A massive flywheel does help keep an engine rotating at slow speeds and under
high-load, low-speed conditions, such as when starting to move a car, due to its
rotational inertia. It does not dampen out vibrations as you described. That is
done by the crankshaft dampener, which has an outer mass ring that is able to
move slightly relative to the crankshaft. It is this counter-movement of the
dampener's mass ring that "damps" out torsional vibrations. A lighter flywheel
will have less rotational inertia and so will be able to slow down and speed up
its rotation more quickly. That can mean a slightly "rougher" idle in some
situations, where the rotational inertial of the flywheel is used to compensate
for pulsing of the crankshaft caused by the delay between firing pulses of the
cylinders. The more cylinders an engine has and the faster it idles, the less
important this facet of flywheel rotational inertia is. The other facet of
rotational inertia is in aiding an engine to move the car from a stand still.
The energy from the rotation of the flywheel (e=MC2) is added to the torque
produced by the engine to help overcome the inertia of the vehicle and get it
moving. You can compensate for a reduction in that rotational inertia caused by
a reduction in mass, by simply raising the engine speed (a reduction in M can be
offset by a corresponding increase in C). So a massive flywheel might help
smooth out the idle a little bit and it will help on takeoff, especially if the
driver tends to let the engine rpm drop by letting the clutch out too quickly or
by not giving enough gas to offset the load applied to the engine. But there is
a price to be paid for these benefits. The mass of the flywheel must be
accelerated by the engine (both positively and negatively) and that takes power,
power that could be used to move the car instead. An engine with a light
flywheel will rev up and down much more quickly and smoothly than one with a
heavier flywheel because the engine has less mass to accelerate. Flywheel mass
does two things, it resists changes in rotational speed and it absorbs heat from
the friction of the clutch. You must make sure the mass of the flywheel is great
enough to absorb the maximum heat load that might be applied by the clutch
without overheating which could cause warpage or cracking. The mass considered
for this purpose should be located as close to the clutch friction surface as
possible. As for mass resisting changes in rotational speed, while that does
help smooth out the idle in some cases and can make it easier to get a car
moving from a stop, it also makes it harder to make changes in engine speed and
makes the changes in speed take longer. But a car engine is not like a
stationary engine, it is constantly changing its speed, and that is why a
massive flywheel is often not such a good idea.
While lots of folks pay attention to the *total* mass of a flywheel, few concern
themselves with the location of that mass on the flywheel itself. The location
of mass on a flywheel has an effect on the rotational inertia and the strength
of the flywheel, as well as on its resistance to overheating. The closer the
mass is to the center or hub of the flywheel, the less effect it has on
rotational inertia and the more it affects the strength of the flywheel. The
torque loads from the engine are applied between the hub, where the flywheel is
attached to the crankshaft, and the area where the clutch cover bolts to the
flywheel, which is where the transmission is "attached" to the flywheel. This
zone is where the flywheel is used to transmit all the torque loads from the
engine to the transmission. Weight outside the clutch cover bolts ads little or
no strength to the flywheel but does add the greatest inertia. So if you lighten
a flywheel by removing the mass mostly from outside the clutch cover and/or
friction surface, you will reduce the rotational inertia without compromising
strength or resistance to warpage/cracking. OTOH mass removed from the
hub/clutch area compromises strength, and mass removed from the friction surface
area compromises the flywheel's resistance to overheating.
sidney raper wrote:
>
> Pardon my ignorance, but why the big fuss about a lightened flywheel?
> Doesn't that induce vibration due to the lack of the dampening effect that
> the heavier one would have? Wouldn't that lead to more fatigue failures on
> a lot of stuff and make driving more nerve wracking?
>
> Not trying to throw cold water on the idea as it is great for racing, but on
> the street (exclusively), it seems to be a waste of money imho.
>
> Am I way off base?
--
Marc Sayer
82 280ZXT
71 510 2.5 Trans Am vintage racer
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