It seems reasonable enough that the rotating and reciprocating assembly in the 96 and 103, being the exact same weight and the same balance, would create the same amount of vibration. One consideration with the ultimately larger displacement 103 not mentioned, is a larger air/fuel charge and thus a more forcefull explosion. A more forcefull explosion transfers more energy to the pistons and the rotating assembly. I'm more inclined to believe that a bigger motor makes a bigger explosion, makes the otherwise same weight engine assembly bounce more, making a bigger vibration, proportionally. The reason two like bikes can have totally different vibration characteristics can be attributed to loose production tolerances for rotating assembly balance standards and tune. The vibration we feel is simply the rapid change of direction of the motor pumping basically up and down on the front mount, Don't forget the engine and tranny are bolted together solid on the Twin Cam, the long, front and top heavy assembly pivots on the rear tranny isolator mount point, and bounces on the front motor mount. Sorry, but I totally spaced out the "rocking coupling" condition thing, it didn't speak to me. Cool topic though.

 

Yes, as I mentioned, driveline forces can produce some of the seat-of-the-pants feel that you get. A "bigger explosion" can account for this. The compensator shock absorber that connects the engine output shaft to the front chain drive sprocket is designed to soak up some of this "shock." Nonetheless, the primary vibration forces are those created by the reciprocating pistons and the spinning crank counterweights.

Regarding the rocking coupling forces, think about it. It's very real. In fact, these forces can cause the crankshaft in some engines to actually deflect and wobble under the forces that are spinning it. Matchless, for one, designed a center crank bearing between its two vertical cylinders to firm-up the crank and hold it tighter and stop it from deflecting.

Again, it's like holding something with both hands and alternately pushing and pulling with both hands. You are trying to rock whatever you are holding.

Rocking coupling is minimized by fitting the con rods close together as on a single crank pin (Guzzi and Ducati). Engines requiring separate crank journals, like the BMW flat twin, exhibit more rocking coupling vibration since the con rods attach to the crank further apart from each other.

This gets much more technical and many ingenious things have been invented throughout mechanical history to deal with these and other errant forces inherent in a reciprocating engine. The vaunted Duesenberg automobiles of the 1930's had Lycoming- (aircraft) built engines that featured voids inside the crank casting that were filled with mercury in an attempt to quell vibrations from the high-output (for the times) power.

 

Alright, well we are talking about a Harley with a single crankpin with homogenous connecting rods, not sure how you can position them any closer than that. Most folks refer to your "rocking coupling" as crank flex and you are absolutely correct, it can create a harmonics type condition. Under load the journal lobes tend to "give" or "flex" a little. Ever heard of a twisted or broken crank, the metal flexes and springs back a little before it actualy bends or breaks. As with my Ranger's German built 60* V-6, it was a true six throw crankshaft, one journal for each connecting rod, not the best choice for a performance build, but I did it anyway. For the crank, I ground and blended all the casting seams and sharp edges and then shot peened it to give the surface more strength and eliminate stress risers, to make it more resilient. V motors with married or paired cylinders, two oppposing cylinders on one journal, exhibit less of this condition, primarily because the crank is much shorter and stiffer, more resistant to flex. Now that we have fully dissected a condition that doesn't really affect Harleys, whats the verdict, does a 103 vibrate more than a 96, all other things being equal?

 

Ok, the primary object of discussion in my original post was not rocking-coupling forces but those greater forces created by the reciprocating pistons and crank weights.

This should be the same, all other things being equal, between the 96 and the 103. The pistons and counterweights are the same.

Having said that, each engine can exhibit slightly different traits. Maunufacturing tolerances can vary enough to be evident. The percent of balance for each piston as obtained in the manufacturing process can vary enough to be evident to the rider.

Also, increased power from the bigger 103 or a hopped-up, re-cammed motor can produce greater shock loads or power pulses on the drive train and be noticeable. This is especially detectable when the motor is being lugged or is under labor.

 

All OTHER THINGS being equal, that implies that at least one thing has changed and that is displacement, with the larger displacement you still have more energy acting on the pistons, driving them down with greater force. More force than the 96, so how could that not impact vibration, regardless of rotating assembly weight and balance. You say the same, I say more vibration, and neither of us have any proof to support our theories. I won't be taking it any further, but if you would like to conduct an experiment and publish the vibrometer results I would be interested in your findings.

 

The MOCO does not balance cranks anymore. They stopped in early 2000 because the engines were counter balanced or rubber mounted. The piston weight on the 96 and 103's are the same. Vibration is in the hand's of the beholder.

 

Why no discussion on crank run-out? Harley has made changes through the years on what is their acceptable run-out. Take the same two bikes, same year and model but, one has more vibration. If a run-out was performed, you would probably find them to be different. Same thing in the heads and the rocker assembly. Some come from the factory with more end play on the rocker assembly than others adding to the vibration factor (and noise). There are many other things within the manufacturing process that can add to one motor being smoother than another. Subtle differences of thousandths of an inch can mean the difference between one motor being smoother than the other, in the same run of motors. This is only a sampling of why one bike can be smoother than others of the same year and model.

The way Harley keeps changing their motors with different parts vendors means we can expect to get something different every year. Just look at what changing to Chinese bearings have done and eliminating Timken bearings (big mistake) on the crank.

Just remember every change made makes a difference no matter how small, they all add up to that quest for a smooth ride.

 

If that CB350 was viberating, it wasn't coming from the engine...just say'n.

 

Very Good write up and follow-ups in this post.

However, the one thing that has not been brought up and this is what makes every engine a little different.

If ever part of the engine was exactly the same in ever engine all the engine would run exactly the same. This being said none of our engines are exactly the same.

Manufacturing of components for and engine always start with a base specification with a plus or minus tolerance. For example a connecting rod has a base spec of 6 inches between centers of the crankshaft hole to the center of the wrist pin hole with a tolerance of +/- .005. So there is deference between one connecting rod coming down the line to the next one coming down the line.

The same applies to every component part of the engine. Even where the hole is put in the piston can vary.

The pressed together crankshaft has a tolerance to how straight the crank pin has to be. This is where you see the run out tolerance at the end of the shaft. If the crank pin is pressed in perfectly you have no run out at the end of the shaft, but if it is off 100th of a degree the run out will increase, a tenth of a degree is a lot more run out.

Now apply that to every part of the motor.

So you end up with a very smooth running motor, this is a result of all the parts in your motor being well matched. Not so smooth, not so very well matched. Rough as hell, you know the answer.

This is the reason for balancing and blue printing motors. This is taking every part of the motor and matching it to its counterparts with a plus or minus tolerance of zero. The weight of every component is exactly the same and the balance is exactly the same. Example, two connecting rods may have the holes in exactly the same place but one may be heavy on the piston pin side and the other heavy on the crankshaft side. Balancing corrects this.

So why hasn’t this been corrected by the manufacturing process. The answer is money!! Who would have guessed? The cost to make the motor perfect every time would double the cost of the motor.

The other problem is the design of the motor itself. The design of the Harley motor hasn’t changed in 60 years, Yes it’s been improved but not changed. If you corrected all the flaws in the designs it would not be a Harley motor, it would not sound like a Harley and it would probably run better than a Harley.

But the mastic of the Harley motor would not be there and what makes a Harley a Harley would be gone.