I've never understood this..

[Huzo]

I see in another post here, the statement that a BMW has a "secondary vibration" that the single crank pin 90 degree V twin does not.
I'm not disputing it because the physics confirms it apparently, but is there a simple explanation ?
I wrongly thought that if the two pistons on the BM were hitting TDC at the same time, then the forces would be nullifying each other.
Not so it seems..
????

[Testarossa]

Good article by Kevin Cameron on just this topic in the new Cycle World.

[Huzo]

Good article by Kevin Cameron on just this topic in the new Cycle World.

That guy is a genius I reckon.
I will try to get hold of that.
Thank you.

[Chuck in Indiana]

I see in another post here, the statement that a BMW has a "secondary vibration" that the single crank pin 90 degree V twin does not.
I'm not disputing it because the physics confirms it apparently, but is there a simple explanation ?
I wrongly thought that if the two pistons on the BM were hitting TDC at the same time, then the forces would be nullifying each other.
Not so it seems..
????

Ideally, that would be correct.. but.. the cylinders are not in line by the width of a crank pin plus enough casting between them to hold stuff. (technical jargon)  :smiley:This causes a twisting vibration which is more pronounced as the engine becomes bigger. BMW eventually went to a balance shaft because of this.

[Wayne Orwig]

I see in another post here, the statement that a BMW has a "secondary vibration" that the single crank pin 90 degree V twin does not.
I'm not disputing it because the physics confirms it apparently, but is there a simple explanation ?
I wrongly thought that if the two pistons on the BM were hitting TDC at the same time, then the forces would be nullifying each other.
Not so it seems..
????

Because the offset in the cylinders. It puts a twist on the crankshaft at TDC and BDC.

[Sheepdog]

The offset crankpins create a �rocking couple� which is responsible for said vibration. It is a very subtle thing and not really an irritant, but present nonetheless.

[Huzo]

Oh.
Ok then. That is a factor that I'd not considered.

[Kev m]

I don't know the if ands or whys, but my R1100RS had a higher frequency vibration that I didn't like at highway speeds.

I don't recall it being the same on my Airheads but they were much less powerful.

[jas67]

The offset crankpins create a �rocking couple� which is responsible for said vibration. It is a very subtle thing and not really an irritant, but present nonetheless.

Guzzis also have rocking couple due to the front/back offset of the cylinders.   Because they share a common crank pin, that offset is less than the boxer, so, I would think the rocking couple would be less as well.

[Huzo]

Ok.
I knew of course that the cylinders were offset longitudinally, but dud not know that was the cause of the "secondary vibration".

[oldbike54]

 Bore and stroke ratio can also come into play, new beemets are pretty over square Guzzis less so,  One of the reasons a BMW R50 which is close square is smoother than an R100 which runs the same stroke as the 500 .

 Dusty

[oldbike54]

Ok.
I knew of course that the cylinders were offset longitudinally, but dud not know that was the cause of the "secondary vibration".

 It isn't directly, single pin vs dual pin.

 Dusty

[Huzo]

It isn't directly, single pin vs dual pin.

 Dusty

It's the crank web or associated structure that exists between the two pins that generates the fore and aft spacing that is greater on the BMW.
It is this greater fore and aft distance between the two centrelines of the cylinders that I'm told is accentuating the secondary vibration.
Am I understanding correctly ?
Conversely, just as a thought exercise, (not considered realistic)
If you had a flat twin (BMW) that had one crankpin and the ends of the con rods bolted together in line, the effect we are discussing would be zero but presumably would rock left/right dramatically because one piston would be TDC when other was BDC.
The Harley apparently has a knife and fork set up, so no offset.
Obviously across the frame crank though.

[ohiorider]

The words "rocking couple"come to mind.

[Huzo]

The words "rocking couple"come to mind.

Apart from the obvious innuendo that one could use to create a bit of toilet humour..
Are there any other words that you could add ?
Conventional wisdom tells me that such a device with pistons moving left to right simultaneously, would be an abomination.
Has it ever been done ?

[kingoffleece]

Good article by Kevin Cameron on just this topic in the new Cycle World.

Bingo.

[Rick4003]

I'm guessing if you did do a common crank pin with knife and fork style conrods, you no longer have a boxer engine but a 180° V-twin.
I'm not sure how that works out balance wise, but I think it would be a bit like a 360°  parallel twin with the pistons always moving the same direction. Of course the firing order won't be the same.

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[kingoffleece]

I wonder who else knows what Rick4003 means-or, to be more precise, what it refers to?

[Rick4003]

It's the crank web or associated structure that exists between the two pins that generates the fore and aft spacing that is greater on the BMW.
It is this greater fore and aft distance between the two centrelines of the cylinders that I'm told is accentuating the secondary vibration.
Am I understanding correctly ?
Conversely, just as a thought exercise, (not considered realistic)
If you had a flat twin (BMW) that had one crankpin and the ends of the con rods bolted together in line, the effect we are discussing would be zero but presumably would rock left/right dramatically because one piston would be TDC when other was BDC.
The Harley apparently has a knife and fork set up, so no offset.
Obviously across the frame crank though.

I'm commenting Huzo's suggestion of a boxer with a common crank pin and knife and fork style conrods.

I'm just saying that if you do make an boxer engine with a common crank pin it is no longer a boxer engine but a flat twin. Similar to Ferrari "boxer" engines in the 512BB, these are actually 180° V-12 engines as they share the same crank pin on the cylinders. Not a true boxer. I think it works with the V-12 as it is essentially two straight sixes bolted together at the sump.

Again I'm not sure how a 180° V-twin would balance, but I'm guessing badly. :)

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[LowRyter]

I'm commenting Huzo's suggestion of a boxer with a common crank pin and knife and fork style conrods.

I'm just saying that if you do make an boxer engine with a common crank pin it is no longer a boxer engine but a flat twin. Similar to Ferrari "boxer" engines in the 512BB, these are actually 180� V-12 engines as they share the same crank pin on the cylinders. Not a true boxer. I think it works with the V-12 as it is essentially two straight sixes bolted together at the sump.

Again I'm not sure how a 180� V-twin would balance, but I'm guessing badly. :)

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[Rick4003]

Precisely

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[blu guzz]

I had an r100rs and then a r1100rs.  The 1100 had an annoying tingle the airhead did not.  I always assumed it was because they changed how they actuated the valves and in so doing, added a lot of mechanical complexity that resonated in a way that the older engine did not.  Guess I was wrong about that.  My 1150RT also did this, but with the 6th gear, wider, weighted bars,  it was hidden better.  I felt that it was a worthwhile tradeoff for the nearly double horsepower, real brakes and the other upgrades.  At 4,000 rpms, the airhead was as smooth as my current rubber mounted 1400.

[oldbike54]

 W/O getting in too deep, engine character depends more on crankshafts phaseing than the way the cylinders are arranged.  A parralell twin can mimic a V -twin by staggering the pins at 76 or 90 degrees, a flat 4 motor can mimic a V -four. The reason why a 90 degree V is smooth is related to relative piston speed, when one piston is at TDC or BDC, the other piston is at maximum piston speed either going up or down.  The crankshaft in a wide angle V has an easier life than a boxer crank because of this.

 Peter, a flat twin with both pistons moving the same direction would be very woogity 

 Dusty

[Huzo]

Damn you to hell Dusty..
There I was understanding everything as well as Phil Irving and Soichiro Honda combined..
And then you introduce the concept of woogitiness.
Back to the drawing board for me..
I'm starting to swallow water.

[oldbike54]

 Woogity is a technical term to describe quinary imbalance Peter,  as in this thing has a greater deal of quinary vibration,  or in layman 's terms it is woogity 

 Dusty

[Paul Brooking]

Woogity is a technical term to describe quinary imbalance Peter,  as in this thing has a greater deal of quinary vibration,  or in layman 's terms it is woogity 

 Dusty

Dusty

I looked up that word in the Australian Edition of "Farkennel's Dictionary of Mechanical Engineering".

Not one mention??

Maybe Huzo has a previous edition which it may feature prominently?
 

[wirespokes]

I think all engines have RPM bands where things don't quite all "mesh". If you know what I mean. Cam shafts are designed to operate the best in a particular band. The exhaust system is supposed to complement the cam. Design of the cylinder head, valves, piston, and everything else are designed to complement the intended use. The engine is smoothest, when it's running in the power band.

And this is still a reciprocating engine with pistons moving one direction, stopping and then moving the other direction. There's vibrations created in all those different reciprocating motions even though the two pistons are balanced. The major motions (pistons and rods) cancel, but there are many smaller ones that may not.

With the airhead, there's a minor vibration about 3200 RPM and another right around 4K. It's just the design of the engine, and I've experienced it on enough of them to know it's true - this isn't something I made up. Careful balancing of the pistons/rods, and tuning to get both sides as closely matched as possible just about eliminates those secondary vibrations.

I think the difference between the airheads and oilheads (besides FI, 4 valves and all that) is a shorter stroke. And that's what I think you felt with the higher rev vibes. I didn't care much for the fuel pump buzz.

What I don't understand is how they can say the Guzzi V twin has a naturally balanced primary vibration. There have been discussions, but so far it still doesn't make sense. I don't see how one piston traveling at right angles to the other can cancel vibrations.

[oldbike54]

 Wire, forget the angle, remember that as either piston is at TDC or BDC the other piston is roughly half way thru its stroke going up or down which means it is at maximum piston speed.  That is what cancels the vibration. It might helpto understand that what causes vibration is the force moving opposite to the piston/rod assembly.

 Dusty

[KiwiKev]

A rotary engine was supposed to overcome all these woogity problems but has pretty much faded into obscurity apart from the odd boy racer types.


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[chuck peterson]

Neat illustration from car and driver