There are several common
questions about rotating mass. Let's look at a few of them.
 If I use a lighter
crankshaft, how much power is gained?
 If cluster gear weight
of a transmission is reduced, how much will acceleration improve?
 Does replacing a heavy
stock steel flywheel with a light weight aluminum flywheel improve
acceleration?
 How much ET can be
gained from using lighter wheels?

These questions can be answered if we know the weight change, the
distance out from the center the weight change occurs at, the speed
(RPM), and the time period over which the RPM change occurs.
What does a rotating mass
actually do?
A
rotating mass does not really consume energy. The mass just stores
energy and eventually returns energy to the system or converts it to
some other form of energy. The energy storage can be helpful, not do
anything at all, or be harmful. With a little time and thought, we can
understand how changes in rotating mass will affect available
horsepower in a vehicle. Available horsepower in turn affects
acceleration in a very predictable manner.
Four things determine
the effect of rotating mass on our vehicles:
 How quickly and often a rotating mass speeds up or
slow down
 How heavy the rotating mass is
 The rotating weight's distance outwards from the
centerline
 How fast the weight spins
Here are the important
things to worry about:
 If
we push energy into the rotating mass and pull energy out several
times, obviously we move more power around than if we made a slow
smooth change in speed.
 The amount of weight is the least important thing! If
we double the weight we only double the stored energy.
 Distance
weight is from the center line is very important, because it determines
the weight's circular velocity (speed)! Stored energy goes up by the
SQUARE of the radius change. If we replace a 4 inch diameter hollow
driveshaft with an eight inch diameter tube of exactly the same weight,
it has four times the stored energy!
 The
faster we spin the weight, the more energy it takes to move it and the
more energy we must remove from that weight to slow it down. If we
double the RPM, we multiply stored energy four times. Again it is a
square of the change, just like number 1 was a square.
If
we reduce mass from twenty pounds to ten pounds keeping the same
distance out and same peak RPM, we reduce stored energy to half the
original amount. Reducing weight is a oneforone change.
If
we reduce diameter by half while keeping the same weight and RPM,
stored energy will be 1/4 the original stored energy. This change is a
square. Twice is a "four times" effect. 2*2=4. Four times is a sixteen
time effect on stored energy. 4*4=16
If
we cut RPM in half, we would reduce stored energy to 1/4 the original
amount. Once again this is a squared change. Change RPM three times,
and the stored energy changes nine times. 3*3=9
We should carefully
think about what this means when we change things. Some changes are
worthwhile, some are not.
Wheel Changes
Let's
assume, just for an example, all of the weight in a wheel is at the
outer edge and remains at the outer edge. If we reduce a wheel's
diameter but keep the overall weight the same, the wheel is a spinning
ring with smaller diameter. The smaller diameter increases the wheel's
RPM at the same vehicle speed. The smaller diameter also moves the
spinning weight closer to the center.
Let's
say we cut diameter in half. Now think about how fast the wheel spins.
RPM will be twice what it was at the same speed. The half size diameter
reduction spins the wheel twice as fast, and that would increase stored
energy to four times the original amount if the weight was the same
distance out.
The
weight isn't the same distance out. The spinning weight is now half
size. This 1/2 size reduction decreases stored energy by four times!
Because
the same weight got closer to the center, but the increase in RPM
increased stored energy, and payback is the same for both nothing
changed.
In this example, we
gained nothing at all with this change. We also lost nothing by the
size change.
Lightening
the tire or wheel would reduce stored energy, especially if the weight
reduction was at the maximum distance out from the center. Here is an
example where we want to make something as light as possible on the
OUTER edge, not near the (wheel) center. Spending money on smaller or
lighter rotors to save rotating weight is not a good use of money,
because the rotating weight is close to the hub of the wheel. Unless
the rotors are huge and we take weight out of the outermost edges of
the rotors, things will not change much. (A light rotor is good
for reducing unsprung weight, and that helps keep our tire's in
contact with the road. It also reduces vehicle weight. But this is a
different problem. Here we are talking about rotation, not the bounce
inertia or "dead weight".)
If
we spent money on the same weight reduction in the wheel, reducing
weight out a little further away from the center, we would do much
better. We would be removing weight further out from the center, where
it does the most good.
Which
brings up an important point we almost never hear mentioned, a lower
weight part might not be lighter at the outside edge. It might be
lighter in the center, where the weight reduction doesn't mean much.
If
we spent our money on a lighter tire we would be getting the very most
return for the weight change. The tire's weight change is mostly
outside between the rim edge and the tread area. We get maximum effect
from the change!
Think
about this carefully. If we buy a lighter tire, we know for sure the
weight comes off the most critical area. If we buy a lighter rotor, it
is close to the center and for the same weight change the return is
much less.
The
wheels also speed up and slow down gradually. With an 11second car, we
have 11 seconds to speed the wheel up. Most of the horsepower pushed
into the wheel is pushed in near the end, when acceleration is least.
Since we have more time to push the bigger amount of energy into the
wheel, it takes less horsepower than we might expect. A little ways
down, I'll show you how to determine the power if you know the speed,
weight, and time.
Drive Shaft Example
Now let's think about a
drive shaft. The driveshaft is a fairly thin hollow tube. Nearly all
drive shaft weight is at the outside, since
it is of course hollow. The shaft also turns at the same RPM no matter
what the driveshaft diameter, because the RPM is set by the rear end
ratio, tire diameter, and vehicle speed. If we make a driveshaft
lighter and keep everything else the same, the vehicle acceleration
change is most often insignificant.
Why insignificant in
most cases?
In
the first place, the drive shaft is small in diameter. With a small
diameter, less energy is stored for a given weight. In the second
place, a driveshaft is really not that heavy. A steel Mustang
driveshaft weighs somewhere around 30 pounds, so we just can't take
that much weight out.
Also,
the driveshaft spins up gradually and smoothly over a long period of
time. It accelerates fastest at slowest speeds, and that is when it
needs the least energy to spin up. Because it has a long time to spin
up, is a small diameter, and doesn't weigh much the driveshaft does not
remove very much horsepower at any instant of time. Despite what we are
told, a change in driveshaft weight has at best a very small effect on
acceleration. Likely any change is immeasurable in a street/strip car.
Now
a lighter shaft certainly can help in a very light vehicle or in a road
race car where instant change in applied power is required, but it
really won't change much in a 3000pound 11second car, except how fast
dollars leave your wallet!
Crankshaft
A
crankshaft is a bit worse than a drive shaft. A crankshaft accelerates
and changes speeds in every gear, so it is constantly storing and
returning energy to the system. In low gears it spins up pretty fast,
spinning up from "launch" RPM to shift RPM. This spin up repeats at
every shift. The crank also has to be heavy to support the pounding and
tugging of the pistons and rods as they accelerate and decelerate, so
we are dealing with some weight.
Fortunately
the crank diameter is small. A 3 inch stroke requires only a 1.5 inch
throw radius. Unless we make a huge change in OUTSIDE weight in the
counterweights, in most engines making the crank lighter makes very
little sense. The dumbest thing to do is hollow out the crankshaft
center because it is the smallest rotating diameter area. Don't believe
this?
Scat has it
100% correct. Many bench racers, and even some crankshaft
manufacturers, exaggerate a good bit! They remove weight where it makes
little difference in stored energy, but might make a difference in
strength. Some transmission experts worry about the wrong thing also.
If we worry about the outside edge weight of the largestdiameter
fastestspinning parts that speed up and slow down at every shift, we
are worrying about the correct parts. If we worry about parts that
speed up at the rate of the driveshaft, we would be wasting our efforts.
The
purpose of the examples was to give you a feel for what to look at
first. Any weight reduction is good for horsepower to weight ratio, but
some weight reduction has a bigger payback. Things that change speed
often, change speed rapidly, and/or are heavy at a large distance out
from the center...make the most difference. Look there first.
The
last "things" to worry about are small diameter "things" that change
speed a smaller amount, change speed over a longer time, and
change speed less often. They will have much less stored energy. If we
want to reduce rotating mass we should look at the heaviest things that
speed up and slow down most often, spin the fastest, and are large in
diameter with most of the weight at the outside edge.
Flywheel
A
flywheel can be fairly heavy, and the weight is a good distance out
from the center. It spins at crankshaft speed, and it has to slow back
down at every upshift. The flywheel can affect acceleration, but it
can affect it two different ways! In a light car with very fast 60foot
times, a lighter wheel can slightly improve 60foot times. This is
because the launch is often at full throttle, the car generally has a
steep gear, and we want to plant the tires hard into the track without
encouraging spin. The tires hook hard, and usually have a very soft
sidewall that absorbs shock. We want the engine to quickly match the
RPM needed to move the rear wheels, and not overpower the available
traction. It is a wide open throttle highRPM launch.
A
typical streetstrip car is different. Generally we can't launch at
wide open throttle, the tires are stiffer walled, the suspension is
heavier, and things just don't hit as hard. We actually want a heavy
wheel (and a heavy crank) to smooth out the power. This lets us have a
much more controlled launch, and smoothes out any sudden application of
throttle. An aluminum wheel, especially when the car is severely
traction limited and heavy, can really hurt 60foot times. A light
aluminum wheel not only makes a street car hard to drive, it hurts at
the track. It is especially bad with a heavy street machine.
Why do things
work this way?
First
we have to understand what power and energy are, and what rotating mass
does with that power or energy. Contrary to popular belief,
rotating mass does not consume energy. A rotating (or moving) mass stores
energy. This effect is very much the same as pouring energy in a
bucket, much like charging a capacitor in an electronics circuit.
Virtually all of the stored energy, except for that lost by conversion
to heat, is still there and available to do work at some time in the
future. That future where energy is returned might be milliseconds
later and help us out, or it could be some considerable time later and
waste energy. This is why time is very important.
One
example of useful energy storage is the flywheel and crankshaft of a
car. The force on the crankshaft is in pulses. A common four cycle V8
has four power cycles per crankshaft revolution, and there are 100
turns of the crank per second. At 6000 RPM an 8cylinder 4cycle has
400 power pulses per second. The flywheel (along with the harmonic
dampener and weight of the rotating assembly) smoothes these pulses out
by storing and releasing the pulsed energy from the explosions in the
cylinders. The result is a smooth rotation that will not tear gears up,
vibrate the car, or beat on bearings.
We
should always remember rotation, or movement of a mass, does not
actually destroy energy. If it did, the earth would have stopped
spinning millions of years ago! The key to understanding how weight
changes affect performance is to understand some very simple basic
energy flow in the system.
Definitions
Energy
Energy
is the capacity of a physical system to perform work. Energy exists in
many forms like heat, mechanical, electrical, and others. According to
the law of conservation of energy, the total energy of a system remains
constant. Energy may be transformed into another form, but it is
constant within a system.
For example, we all know
two pool balls eventually come to rest after colliding. They stop
moving only because the applied energy
(from moving the cue stick) is eventually converted to heat (from
friction with air and the table) and sound (which is not very much of
the energy loss). The ball movement along the table's felt
surface and through the air transfers energy outside the two
moving balls to the air and environment around the table and into the
table itself. The temperature of the table and air rises ever so
slightly, because the applied energy moves outside the system we "see"!
Since the heat energy is spread all around in a very large area, we
don't notice the temperature rise. We just notice the balls quickly
quit moving.
Another
example is our car's brakes. The energy stored in the moving weight of
the car is converted to heat by friction of brake pads rubbing against
metal rotors attached to the rotating wheels. This converts stored
energy (the engine put into the weight of the vehicle) into heat, and
the heat (containing all of that energy) radiates out into the air.
Most of what we actually do in a car is move heat around.
Newton's first law
A
mass continues in its state of rest, or continues uniform motion in a
straight line, unless it is compelled to change that state by forces
impressed upon it.
Old
guys like Newton sure had a lot of time on their hands to think about
simple things, but they got it right. A rocket coasting through outer
space is a good example. It will go on forever in a straight line
unless it hits something, or unless gravity or some other force
pulls it in a new direction.The earth wants to move in a
straight line, except gravitational attraction to the sun bends its
path constantly. A bullet reacts the same way, except friction with air
and gravity changes the direction and speed gradually over distance.
Newton's second law
The
acceleration produced by a particular force acting on a body is
directly proportional to the magnitude of the force and inversely
proportional to the mass of the body.
We
push harder and/or longer, and something moves faster. If it is
heavier, we need to push longer or harder (or both) to obtain the same
speed. It takes more energy to accelerate a heavier object to the same
speed as we might move a lighter object to that same speed. We can
either apply more force or apply the same force over a longer time to
make something move faster. It is all about TIME times the POWER, or
the amount of TIME an amount of POWER is applied. This is why those big
showoffs can eventually move a large boat, a railroad car, or an
airplane. All it takes is low friction and enough time and someone who
can't move a Volkswagen with two flat tires can roll a 10ton railroad
car.
Acceleration, Energy,
and Power
Acceleration,
by definition, is a change in direction or
speed. If we slow something down it is acceleration, just in a negative
direction. If we turn a vehicle or any other mass in a new direction,
it is really acceleration at a new angle or in a new direction. This is
why we can compare or define braking and cornering in Gforce (g's),
just as we do with "taking off" acceleration.
We
apply force (and this means we apply energy) over time (force applied
over time is power) to accelerate an object. If we want to spin a top,
we apply force offcenter from the axis and at right angles to the
axis. The top stores the energy we apply, and continues to rotate. Over
time the stored force is converted to heat from friction and the top
gradually slows until it finally stops.
Force is pressure or
energy. The product of the time we apply the force and the amount
of force
is the power. Power over time is a very useful thing to us because it
means we can do work with it. Power alone, without time it is applied,
is not not so useful. Let me give some examples:
"Watts"
are a measure of power, much like horsepower. "Watts" alone are not
speed, because a watt does not include a defined application time. A
watt is only power level, or work level, of energy over an undefined
time.
If
we include one hour's time we would have a watthour. Kilowatthours,
wattseconds, watthours, and other combinations of power level and
time define electrical energy or work. This is why we billed for
kilowatthours at our homes! If we were billed for plain old "watts",
it would not tell anyone how much "work" we bought. Watts are a true
scalar (single dimension) measure of ability to do work, just as
horsepower is. Both indicate a force or the ability to do work, but
both lack any inclusion of work time, so we have no idea how much work
was done, or could be done.
Horsepower
is a function of RPM and torque, just like watts are volts times
amperes. Horsepower is an ability to do useful work, but doing actual
work requires time. Torque is pressure, and since it does not
include speed it is not a very useful measure of system power or the
ability to accelerate or move weight. Despite what we hear, crankshaft
torque
is not directly related to moving something off the line or pulling a
heavy load. Up at the engine, it is really all about horsepower. The
horsepower (torque at a certain RPM) is eventually converted through
gears and other mechanical devices to a new torque value at a different
RPM. Eventually all we care about is the rotational pressure on the
contact patch of our tires that thrusts our car forward. A 800 lb/ft
torque at 2000 RPM engine does not accelerate a vehicle as well as a
400 lb/ft engine at 5000 RPM, because horsepower is a product of torque
and RPM. The higher RPM engine can be geared to provide more forward
pressure at the wheels. The higher RPM engine, with less torque, has
more horsepower.
If
you notice, ET calculators don't ask for torque. This is because torque
does not quantify the ability to do work. ET calculators ask for
horsepower, because horsepower clearly defines an ability to do work.
Joules
are another common measure of ability to do work. A joule includes both
time and force (pressure). A single joule is one wattsecond, or the
equivalent of one watt applied for one second. A single joule could be
10 watts applied for 1/10th of a second (10*1/10 = 1), the product of
time and force only has to be ONE wattsecond to make one joule. If we
applied TWO watts for 1/2 second, we have the same work. Two watts for
1/2 second is one joule (2*1/2=1).
Horsepower
can also be stated in kilowatts. One horsepower is approximately 0.7457
kilowatts, or 745.7 watts (the exact value is 0.745699872 kilowatts).
This means 746 watts for one second is 746 joules
and that is one horsepowersecond! One kilowatt is 1.341 horsepower.
Many
European engines are rated in kilowatts instead of horsepower, you've
probably seen that. A 300horsepower engine would be about 223.7
kilowatts. Your house probably consumes between 2 to 5 kilowatts of
average power, depending on how large it is and how you heat or cool.
This is somewhere between 21/2 to 7 horsepower of average
power. Think of what would happen to the power grid if we
converted all our cars and trucks, like the Greenies want, to run on
electricity! We would run out of electricity very quickly.
How
many joules are in 1492 watts when applied for 1/2 second? 1/2 times
1492 or 746 joules! 746 joules is one horsepowersecond. We could rate
our engines in joules if we needed to include both power and time.
Horsepower and
Acceleration
We
know horsepower alone is not a measure of useful work results, we must
know the time a certain horsepower is applied (or removed) to know how
it affects acceleration. Fortunately there are horsepower calculators
that predict ET for a given power. These calculators work because they
know the distance, they know the applied horsepower (they assume it is
constant), and from that they can calculate speed and elapsed time.
They do this because they assume the power is applied constantly and
they calculate the speed change over time. From the speed and time,
they get the distance. When they see 1/4 mile (or 1/8th mile) they stop
calculating and display the speed and the time taken to reach that
speed and distance.
Now
here is an interesting thing. It takes a certain number of
horsepowerseconds (certain energy applied) to reach a certain speed
for a given weight. If we make the vehicle twice as heavy, it takes
twice as many horsepowerseconds (twice as much energy) to go the very
same speed.
Now
let's apply 100 HP to go 1/4 mile in a 1000 pound vehicle. We went
108.6 MPH in 12.55 seconds. Now let's say we have a 2000 pound car. To
have the same speed and time, we have to also double the applied force.
If we apply 200 HP in our 2000 pound car we have exactly the same ET
and MPH! Now we know why insurance companies, in the late 60's, often
limited insurance to a car with 10:1 weight to horsepower ratio or
more. They didn't care if it was a 4,400 pound Super Bee Dodge with a
425 HP hemi or a 315 HP 3200 pound Hurst Rambler Scrambler, the
insurance companies wanted weight to power over 10:1 ratio or you could
not buy insurance. 10:1 weighthorsepower is at very best a 108.6
MPH at 12.55 seconds car! My American Motors 10:1 WeightHP Hurst S/C
Rambler, as a documented fact, set a new national ET record of 12.54
seconds in the 1/4 mile back around 1970.
Rotating Mass
Let's
say we want to change the drive shaft rotating mass to improve power
available to the rear wheels. We all know most of the weight in a
driveshaft is at the outer edge. It is a hollow tube. Let's say the
original shaft weighed 30 pounds, and we want to change it to a 15
pound aluminum shaft. The drive shaft is 3.5 inches in diameter.
We
can go to another calculator to find the joules stored in the
driveshaft! When we know the joules, we know the horsepowerseconds
sapped from moving the car. Let's say the engine peaks at 6000 RPM at
the end of the 1/4 mile, and that took 13 seconds.
Go to this calculator:
http://www.botlanta.org/converters/dalecalc/flywheel.html
The
original driveshaft weighed 30 pounds and we had to spin it to 6000
RPM. If we input that, we see it consumed (and stored) 5310 joules. 480
ounces in a 3.5 inch diameter RING (hollow center) and 6000 RPM.
That
is 5310/746 = 7.12 horsepowerseconds to spin the shaft to 6000.
Since the time was 13 seconds, the shaft soaked up 0.548 horsepower
distributed over that 13 seconds.
Now
we change to the aluminum shaft. Everything is the same except the
weight, it is now 15 pounds or 240 ounces. Using that flywheel
calculator we find we used 2655 joules. This is 2655/746 = 3.56
horsepowerseconds. Over 13 seconds, we "stored" .274 horsepower. The
net gain in available energy over 13 seconds was about 1/4 horsepower.
Here is the real rule of
how this works....
If
we are spinning up a very large diameter mass, or a very heavy mass,
and we do it rapidly, we sacrifice a lot of available power. If we are
spinning up a very small diameter mass, especially over a longer period
of time, we give up less power at any instant.
The
change from an aluminum flywheel to a steel flywheel is much more
pronounced than the change of the same weight in a driveshaft because
the aluminum wheel is much larger in diameter. We also speed and slow
the flywheel as we accelerate and shift, instead of smoothly spinning
the thing up like a driveshaft.
The
truth is for drag racing, unless we have a Godawful fast car or a road
race car where we have to instantly change power, an aluminum wheel
barely makes a perceptible change over a steel flywheel. The aluminum
wheel can actually be slower in a drag car, because the applied power
is not as smooth. It is harder to get a light aluminum flywheel out of
the hole, and that can easily offset any small "available power" change.
Summary
This
is an approximation designed to give you a reasonable feel for how a
change in rotating mass affects acceleration. We can see the power
extracted to spin a weight up is not very much if we do not spin it up
too quickly, or if what we spin is not very heavy and/or very large in
diameter. The "feeling" most people cling to (and parrot) is that
"heavier rotating mass kills acceleration". This is generally not
true at all for big heavy cars, although it can be true. Most things we
fret about make no appreciable difference in the grand scheme of
things. I would never bother changing from steel to an aluminum
driveshaft in my car, because my car takes 11 seconds to go 1/4 mile.
The car weighs 3000 pounds, and this means I might save 20 pounds of
weight and 1/2 horsepower lost to spinning that weight over the length
of the track. $400 is not a good investment at all for 1/2 horsepower
over the length of the track, or the extra 1/2 horsepower applied for
11 seconds I have to extract at the end and convert back to heat with
my brakes.
I
don't really have to worry about how fast things spin up at this point.
I don't care if the crank is 12 pounds lighter out of 50 pounds. I
don't care if the driveshaft is 15 pounds lighter out of 30 pounds!
Right now that $400 to $1000 would go a lot further if it made 20 more
engine horsepower, or removed 60 pounds of static weight. When I start
running out of easy power, then I will spend money making expensive
things lighter. The big problem right now is traction, so right now I
want to smooth the power out. The last thing I need is to make the car
more critical for launch RPM by using a lighter flywheel or shock the
tires more by using a lighter driveshaft. The first major weight
reduction will be the front K members, because that would remove weight
from the front and effectively add a larger percentage of weight to the
rear wheels! The last weight reduction for my car will be an aluminum
flywheel or driveshaft.
