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The new Horex motorcycle with VR6 engine: A Technical Analysis

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.

51 Comments

  1. Keith says:

    Hopefully the Kompressor will be an option…well, one can HOPE.

  2. Biker Cat says:

    I current drive a 2008 VW R32 with the VR6 motor, with 250HP and 245Lbs of torque it’s smooth with that torque down low. I love this motor and it has that rare exhaust note which is music to a motor head. It’s pretty cool someone also loves this motor and designed a motorcycle using this motor. I would like to see a sport touring model with a shaft drive.

  3. takehikes says:

    engine is interesting. bike like so many others I will not buy. At this moment I want a new motorcycle that I can afford. When a bike is in range of very nice 4 wheel vehicles I can’t go for the bike. How about the original Honda 750 with today’s components for under $10k? That I’d buy and they would sell the hell out of.

    • zuki says:

      If you wouldn’t go for a motorcycle in this price range over a car if you had the money then you just aren’t a true motorcyclist and just don’t get it… in my opinion! In this price range what would you get? A Civic? How fun! Your bonus is you don’t have to worry about bad weather? Really, how boring! Same rings true with the complaints about the Motus’ price!

      • Norm G. says:

        re: “If you wouldn’t go for a motorcycle in this price range over a car if you had the money then you just aren’t a true motorcyclist”

        correct. he is nothing more than a consumer MASQUERADING as a “Motorcyclist”. we have a lot of that now in modern day. not that there’s anything wrong with that for that’s how most start out, but we’ve gotten to the point where we’re a little too “fast and loose” with that label. not all motorcyclists are “Motorcyclists” (capital M).

  4. PN says:

    I love it. That’s beautifully done computer modeling. I’d love to see Horex make it. Showa bought the rights to the old Horex 500 vertical twin in the 1950s, then Yamaha bought Showa and its engineers blew up that 500 twin to the XS650. Pretty cool.

  5. Oz says:

    Execpt for the additional jug, how is this different than the RC211V that Rossi raced in ’02?

  6. 808rider says:

    …”oversquare…” “This bore/stoke ratio favors engine torque at low rpm, at the expense of engine torque at high rpm…”

    Since when is this true? Always understood that undersquare engines favor torque. Oversquare allow for higher piston speeds and HP.

    • Dirck Edge says:

      The reference is to “slightly oversquare”, i.e., a longer stroke than most modern engines of similar displacement.

    • Goose says:

      The bore/ stroke ratio has nothing to do with the power output. Longer stokes just lower the maximum safe RPM. Smaller bores limit valve size. It is difficult to make a long stoke engine a “rever” but it is easy to make a short stroke engine a torque monster. The myth that long stokes produce more torque at lower RPMs is so well ingrained in gear head mythology it will never go away, that doesn’t make it true.

      What long strokes do cause, all other things being equal, is more flywheel effect. The engine will gain and lose RPM more slowly. A good thing for the street (well, IMHO) not so good for racetracks.

      I’d guess a longer stoke would be good in the VR configuration in that it makes it easier to cram the cylinders close together. It might also slightly reduce the higher frictional losses inherent in the design.

      Goose

      • Norm G. says:

        re: “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.”

        dirck’s statement above is 100% correct as written.

        • Goose says:

          My point was that bore/ stroke doesn’t effect power out put at a given RPM. You are welcome to believe otherwise, that doesn’t change facts.

          Goose

      • John Tuttle says:

        Piston stroke affects more than the maximum safe rpm. It determines how far the air that has entered the chamber has to travel to make the roundtrip from the intake valve to the piston face (when the piston is close to BDC) and back to the intake valve.

        Engine torque at a given rpm is linearly bound to the quantity of air that the engine traps per individual intake stroke, at that rpm. If a factor that influences this quantity of air does so in a manner that is interdependent with rpm, it will influence the overall shape of the torque curve. This is the only means by which a factor can ordinarily have any appreciable influence on the overall shape of the engine torque curve.

        The question, of whether piston stroke has appreciable influence on engine torque at low rpm, reduces to the question of whether, for a given cylinder volume displacement and for a given duration of the intake stroke, the volume of air trapped per individual intake stroke is appreciably the same for a cylinder that is long and skinny as it is for a cylinder that is shaped more like a disc.

        For most of the intake stroke, the air entering the chamber is trying to catch up with the piston. Sometime before the piston comes to a full stop, the air smacks against the face of the piston and bounces off, then races just as fast, but now moving in the other direction. The amount of air that escapes through the intake valve before it closes depends on several factors, to include the speed at which the air is moving, and the distance it has to travel, and how much time is available, which is different at different rpm. These factors are interdependent. The optimal distance for the air to have to travel depends on the amount of time that is available. For a given amount of time that is available (corresponding to a specific rpm), and for a given cylinder volume displacement, and all else being equal, the quantity of air that gets trapped within the cylinder is different for different bore/stroke ratios.

        “You are welcome to believe otherwise, that doesn’t change the facts.”

        Alas, the facts are often not in agreement with what one chooses to believe. We all run into this from time to time, so there is nothing to be embarrassed about, unless perhaps the tone you used was unduly assertive.

  7. Norm G. says:

    that video is the epitome of parametric modeling. i’d like to see them go on to ultimately use this 6-pot in something other than a standard (their range topper), then diversify the line with a smaller, “racier” (is that even a word?) 4 cylinder or 3 cylinder…? like yam’s crossplane, this format will define the horex brand and further add to the great spectacle of engine choice/character that is UNIQUE to our beloved motorcyling. bmw fancys boxers, triumph has triples, ducati runs RVT’s, and harley does “potatoes”. now that the VR is finally being tackled, i just want to point out to anyone inclined, the emissions compliant, direct injected 2-stroke idea is still on the table. :) PS: don’t hold your breath for the homogenized world of car-side to do any of this.

    • Jake says:

      Norm G. says: “…now that the VR is finally being tackled…”

      In the “Nothing new under the sun” department: Let us not forget the Matchless Silver Arrow 400cc narrow-angle V-Twins and Silver Hawk 600cc narrow-angle V-4s of the 1930s.

  8. rapier says:

    An additional complication is that the port lengths are different so to equalize the output of the cylinders various strategies must be employed on the intake and exhaust plumbing and even valve timing. Obviously all doable but along with all the other oddities like the beveled pistons it all strikes me as an answer to a question that nobody asked.

    While in theory the more cylinders engines of the same displacement have the more power they can make I bet the compromises in this engine would make it incapable of out powering a stock, say Kawasaki 1200, if tuned to the bleeding edge. If it’s smoother for the rider than a balance shaft big Japanese 4 it probably isn’t by much. In other words what’s the point?

    Hey, I like different for different sake really and the engineering to go along with it so don’t get me wrong. If it makes for a nice bike that the company can make a profit on great.

    What happened to the supercharger however? I’ve been waiting for forced induction to reappear on bikes, of smaller displacement, and am almost shocked it hasn’t happened yet.

  9. Rocky V says:

    I would rather see a Gsxr 600-6

    or a Zrx 1200-6

    how about a Zrx 1200-6 Turbo Diesel

    • Norm G. says:

      under the heading of “talkin’ crazy”, let’s just make a baby veyron engine. a 1 liter W16 replete with 4 turbos.

  10. Bones says:

    Nice looking bike. Cool (albeit straightforward) engine. More coin than I’d want to spend for a naked, even a cool looking one. Hey, it’s way cheaper than a MOTUS.

    I wish them luck.

  11. Reinhart says:

    So they want $25,000 for an untested, chain driven motorcycle that has as it’s biggest virtue an engine that is more compact and offers a bit less vibration? Once again the new players in the motorcycle arena are trying to “Motus” us into spending big money for something that is unique, but no better. How about someone coming out with a new bike that doesn’t do anything special but is appealing and affordable? That would be making great strides in the motorcycling world IMHO.

    • Denny says:

      Exactly: they are just looking at selfpleasing stunts. Similar, if not worse is Czech (supposed to be) built Midalu 2.5l V6. Who needs that c-ap?

      BTW you hit it right pointing out chain as means of final transmission. Cheap and cinchi. V6 should always come with shaft, especially something so plush like this.

    • Tom R says:

      Yeah, I get it. A six-cylinder engine that appears about as wide as an in-line four. This is their gimmick or “schtick”, and I understand the appeal. But bolting it up to a low rent chain-and-sprockets final drive is laughable hypocracy for this price range and supposedly high-tech territory.

  12. Stinky says:

    Very interesting, not much to me, call me curious. I’ll bet Jay Leno gets one and hope it goes bellyup after just a few units. Probably a nice bike as many people are tolerant of barges. Touring behemoths of 700+lbs, dirtbikes of 260+lbs, when in motion they might seem lighter, but physics always seems to raise it’s ugly head when I’m astride backin’ or pickin’ ‘em up!

  13. Pat Walker says:

    They could make it almost as narrow as an inline if they used
    triangular pistons with 3 valves per cylinder (1 exhaust valve
    and two intake valves).

    • Kentucky Red says:

      Thats a good idea, and it seems like a simple idea in the days of CNC machining and precision milling, but developing an engine with that configuration would certainly be an engineering nightmare, and would be very cost prohibitive, and very likely less reliable. Think about the piston seals; how would you be able to create the same pressure against the cylinder wall at the corners and along the straights?

      Oval pistons were used in the NR750 and it would up being the most expensively developed motorcycle ever. It was also pretty unreliable. If you look at the engine configuration, it is actually pretty simple; basically a V-8 with the outside cylinders connected to the inside cylinders. Much simpler than a triangle piston engine, and it was still a money-loser.

      My bet: electric motorcycles will take over the market before we ever see a combustion engine with 3-sided pistons.

  14. harry says:

    An eight (8) cyl Horex of equal bore & stroke would be a 1.63 liter mill and have approaching 215 hp everything being linear (they are not) and would generate a pile of hair three miles high in california alone as hog riders simultaneously violently scratch their heads.

    • Tom R says:

      “…would generate a pile of hair three miles high in california alone as hog riders simultaneously violently scratch their heads.”

      Almost as funny as it is true.

  15. Dave says:

    Good for them. the world needs more interesting motorcycles. Without people and companies willing to take risks, think up new stuff, etc. we’d all be riding push-rod twins with sprung seats. Think about how awesome your bike would be if everyone decided that bikes were ‘good enough’ in 1948*.

    *unless you’re a Harley rider, in which case maybe post up here and let the rest of us know what it _is_ like for that to happen :)

  16. MGNorge says:

    “Given that Volkswagen developed this engine design over two decades ago..”

    I don’t think what VW did with their engine really lends anything toward what owners of this bike will see. Cylinder layout similarity is the only thing, otherwise, they are two different animals. Take it on its own merits. One cannot compared all inline fours as being the “same” just because they share the same cylinder layout.

  17. T. Rollie says:

    Very smooth and a sweet-six sound are good enough reasons to buy this vs. an in-line four. At least for those with excess cash lying around. Maintenance may actually be lower in the long-run because of the reduction in stressful vibrations and the shorter, stronger crankshaft. It’s good for the motorcycle industry.
    But the liquid-asset-challenged among us will always retreat to a used Honda or Suzuki to get our kicks. We’re not their target customer.

  18. Mike Johnson says:

    This is a great engine concept that is just TOO big. The entire purpose of going for more cylinders is to boost performance and smoothness in a smaller, lighter package.

    This would make a killer 800 engine and still do 150mph- maybe even 600 cc

    You need tugboat displacement when tech and metallurgy is Stone Age. As a concept the Rocket 3 is very interesting at 10-1200 cc and might be a cool shaft tourer but instead it is an overpowered boat anchor.

    Here, this week guy buys a new Harley passes an unmarked police car gets 1000. fine and 7 day impound + fees- That is on a Harley- presumably the Horex can earn 5X that in one day- now that’s thinking straight!

  19. soi cowboy says:

    A feat of engineering, but it is complexity for the sake of complexity. 90% of the late model used bikes are v-twins. How many of the v6′s are they going to sell each year? For all that effort, they could have built a parallel twin with a turbo. Imagine an sv650 or en650 with 150hp.

    • Dave says:

      What is the complexity you refer to? It doesn’t seem any more complex than any other multi-cyl. motor…? Should a C14 be a turbo twin, because that’s somehow less complex than a VVT 4 cyl? I don’t get what you’re saying here…

      • Steve says:

        easy there soi cowboy….

        sounds like Dave is referring to the engineering technology & you are referring to sales & marketing…
        Dave is just asking you for the specifics behind your opinion… he’s not saying there’s anything wrong with your opinion…

  20. Denny says:

    Good writeup, but at the end one has to ask – is it worth the effort? Not in my mind, other than as feat of technical supremacy. It is not gonna be light(bellow 200kg wet) machine and, it is not cheap a cannot be. So, where is the prime purpose of motorcycle? Built quality is going to be likely high – it’s German after all; it’s gonna be a conversation piece for sure. That’s about it. As for myself, I learn over and over again what kind of ‘jewel’ I have in my Hornet’s CBR900 based engine. But variety is good thing; so guys go ahead and show it to the world!

    • anon says:

      If you think of this bike as going for the Munch Mammut market, this bike makes a lot more sense than if you put it up against more conventional offerings. The Munch is just sort of seen as an oddball in America, but it has legend/cult status in Germany.

    • Norm G. says:

      re: “Good writeup, but at the end one has to ask – is it worth the effort? Not in my mind, other than as feat of technical supremacy.”

      WELCOME TO GERMANY…!!!

      re: “it is not cheap a (it) cannot be”

      here’s a BETTER QUESTION to ask, why is it the default position of the motorcycle set, that every bike created be “cheap”…?

      re: “So, where is the prime purpose of motorcycle? Built quality is going to be likely high – it’s German after all; it’s gonna be a conversation piece”

      you’ve answered your own question.

  21. Fred M. says:

    I’m generally the engineering skeptic, but this is a proven design that VW has used successfully for years. In fact, this Horex has the exact same V angle chosen by VW: 15 degrees. It’s really a conservative design rather than a solution looking for a problem (like so many radical new engine designs for never-to-actually-be-sold motorcycles).

    It has real possibilities for short-stroke engines with abbreviated piston skirts. The shorter cylinder walls mean that the cylinder could be canted such that they were directly in line with the crank centerline.

    What the video fails to address is intake and exhaust. In a conventional V motorcycle engine (Ducati, Buell/Rotax, Aprilia), there is room between the cylinders for the throttle bodies. On the outsides are the exhausts. This is looking like something that will either be very tall by the time they route the intakes and exhausts.

    • soi cowboy says:

      Probably why they planned to use a supercharger.

    • Bud says:

      In the photo at top all of the exhausts exit at the front of the motor.

    • Norm G. says:

      in a VR motor (at least V-dubs design), it also used a single piece head. all the exhausts exit on the same side by virtue of different length ports. the 3 inboards were longer. the 3 outboards shorter. the reverse for the intake. in this respect it’s not unlike a typical gsxr. valve lift all accomplished with just 2 cams (not 4 as you might think). if i recall, rocker arms were used to “reach out and touch” the valves on the adjacent cylinder bank so intake and exhaust lift could remain on the same “stick” and retain freedom/flexibility of seperate timing.