Free Power! offset cylinders explained

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We’ve been reading a lot about offset, or désaxé, cylinders this year, they seem to be the flavor-of-the-month for motorcycle engine technology, helping to explain the unprecedentedly high outputs from bikes like the 2011 Kawasaki ZX-10R. But, we’ve been unable to translate the relatively simple concept into anything less than very complicated words. This drawing does that for us. As you can see, the design locates the piston to the side of the crank centerline, creating a straighter path for the connecting rod during the power stroke and reducing its sideways force on the piston as it slides up and down in the cylinder. This means less frictional losses and an easier life for the pistons and rods, which can be made lighter, thereby reducing reciprocating mass. That helps the Ninja make a record 197bhp, but bikes like the Honda CBR250R and even the Horex VR6 benefit too.

This drawing actually comes from Honda, included as part of the tech material on the CBR250R (more on that soon), but we wanted to highlight offset cylinder technology on its own as it looks like more and more motorcycle engines will be using the technology.

Actually, “technology” is something of a misnomer. Offset cylinders have been around as long as internal combustion engines. It doesn’t take radical new materials or know-how to take advantage of offset cylinders, just engineering them into a new engine.

The concept is simplicity itself. In a normally-configured engine, the conrod sits at an angle, pushing the piston against the cylinder wall. Increased friction saps power and requires stronger components, increasing reciprocating mass, which also wastes energy during the power stroke, also reducing power. Reposition the cylinder (the CBR and Ninja move it forwards a couple millimeters) and that conrod now travels straight up and down during the power stroke. Voila, less force pushing the piston sideways and therefore less friction. As a result, the length of the piston skirts can be reduced and the size of the rods, pistons and bearings can also be decreased.

Discussing the benefits, Honda says, “Another slick bit of design work further reduces engine friction: The cylinder centerline is offset from the center of the crankshaft 4mm toward the exhaust side. Doing so reduces the lateral resistance generated between the piston and the cylinder during the power stroke. Granted, it’s a small increment, but it’s an ingenious design element exemplifying attention to detail that yields free benefits.”

The Horex VR6 not for frictional benefits, but for packaging purposes. By staggering the offset front to rear, the VR6 engine configuration allows six parallel cylinders to fit into a package more or less the size of an inline-four. Check out this illustration; the first cylinder is offset in one direction, the next in the other, allowing the cylinders to be placed very close together and in parallel instead of the more conventional V arrangement. Horex actually goes one step further to reduce friction, adding a slight V back in to allow for an even straighter load path during the power stroke.

Kawasaki references offset cylinders in the new ZX-10R’s press material, saying, “Offset cylinder bores (relative to the crankshaft) are positioned 2mm toward the exhaust side of the engine, resulting in reduced lateral piston force at the point of maximum combustion pressure, reduced mechanical friction and lower piston loads – allowing the use of lighter pistons.”

So why don’t all engines offset their cylinders, reaping frictional and reciprocating mass benefits? According to Kevin Ash, the arrangement does have its disadvantages. Kev writes, “The power stroke takes longer, which is good for a high revving engine as it allows more time for efficient combustion, but it also makes controlling vibration more difficult as the secondary out-of-balance forces become more complex and asymmetric.”

A longer stroke is necessitated and the power stroke requires more time, which can lead to vibration problems. The v-twin in the Hesketh Vulcan used offset cylinders, which were partly to blame for that bike’s significant vibes.

  • jpenney

    Timely post as I was thinking about this last night. I hadn’t looked in to which way they were offset, for the down or up stroke.

    This is making it into cars too. The 2011 Hyundai Elantra has an offset crank.

  • Joel

    My physics is weak so I’m probably missing something, but wouldn’t you incur more lateral resistance on the return stroke on the opposite cylinder wall because the return angle is increased?

    I could see that perhaps the increase in lateral resistance on the return stroke is less than the decrease on the power stroke, so there’s a net gain?

    • Wes Siler

      I think that’s right. It maximizes the power in the power stroke so there’s a net gain over increased friction on the return stroke, when there’s less stress on the piston and rod anyways.

      An engine builder can probably explain this better than I can.

      • seanslides

        There’s definitely more pressure against the exhaust side cylinder wall during the exhaust and compression strokes, but in the grand scheme of things, it’s relatively minor. The benefit of not trying to pump horsepower sideways into the cylinder wall far outweighs the increased friction during compression.

        Another way to think of it: The force of the fiery expanding air/fuel charge is quite a bit more than it’s resistance to compression before it’s ignited.

  • wwalkersd

    I think I see one downside from a manufacturing standpoint. The offset needs to be proportional to the stroke, i.e., the offset of the crankshaft journals. So a future change in stroke requires a new block with a different cylinder offset, which is very expensive.

    • seanslides

      Nope, you’re over-thinking it. If I understand you right, you’re concerned with the effect an offset crank has on rod angle. It’s not really a big deal, but there are other ways of dealing with that issue. First, the crank is only offset by a few millimeters, the KTM 250 motocross motors are around 8 for example. What this means is that the effect of rod angle on the compression and exhaust strokes isn’t drastically altered. Secondly, when an engine builder changes the stroke in one of these motors, you can bet he’s got a few tricks up his sleeve to keep everything in the bottom end happy.

      Production pistons are designed to be produced as cheaply as possible. As such, there’s almost always room to move the pin up a few millimeters, often enough to allow an engine builder to use the same length, or longer, rod as before. To get even trickier, an engine builder can spec a piston that’s seemingly impossible: One where the oil ring is lower than the top of the pin. With a little trick called ring rails, they work fine. Another benefit of raising the wrist pin is the option to make the skirt shorter.

      If all of that makes your head spin, forget I ever said anything, and leave the engine building to the pros ;)

  • rohorn

    This is also done by offsetting the wrist pin in the piston – which has been done for ages. That’s why seemingly symmetrical pistons have directional arrows on them. Sometimes.

    Funny what becomes “innovation” when the myth of being innovative demands it. Pretty soon, such exciting developments as “ball bearings”, “pneumatic tires”, or “new paint and chrome” will generate press releases…

    • Wes Siler

      Ha, there really wasn’t any specific push of its mention beyond the two obscure paragraphs I quoted. Just something I geeked out on, so have been researching.

    • seanslides

      Sometimes they only seem symmetrical, and those valve pockets are in fact different sizes and in different locations. I’ve had at least 10 phone calls from rednecks who put their pistons backwards who just cant figure out why the motor is “makin’ a clickin’ sound.”

      Offset wrist pins aren’t too common anymore because piston skirts have become too small to keep a piston with the full force of combustion behind it nice and level.

      Offset crank’s are certainly not new, or even high-tech (pretty much every KTM 4-stroke has been using them sine the early 2000′s), but it makes for a more interesting press release for something lackluster like a CBR250R.

      • rohorn

        The pistons I’m referring to didn’t have valve pockets – otherwise, you’re right. Yes, they had huge skirts.

        Wrenching can get one some interesting phone calls…

        • seanslides

          That clickin’ sound turned out to be worth about $600 bucks for us.

  • rohorn

    Researching is cool – sharing is good!

    I just had to try out the dry humor/sarcastic asshole thing out for myself. For the first time.

  • seanslides

    For even more engine geek out, research rod ratio. Turns out that Rod Length over Stroke affects the way a motor makes power.

    • Ducky

      That’s sort of a given, no? Same as how certain multicylinder V-engines work best only at certain V-angles

      • seanslides

        It is, but not too many people know how it affects piston acceleration and speed, and how those things affect the power stroke and intake charge.

  • Your_Mom

    So – Wes and/or Grant did not make high marks in physics? This is basic statics….

    • Wes Siler

      I’m sorry, is there an error?

  • mugget

    Huh. That diagram makes sense. Haven’t heard off offset crank before – but I guess I will more and more. At least now I know what they’re talking about!

  • James

    Aren’t there other disadvantages in the unequal paths in the flow of intake and exhaust gases? I remember hearing somewhere that the unequal port lengths are detrimental for some reason, could someone elaborate? I’m not an engineer.