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Crankshaft Balancing

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Cotten

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Posts: 2674

Joined: Thu Sep 30, 1999 12:01 am

Location: Central Illinois, USA

Post Tue Jan 25, 2011 7:49 pm

Re: Crankshaft Balancing

Sidewinder wrote:When I run my U the first year it was running smooth only on low rpm. I seamed to pull happily up hills in high gear and low rpm. In highway-speeds it was not smooth. The vibrations where so bad that my boots kept sliding of the footboards. The two front tubes actually cracket above the sidecar loops under previous ownership. And I kept loosing nuts and bolts. The balance factor was lower than 50% with aftermarket pistons (cant remember the exact figure anymore).

During rebuild the flywheels where rebalanced to 55%. No change in cylinder/top end/cams during rebuild. The motor now runs smooth at any rpm you dare to take her to. Maybe a litle bit less smooth at very low rpm than the previous setup. And I have not retightened a singel nut in 10 seasons now (and no nylock or tread locker anywhere).

This is my experience, not a scientific evaluation. Might be some factor I have not taken in concideration. But I think it is safe to say that the higher balance factor had no bad effects on my motor.


Sidewinder!

My contention is that the bad vibrations that were shaking your chassis apart were "mechanical" and not a balancing issue, such as the flywheels loosing their trueness. The vibrations came out in the wash of the rebuild, and I suspect that it would have even without the re-balancing.
Even if the aftermarket pistons were forged, the added weight lowers the factor perhaps a couple of percentage points at most.

....Cotten
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Sidewinder

Posts: 89

Joined: Tue Dec 25, 2007 4:04 pm

Location: Norway

Post Wed Jan 26, 2011 1:35 pm

Re: Crankshaft Balancing

Cotton...yes, that is possible. Flywheels was not checked for trueness before rebuild.
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47knuck

Posts: 530

Joined: Sat Apr 02, 2005 1:01 am

Location: Ogden, Utah, USA

Post Sun Feb 13, 2011 3:06 pm

Re: Crankshaft Balancing

OK time for a dumb observation. All the different "balance" factors seem to be related to frame strength and geometry. We have one advantage with these "old" motors. Only one plane as the cylinders are in line. No rocking due to offset con rods. So is there a big difference in frame stiffness front to rear and up down? Is the frame able to absorb variable imbalances better in one direction than another? Single down tube frames like the 45 and VL would be different than double down tubes like than EL and FL? And would this change again with the double cradle of a free arm frame? Or am I just lost again? Balance for a V-twin is very much like balance for a very big single I think. Any one that cares to try and enlighten me? Thanks, Steve
Steve H
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Cotten

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Posts: 2674

Joined: Thu Sep 30, 1999 12:01 am

Location: Central Illinois, USA

Post Sun Feb 13, 2011 6:52 pm

Re: Crankshaft Balancing

47knuck wrote:OK time for a dumb observation. All the different "balance" factors seem to be related to frame strength and geometry. We have one advantage with these "old" motors. Only one plane as the cylinders are in line. No rocking due to offset con rods. So is there a big difference in frame stiffness front to rear and up down? Is the frame able to absorb variable imbalances better in one direction than another? Single down tube frames like the 45 and VL would be different than double down tubes like than EL and FL? And would this change again with the double cradle of a free arm frame? Or am I just lost again? Balance for a V-twin is very much like balance for a very big single I think. Any one that cares to try and enlighten me? Thanks, Steve


Steve!

As I noted earlier, the VL had a much lower factor.
I have never back-calculated a 45".

Since these machines operate well over such a large range of factors, choosing a factor becomes a tuning practice.
A high factor provides more dampening from the flymass itself, but resists changes in RPM by virtue of inertia.
Low factor provides more leverage for the pistons over the wheels, making for a motor that likes to accellerate quickly, but never really wants to quit accelerating.

Thus a very light tavern-to-tavern sporting machine can enjoy a lower factor, and a heavy machine for cruising can enjoy a higher factor.

...Cotten
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47knuck

Posts: 530

Joined: Sat Apr 02, 2005 1:01 am

Location: Ogden, Utah, USA

Post Mon Feb 14, 2011 11:21 am

Re: Crankshaft Balancing

Cotten: Never thought about balance as a tuning choice. Just considered that the motor would live longer. With weight shifts effecting acceleration of the crankshaft and the timing of when that would happen helps to "see" the dynamic but it may be over my head to understand. I will keep thinking if you will keep helping. Thanks, Steve
Steve H
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Cotten

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Posts: 2674

Joined: Thu Sep 30, 1999 12:01 am

Location: Central Illinois, USA

Post Mon Feb 14, 2011 9:18 pm

Re: Crankshaft Balancing

Steve!

It was only my gut instinct until Herr Elvenkemper validated it at http://virtualindian.org/1techflywheel.htm.

It must be reiterated that this only applies to single-throw V-twins, and ranks as a major reason why the design has ruled American motorcycling for generations.

...Cotten
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