When we think of a modern motorcycle with a vibration-free straight-six engine, we likely think of BMW’s new K1600. But there is another German motorcycle about to enter production that uses a newly-designed, six-cylinder engine, that for intents and purposes is also a straight-six.
Horex is an old German motorcycle company that ceased production more than a half-century ago. Few current motorcyclists had even heard of Horex when the rights to the name were purchased about four years ago. If all goes according to plan, motorcyclists everywhere will soon know more about Horex. The company’s credentials are impressive. Clemens Neese, the CEO and General Manager, has both a mechanical engineering degree (TU Munich) and an MBA (Columbia University, New York). It was Neese who decided that there should be motorcycles using an engine like the Volkswagen VR6. He patented the idea of using that sort of engine in a single-track vehicle, then started the new company and bought the rights to the Horex name.
For intents and purposes, Volkswagen’s VR6 engine is more a straight six than a V-6. A better name would be “staggered six”. Volkswagen wanted a six-cylinder engine that would fit transversely under the hood of small front-wheel-drive cars. They could have settled on a common V-6, but they didn’t. The VR6 is lighter and more compact overall than a V-6, and exhibits far less vibration than a V-6. As compared to a true straight-six, the main advantage of the VR6 is its shorter length.
Straight-six engines are exceptional. Every other engine type exhibits at least one of two distinct sorts of vibration: rectilinear vibration, i.e., back and forth in a straight line, and rotational vibration, where the crankshaft rocks in an end-over-end fashion. The straight-six is the only basic type of engine that does not exhibit either of these two fundamental types of vibration. It is equivalent to two in-line triples joined end-to-end. In-line triples are like boxer twins in that rectilinear piston motion cancels while rocking motion remains. When you put two of them end-to-end, the rocking motion cancels.
Volkswagen realized that by using a single cylinder bank and by staggering the cylinders slightly, the engine would not be much longer than an in-line four of comparable displacement. This begs the question of what determines whether a narrow-angle V6 (what VW calls the VR6) is really a V6 or a straight six. With either crankshaft, the crankpins are all collected into three angular positions located 120 degrees apart from one another. In a V6, each cylinder bank is in essence an in-line triple. The two pistons closest to one end belong to different cylinder banks but share a crankpin. The same goes for the two cylinders at the other end, and likewise for the two cylinders in the middle. In a straight six, the crankpins at the far ends of the crank (cylinders 1 and 6) share the same angular position. The crankpins for cylinders 2 and 5 similarly share one of the three angular positions, and likewise for 3and 4. In the Volkswagen VR6, there is a small angular offset in the crankpins for cylinders 1 and 6, and ditto for 2 and 5, and ditto for 3 and 4. The offset crankpins compensate for the offset in piston phase that occurs due to the narrow V.
As the “V” angle becomes more narrow, the bottoms of the cylinders eventually get in the way of each other (the Horex video below illustrates this, and other aspects of the design, quite well). There are two ways to mitigate this. You can make the connecting rods longer, thereby pushing the cylinders further away from the apex of the V. Alternatively, you can spread the cylinders in each bank further apart along the row, thereby making more room for the cylinders in the other bank. A three-way trade off thus exists among the length, the width, and the height. By increasing the height, you can make the “V” more narrow, or you can shorten the engine lengthwise.
There is a way to make the engine taller, in effect, without actually making the engine physically taller. Ordinarily you expect to find the crankshaft located on the line of intersection for the two planes associated with the cylinder banks. In the Horex VR6, the crankshaft is located several inches higher than the point of intersection of those two planes. Relative to a given actual height, the bottoms of the cylinders are moved further apart from the cylinders on the other bank. The benefit is that for a given cylinder bore/stroke ratio, either the “V” angle can be made more narrow, or else the crankshaft can be shortened. Horex ultimately settled on a quite narrow 15 degree “V” angle. Again, the video below helps illustrate the design options.
The new Horex motorcycle is not at all the same type of motorcycle as the BMW K1600, and the displacement is not at all the same, so comparing the two is sort of like comparing apples to oranges. But comparing the numbers is still revealing, keeping in mind that a 1.22 liter Horex engine is being compared to a 1.65 liter BMW engine.
In most modern motorcycle engines, the cylinders are “oversquare”, meaning that the bore is greater than the stroke. The cylinders in BMW’s K1600 are just slightly oversquare, and this is the main reason that the engine is as short as it is (“short” in terms of length side-to-side, as opposed to height). This bore/stoke ratio favors engine torque at low rpm, at the expense of engine torque at high rpm, and therefore at the expense of peak power. Peak power is quoted as 160 hp, which is good but not exceptional for a modern 1.65 liter motorcycle engine. The comparatively flat power curve is well-suited to a touring motorcycle, however.
At the cylinder head, the Horex engine is about five inches shorter in length than the BMW engine. Horex uses a bore/stroke ratio of 68/55 (mm), which is still conservative by modern standards, and which ought to lead to a good balance between low-rpm and high-rpm performance. They recently published final numbers for the production engine (normally aspirated version, similar to the BMW): 161 hp. The 1hp advantage over the k1600 engine is not likely a coincidence.
Production schedules have slipped, no surprise, and supply problems were blamed. Originally, there was no mention of a normally aspirated version – the bike was to be available only with a supercharger. Per the revised production plan, the initial production model will forgo the supercharger. The only other major change seems to be the switch to chain final drive, whereas belt drive was originally planned.
Converted to U.S. dollars, the pricing upon release this spring in Germany and neighboring countries is expected to be about $25,000. The styling is naked and somewhat retro, but still unique and cleaner in appearance than many other modern naked bikes. If it does well in Europe, perhaps we’ll see them in the U.S. market in a few years. Take a look at the Horex web site if you want more details.
Personally, I think the new Horex is a very cool bike, and 161 hp is plenty (no need for a supercharged version). If it ever makes it to the U.S. market, I hope the price drops considerably. No matter what happens with the new Horex, maybe this type of engine will catch on with other manufacturers (subject to the Horex patent rights, whatever they may be). Given that Volkswagen developed this engine design over two decades ago, and given that there was a four-cylinder version of the same basic approach even earlier than that, you have to wonder why it has taken this long for motorcycles to benefit from this space saving layout.