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## Crank Balance

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• Guys, I ve got a problem (I think!) A mate has given me an Aermacchi race crank to balance ....... and he wants 26% balance factor. I was somewhat surprised
Message 1 of 12 , Oct 5, 2005
Guys,
I've got a problem (I think!)
A mate has given me an Aermacchi race crank to balance ....... and he
wants 26% balance factor.
I was somewhat surprised at how low this was but was assured that this
is the recommended figure by Aermacchi race engine builders so I will
bow to experience, until I hear otherwise...lol.
I've balanced a few cranks before with no problems but this one is
different.

Please tell me if I am doing something wrong here:
Weight of Piston, rings, gudgeon pin = 375gm
Weight of little end of conrod, when supported horizontally from the big
end = 147gm
Total weight to be balanced = 375 + 147 = 522gm
Apply balance factor of 26% = 136gm
Weight to be hung from little end to balance crank = 136gm - 147 gm = - 11gm

Interesting, all I have to do is fill a balloon with helium or hydrogen
till it exerts and upward 11gm force and attach it to the little end
.... and Bob's my Uncle?
....lol.....

No, seriously, instead of attaching a balance weight to the little end I
guess I have to attach 11gm to the edge of the flywheel _exactly
opposite_ the big end and drill wheels to balance as usual?
Am I missing something here? ...and what other bike engines run sub-50%
balance factors?
Thanks,
Grog
• ... Yep. Or remove weight from the crankpin side of the crank. Is the Aermacchi a horizontal cylinder engine? The odd balance factor was probably to reduce
Message 2 of 12 , Oct 5, 2005
Greg Summerton wrote:

> No, seriously, instead of attaching a balance weight to the little end I
> guess I have to attach 11gm to the edge of the flywheel _exactly
> opposite_ the big end and drill wheels to balance as usual?

Yep. Or remove weight from the crankpin side of the crank.

Is the Aermacchi a horizontal cylinder engine? The odd balance factor
was probably to reduce 'felt' vibration at the handlebars; forward-back
vibration is less objectionable than up-down to most people.

--
===ronin@... (Dave Williams)===================DoD #978===
== "Peace Through Superior Firepower" USAF Strategic Air Command ==
========================== http://www.bacomatic.org/~dw/index.htm ==
• Probably others that have horizontal cylinders?
Message 3 of 12 , Oct 5, 2005
Probably others that have horizontal cylinders?

> From: Greg Summerton <gregss@...>
>
> ...and what other bike engines run sub-50%
> balance factors?
>
• ... Yes, I sort of guessed the horizontal mounting would have something to do with it, but am still surprised at the 26% figure, it feels wrong ! I guess
Message 4 of 12 , Oct 5, 2005
Dave wrote:

> <>Is the Aermacchi a horizontal cylinder engine? The odd balance factor
> was probably to reduce 'felt' vibration at the handlebars; forward-back
> vibration is less objectionable than up-down to most people.

Yes, I sort of guessed the horizontal mounting would have something to
do with it, but am still surprised at the 26% figure, it 'feels wrong'!
I guess balance is a bit of a subjective thing.
I'm trying to get it into my head what relative importance the actual
'comfort' factor is compared to the destructive nature of the vibration.

I suppose the destructive power of the vibration induced in the frame
depends on the natural 'resonance' and and 'Q factor" of the assembly.
I am borrowing an electronic engineering term here, by Q factor I mean
the effect that any inherent damping in the whole assembly has on the
induced vibration.
It is a bit like striking a glass with a fork and listening to the
induced resonant tone compared to striking a glass that has a large dob
of mashed potato stuck to the side...this one would still resonate at
it's resonant frequency but give a softer dull note that did not ring
for a similar time as the clean glass.

As I write I guess it becomes a little clearer to me.....
A motor will produce both horizontal and vertical vibration components
and, I am guessing here, but probably the horizontal vibration feels
less objectionable to the rider than the vertical component?
But the magnitude of the change from 'normal' balance factors (50-75%?)
to 26% still seems abnormally large....but maybe the resonance and Q are
different in the horizontal plane compared to that induced in a vertical
plane and this would be why the factor needs to be changed so
drastically so that the induced vibration is in the direction of least Q?
Am I on the right path?
Thanks,
Greg
• Greg, This is not the way engine balancing is normally described but what you mentioned below may not be in regard to normal universal procedure and may be
Message 5 of 12 , Oct 6, 2005
Greg,

This is not the way engine balancing is normally
described but what you mentioned below may not be in
regard to normal universal procedure and may be
particular to some specific technique used on that
particular engine. Indeed, I can envision it working
in a particular case with a method I had derived for
another engine with a pressed up crankshaft which I
didn't want to disassemble.
Anyway, you now have calculated what would
normally have been the reciprocating part of the
balance weight term but must add 100% of the lower end
of the rod plus whatever bearings and thrust washers
and the weight of any oil assumed to be inside the
crank pin then make up a bob weight with the total
weight equal to the two terms. You would then
normally be required to remove the rod and its
bearings then hang the assembled bob weight on the
crank pin and drill, grind, spin and repeat until you
are satisfied that the whole temporary assembly is
balanced with no tendency to settle in the knife edge
rollers. Then remove the bob weight and reassemble
the crank with its rod and bearings.
Long ago in a galaxy far away I had once derived a
method of doing this without disassembling pressed
together cranks which yielded balanced crankshafts and
will describe it below but you may have mentioned a
hybrid method in your post which is peculiar to your
engine and the 26% factor mentioned may easily
represent what would normally be the component of
reciprocating weight required to correct for the
component you can't independently measure.
In the meantime, if you place the crank assembly
on knife edge rollers and hold the rod horizontal, you
will be resolving the required 100% of the rod/ big
end bearing weight into the system and if you can
accurately place the CONVENTIONALLY factored
weight of the reciprocating component DIRECTLY on
the center of the rod big end, you should be able to
see if the crank counterweight is grossly out of
specification. You can't spin balance it and it will
be a tricky balancing act but that would be
theoretically correct way to achieve the static
balancing process. However, 26% seems far too low a
balance percentage to me if the standard definition of
balance and balance factoring is applied. Indeed, a
comment about horizontal and vertical stroke axes and
perceived balance has been posted in this forum which
applies to the orientation of human spine and sight
axes but the comment was accurate for what most people
perceive.
The balance method I had derived and mentioned
above involves weighing the light end of the rod as it
pivots HORIZONTALLY from its crank pin and
SUBTRACTING 100% of that weight from whatever the
CONVENTIONAL reciprocating component would be to
produce a Modified reciprocating component weight.
This weight is then placed on the wrist pin and the
rod allowed to then dangle directly below the crank
pin as it is held on the knife edges with the crank
pin.

In your example :
Weight of Piston, rings, gudgeon pin = 375gm
Weight of little end of con rod, supported
horizontally from the big end = 147gm
Total weight to be balanced = 375 + 147 = 522gm
Initial balance factor I would choose = 50%
Initial balance factor I would choose x 522 gm = .50 x
522 = 261gm
Since this is the total and we can't remove the light
end of the rod from the crank, just consider that the
total should be 261gm which includes the 147gm we
can't remove so we need

261gm - 147gm = 114gm more weight to give the
conventional reciprocating balance weight component.
Since we also need 100% of the big end weight and as
it is already included when hanging there, just ignore
it and hang another 114 gm from the rod eye.
Since the gudgeon pin hopefully weighs less than
114gm, subtract whatever it weighs from the 114 gm to
yield a tertiary hanging weight, make up a washer of
the weight required and stick that washer and the
gudgeon pin on the rod as it will then be properly
centered. Let the rod hang vertically downward with
the crank pin placed in the horizontal position and
remove metal from the crankshaft wheels until it
statically balances.
As for the choice of balance factor, I am not sure
what to use with the Aermacci horizontal stroke axis
but I use a significantly higher balance factor in my
T-140 which runs smoothly but I have lightened the
reciprocating masses as much as I can. Good luck but
please keep the forum posted with your results if you
can. Thanx.

Ken Augustine
Kinetic Analysis
53 Mark Dr San Rafael CA 94903
415 472 4952 h, w & fax
kineticanalysis@...

--- Greg Summerton <gregss@...> wrote:

> Guys,
> I've got a problem (I think!)
> A mate has given me an Aermacchi race crank to
> balance ....... and he
> wants 26% balance factor.
> I was somewhat surprised at how low this was but was
> assured that this
> is the recommended figure by Aermacchi race engine
> builders so I will
> bow to experience, until I hear otherwise...lol.
> I've balanced a few cranks before with no problems
> but this one is
> different.
>
> Please tell me if I am doing something wrong here:
> Weight of Piston, rings, gudgeon pin = 375gm
> Weight of little end of conrod, when supported
> horizontally from the big
> end = 147gm
> Total weight to be balanced = 375 + 147 = 522gm
> Apply balance factor of 26% = 136gm
> Weight to be hung from little end to balance crank =
> 136gm - 147 gm = - 11gm
>
> Interesting, all I have to do is fill a balloon with
> helium or hydrogen
> till it exerts and upward 11gm force and attach it
> to the little end
> .... and Bob's my Uncle?
> ....lol.....
>
> No, seriously, instead of attaching a balance weight
> to the little end I
> guess I have to attach 11gm to the edge of the
> flywheel _exactly
> opposite_ the big end and drill wheels to balance as
> usual?
> Am I missing something here? ...and what other bike
> engines run sub-50%
> balance factors?
> Thanks,
> Greg

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• Ken, Thanks for the comments, I should have stated that the method I described in my original post is one that I have often used to balance an assembled crank,
Message 6 of 12 , Oct 6, 2005
Ken,
Thanks for the comments, I should have stated that the method I
described in my original post is one that I have often used to balance
an assembled crank, since they often come to me assembled.
I too, hate to press them apart just for balancing.
I prefer to receive them unassembled and accurately balance EACH
flywheel...then, being somewhat anal I always check the final assembly
using the method described below, however, it is not a method peculiar
to this engine!
This method is described in the 1936 (fourth) edition of "Speed and how
to Obtain it", Iliffe and Sons.
It was also advocated by the late and great Phil Irving in his sacred
handbook, "Tuning for Speed"

The method as described in both books:
Measure the weight of the little end of the rod while still attached to
the crank assembly.
The rod must be positioned horizontally with the little end resting on
the scale with the contact point directly below the centre of the piston
pin hole..
Add the weight of the piston assembly, apply the required balance factor
then subtract the weight of the little end from this, because the little
end will be used in the balance checking operation.
Hang this resultant weight from the little end on a thread then set the
wheels up on horizontal straight edges and check that it is balanced.
Roll the assembly repeatedly to check that it comes to rest in a random
position.

As I understand it, Balance Factor is always expressed as a percentage
of the total reciprocating weight...am I correct?
I think that the method that Irving and others have described applies to
this conventional method of specifying BFs.
Therefore I believe that I should be okay in using it, because as far as
I know the 26% was stated as a standard BF specification.
But then I may be missing something here, that is one of the reasons why
I posted the question, my ignorance may be the other 'factor' to be
balanced...lol...(sorry)

The crank in question is to be balanced to 26% of the reciprocating
weight so I was going to use this same method that I have successfully
used before
I did question the 26% 'instruction' directly with my mate but do not
want to raise the subject again and get on with the job ..... to avoid
any 'awkwardness' ....

However, the main problem I struck was when the math gave me a negative
number!
So, assuming the method is correct and assuming the 26% is correct, how
do I use this negative number?
I am of the opinion that I need to superglue 5.5gms to each flywheel on
the opposite side of the mainshafts to the crankpin, in a position that
corresponds to the centre of the crankpin..
Then I can place the crank on straight edges and drill to balance it.

So two questions remain:
Do I stick 11 gms in the exact mirror position to the centre of the
crankpin to achieve this?
Is 26% possibly correct?

Thanks for your comments Ken and others, I hope we can elicit a
definitive answer.
Greg

>This is not the way engine balancing is normally
>described but what you mentioned below may not be in
>regard to normal universal procedure and may be
>particular to some specific technique used on that
>particular engine. Indeed, I can envision it working
>in a particular case with a method I had derived for
>another engine with a pressed up crankshaft which I
>didn't want to disassemble.
> Anyway, you now have calculated what would
>normally have been the reciprocating part of the
>balance weight term but must add 100% of the lower end
>of the rod plus whatever bearings and thrust washers
>and the weight of any oil assumed to be inside the
>crank pin then make up a bob weight with the total
>weight equal to the two terms. You would then
>normally be required to remove the rod and its
>bearings then hang the assembled bob weight on the
>crank pin and drill, grind, spin and repeat until you
>are satisfied that the whole temporary assembly is
>balanced with no tendency to settle in the knife edge
>rollers. Then remove the bob weight and reassemble
>the crank with its rod and bearings.
> Long ago in a galaxy far away I had once derived a
>method of doing this without disassembling pressed
>together cranks which yielded balanced crankshafts and
>will describe it below but you may have mentioned a
>hybrid method in your post which is peculiar to your
>engine and the 26% factor mentioned may easily
>represent what would normally be the component of
>reciprocating weight required to correct for the
>component you can't independently measure.
> In the meantime, if you place the crank assembly
>on knife edge rollers and hold the rod horizontal, you
>will be resolving the required 100% of the rod/ big
>end bearing weight into the system and if you can
>accurately place the CONVENTIONALLY factored
>weight of the reciprocating component DIRECTLY on
>the center of the rod big end, you should be able to
>see if the crank counterweight is grossly out of
>specification. You can't spin balance it and it will
>be a tricky balancing act but that would be
>theoretically correct way to achieve the static
>balancing process. However, 26% seems far too low a
>balance percentage to me if the standard definition of
>balance and balance factoring is applied. Indeed, a
>comment about horizontal and vertical stroke axes and
>perceived balance has been posted in this forum which
>applies to the orientation of human spine and sight
>axes but the comment was accurate for what most people
>perceive.
> The balance method I had derived and mentioned
>above involves weighing the light end of the rod as it
>pivots HORIZONTALLY from its crank pin and
>SUBTRACTING 100% of that weight from whatever the
>CONVENTIONAL reciprocating component would be to
>produce a Modified reciprocating component weight.
> This weight is then placed on the wrist pin and the
>rod allowed to then dangle directly below the crank
>pin as it is held on the knife edges with the crank
>pin.
>
> In your example :
>Weight of Piston, rings, gudgeon pin = 375gm
>Weight of little end of con rod, supported
>horizontally from the big end = 147gm
>Total weight to be balanced = 375 + 147 = 522gm
>Initial balance factor I would choose = 50%
>Initial balance factor I would choose x 522 gm = .50 x
>522 = 261gm
>Since this is the total and we can't remove the light
>end of the rod from the crank, just consider that the
>total should be 261gm which includes the 147gm we
>can't remove so we need
>
>261gm - 147gm = 114gm more weight to give the
>conventional reciprocating balance weight component.
>Since we also need 100% of the big end weight and as
>it is already included when hanging there, just ignore
>it and hang another 114 gm from the rod eye.
>Since the gudgeon pin hopefully weighs less than
>114gm, subtract whatever it weighs from the 114 gm to
>yield a tertiary hanging weight, make up a washer of
>the weight required and stick that washer and the
>gudgeon pin on the rod as it will then be properly
>centered. Let the rod hang vertically downward with
>the crank pin placed in the horizontal position and
>remove metal from the crankshaft wheels until it
>statically balances.
> As for the choice of balance factor, I am not sure
>what to use with the Aermacci horizontal stroke axis
>but I use a significantly higher balance factor in my
>T-140 which runs smoothly but I have lightened the
>reciprocating masses as much as I can. Good luck but
>please keep the forum posted with your results if you
>can. Thanx.
>Ken Augustine
>
>
>
>> <>Guys,
>> I've got a problem (I think!)
>> A mate has given me an Aermacchi race crank to
>> balance ....... and he
>> wants 26% balance factor.
>> I was somewhat surprised at how low this was but was
>> assured that this
>> is the recommended figure by Aermacchi race engine
>> builders so I will
>> bow to experience, until I hear otherwise...lol.
>> I've balanced a few cranks before with no problems
>> but this one is
>> different.
>>
>> Please tell me if I am doing something wrong here:
>> Weight of Piston, rings, gudgeon pin = 375gm
>> Weight of little end of conrod, when supported
>> horizontally from the big
>> end = 147gm
>> Total weight to be balanced = 375 + 147 = 522gm
>> Apply balance factor of 26% = 136gm
>> Weight to be hung from little end to balance crank =
>> 136gm - 147 gm = - 11gm
>>
>> Interesting, all I have to do is fill a balloon with
>> helium or hydrogen
>> till it exerts and upward 11gm force and attach it
>> to the little end
>> .... and Bob's my Uncle?
>> ....lol.....
>>
>> No, seriously, instead of attaching a balance weight
>> to the little end I
>> guess I have to attach 11gm to the edge of the
>> flywheel _exactly
>> opposite_ the big end and drill wheels to balance as
>> usual?
>> Am I missing something here? ...and what other bike
>> engines run sub-50%
>> balance factors?
>> Thanks,
>> Greg
>
• ... Yep - recip weight is the sliding bits ( piston, p.pin, rings, circlips and the little end of the conrod [ as weighed with conrod horiz.] ). ... 26% is
Message 7 of 12 , Oct 7, 2005
>As I understand it, Balance Factor is always expressed as a percentage
>of the total reciprocating weight...am I correct?

Yep - recip weight is the sliding bits ( piston, p.pin, rings, circlips and
the "little end" of the conrod [ as weighed with conrod horiz.] ).

>I did question the 26% 'instruction'

26% is lowish, but down to 0% is used in some applications.

>However, the main problem I struck was when the math gave me a negative
>number!
>So, assuming the method is correct and assuming the 26% is correct, how
>do I use this negative number?
>I am of the opinion that I need to superglue 5.5gms to each flywheel on
>the opposite side of the mainshafts to the crankpin, in a position that
>corresponds to the centre of the crankpin..
>Then I can place the crank on straight edges and drill to balance it.
>
>So two questions remain:
>Do I stick 11 gms in the exact mirror position to the centre of the
>crankpin to achieve this?

No - you drill a hole and fill with "heavy metal", also called "mallory metal".
It is pretty well double the density of steel, so you drill the same sized hole
as if you had a "positive" figure, then fill it with mallory metal.

There are weldable grades around, they machine well too. If you can't get
a weldable grade, you can stake them in, or drill deeper and put a steel
plug in the top, which you then TIG in.

>Is 26% possibly correct?

Possibly.

Cheers IAN

--
No virus found in this outgoing message.
Checked by AVG Anti-Virus.
Version: 7.0.344 / Virus Database: 267.11.13/123 - Release Date: 6/10/2005
• D.M.Co., Just don t forget to include the weight of the reciprocating end of the rod and when you make a bobweight that you have to also include 100% of the
Message 8 of 12 , Oct 7, 2005
D.M.Co.,

Just don't forget to include the weight of the
reciprocating end of the rod and when you make a
bobweight that you have to also include 100% of the
rotating end of the rod and whatever oil can be
assumed to be inside the crankpin.

KA

--- "D.M.Co." <iwd@...> wrote:

>
> >As I understand it, Balance Factor is always
> expressed as a percentage
> >of the total reciprocating weight...am I correct?
>
>
> Yep - recip weight is the sliding bits ( piston,
> p.pin, rings, circlips and
> the "little end" of the conrod [ as weighed with
> conrod horiz.] ).
>
>
> >I did question the 26% 'instruction'
>
>
> 26% is lowish, but down to 0% is used in some
> applications.
>
>
> >However, the main problem I struck was when the
> math gave me a negative
> >number!
> >So, assuming the method is correct and assuming
> the 26% is correct, how
> >do I use this negative number?
> >I am of the opinion that I need to superglue 5.5gms
> to each flywheel on
> >the opposite side of the mainshafts to the
> crankpin, in a position that
> >corresponds to the centre of the crankpin..
> >Then I can place the crank on straight edges and
> drill to balance it.
> >
> >So two questions remain:
> >Do I stick 11 gms in the exact mirror position to
> the centre of the
> >crankpin to achieve this?
>
>
> No - you drill a hole and fill with "heavy metal",
> also called "mallory metal".
> It is pretty well double the density of steel, so
> you drill the same sized hole
> as if you had a "positive" figure, then fill it with
> mallory metal.
>
> There are weldable grades around, they machine well
> too. If you can't get
> a weldable grade, you can stake them in, or drill
> deeper and put a steel
> plug in the top, which you then TIG in.
>
>
> >Is 26% possibly correct?
>
>
> Possibly.
>
>
> Cheers IAN
>
>
>
> --
> No virus found in this outgoing message.
> Checked by AVG Anti-Virus.
> Version: 7.0.344 / Virus Database: 267.11.13/123 -
> Release Date: 6/10/2005
>
>
>

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• Whoops! Ian, maybe I did make myself sufficiently clear. I did not mean that I would/should PERMANENTLY add 11 gms of steel to the crank web opposite the
Message 9 of 12 , Oct 8, 2005
Whoops!
Ian, maybe I did make myself sufficiently clear.
I did not mean that I would/should PERMANENTLY add 11 gms of steel to
the crank web opposite the crankpin in order to achieve a FINAL
balance....this weight is only the weight used in the balance setting
procedure.
What I meant to ask was:
Do I temporariy glue 11 gms of steel (that is PURELY FOR THE PURPOSES
OF FINDING THE BALANCE POINT) in this position so I can then proceed
to REMOVE metal from where I need to get it to balance?

I would then remove the glued on weight.
The crank should then be balanced to 26% of the reciprocating weight
(which was -11gms)

I know you guys have given me your opinion and I appreciate it, but I
am still not convinced of "WHY" I should add metal at this point
of the procedure .. so I ask you to please consider this:

The negative figure for setting the percentage of the reciprocating
weight is confusing things a bit so lets say that if I had wanted a
30.3% BF I would have arrived at a POSITIVE weight, for balance
purposes, of 11 gms.
Would I not then hang this 11 gms from the little end so that I could
determine from where I needed to remove metal to achieve
balance?...(removal might be from either the pin side of the flywheel,
or the side opposite the pin, depending on the crank construction)

So, because I have used a BF that gives me a NEGATIVE weight for
balance purposes, would I not hang/stick this weight in a position
that is exactly on the opposite side of the centre line of the crank
from the crank pin?

I probably should sleep on it before I send this....(I hate being
wrong and looking like an idiot)
However, since I am only temporarily sticking the weight in this
position simply to set a balance point I would not need to use mallory.
All I need to do is drill to remove metal from wherever I have to in
order to achieve final balance(?)
Adding Mallory on the opposite side would only be performed if for
some reason I could not add sufficient holes on the opposite side?
...or if I did not want it to look like a swiss cheese.

I hope I explained this question clearly.
I've almost convinced myself I am correct!...lol
Thanks for your patience guys,but I would like your opinion on this
perhaps misunderstood question,
Greg

> >Do I stick 11 gms in the exact mirror position to the centre of the
> >crankpin to achieve this?
>
>
> No - you drill a hole and fill with "heavy metal", also called
"mallory metal".
• Ian, I replied to your last post late last night and I have sort of taken my own advice and slept on the problem....and see it a little more clearly now, I
Message 10 of 12 , Oct 8, 2005
Ian,
I replied to your last post late last night and I have sort of taken my
own advice and slept on the problem....and see it a little more clearly
now, I think.
Please excuse me if I am repeating myself but often it helps to state
something in a different way so that the full meaning is clearer.

Can anyone please explain to me if and where I am wrong as I get back to
basics:

What we are trying to do is balance a portion or percentage of the
reciprocating mass with an imbalance in the rotating mass.
Or...another way of stating it, we are balancing the reciprocating mass
with a DIFFERENCE in rotating masses on either side of the mainshaft.
(i.e. crankpin side and non-crankpin side difference)
I think 'difference' is the key word here.

The method described calculates a mass that is equal to the percentage
of reciprocating mass that we wish to balance for.
Then we subtract the mass of the little end, because it is already
present in the test/setup method we are employing.
Then we normally add it to the little end and check to see where the
flywheel assy comes to rest and drill metal from the flywheels on
whichever side of the mainshaft is required to balance it.
We achieve a difference in rotating mass on either side of the mainshaft
that balances the required percentage of the reciprocating mass.

Normally, a relatively large BF is used.
Normally, when it is applied to the total reciprocating mass we get a
sufficiently large figure from which we subtract the little end mass
(because it is already present when we test for balance) ...and we get a
positive value.
Easy.

However, if the BF low enough we get a low proportion of the total
reciprocating mass that we need to balance.
This can be so low that when we subtract the already present little end
portion of the reciprocating weight we get a NEGATIVE figure.

This means that the little end of the rod already has more than the mass
that needs to be balanced by the DIFFERENCE in the halves of the
rotating mass.
Therefore we cannot add a weight to the little end so that we can set
the difference in halves of the flywheel to balance it.
The only answer I can see is to add a weight OPPOSITE the crankpin side
then drill flywheels to get balance.

It matters not if we add mass to one side or remove mass from the other
side, what we are doing is achieving a DIFFERENCE in rotating mass, an
imbalance in the rotating mass to balance a portion of the reciprocating
mass.

Am I correct?
If so then I believe I need to add a temporary weight of 11gms at the
same distance from the centre of the crank but on the exact opposite
side of the mainshaft to the crankpin then drill form whichever side I
have to in order to achieve balance?

Thanks,
Greg
• ... SNIP ... Now you re confusing me !! Here s how you do it for a single cylinder pressed up crank : - weigh up piston, pin, rings, circlips. - weigh the
Message 11 of 12 , Oct 12, 2005
> I replied to your last post late last night and I have sort of taken my
> own advice and slept on the problem....and see it a little more clearly
> now, I think.

SNIP

> It matters not if we add mass to one side or remove mass from the other
> side, what we are doing is achieving a DIFFERENCE in rotating mass, an
> imbalance in the rotating mass to balance a portion of the reciprocating
> mass.

Now you're confusing me !!

Here's how you do it for a single cylinder pressed up crank :

- weigh up piston, pin, rings, circlips.
- weigh the "little end" of the conrod.
- add 2 together - this is your "reciprocating mass".

Set the crank up between centres ( as long as they are very "free" )
or on overhanging vee blocks on the main bearings (wash any heavy
oil out of bearings ).

Let the rod hang free, you now need to hang stuff from the LE until
the crank is in equilibrium. i.e. - it shows no tendency to stop in any
one position. Note, a piece of solid bar works well too, just cut it
long and keep cutting it down until you get balance.
Now weigh this "stuff" and add the weight of the LE - this is the
total weight required to get the crank into equilibrium.

Your balance factor is now this "eqilibrium mass" divided by the
"total reciprocating mass".

If the crank will reach equilibrium with the piston / rings attached
and hanging freely, then you have a 100% BF. If the crank will
reach equilibrium with you gently supporting the piston horizontally,
then you have a 0% BF.

To get a desired BF, you work backwards, you work out the
weight to be hung off the hanging rod, and drill the crank at the
lowest point when it settles.

Using a dynamic balancer with suitable jigs to guide the rod will be
more accurate, but this static method is good for 1/2 a BF
percentage point I reckon - even better with practice.

Cheers IAN

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AUSTRALIA
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• ... I m good at that! ... I agree with and understand all that. ... Yes, that is what I have done before on various cranks and what I was attempting to do on
Message 12 of 12 , Oct 17, 2005
Ian wrote:

>Now you're confusing me !!
>
>
I'm good at that!

>Here's how you do it for a single cylinder pressed up crank :
>
>- weigh up piston, pin, rings, circlips.
>- weigh the "little end" of the conrod.
>- add 2 together - this is your "reciprocating mass".
>
>Set the crank up between centres ( as long as they are very "free" )
>or on overhanging vee blocks on the main bearings (wash any heavy
>oil out of bearings ).
>
>Let the rod hang free, you now need to hang stuff from the LE until
>the crank is in equilibrium. i.e. - it shows no tendency to stop in any
>one position. Note, a piece of solid bar works well too, just cut it
>long and keep cutting it down until you get balance.
>Now weigh this "stuff" and add the weight of the LE - this is the
>total weight required to get the crank into equilibrium.
>
>Your balance factor is now this "eqilibrium mass" divided by the
>"total reciprocating mass".
>
>If the crank will reach equilibrium with the piston / rings attached
>and hanging freely, then you have a 100% BF. If the crank will
>reach equilibrium with you gently supporting the piston horizontally,
>then you have a 0% BF.
>
>
>
I agree with and understand all that.

>To get a desired BF, you work backwards, you work out the
>weight to be hung off the hanging rod, and drill the crank at the
>lowest point when it settles.
>
>
Yes, that is what I have done before on various cranks and what I was
attempting to do on this particular crank

As you say, to work out what weight to hang from the crank you multiply
the total reciprocating mass by the desired BF then subtract the weight
of the little end of the conrod.
Then you drill the crank as you stated to reach equilibrium.
However, IF the weight of the little end is GREATER than the value of BF
multiplied by the total reciprocating mass you will have a NEGATIVE
weight.....and this is where the whole 'question' began.:
How to you treat this negative value?
If it was a positive' weight you would hang it from the rod and drill
the flywheel to reach equilibrium.
Since it is negative you need to hang a 'negative weight' from the
little end......helium balloons? (Just joking)

If it is a negative value then you place a temporary balance weight on
the flywheel side face in a position that is exactly opposite the centre
of the big end pin on the other side of the flywheel centre.
(In other words, in a position that is on a line drawn through the
centre of the big end pin and the centre of the flywheel, at exactly the
same distance from the flywheel centre as the big end pin)
Then you drill the flywheel at the lowest settling point until it
reaches equilibrium.

I was asking if this is correct. (Hope so, cos that's what I've done!)

I hope I have succeeded in explaining my question properly this time
...... and apologise for the confusion.
I am seeking confirmation, one way or another, so I can sleep when this
crank is installed!

I was late in replying to your comment because I was enjoying myself in
beautiful and perfect spring weather at Phillip Island watching the
latest MotoGP.
I hope you were there Ian?
Of course the result of the main event was predictable, but entertaining
none the less.
The large fields in local support races held each day were arguably the
best racing of the weekend (as usual), and again illustrated the modest
numbers but considerable depth of talent we have in this lucky country.
For those not lucky to have attended this brilliant venue, the circuit's
excellent natural amphitheatre affords brilliant views of the racing.
Combined with scenic ocean views from the clifftop location the
brilliant spring atmosphere was complete.
A great weekend for sure, it's a pity all you motorcycle nuts were not
there to enjoy it, sheer magic.
Thanks,
Greg
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