re: Subject: formulas to relate diameter of rotating mass to HP

["Mark Dean" <osterbri@xxxxxxxxx>]

>Date: Fri, 15 Jan 1999 19:39:35 -0500 
>From: mark kibort <mkibort@domain.elided> 
>Subject: formulas to relate diameter of rotating mass to HP 

>Message text written by INTERNET:bmw@domain.elided 
>>Any formulas for how to relate the mass of those "rotating thingy"s to
the 
>>power drain??  If I reduce that mass by 5%, can I assume a 5% lower
drain? 
 

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

A one foot diameter steel flywheel would be about 3mm thick, an aluminum
one would be 9mm thick.  I think your estimate is a little off.  Unless
you're
talking about a pulley.

>and you race somone the acceleration from 60 to 100 takes about 9 seconds
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.

>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.
Funny how you say a lightened flywheel is a waste of money when it provides
up to
three times (plus) the performance advantage that you claim for your thing.

>Nascars do it to drop total weight, and increase reving responsiveness and
>shiftability as they dont use clutches all the time and the engine speeds 
>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.

>(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 car
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.

>ALSO,  reducing the pulley  sizes  for reducing accesory speeds is also a
>waste.  alternator will produe the same amps that needs to produce at 
>any speed, thus pulling the same HP.  Water pumps could gain and so could 
>driven fans, maybe PS pumps.  A/C is usually off anyway and is free
wheeling 

>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.  For something like a crank or a wheel, it
depends 
on where the mass is removed, and isn't so easy to calculate.  In general,
the
further from the axis of rotation that the mass is removed, the more
benefit it
provides.

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