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I'm too tired to even try to think of which way the 7% would pull, maybe you can do a little math on it?

I'm still intrigued by Cotten's post on how balance factors altered the "feel" and power delivery of similar engines. As I read it, he wasn't talking about the obvious differences in smooth running rpm ranges. Perhaps this is a phenomenon aking to the way certain firing orders on similar engines "hook up" on dirt tracks better than others? (like 360 degree parallel twins being better than 180 degree ones, and both-barrels-together" thumbers being good for certain conditions.

Plane of the vibrations:
Or could it have something to do with planes (or directions, if you will) of vibration, and the damping or resonating of the chassis (frame & other  parts)? Again as I see it, what basically happens when you change balance factors, is that you move the plane of the vibrations. The engine will "shake" with the same energy no matter what balance factor is employed (no?).  If you balance rotating weight only, the bike will shake "up" when the piston(s) shakes "down".

If the reciprocating weight is balanced 100% then the counterweight and the reciprocating parts will balance out at tdc & bdc, but midway there will be nothing to balance the counterweights, resulting (in a single or "narrow V-twin" such as ours) in the engine shaking just as bad as the unbalanced one, but at 90 degrees to the cylinder axis (or for an Indian presumably 90 degrees to vertical = horizontal).

Everybody clear on this? When the piston is travelling halfway down the bore, the counterweights are travelling roughly horizontally. Any balance factor between 0 and 100% of the reciprocating weight (the piston & part of the rod) will result in some "up & down" vibration + some "back & forth" vibration, which will a up to a "total vibration" in a direction between vertical and horizontal. Which is where the chassis enters.

Moto(r)cycle frames are made the way they are for a number of reasons. I don't think these includes stiffness, at least not as a primary objective. I have a sneaking suspicion, that the major reason we all sit on top of our bikes, is because they evolved from pedal bikes (where such a position was favorable to treading the pedals), and someone forgot to adjust the riding postion when they removed the pedals... Suffice to say that bike frames are not the stiffest structures known to man, and furthermore they are not equally stiff in all directions (as seen from the perspective of the engine).

When the engine shakes "up", the vertical tubes (saddle tube, front tubes) don't get very exited (engineering pun), as they are pretty stiff in the end-to-end direction. The bottom & top tubes, running at right angles to "up", do not have great stiffness when prodded in their sides, and they will easier start to vibrate in sympathy with the engine.

When the engine shakes "back" (or "forth"), the picture is the reverse of above.

In any motorcycle chassis there is a direction where the most tubes are in a "best" position to resist vibration. And another where they're least able to.

These directions could be said to determine the best "overall balance factor". The stiffness of the chassis do not change with the frequency of the engine vibration (= rpm) and thus it should be possible to find a direction (or balance factor) where these match the stiffest plane of the chassis. But...

Resonance:
On top of this, frames are composed of a collection of thin metal tubes, each with their own resonance frequency (they "ring" when you hit them, and if you can sing that note loud enough at them, they will ring without you touching them). There's a number of intentionally placed springs on our bikes too, and a few pivots for good measure... (not to mention parts like tanks, handlebars & Motolamps)

These all have their own resonance frequencies (and harmonics thereoff; octave above & below etc etc), and they add up to sort of a symphony when the bike is fully assembled.
The parts all interact. That is to say they dampen or reinforce each other's natural resonances. In our bike chassis, this means that it is easier to make the whole (or parts of the) chassis vibrate when excited in some directions, then in others (much like the stiffness above).
The "chord" (mix of frequencies) it takes to make your Indian sing depends on the direction you shout at if from...(I like that statement. It may not be strictly true, but it's quite poetic...).

OK. Now rpm:
As the rpm translates into "frequency of vibration" via the crank mechanism (translating rotational movement into reciprocating), the rpm corresponds directly with the frequency our engine shakes with.

As the bike chassis is composed of many parts each with their own resonance frequence, AND as these parts are more or less sensitive to vibration (prone to start ringing) according to the way they're hit (end-on etc), AND as the parts are dispersed in various directions on the bike, it follows that the bike is unequally sensitive to vibration in any direction you choose for a given frequency (engine rpm). Or, put another way, the bike is unequally sensitive at a given frequency (= rpm) -depending on the direction you "attack" it from!

It follows naturally from that, that any rpm (or range of rpm, as resonances work over a certain range of frequencies) will have a direction (= a balance factor!) where they will meet the most resistance from the rest of the bike, and thus shake less.

Finally; balancing:
I believe the only way of finding these directions -or rather (& much more interesting!) finding the balance factor CORRESPONDING with these directions, is to do what S&S did. And, I bet, what the Springfield factory did when they first tested & built our bikes.

So, on stock(-chassis) Indians, go with the factory balancing factor, or with something someone else tested out.

OR take the more sophisticated approach, that the stock balancing factor came about from a combination of the stiffness of the chassis + the resonances (and directions of them) in the chassis + the typical rpm range envisaged.

As I mentioned above (just before "Resonances"), the stiffness could be said to determine an "overall" balance factor, while the resonances determine a "modified factor" for smooth rpm ranges. This means that if you go faster (overall) than the factory intended, it may be a good idea to re-balance the engine accordingly. Unfortunately mere speculation can not say if the factor needs to go up or down (or at least my speculation can not!), so either the S&S approach is called for, or the advice of someone who has been here before & done the testing necessary to determine the best balance.

Dynamically balancing the flywheels is certainly a help. As Mat said, static balancing can not detect different centres of gravity in the 2 flywheels (resulting in side-to-side shakes, the direction where narrow bike frames are the absolutely least stiff), but the factor should remain as stock (unless you go for the resonance theory...).

On non-stock chassis Indians, you're on your own (but the dynamic balancing is still a good idea). I don't believe many US listers have Indians in "special" frames, but I mention it because I know of at least 4 people who have (or are building) V-twin Indian engines in a variety of british bike frames. An off-the-wall guess would be to look at the balance factor of the engine that came in the frame originally. Engine mounting points would seem to be important, too; both as frame stiffeners (the engine as a "brace") and from the vibration feed-in perspective.

I'm not sure I can see why a stroked engine in a stock chassis would need another balance factor, but maybe I'm missing something?
 
 
From: Guy <guyiii@home.com>
Thanks, Moen - good job, ... just wanted to comment on a couple things below:

> No doubt their (S&S) hands-on approach to figuring out balance
> factors is a surefire way to go, if a little impractical for
> most of us. However, it may really be the only way to
> go -save troublesome finite element analysis & stuff
> like that (which will only work up to a point, depending
> on the accuracy of the input data, bringing us back to
> empirical hands-on testing) for determining the ideal
> balancing factor on non-stock Indians.

One idea which occurred to me was that a company sells "dynamic balancers" for HDs (viscous damping??) - I don't know much about them, but there may be a partial solution here for us that potentially gives us "real time" dynamic balancing....(or maybe we should add a piston supercharger to change the balance characteristics....)....or, maybe carbon fiber lightweight parts would turn the trick....

> There are some theoretical considerations that might bring us a little
> closer to the answer, however.
> I'm too tired to even try to think of which way the 7%
> would pull, maybe you can do a little math on it?

Welll, I don't immediately know how to calc it...but the narrower included angle of our beloved Indians would fairly obviously tend to accentuate the vertical vs horizontal difference in rod/piston momentum which causes our balancing dilemma.....
 
>Perhaps this is a phenomenon akin to the way certain firing orders on >similar engines "hook up" on dirt tracks better than others? (like 360 >degree parallel twins being better than 180 degree ones, and >"both-barrels-together" thumbers being good > for certain conditions.

Ummmm - the analysis I've read says that its the uneven firing pulses of the engines delivering variable force to the rear tire - that pro riders are so good that they can get up to peak traction and keep it there by sensing when it will be lost (tire slips, tire catches, tire slips....) vs a four that has a smooother power delivery - when the tire slips, it may be too late (I say MAY because these guys can ride a slipping rear tire AND a sliding front tire)

> I believe the only way of finding these directions -or rather
> (& much more interesting!) finding the balance factor COR-
> RESPONDING with these directions, is to do what S&S did.
> And, I bet, what the Springfield factory did when they first
> tested & built our bikes.

How about some suppliers (S&S, flatheadpower,??), restoration companies, etc, testing non-destructively via accelerometers & dynos existing engines for which the balance factors are known??

> Unfortunately mere
> speculation can not say if the factor needs to go up or
> down (or at least my speculation can not!), so either
> the S&S approach is called for, or the advice of some-
> one who has been here before & done the testing ne-
> cessary to determine the best balance.

Now that I've considered it more, I can speculate based upon the narrower included angle that the balance factor should go up for Indian compared to HD....so if 60% is good for HD, the 65% someone else mentioned may be good for Injuns...

> Dynamically balancing the flywheels is certainly a help.
> As Mat said, static balancing can not detect different
> centres of gravity in the 2 flywheels (resulting in side-
> to-side shakes, the direction where narrow bike frames
> are the absolutely least stiff), but the factor should remain
> as stock (unless you go for the resonance theory...).

I don't really follow this difference in CG stuff, if the wheels are independently balanced with half of the "appropriate" mass, the CGs are co-axial, yes??
 
 
From: "Moen" <moen@get2net.dk>
> Now that I've considered it more, I can speculate based upon the
> narrower included angle that the balance factor should go up for
> Indian compared to HD....so if 60% is good for HD, the 65%
> someone else mentioned may be good for Injuns...

You are of course totally right. The wider the V, the more of a horizontal component in our "ideal vertical piston" we get: And of course less of a vertical component, so for Wide Vees the factor should go down (under identical circumstances etc).
 
> I don't really follow this difference in CG stuff, if the wheels are
> independently balanced with half of the "appropriate" mass, the
> CGs are co-axial, yes??

Hmmm. Are they? Can't we have a situation where one wheel is heavier at the rim & lighter in the... No, to my horror, I think you may be right, and I may have been trying to mislead y'all by mindlessly passing on what I've heard. We shall grill Mat on this when he comes online next!