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Motorhead Memo: V-twin vibe angles

By Kip Woodring

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The basic architecture of the V-twin engine is the second-oldest layout in existence. Like that other enduring specimen, sharks, they seem to be an ultimate survivor compared to any other motorcycle powerplant and in their own way are just as capable a predator. Sticking with the analogy one step further, after all this time there are several “species” of V-twin. In fact, most other engine layouts usable in motorcycle chassis have so little true variation within the layout that they are positively boring by comparison. The variations that do exist tend to add complex gimmickry, largely unnecessary for daily use. That, in my opinion, is exactly why V-twins are still with us and remain arguably the best compromise for two-wheel motive power.

The history of motorcycling is rife with engine designs, from the ever-so-basic single, to parallel twins, to triples, to ubiquitous transverse fours, to glamorous in-line fours and V4’s, to the radial five Megola, to the Guzzi and Morbidelli V8s, to in-line and Vee six-cylinder stuff, to rotary Suzukis and more. Honda has even tried V5 engines! None, not even the single, has stuck with us like the V-twin. Ask any motorcycle freak to name six bikes he’d kill to have, from any point in history, and it’d be a miracle if a V-twin or two… or more… weren’t on the list! Why do you suppose that is?

First, after the sheer heritage of the design is its elegance. In engineering (as in math) elegance doesn’t mean luxury or style, but the best solution to a given problem. Maybe easier to recognize than define, but whatever it is, V-twins have it. The center of gravity is low, and the engine is narrow, fitting in the frame so nothing much gets ripped apart in the event of an accident and plenty of usable power can be had.

Secondly, these engines, properly done, are relatively simple. That ties into the concept of elegance, but mostly that means practical. Most V-twins are readily comprehensible to the average mechanic; sometimes even the owner. Sadly, inch by inch, law by law, marketing motivation by marketing motivation, this is less and less true as time goes on.

Finally, the whole configuration looks and sounds right. Not just a matter of mere aesthetics, it proves the old engineering saying that “if it looks right, it probably is right.” Ah, but which V-twin is “rightest?” Ask the Ducati guys and you’ll get a diatribe advocating the “L” version. Ask a Guzzi freak and they’ll inform you that the transverse fitment is most desirable. What about Indian riders, with first a 42-degree Vee and now a 49-degree Vee? Rabid fans of esoteric stuff like Morini and Matchless, both of whom built decidedly odd V-twins, can make a good case for the details of their pet designs. Even Harley has managed to come up with vastly different interpretations of the V-twin engine. What’s the story with all this Vee variation anyway? Well, to oversimplify a touch, it’s a textbook argument for “horses for courses,” national engineering “identity” (or prejudice), and refined traditions.

V-twins—what is the angle anyway?

A quick rundown: Four-stroke single-cylinder engines shake! They have a firing interval that offers only one power stroke every 720 degrees of crankshaft rotation. There are clear limits to how big you can make one too. So the easiest way to get around these issues is add more cylinders. First step, just add one to the engine you already have. Everybody started out with a single, so most of ’em just added one to the front or back of the existing setup. All you needed were new crankcases and a few other minor mods, right? Hell, the thing needed to fit in frames already being used, so the narrow-angle V-twin was a natural. The unnatural versions came later and one of the first of those was a real game changer, in effect a “zero-degree” V-twin known to posterity as the “parallel” twin. Made sense actually, because without adding any real mass, you got twice as many power strokes for the same displacement, and in the beginning when compression was low and sizes were small, not much vibration. Failing to observe the true advantages and grossly underestimating the disadvantages led to the almost total eradication of the best qualities of traditional parallel twins. Whether 180-degree (one piston up and the other down) or 360-degree (both together) cranks were employed, firing intervals and unbalanced masses meant there were (and are) clear limits to what could be expected from this layout. The modern versions that exist today address the limitations through engineering trickery and complexity, not to mention the technical Band-aid known as the counter balancer, all moving the design away from its original elegant simplicity.

A considerably more brilliant notion was to stick the extra barrel opposite the first, forming a 180-degree V, commonly known today as a “flat” or “boxer” engine. The British came up with that one, long before the German brand we all know and respect did, but it has become so “identified” with BMW that most folks can’t name anyone else who did the engine, ever. Ironically, Harley-Davidson built a flat twin, albeit longitudinally mounted, rather than the better-known transverse mounting, half a decade before BMW. Be that as it may, boxer twins have all three of the preferred characteristics of a properly engineered twin. Namely, perfect primary balance, secondary balance and firing order! In between these zero-to-180 extremes, as I’m sure you’ve figured out, is, well, practically everything to a degree (or fraction thereof).

The firing squad

Hereabouts you’ll find a Wikipedia graphic of the multitude of angles that V-twin engines have employed over the years. None were conceived and executed for no good reason, although for plenty it was a matter of convenience or expedience. It’s equally fair to say that most came to no good and some turned out brilliantly. For instance, though few remember now, for many reasons one of the best of the best (and a major success) was the Guzzi 120-degree 500 V-twin GP racer. On the other hand the 26-degree Matchless was ahead of its time and failed miserably. For our purposes (and lack of space) we will only look more closely at the more popular angles. Mostly, that means we’re talking about 90-degree and 42–60-degree engine configurations.

So much nonsense has been bandied about regarding V-twin vibration that it’s easy to lose sight of other important elements of the design like firing angle, single pin cranks, etc. But let’s come back to that after we tackle the idea of keeping our V-twin balance.

On paper, the 90-degree Vee engine has several advantages that make it an obvious and desirable design. The cylinders are well separated and cooling is no problem. There’s plenty of room for carbs or EFI induction and exhaust systems. More to our point, it is easy to balance the so-called primary forces caused by the rise and fall of the pistons and related reciprocating masses, by adding a counterweight equal to the weight of one piston assembly plus the small end of the rod. (And we all know how great it is to have perfect primary balance, eh?) All the same, trouble in paradise shows up in the form of irreducible and powerful secondary vibration, occurring at twice engine speed, acting at right angles to a line bisecting the cylinder angle. Most mount the engine fore and aft where it won’t really be a nuisance unless it’s a big-inch engine—which most are these days! Oh, and there’s that pesky firing interval.

If overall engine balance includes the firing interval, which technically it doesn’t but in reality it very much does, a conventional single-pin crank 90-degree engine actually suffers from quite a lack of balance! (Ducati goes to the trouble of having most of their engines rotate backwards in the chassis to mask this, but it’s still there.) You see, the short firing angle is 270 degrees (3/4 crank revolution) and the long one 450 degrees (one and a quarter revolutions). That is a long ways from the desired perfection of 180 degrees! As most will know, even Harley’s 45-degree V-Twins have a more even firing interval (of 315 degrees and 405 degrees).

Because of the drawbacks of the 90-degree V-twin, and not least because cooling lower-powered engines of the past was less of a problem, we get back to the traditional narrow-angle V-twin. The most common examples of these are obviously from H-D with the legendary 45-degree angle, but plenty of engines from others have tweaked the angle a bit. Original Indians worked with 42 degrees, Vincent used 47.5 at first then switched to 50 and so it goes, with some companies moving clear to 60 degrees. Thing is, virtually all of them used a single crank pin! Some, like Harley, run both rods in a “knife and fork” arrangement, others run the rods side by side (introducing another kind of wobbling imbalance known as “rocking couple”) but with proper counter-weighting (usually 60 percent for Harley cranks) and/or rubber mounting, vibration can easily be controlled. Additionally, while one piston is at a dead-center point (either top or bottom of the piston travel) and reversing its direction, the other is traveling at nearly maximum velocity. Meaning the kinetic energy stored in those pistons is much more nearly constant than with other Vee configurations, so the whole reciprocating mass contributes to the flywheel effect. In short, the only real drawback to the narrow Vee—but it’s a serious one—is cooling. We’ll get back to that…

This being the 21st century, not the 20th or even 19th, you need not look too far to see some prime examples of workarounds that try to refine the V-twin basics. We’ve already touched on balance shafts and rubber mounts, but some companies go to a lot more trouble striving for perfection. Honda came up with a damn clever notion some years back in the form of a narrow (52-degree) V-twin which used not a single but a pair of crank pins. This dual offset (at 72 degrees) pin setup was a way to fake the balance and firing interval of a smoother engine, effectively of 90 degrees! Slick in the sense that it kept the (water-cooled) engine compact but a lot less vibratory.

To find firing interval for various Vee angles on single pin engines: 360- (degree angle of Vee) for one cylinder and 360 + (degree angle of Vee) for the other. For example a 60-degree V-Rod: 360-60=300-degree firing interval and 360+60=420-degree firing interval. NOTE: This won’t work for engines with dual crank pins or some XR750’s that have had firing intervals intentionally modified.

To find firing interval for various Vee angles on single pin engines: 360- (degree angle of Vee) for one cylinder and 360 + (degree angle of Vee) for the other. For example a 60-degree V-Rod: 360-60=300-degree firing interval and 360+60=420-degree firing interval.
NOTE: This won’t work for engines with dual crank pins or some XR750’s that have had firing intervals intentionally modified.

 

Something to keep in mind: No matter what configuration an engine has, no matter how few or how many cylinders, and no matter what the balance and firing order in theory, in practice it’s the start-speed up-stop nature of reciprocating piston engines that is the root of all evil. Once it starts on its way up the cylinder, the piston (and its related components) tries to keep on going, its motion stopped and reversed by the connecting rod and the momentum of the crank. As you can imagine there are serious forces at work in this scenario! Take a 16-ounce piston assembly in an engine with a three-inch stroke, running at 6000 rpm. That one-pound piston heading for TDC suddenly weighs nearly one ton (1,917 pounds to be more exact) and 1,151 pounds heading back down! Think about it! This can amount to several times the weight of the whole motorcycle and it is happening one hundred times a second! Then there’s the acceleration and speed of the thing (left unchecked), which is over five G’s and 1,200 mph in under 10 seconds! By which time this piston would be nearly a mile and three-quarters into the air! But that piston is checked and the strain of the restraint is the main reason all engines vibrate. Not that there aren’t others!

We’ll talk more about that, V-twins, cooling and the other things that affect the shakes you feel from the saddle next time!

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