Message to Mark Dean regarding rotating mass. Big errors!

[mark kibort <mkibort@xxxxxxxxxxxxxx>]

>From: "Mark Dean" <osterbri@domain.elided>
Subject: re: Subject: formulas to relate diameter of rotating mass to HP =


My replies (mark kibort) are encased by the >>>>>>>>>>>>>>>>>>>

>Ok, if you have a 1 foot diameter flywheel and it weighs 4 lbs

>>>>>>>>>>>>>>>>yes , we are talking about a pulley . Just an
example>>>>>>>>>>>>>>>>

>and you race somone the acceleration from 60 to 100 takes about 9 second=
s
in
>a 200 hp 2600 lbs car. each 1000 rpm segment is between 3 and 5 seconds =

>depending on the HP to weight of the car. 3 seconds to accelerate 4 lbs,=
 =

>one foot in diameter, to a difference of speed of 1000 rpm, is less that=

.5 hp.
>(more like a .25 hp depending on the shape of the pulley of course)

I would *love* to see how you figured this out.
What assumptions did you make?  Just pulling some numbers out of the air
for the flywheel of 12 inches in diameter and 1 inch thick, the power
consumed
in your example is more like 2 to 15 hp, depending on the material and
speed range.
After looking at the weights of some BMW flywheels (11 to 25+ pounds), I
think
my estimate is a little more accurate.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>I figured this out using simple physics.=
 =

REMEMBER, the
HP needed to accelerate an inertial load is dependant on the speed of
acceleration.  Now, Im telling
you the assumptions.   4 lbs , 12" diameter pulley  (sorry , not flywheel=
 ,
no difference though)
IT IS NOT 12 HP TO ACCELERATE  A 12" DIAM. PULLEY , 4 LBS OVER 1000 RPMS =
IN
3 SECONDS
The numbers were not "pulled out of the air
they were used as an example that in a real life application , even
removing =

a 4 lb pulley ,will not regester on a dyno.  Do you understand how a
chassis dyno works.
you run your car over a 12 second run over a speed range of 3000 to 6000
RPM. =

since the most acceleration  is at the peak torque area , we look at the
fastest 1000rpm change.
at 3 seconds for  a 1000 rpm change on a 300 hp car,  you get less than 1=

HP, YES, about a 1/2HP.
Now, figure that out and tell me that Im all wet.    Before you go on,
figure that out.

a human being can accelerate a 4 lb , 1 foot diameter mass over 3 seconds=

to 1000 rpm.  this means it is =

less than 1 hp.   I hp is 550 lbs in 1 second , lifted 1 foot.


>It is not even a factor , so dont waste any money on lighter flyweels. =


It IS a factor, especially in lower gears, e.g. accelerating away from a
stoplight on
the street, where most people drive and will feel a performance
improvement.
>>>>>>>>>>>>>Yes, i will be more of a factor in low gears as the engine
revs faster,
thus increasing the HP needed to accelerated the mass.  Yes, now how much=

is very easy to =

calculate.     How long are you in 1st gear.   3000 rpm to 7000 in 1.5
second or something.  Then ,
the HP is much greater, but you cant determine this on a chassis dyno.

Funny how you say a lightened flywheel is a waste of money when it provid=
es
up to
three times (plus) the performance advantage that you claim for your thin=
g.
<<<<<<<<<<<<<<<<<<<<<<The eRAM produced 10 hp on all  cars having 200 hp =
or
more
baseline.   Now a performance flywheel is saving how much, 12" 4 lbs???  =

do the math.
I will however, say that it does make shiftabilility better as you have
much more responsiveness
of rev's, dragersters would have the most to gain here, not road racing ,=

where you are in 3rd and 4th gear for 5 to 8 seconds
Time is everything here.  This is my point, if you are racing someone on
the street from 50mph to 100 mph
it will take you about 10 seonds or a little more .   3seconds for 1000
rpms of speed change .  this is real life.
DYNOS will not show this.  a stand alone engine water brake dyno can thou=
gh
as the test would have to be reving the engine
over short amouts of time.

>Nascars do it to drop total weight, and increase reving responsiveness a=
nd
>shiftability as they dont use clutches all the time and the engine speed=
s =

>change very quickly. =


Who ever told you this is either a liar or a fool.  Winston Cup cars have=
 a
high minimum
weight, and most need to add lead ballast to make it.  They use lightened=

rotating
components to reduce the effective mass that the engine must accelerate,
looking
for any edge they can get in their highly competitive field.  They also
(except for a
handful of roadraces) don't shift a whole lot in the race.
>>>>>>>>>>>>>>>>>>>>>>>>>>>So, if they are in a top gear , how much time
does it take to accelerate
from 80 to 180 mph?????  Let me say that it is over 5 seconds and they ha=
ve
over 600 hp.
yes, the hight and weight is controled, but they can shift every ounce th=
ey
can off the rotating mass for the above reasons.

>(ie if you rev in idle from 1500 to 7000rpm within a 1/2 second, that =

>becomes big hp. ) =


>Anyway, chassis dynos cannot measure a gain because it acts like your ca=
r
is on the
>road.  typical dyno runs last 10 seconds over a speed range of 3000 to
6000 rpm.
>do you thing it could measure , or is there a diff???????? =


An INTERTIAL chassis dyno can measure the effects of lightened components=
.
Of course a competent engine designer doesn't need to measure it, he will=

calculate the effect of the weight reduction before building the part.
>>>>>>>>>>>>>>>>>>>>>No, a inertial Chassis dyno like a Dyno Jet cannot
measure
lighted components as the runs are too long in  time to accelerate..  Yes=
 a
competent
engineer will know the effects lightened components.  So , on street
application, or road race =

where I spend the majority of my time, it makes no sense to lighten
flywheels as my cars engine
is at 4000 to 6000 rpm the majority of race or event  and it takes 8
seconds to go from 3500 rpm
to 6200rpm   60 to 100 mph.   at 300 hp for the engine, what do you think=

it takes to do that at =

4lbs , 1 foot diameter.   GET BACK TO ME AND LET ME KNOW WHAT YOU FIND OU=
T.
IVE DONE THE CACULATIONS, HAVE YOU??

>Mark Kibort

To answer the original question, it depends :).  What is really important=

is the
mass moment of inertia, which depends on the mass of the object, and its
shape.
for something simple, like a disk shaped flywheel that is lightened by
reducing
its thickness, a 5% reduction in mass will produce a 5% reduction in the
power
required to accelerate it. =

>>>>>>>>>>>>>>YOU LEFT OUT THE MOST IMPORTANT FACTOR IN THE FOMULA:
TIME,   TIME IS THE CRITICAL ISSUSE HERE. OVER WHAT TIME.
On a dyno, it takes several seconds to accelerated the inertial drums tha=
t
weigh 3000 lbs.
the time it takes to accelerate that known weight over small periods of
time, is the HP !!!
If you ignore this factor , you forget that if I was to accelerate the 4 =
lb
flyweel or pulley in infinitely =

small time, the HP would be infinitely HIGH, 100000000s of HP depending o=
n
TIME!!!

 For something like a crank or a wheel, it
depends =

on where the mass is removed, and isn't so easy to calculate. =

>>>>>>>>>>>Doesnt seem that easy for you .  Do us all a favor, call a mat=
h
teacher
or look at an old highschool physics book , You will see that Im right!!

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