Flywheel balancing

[carpenterbill]

Hey guys got a question concerning balancing. I would like to try pop-up pistons in one of my flatheads and plan to use K sportster piston and stroker plates to accomplish this. When making the switch from stock pistons to the taller sportster piston, how much is this going to throw the motor out of balance? Gonna require a complete teardown? Thanks.

[Rubone]

If you match the weight of the new pistons to the ones you take out it won't change a thing.
Robbie

[T. Cotten]

A single-throw V-twin cannot be "out of balance";
It can only be poorly balanced for a particular application.

Heavier pistons lower the factor.
But do you know what the factor was?

Do you know what factor you really want?

You will get a very, very wide range of opinions and endorsements for factors.

Any one of them will work just fine!

(So will ignoring it altogether.)

....Cotten

[billpedalino]

I agree with Cotton. The balance factor used only shifts the rpm at which optimal dynamic balance (the 'sweet spot') will occur. Actually, the sweet spot occurs mathematically at many rpms (resonant frequencies), but physically, only one sweet spot is in the physical range of the motor assembly.

Bill

[Slojo]

A single-throw V-twin cannot be "out of balance";
It can only be poorly balanced for a particular application.

Heavier pistons lower the factor.
But do you know what the factor was?

Do you know what factor you really want?

You will get a very, very wide range of opinions and endorsements for factors.

Any one of them will work just fine!

(So will ignoring it altogether.)

....Cotten

Cotton you have me laughing, good comments "(So will ignoring it altogether.)". I think you meant a single throw V twin can never be in balance (without auxiliary balance shafts). Actually a single throw V twin can have perfect primary balance if it is a 90 degree V twin such as a Ducati 90 degree twin that can be revved to the moon without an annoying primary vibration.
The 45 degree twin presents its own problematic characteristic. It would take up way to much space to explain the problems with our beloved 45 degree twins balance deficiencies. Simply put the mass of the rotating and reciprocation parts coupled with the mass of the motorcycle and the desired RPM range interfaced with the tolerance of the rider all make up the appropriate balance factor.
The higher the balance factor ratio the higher the RPM range intended for the motor. I would set the factor/ratio to a higher numerical value say about 55 percent on your not so fast revving street flatty. If the ratio is a high number you will have low frequency high amplitude vibration at the lower common rev range, little buzz lots of shake. At higher rev ranges you will have higher frequency lower amplitude vibration more minor buzz less violent shake. Personally, the high frequency is what tears up thing so a better balance up high is desired than a low rpm based balance unless you are marking tires while driving a left hand throttle right hand shift servicar. Advise is worth what you pay for it send me a nickel please.
Joe

[T. Cotten]

Joe!

The higher factor for higher RPMs doesn't plug into the theory, although it sure seems that way in practice!

Matt Elvenkemper made it as simple as possible at http://virtualindian.org/Flywheeltheory.htm

Its all about how the motor is secured.

....Cotten

[Slojo]

Cotton
Ohhhh Now I think I am getting a head ache. You were right there are many differing opinions and the one that works is difficult to find unless special efforts are made to balance test run and re-balance until the desired affect takes place. Probably what I will do with my 1914 cannonballer motor.
I can balance the motor and then check by sophisticated equipment while it is running in the chassis and then make alterations if not pleased with the results.
My 48 twin carb. pan is balanced at 53 percent. It was built to look fast and out run your average pan head with its mild cam and poor ports common on early pans (yes I did use 48 heads for authenticity not performance) . The bike is pleasantly smooth until about eighty to one hundred miles per hour where the shake becomes unpleasant. At 100 miles per hour the 48 appears to stop accelerating I think that is because of the bomb site venturies. The venturies offer fuel injection like throttle response but prohibit top end breathing.
I will leave the 48 as it is for now I am happy with its performance. Perhaps in the future I will find the time to revisit the performance issue of the 48. If I feel the need for speed I will hop on a Ducati. Besides a future project on the agenda is a high performance unorthodox Knuckle to run at the Maxton (sp?) mile and perhaps the salt also.
Joe

[fillibuster]

I 5-penny-d my Chief, and it runs fine at 120, but I want 130! do I 6-penny it?

[T. Cotten]

Joe!

The bad vibes you feel have little to do with motor balancing.

Motor balancing techniques only address what the motor feels. There nothing in the equation that considers the rider.
Please review http://virtualindian.org/Flywheeltheory3.htm

The circumstances where you feel the most vibration could be those where the frame is doing its job the best, by absorbing them from the motor.

Phil!

USA pennies, or Australian, or...?

....Cotten

[Slojo]

Cotton
I am aware of the chassis playing a big role in the feel of the overall package. I will review the information when the motor for my 1914 cannonballer is in process so the information will be fresh in my mind. Right now the chassis is my primary focus. Thank you for the lead.
Joe