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Motorhead Memo: Reciprocal Arrangement

By Kip Woodring

Motorhead_Memo

In case it’s news to anybody, let alone those who might have read last month’s column, all reciprocating engines vibrate. Thing is, some do more of it and some do it in sneaky ways… or don’t. We’ve been through the facts that a single cylinder is worst, an in-line six is best and everything else falls somewhere in between, at least in terms of purest, most basic layouts. With twin cylinders it goes something like this: 180-degree (opposed) are best, having “perfect” so-called “primary” balance in regard to the monkey motion of the masses, plus a perfect firing order, conveniently also 180-degrees. At the other end of the scale, the worst is a 360-degree parallel twin, because it shakes exactly like a single, saved only if the slinging mass is less and by the firing order, each slug is going bang on opposite trips to the top. A 180-degree version is smoother but introduces a side-by-side urge in the crankshaft to wobble or rock… rocking couple, as it’s known. Among V-twins, we still get a smattering of all this stuff to a greater or lesser degree with the 90-degree version, allegedly the best compromise because it offers one advantage—perfect primary balance. Harley’s 45-degree engines, with the traditional (and now virtually unique) “knife and fork” rods occupying the same spot on the crank pin, do not create a rocking couple. We already covered this, so let’s stop right there!

What’s more to our purpose and advantage is trying to get at the heart of this whole “balance” thing. After all, the dictionary says it’s a “state of equilibrium” and/or “equal distribution of weight.” Just how the hell do we achieve that in a running engine anyway? The truth is we don’t! But we try. Real motor men seem to have a pretty fair grasp of the bifurcated nature of the concept, and it has to do with the parts inside the engine that go up and down and how that works with the parts that go ’round and ’round. The obvious ’round and ’round bit is the crankshaft flywheels, but for the record there are others: the clutch, camshafts, alternator, and appearing on more and more machines, variations of a truly antiquarian gadget of pure genius—the Lanchester Balancer. (We’ll talk more about balancers, and isolating rubber mount strategies, later on.) Each and every one needs to be “balanced” but there are two versions (of the two definitions) to deal with… namely static and dynamic.

The wheels in the engine go ’round, and ’round

The task itself is fundamentally the same as balancing a wheel and tire. The test for static balance is to spin the wheel. If it’s out of balance it will come to rest with the heavy spot down every time. You add weight to specific areas of the wheel until that behavior ceases, and the wheel stops spinning in a different spot… every time. All of which has been done somewhat successfully for a long damn time. Dynamic balancing is almost the opposite in that it pays off at extreme rotational speeds and weight is added, subtracted or rearranged and relocated as required. The best results today come from doing it with computers on high-tech machines.

So, our first insight should be: a wheel (or flywheel) that has been statically balanced may not be dynamically balanced, but anything that has been dynamically balanced is automatically statically balanced. Why does this matter? Well, for one thing, the bearings that support your crankshaft assembly and in which unbalanced flywheels may rumble, will live longer and, yup, roll more smoothly! That, and any harmonic vibes that could emigrate to most other areas of the powerplant… won’t! Sounds good, works good, and it’s simple enough as far as it goes, but there’s this other thing…

The up-and-down part of the engine balance equation consists mostly of the piston and rings, but I choose to include pushrods, rocker arms and tappets as well. The piston and rings travel up and down, although not so much in a perfectly straight line, but more like a rather sinusoidal pattern (like a salmon swimming upstream) constrained by a (hopefully straight) cylinder. The tappets tend to behave that way too. Not really the rockers and pushrods, although sadly there’s not that much to be done with those items beyond making sure they are light, stiff, strong and straight as they can be. This is where the “equal in weight” definition comes in handy, especially where the pistons and rings are concerned! Mostly, because in a running engine they never hold still and never move at a constant velocity, always speeding up, slowing down, and screeching to a halt. All reciprocating parts are not only accelerating and decelerating but flexing and wobbling along the way! All this reciprocating force is proportional to the square of the rpm! A piston weighing ounces at rest can generate a reciprocating force at max rpm that exceeds the entire weight of your bike and the people aboard it! By 3–5 times! Hardly surprising that vibration can be a big problem if there’s any imbalance. So the concept, if not the actual practice, of matching masses very closely is also simple enough as far as it goes.

But wait—there’s one more thing!

 

Motorhead Memo

These days we simply do not need to worry overmuch about vibration from our Harleys.
It wasn’t always so. Mostly, that’s due to progress along these lines: rubber mounted
in splendid isolation, we stand on the shoulders of giants, or sit on the motorcycles of
genius, whichever way you prefer. Erik Buell is often given credit for this whole rubbermount
phenomenon. To be sure, he helped, but it wasn’t him alone. Starting with what
Bernard Hooper wrought in the 60’s, the team at H-D, headed by Ray Miennert, upped
the ante considerably with the development of the FLT in the late 70’s. By 1982 the FXR
followed and even later, Buells (as shown in this patent drawing)… all using some form
of the linear-controlled chaos we now call by several names. It boils down to the same
wonderful thing though… smooth motorcycles in spite of inherently vibratory engines.

How about a riddle? OK; What doesn’t really go ’round and ’round or up and down… but does both? Answer? Connecting rods! The big end where it wraps around the crank pin clearly rotates with the crankshaft and hence can be correctly balanced by a counterweight to the crankshaft on the opposite side. The mass of the small end of the rod surrounding the wrist pin moves in the same manner as the piston (assembly), so it can be directly added to the total reciprocating mass of the piston, rings and wrist pin, but the straight section of the rod that “connects” the big and small ends operates in a combination of rotational and reciprocating motion. Tricky that! Just how much of each part of the complete rod should be “assigned” to the different functions and motions involved in order to come up with a workable overall balance? Nowadays computers do most of the (pardon the expression) heavy lifting when it comes to calculating it correctly, which should serve as a clue to how complex it can be to get right. A lot depends on how long the rod is, relative to stroke and flywheel diameter. Some of it depends on how heavy the rod is and its angularity through its travel. The list goes on… and no matter how you fuss over it… it is all impossible!

The best thing you can hope for is one of many engineering compromises, because an engine balanced in one plane will be out of balance in another, to a greater or lesser degree. Then there are the sympathetic vibrations… usually grouped under the heading of “harmonics” and/or “resonance.” They may come from the engine itself, any related part of the engine (the drive train in general), or engine mounts, or the chassis, or the stuff you bolted or packed onto the motorcycle! Plus, no matter how smooth things get (or can be made) at one engine speed, it will not be as smooth, if smooth at all, at any other. At the end of the day, whenever a periodic vibration persists, altering the so-called “balance factor” only chases the vibe up or down the rpm range. The best approach is to try to move the vibes and resonance to a rev range beyond what you’ll ever use. On the road that means as smooth as possible in the 3,500–5,000 rpm range and who cares if it shakes apart at 7,500? A race bike, on the other hand needs to be a sewing machine at (or near) red line, and if it buzzes badly at half throttle… it don’t matter!

Solid, shaky or flailing?

So far, the folks who own older Harleys have probably been reading with their heads bobbing—either up and down or side to side as they read. They have greater vested interests in keeping engines smoother, for reasons ranging from numb body extremities to durability. I’d like to think they represent the “solid” majority… heh! heh! The rest of us ride Harleys that have been isolated from high vibes via one of two methods. It’s a matter of record that both work—differently but pretty well. So, how dey do dat?

Motorhead Memo

Though they add a layer of complexity to the basic V-twin layout and include apparatus that
has nothing to do with increased power, there are obvious, notable benefits to the “built-in”
vibration cancellation offered by mechanical balancers. As seen here from the patent, on
paper, the factory balancers might appear to be a bit compromised, but the reality is the
patented design used in the Twin Cam “B” engine works even better than rubber mounts,
since these engines generate fundamentally smoother running at any rpm, from idle to
redline!

Mind you, to my mind, these “solutions” can lull you into a false sense of security, because they do not cure vibration, they just keep it away from you. Lots of engines that should/would/could benefit from attention to the things we discussed here never get it because the rider doesn’t realize they need it. If ya can’t feel it, it doesn’t exist, right? Wrong!

OK; that part off my chest, we come to the really rather brilliant idea, employed by Harley engineers as far back as 1980, of rubber mounting. OK, more accurately H-D adapted a system that kept vibration in single plane and isolated the rider from it (using a lot more than just rubber) from the Brit that came up with the notion originally. A genius gent named Bernard Hooper had the original idea and used it to save Norton’s paint-shaker parallel twin. The result was the legendary Commando and folks at The Motor Company; facing similar problems, took a very hard look at this solution! When their version arrived in showrooms, it involved transverse links, specialized rubber sandwiched mounts, a new five-speed tranny to hold the new swingarm and kept separate from the all-new frame! It worked so amazingly well that the setup lasted, unchanged, for almost 20 years. The Dyna, 2004 and later Sportster, and the new TC bagger chassis use variations of the system; similar in concept, but benefitting from almost a quarter century of refinement, not to mention computer modeling, something the original system lacked, being basically pre-personal computer. A singular characteristic of vibe-isolation by rubber mount is that you still get the shakes at very low engine speeds, but as you go faster it gets smoother.

Curiously, when it was time to redesign the last of the solid-mount Big Twin chassis, the Softail did not get rubber mounts! Instead, again with help from massive computer input and some clever conceptual thinking from a guy at Harley named Dave Safarik and his crew, the new Twin Cam “B” engine incorporated a pair of mechanical balancers. The basics of balancers are nothing new BTW, originating well over 100 years ago with another “Brill Brit” by the name of Frederick Lanchester. This tells us a couple of things. First, the physics of engine vibration was well understood even back then, and secondly the brightest guy around at the time came up with a solution so sound and so basic we still use it! All the mumbo jumbo we have covered regarding primary balance, secondary balance, harmonics and all… is neatly canceled out by what amounts to weights flailing around inside the engine, rotating in the opposite direction of, but twice as fast as, the crankshaft. Variations abound, typically unique to a given engine layout and cylinder count, as you’d imagine. As far as 45-degree V-twins are concerned… the factory’s balancer design is specifically tailored… to a “T”! (Or should that be to a “V”?) Anyway, for those who’ve peeked at a parts book or studied the patent drawings, it’s pretty clear that the “B” balancer setup is as simple, compact and efficient as is possible, given the state of the art. Purists might cringe a bit at the whole shebang (chains and all) being contained on one side of the engine, but the tradeoffs in serviceability and reliability more than compensate. The net effect of a bit of extra mass and complexity, properly placed and “weighted” to counteract the negative vibes of a Harley’s massive crankshaft, makes for one damn smooth engine… at all usable engines speeds! ’Nuff said!

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