Atlas balance factor

I'm in the process of rebuilding a motor for an Atlas. It is a mix of parts like you will sometimes find on old bikes. The drive side case is a P11, timing side maybe Atlas, crank turned .0020 under, mismatched rods due to blow up. Since a lot of parts are of unknown quality, and will be replaced, I will have the crank, etc. balanced. I have read threads here that expound every different balance factor you can imagine! This engine will not be used hard, so I am more concerned with a factor that will keep it smoother below say 5K RPMs. Also, I am looking for a factor from someone who has built a similar engine and knows what works. Last but not least, a DRY balance factor, as a wet factor does no good when the parts are in a shop and not filled with oil.
Thanks in advance, Bill

And what compression ratio pistons do you want to use?

Bernhard said:

And what compression ratio pistons do you want to use?

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Of course I would like to use the original 7.5/1 pistons, but Commando pistons with a plate under the cylinders will have to do. Figure about 8/1.

hello for an Atlas balance factor should around 30% the mass of the top end I have a 1963 motor to do myself of a mate

i have NEVER heard of a balance factor expressed that way. it is ALWAYS 100% rotating and a percentage of reciprocating. IIRC an atlas is 70%

annajeannette said:

hello for an Atlas balance factor should around 30% the mass of the top end I have a 1963 motor to do myself of a mate

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bill said:

i have NEVER heard of a balance factor expressed that way. it is ALWAYS 100% rotating and a percentage of reciprocating. IIRC an atlas is 70%

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I have seen 70% as well, the question is, is that a dry or wet #? If wet, you would probably want to balance it dry to about 81 or 82%.

well mass mean the total weight of the piston and ring and wrist pin and rods and shells and bolts and nuts have to be taken in the the recipiating mass at speed of rotation and that how you find your balance factor which you never get 100% or any where near 30% is more realistic

annajeannette said:

well mass mean the total weight of the piston and ring and wrist pin and rods and shells and bolts and nuts have to be taken in the the recipiating mass at speed of rotation and that how you find your balance factor which you never get 100% or any where near 30% is more realistic

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SPACE SHOT>>>anyone thinks she has a clue? You're in trouble if you do !

"hello for an Atlas balance factor should around 30% the mass of the top end I have a 1963 motor to do myself of a mate"

He has my sincere sympathies....

When I was building my solid mounted race motors, I believe I was using 68% though my Compression ratio was 8.3 to one using methanol feeding a Sherock supercharger.
Al

Below are the balance reports (static and dynamic) done by Brisbane Engine Balancing on my 650ss - done to 65% dry
Quite a difference made on dynamic - down from 20g to ~0.2 g both left and right.
Might by interesting/useful for some?

That's quite a difference. Could you feel the difference before and after being balanced?

Nice to see a print out.
If it was left alone with the components used as is, the balance would have been 70% BF = 264/375 .
The frame vibration comes from the residual reciprocating 70% mass induced shake 375-264=111 grams.
The frame vibration comes from the residual reciprocating 65% mass induced shake 375-244=131 grams.

The big picture is that you can not actually "balance" a single or a parallel twin which acts as a two piston single for balance purposes...you are only calibrating the amount of IMBALANCE.
One goal is to reduce or eliminate rocking vibration which is a minor considering the big picture.
added:It's been a long time since I did this so I'm gonna rethink and check my numbers to make sure it is right.

Here are some recommendations for balance factor from various sources that I've collected over the years. Not sure which are wet and which are dry, except the 62% recommended by Milliken is wet.



Ken

the theory is the higher you go over 50% it moves it from a vertical shake to a horizontal shake and with a horizontal shake being less noticeable to the rider, but this is also dependent on the RPM range you run it at. as dyno dave stated it will NEVER be in balance like a tire as an example.

??????????? this shows you have NO CLUE.

annajeannette said:

well mass mean the total weight of the piston and ring and wrist pin and rods and shells and bolts and nuts have to be taken in the the recipiating mass at speed of rotation and that how you find your balance factor which you never get 100% or any where near 30% is more realistic

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The frame the engine gets fitted to has an effect too, the harmonics of the frame structure can mean a balance factor that works in one frame could be terrible in another.

lcrken said:

Here are some recommendations for balance factor from various sources that I've collected over the years. Not sure which are wet and which are dry, except the 62% recommended by Milliken is wet.

View attachment 8409

Ken

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Thanks Ken. Neat to see all the differing numbers, which is exactly why I posed the question originally! Talked to Jim Comstock and he recommended 70% adding 150 grams for oil in the crank. I realize that you will never balance a 360 degree vertical twin perfectly, you just move the vibes up or down the rpm range. The balance factor I ended up with for the Norton I road raced is not what you want for a gently ridden street Atlas. I just had an 850 dynamically balanced at 63% dry, per the factory MkIII manual. He noted that is was pretty close to that as it was, which leads me to believe that the earlier factory manual that listed the balance factor as 52% dry was an error that was corrected later in the MkIII manual. Commando's aren't as picky as the Atlas due to the rubber mountings, but they are sweeter when they are right.

milfordite said:

, you just move the vibes up or down the rpm range.

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The balance % of the usually unmentioned mass is what controls the ever present energy in the front-back and up-down plane.
The changing rpm of the engine is therefore the frequency of the energy source imparted to the frame. The mechanical attributes of the frame is what is controlling the sympathetic resonating frequency. That is what the rider feels.
It is an extremely complex physics concept and is the reason few people understand it.

If the frame were left out of the equation the vibration would be linearly increasing from idle to full RPM...without spot rpm resonances.

This is the device where, as a field service engineer in the 90's, where I learned about vibration analysis in very expensive Coriolis mass flow meter installations. They were about $3000 a piece in the 90's and are now around $4500 ea for todays replacements. After searching for about 5 years I finally got a pair for $$
They work using FFT fast Fourier transform analysis to display the resonant frequencies that the pick-ups detect. A cheap vibration detector does only that...display the (dumb) amplitude of the vibration.
These mounted on one of my featherbed engines/bikes, on my chassis dyno will provide hours of fun research....one for the up-down and one for the front-back planes.

annajeannette said:

well mass mean the total weight of the piston and ring and wrist pin and rods and shells and bolts and nuts have to be taken in the the recipiating mass at speed of rotation and that how you find your balance factor which you never get 100% or any where near 30% is more realistic

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Sorry annajeanette, but your information is utterly wrong. The common rule is 100% of the mass of the complete piston, and 33% of the mass of the rotating/translating part which is the conrod (to be exact, it's the weight of the small end measured in a device supporting the conrod's small and big ends on knives). The total isn't compensated by counterweights to 100% - a lower figure is applied, usually between 60 and 75%.
The reason for this is that balancing has to cater for one revolution of the crankshaft. While piston and conrod are vertical to the crankshaft centerline at TDC and BDC, this is not the case for piston positions in between, hence an eccentric factor occurs. Furthermore, the mass balance is influenced upon by the acceleration and deceleration at TDC and BDC (flywheel counterweights do not contribute), hence the revs come into play. It is not possible to determine the balance factor analytically, it rather has to be determined in practical trials using the most probable rev bands. These trials are a tedious process conducted by the OEM factory. The way the engine is supported (i.e., rigidly or elastically mounted) affects the findings of course. Hence, the balance factor is a subjective compromise between various objectives.

I hope this helps.

-Knut