George raised this question the other day, and it is a good one, with
considerable practical and theoretical interest. Tom made an
interesting observation about his experience with it.
As we all know, objects at rest and in motion both resist changes to
their speed.
It seems impossible that flywheel weight could have any effect on
horsepower and torque that any engine could produce, but it can change
certain characteristics of a bike or car, for better or worse.
Looking at one and two-cylinder bikes, fairly heavy flywheels are
necessary on them to get acceptable smoothness of low and mid-range
pulling. The selection of flywheel weight is something of an art with
these. The Vincent 1000cc twin had its flywheel weight selected by the
great designer, Irving, and as you would expect, it is perfect. In the
sense that you are never conscious when riding one of either "heavy"
flywheels that give a sluggish feeling, nor "light" flywheels that give
a lumpy feel to the drive. A couple of years after the twins came into
being in the late '40s, the factory came out with a 500cc single, using
many of the twin parts, to expand their range of market offerings, AND
used the twin flywheels (altered slightly for the different balance
factor, natch). So forever afterwards the 500cc model suffered from a
flywheel weight that was slightly too heavy, and gave a slightly
sluggish feel to the bike. On the other hand, should the 500cc model
chance to have a sidecar fitted, which was not unusual in the period,
the extra flywheel effect would be beneficial in getting the whole works
moving.
In riding a new Harley Sportster several hundred miles not long ago, it
was clear that Harley likes heavy flywheels on their road bikes. This
is good for two reasons, first it improves smoothness of low and
mid-range pulling, which is what riders are using almost all the time,
second, every time the rider starts off from rest in first gear, the
bike wants to jump away and the rider thinks "Wow!"
No doubt flywheels on the heavy side help a racing bike or car start
harder from rest because of the stored-up inertia of the flywheel, but
the trade-off is that the engine will have to work harder to build rpms
further down the course since the flywheel will be resisting speed
increase. 40 years ago we were told that high-gear-only dragsters liked
heavy steel flywheels (maybe 60-70 lbs)which helped get the car weight
off the line. I imagine the driver would be holding 4000-5000 revs, and
start by applying throttle and slipping the clutch to get the amount of
wheelspin he wanted. But further down the course the engine had to work
harder to get the flywheel to rev up to max, subtracting power available
to accelerate the car. With a car engine, of course, the crankshaft,
clutch or torque converter and the whole rotating assembly is part of
the flywheel weight.
I think with racing bikes where there are only one or two cylinders, the
feel of the engine could easily be ruined by taking too much flywheel
weight off. Needless to say, the "correct" balance factor has to be
retained (whatever it is, varies by engine) to minimize vibration.
However, when converting road bikes for racing, probably some weight
could be removed, but in my mind the main benefit would be just weight
reduction of the whole machine, rather than any performance improvement
to the engine.
The whole history of the Japanese multi-cylinder motorcycle movement has
been "light flywheels". With the fours particularly, the crank itself
is most of the flywheel effect, but the engines are so smooth-pulling
that the rider quickly learns to buzz it and give it a little throttle
for smooth starting.
My Ardun has a light aluminum flywheel and this works fine for speed
trials, giving no particular feel one way or the other. On the other
hand, Elmo Gillette prefers a 60 lb steel flywheel on his Arduns at
Bonneville, to ease push starts in high gear. He did tell me that the
downside of the heavy wheel is that you have to be careful not to snap
the throttle shut at the end of a run because it tends to keep the car
running fast longer, and this is the time when you are most likely to
break con-rods. So you ease the throttle shut to slow the car and save
your rods. The reason being, of course, that the gas pressure atop your
pistons with the throttle open tends to offset the weight of the pistons
which wants to break the rods.
Another detail with Arduns is that with his heavy steel flywheel Elmo
can start the engine at full advance of 45 degrees on a fixed magneto
without kicking-back and bending the starter shaft, whereas with my
light flywheel I would (I found this out using an H/C mag with fixed
timing at first) get kick-back and a bent starter. So I must use a
Vertex with an advancer and start my Ardun at 20 degrees only. But this
is no handicap.
One more consideration would be that using a gearbox and starting hard
from rest with a heavy flywheel, would be more punishing to the box and
more likely to break its internals.
Cheers Bill
///
/// land-speed@autox.team.net mailing list
/// To unsubscribe send a plain text message to majordomo@autox.team.net
/// with nothing in it but
///
/// unsubscribe land-speed
///
///
|