Practically speaking, there should be no vibration difference between the 96 and the 103 since the engine components weigh the same. The 103 has the same stroke as the 96 and its slightly larger pistons are produced to weigh the same as the smaller 96 inch pistons. Theoretically, two same engines with the same stroke and the same weight pistons should have the same vibration characteristics.

There may be some driveline forces evident from the increased power but the engine vibration should be near equal. On the other hand, an H-D motor with a longer stroke IS likely to exhibit more vibration, even when properly balanced, because it creates more residual inertia with it’s larger crank/flywheel.

I hate to bore anybody with an explanation of engine balance. It can get complicated. I’ll try to do it simplistically.

The motion of a piston moving up and down in a cylinder creates a “shake” as the piston gets going in one direction and its force wants to keep it going in that direction. At the top of the stroke, the piston changes directions and heads back down the cylinder. At the bottom of the stroke, it changes direction again and heads back up the cylinder. That creates a “shake.”

Ball your fist and start punching it in and out in front of you. Your body feels the same sort of reaction to the change of motion of your fists. This is what creates vibration.

To neutralize this effect, an equal weight can be set in motion in exactly the opposite direction from that of the piston to counteract the force of the piston and equalize these forces and reduce the vibration. This is the function of crankshaft or flywheel counterweights. Extra weight is positioned on the flywheel so that when the wheel spins that weight will end up at the bottom of the upstroke of the piston and at the top of the downstroke of the piston. Hence, an equal and offsetting force to the motion of the piston is created that tends to offset the inertial forces of the piston that create vibration.

Another beautiful way of offsetting the inertial forces of a stroking piston is to have another piston of equal weight moving in an equal but exactly opposite stroke. The vibration-causing forces of each piston are cancelled out by the same but opposite forces of the other piston. This is exactly the design of the BMW “boxer” engine.

It’s sorta like getting two fat women back in a kayak once they’ve capsized (I’ve done this). If only one tries to pull themselves up into the boat, their weight or force will cause the boat to capsize again. If, however, you get both to do it in unison, they cancel the force of each other and will likely land in the boat like two flopping fish if your timing is right.

So, the flywheel counterweights work to create equal and opposite forces to cancel out one another. That’s all well and good but the problem is that the flywheel counterweight is spinning in a circle whereas the piston is rising up and down. The flywheel weight is correctly positioned when the piston is at the top of its stroke and when it’s at the bottom of its stroke. However, while the piston is in the middle part of its stroke that same flywheel weight is creating inertial force “sideways” from the motion of the piston as the weight rounds the circle while turning. This results in other inertial forces directed at angles away from the direction of travel of the piston.

Now, if you have a 90 degree twin such as a Ducati or Moto Guzzi, you cant get a little help. As that flywheel counterweight rounds up its circle away from the top or bottom of the number one piston stroke, it comes into the top or bottom of the number two piston stroke. The two pistons provide a weight-counterweight relationship in two different planes. The flywheel or crank counterweight is not left alone to spin off at 90 degrees from and create a side force that is not counteracted by another piston. Similar to the boxer twin (BMW), the 90 degree twin (Guzzi, Ducati) is inherently better designed to eliminate inertial forces and vibration.

So, the architecture of the Harley 45 degree, single pin crank motor defies total and perfect balancing since pistons and counterweights are never positioned to exactly offset each other's inertial forces. I might note, however, that H-D has constructed their "B" motor (that is used in the rigid-mount frames) employing a separate spinning balance shaft with weights designed to counteract the errant forces coming from the basic motor configuration.

Now, everything seems so simple, right ? Well, there is another force effect that comes into play : “rocking coupling.” Going back to our kayaking analogy, consider the paddle. If you take both hands and grasp them side-by-side on the paddle, alternating forces to your hands will tend to rock the paddle back and forth. This is a “rocking coupling” force.

The same holds true when attaching pistons to their crank pins. If the piston connecting rods are attached side by side to one or more than one crank pin, the same rocking coupling motion is induced as the pistons exert force on the crank pin(s).

Although the H-D 45 degree single crank pin engine design inherently has the previously-mentioned balancing problems, it does not have the rocking coupling issue that can also cause some vibration. The Harley rods are attached in a “knife-and-fork” fashion to their single crank pin so there is no rocking coupling.

The other 90 degree engines, Guzzi and Ducati, as well as the 180 degree BMW, have side-by-side connecting rod arrangements that do create such forces that result in some (lesser magnitude) vibration. If you look at a BMW boxer engine, you will see that one cylinder sits further forward than the other. The same is true for the Guzzi motor. Likewise, the Ducati 90 degree twin, has one cylinder offset further to one side than the other. This indicates the configuration of the relative con rods on the crank.

Can a 90 degree twin or any other engine employ a knife and fork set-up like the H-D ? Sure. In fact, the old radial airplane engines have a lot of similarities with the H-D motor. They too had a King Pin on the crank with an ingenious way of attaching the various connecting rods in a fashion that was not side-by-side. All the cylinders were attached to the crankcase in the same plane as with the H-D motor.

The 180 degree boxer motor is a little different and, since both pistons move in exact opposite directions as the crank turns, staggered crank pins are required. Some rocking coupling force is inevitable with this configuration. This is why you can still feel a slight buzz with a BMW twin. It aint bad but it's there.

Why would someone use a side-by-side con rod ? This arrangement can result in a stronger rod/crank assembly that can withstand higher revs than the H-D normally ever sees. A side note is that at least one aftermarket Harley-clone engine manufacturer used to build a motor with side-by-side rods on a one-piece crank. This leads into one-piece rods and cranks versus component rods and cranks and on and on.

Anyway, I hope this simplistic digression is helpful in explaining engine balance. It may be lacking and I’m sure can withstand polishing. But, back to the basic question – I don’t think the 103 should be much more of a vibrator than the 96 considering it’s construction. I would say the same for the 106 and 107 variants. If you go to a longer stroke as in a 110, then that’s another story.

 

Interesting,nice write up.

 

awesome write up....thanks for taking the time to explain. Things like this and the people who take this kind of time to explain stuff make this forum so worth while!

 

My favorite part of the whole write-up! No really, good job.

 

Excellent, informative write-up.

That said, it seems no two Harley's are exactly alike. If not the motor itself, then other factors such as tolerances, torque specs, bushings, proper set-up, etc and whatever must come into play.

When I bought my 103 over a 96 at the dealership, there was no question the former was shaking a hell of lot more than the latter. The salesman attributed it to the larger motor, but I now think there were other factors involved.

So much for listening to a salesman.

 

Originally Posted by leafman60
It’s sorta like getting two fat women back in a kayak once they’ve capsized (I’ve done this).

They are a lot lighter if you leave them in the water. (or so I am told)

 

Whatever the dynamics involved, my 2010 Limited shakes less that my friends 2009 Ultra. I gave my wife a ride on my friends Ultra spring before last and we hadn't gone a mile when she said that she didn't like it at all, there was just too much vibration. Two weeks later, with some dread, she agreed too take a test ride on a Limited. After a couple of mile, she said that this bike was completly different, it didn't have all that vibration that our friends 96 had. We brought the Limited home with us. With almost 16K on the Limited now and a trip to Nova Scotia, and trip to the Keys and a lot of local riding, my wife has yet to complain about any vibration. The ergos were terrible for her, she wouldn't hardly ride the bike after the Nova Scotia trip, but a 2 1/2 inch set back of the tour pack and a set of CVO board mounts that let us move her floorboards two inchs forward and she now says it is every bit as comfortable as our Goldwing and almost as smooth. After I bought the Limited, my friend and I trade bikes several times so he could see how the 103 compared to his 96. Going directly from one bike to the other, I have to agree that she was right, our 103 is much smoother than our friends 96. Seeing as all the other 96s I have rode were B motors, our friends Ultra is the only rubber mounted 96 I have any experience with, so his may be and anomaly, but his for sure is not as smooth as our 103.

 

That's what I meant by no two Harley's are exactly alike, seeing I had the exact opposite experience.

It's also part of the charm too.

I'm very happy with my bike, but if I were buying another Harley in the future, I would definitely try to test 2 of the same model if possible.

 

I see no charm what so ever in having a bike that vibrates to the point of being uncomfortable. Years ago I rode the back of my brother-in-laws CB350 Honda 50 miles to pick up my bike which was in the shop. The bike vibrated so much that my bottom felt like it has been attacked by fire ants. Swore I'd never ride it again and didn't. I guess I'm lucky that I got a very smooth Harley 103. If I ever buy another new one, doughtful considering the problems I've had getting any warrenty work done on this one, I'll take your advise and test ride more than one.

 

No doubt.

I meant the charm of Harley's not being exactly cookie-cutter alike as were a couple of sport bikes I owned over the years.

Sorry to hear about your warranty issues. Trust me, I had a couple issues with my bike that I found less than charming too.

Hope it all works out,
Murph