well. Some
people have reported issues with engine bogging out of the hole.
a larger effect on harmonics. I don't know how to quantify the differences,
but in the above case the mass plays a role in reducing harmonics.
With the driveline engaged, the difference in flywheel mass has more to do
with accelerating, than it does with harmonics. In this case, it's the crank
arm furthest from the load which produces the most deflection.
Other Points of Interest: Drag Racing
I know there are a few threads on here pertaining to launching a car with
reduced flywheel mass. You might want to check those out as well. Some
people have reported issues with engine bogging out of the hole.
10-23-2006, 11:29 PM
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GM has used dual mass flywheels with several applications. These are relatively heavy flywheels that were implemented to reduce driveline (transmission) noise in several cars. The heavy mass flywheel reduces the change in engine rotation when engine is at idle or an overdrive gear between cylinder firings. The engine actually speeds up and slows down minutely as a cylinder fires or one comes up on compression. This phenomemon is responsible for gear chatter in some manuals and some centrifugal superchargers.
In a related area I once ran a 283 hub on a 350 sbc. The hub just contained a timing mark, but no outer ring mass as on a traditional balancer. At about 6000 rpm I could hear a ringing noise. Until I replaced the hub with an 8" balancer it never dawned on me what that noise was caused by. The 8" dampener cured the ringing sound but was noticeably more sluggish in throttle response. I'm not sure if maybe the steel pulley was ringing or if it actally came from the crankshaft but I swapped back to the hub and the sound came back.
I have twice ran an aluminum 15# flywheel on deep geared sbc's as well as ran 30# and 40# steel flywheels. For all around driving I've concluded that I prefer a heavy flywheel. With the right gear/tire combination leaving a stoplight is totally effortless and there is less driveline vibration. I've always left harder with the heavy flywheel and I think its much easier on the clutch. While there is no denying that a light flywheel can accellerate faster the heavy flywheel recovers from shifts better for me and I really think its a better driving package.
All of the above may change however as my car's latest setup has an adjustable 10.5" Ram sintered iron clutch and I will once again try the aluminum flywheel as the new setup shouldn't hit the motor as hard on the dump.
In a related area I once ran a 283 hub on a 350 sbc. The hub just contained a timing mark, but no outer ring mass as on a traditional balancer. At about 6000 rpm I could hear a ringing noise. Until I replaced the hub with an 8" balancer it never dawned on me what that noise was caused by. The 8" dampener cured the ringing sound but was noticeably more sluggish in throttle response. I'm not sure if maybe the steel pulley was ringing or if it actally came from the crankshaft but I swapped back to the hub and the sound came back.
I have twice ran an aluminum 15# flywheel on deep geared sbc's as well as ran 30# and 40# steel flywheels. For all around driving I've concluded that I prefer a heavy flywheel. With the right gear/tire combination leaving a stoplight is totally effortless and there is less driveline vibration. I've always left harder with the heavy flywheel and I think its much easier on the clutch. While there is no denying that a light flywheel can accellerate faster the heavy flywheel recovers from shifts better for me and I really think its a better driving package.
All of the above may change however as my car's latest setup has an adjustable 10.5" Ram sintered iron clutch and I will once again try the aluminum flywheel as the new setup shouldn't hit the motor as hard on the dump.
11-09-2006, 07:01 PM
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Flywheels will have an extremely small if any affect on crankshaft harmonics because they are on the centerline of the cranshaft and are designed to maintain cranshaft momentum not factor out crankshaft harmonics. A heavier flywheel will have more more momentum when the rpm slows versus a lighter flywheel. Conversely, a heavier flywheel will overcome inertia more slowly when accelerating which would cause the vehicle to accelerate more slowly. A lighter flywheel will accelerate much quicker since there is not as much inertia to overcome. However, since the lighter flywheel does not have as much mass to move the car, more rpm's are required to get the vehicle moving.
