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Discussion Starter #1
Did a search regarding a vibration that turned up between 5-6k rpm and saw MonstaS4's thread from 8/18.

Monsta, did you find anything?

I mounted a 40 tooth sprocket and raised the rear 3/8" and now, slightly in 4th but mostly in 5th and top gear at around 5-6k, it's a thudding vibration almost as if I were lugging it ???

Throttle synch is spot on, everything's tight, lined up...hot straight and normal!

Anyone?
 

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I haven't come across anything yet. After reading Speeddog's vibration post, at least now I know it isn't just me. I was thinking the throttle sync but if yours is sync'ed and still doing it, that has to point to something else.

It is kind of freaky when you are coming out of a right hander between 4000 and 5000 rpm's doing 70 and the bike starts to vibrate.

I am going to take it into the shop to see if they can find anything wrong with the S4. If I hear anything, I'll let you know. I didn't REALLY notice it until I took the rear end up. I still have the stock sprocket, FYI.
 

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Man, I feel better already.
I thought I was imagining it when it started on my S4, and then MonstaS4 posted.

I think it's just the "nature of the beast", but I'm not completely convinced yet.

Seems to happen when the bike is hot, and when the fuel level is low. But, the first time I noticed it, I filled up shortly afterward and it was still there. It's similar to if I'm riding uphill or have a headwind on the freeway, throttle's open further but still going 85 mph or so at 5000 rpm.

I'm thinking it may be a combination of air temperature and engine temperature, and the ECU is applying a less than optimum map. But I'm running a PC III, although it may not be modifying the mixture under those conditions.

I'm running stock gearing, and I lowered the rear a bit to get the sag right, but that was 500 miles before I noticed the vibration.

MonstaS4, let us know if the shop finds anything. I'nm still stumped.
 
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Discussion Starter #4
MonstaS4.....I would love to ride the S4 and check it for you....
I remember a few things from riding on back....would love to get an better idea of what you are talking about vibration wise.....
 

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Sorry Retro,

I look at my bike like I look at my wife, only one man rides her.*

* - Service guys don't count in the above analogy
 
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Discussion Starter #7
I'm trying to think of commonalities between your S4 and my air cooled and, as far as vibe sources, seems like damned few other than the basic architecture and elevated rear.

And I don't recall this at all in my '94 SS.

I'll check in with BCM. They remapped the ECU, maybe they've seen this before.

later..
 
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Well.....no response from BCM. ???

I guess the price of just a remap doesn't buy much "Ask The Doctor".

Maybe someone else will have better luck but this vibration is drivin' me nuts.

Just as long as it drives me home then, eh? ;)
 

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Well.....no response from BCM. ???
I guess the price of just a remap doesn't buy much "Ask The Doctor".
Maybe someone else will have better luck but this vibration is drivin' me nuts.
Just as long as it drives me home then, eh? ;)
You know, my M750ie has some of the same thing... It's either gotten better in the past 1000 miles, or else I've gotten used to it. It's still noticeable, but not nearly so much. But yeah, there's a funny shaky rumble, sort of like lugging.

I have a theory on this one, actually... I have a car with a fresh, somewhat tweaky engine (old-skool BMW M3, bored stroked and cammed), and I've found that the only speed where that engine has any vibration or roughness other than the inherent second-order buzz of a 4cyl is at about 4700-5000 rpm, which is right where that engine REALLY comes on the cam. My M750 feels like it's coming on the cam at about the RPM where it has that rumble in 5th (roughly 5500rpm on mine)...

My theory (which is informed by some work with piston-powered airplanes - I'll leave that out as I'm trying not to be too verbose here :) ) goes like this: Right at the torque peak, the engine is operating at or near maximum volumetric efficiency. What that could mean is that any small variation in the mixture, temperature, cam timing, valve clearances, etc. between the two cylinders at that point will result in a wider disparity between the amount of power each cylinder produces than you'd see under most other operating regimes - one cylinder could "get on the cam" a hundred RPM before the other. A difference in the amount of power produced could certainly account for a funny, rumbly, luggy vibration. I'd expect this to be less severe with the water-cooled engine, where there should be much less difference in temperature etc. between the two cylinders, but hey, if the handcrafted German jewel in my M3 does it... :)

It's a theory. Shrug. In terms of useful info, if my experience is any guide, it gets better with some miles, and you get used to it too.

M.
 
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Discussion Starter #10
One reason I wanted to drive Monsta's bike, as I have ridden on back is I wanted to check where the vibration occured
and if it was always there at a certain rpm. I have noticed
the same question on another list also, on a 900ss.....my rudimentary guess is something going on with the tuning......does anyone with this problem have a dyno chart? Monsta perhaps it would be good to talk to Doug Lofgren about this one and let him have a look on the dyno and see if he can find something. I was planning on getting over there myself next week? Let me know.
 

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----------------------------------------------------------------My theory (which is informed by some work with piston-powered airplanes - I'll leave that out as I'm trying not to be too verbose here :) ) goes like this:
 

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I would like to get a professional opinion on this. Let me know when you are going to Doug Lofgren's. I asked Delano and Primo and both gave me the discouraging shoulder shrug.
 

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Good hypothesis. Please describe the similarities you found with piston aircraft, I'm curious.
It's a pretty loose connection, and this is going to go on way too long, but...

On most piston aircraft, the pilot has direct manual control over the air-fuel mixture; thus, he can at any time set the mixture anywhere from somewhat too rich to way too lean (in fact, the normal way to shut down the engine in a plane is to pull the mixture control past lean to "cutoff" and wait a second for the engine to quit).

One thing that we find is that most small aircraft engines will not smoothly run at a leaner than stoichiometric air-fuel ratio - their carbs or FI systems and intake manifolds are very primitive, and the mixture varies from one cylinder to the next. Running richer than stoichiometric (known as rich-of-peak because you read the EGT to determine what the mixture is, and the EGT peaks right near stoichiometric), the limiting reagent in combustion is air - adding a little more or less fuel won't have much impact on the power produced. But when running lean of peak, the limiting reagent is fuel, and thus a small change in the amount of fuel delivered will have a significant effect on the amount of power generated. If the four (or six or eight) cylinders in the airplane aren't each getting pretty close to exactly the same amount of fuel, they'll each generate a meaningfully different amount of power, and the result is an unpleasant, snatchy, rumbly vibration - not unlike what my M750ie does right where it's getting up on the cam. People who want to save fuel and make their aero engines last longer will spend quite a bit of money on balanced fuel injectors and other such to even out the fuel flow and enable their engines to run smoothly lean-of-peak.

I guess the only real connection here is that the aero engine gets the same kind of vibration when it's operating in a regime where we know that we're generating significantly different amounts of power from each cylinder. But it's seriously, like, almost exactly the same feel...

M.
 
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Discussion Starter #15
Chaf, I understand what you're saying, but it doesn't explain (or I missed it) why this anomaly is so pronounced in some bikes and recessive or nonexistant in others.

And I can understand why it should be more pronounced in a twin than a multi since more cylinders would smooth out power delivery, but even on a twin the counterweights should absorb most uneven power delivery between cylinders ( especially a 90 degree twin) unless a significant differential existed which should probably be evident all the way through the operating range, correct?

Then, your explanation had me thinking perhaps some combination of factors is creating a resonant frequency at that particular rpm, but it really only happens in the top 3 gears and doesn't seem to be present at all when standing.

What am I missing? ???
 

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Well, let me preface by saying (once again) that at least with respect to the bikes I'm promoting an untested theory that has some parallel in a known and tested phenomenon (the airplane thing).

Basically what I'm thinking is that on any of these engines, as on many higher-strung piston engines, there's a spot where the engine "comes on" to some degree - in my M750, maybe around 5500rpm or so. Now, when I say the engine "comes on" what I'm specifically referring to is the fact that it makes a good deal more grunt at 5600rpm than it does at 5400rpm. (I'm guessing these numbers, since my tach still doesn't work :p) But the amount that the engine's output increases between 5600 and 5800 is far less than the amount it increases between 5400 and 5600. OK.

Now, here's the kicker - suppose that the cam timing or valve clearance or something of, say, the vertical cylinder "comes on" at 5600rpm, whereas the stuff for the horizontal cylinder "comes on" at 5500rpm. Given what I've heard about the way the two cylinders are built to cope with different temperatures etc., on the air-cooled engines at least, this seems plausible... Anyway, so now you're at 5550 rpm, the horizontal cylinder is "on the cam" and the vertical cylinder isn't, and that makes a significant difference in the amount of oomph each cylinder produces. Once you're up at 5700rpm, both cylinders have made the transition, and the discrepancy is much smaller and the engine gets smoother again. I would guess that the reason it's more pronounced in the higher gears (which is the case with mine as well) is because the higher the gear, the more load there is on the engine, and thus the more difference you'll see based on RPM. In the lower gears, any significant load and it'll pass through the shaky zone too quickly to notice. It'd be interesting to throw one on a dyno and see if it has the same vibe at 1/2 throttle in first gear if it's held down to that speed. OK, so the reason you'd expect the whole effect to be more pronounced on one bike than another would be production variations - ignition trigger points, cam grinds, belt tensions, valve clearances, piston deck heights, combustion chamber volume in the head... there's a million things that could be just a little different and change the power curve of only one cylinder, or might on another machine be the same and not have much effect.

As for the flywheel question, these engines really don't have that much in the way of flywheel, even stock. The counterweights serve primarily to undo the inherent imbalance of the 90-degree V-twin, and thus they shouldn't have much if any more mass effect than the pistons and rods themselves. By comparison, the Cessna engine from my earlier long-winded example has a crankshaft that weighs about 100 lbs and a propeller on the front that weighs about 100 lbs, and still you get the same effect when the jugs aren't quite working as a team...

Jeez, I'm rambling now, sorry :-/ Anyway, OK, so this is just a considered theory presented by someone who tried to major in English literature, dropped out of college, and now writes software for a living. You might want to ask a real engineer for a second opinion :)

M.
 
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Discussion Starter #17
Chaf,
You are right on the money! There is a differance between vibration from inbalance and vibration from uneven power pulses. Not everyone gets this. These engines are reasonably balanced assemblies as concerns the reciprocating weight. But at certain RPMs and loads there is an uneveness of the power pulses. Many times this effect can be more or less depending on chain condition. A chain with tight and loose spots will keep the power from flowing in an even fashion.
 

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Chaf, thanks for the detailed response.

As a "real engineer", to me it sounds like a possible explanation. Next time I notice the vibration, I'll try slightly higher or lower speeds/RPM to see what it's like.

BTW, this is the "tech" section, so rambling on is allowed and even encouraged. ;D
 
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By all means- ramble on!!

Oh wait...Led did that already. ;)

Indeed, thanks for the further explanation.

I've always been a proponent of "blue printing" and agree that conditions in seperate stock cylinders are far from identical or optimal and, with so many intrinsic and extrinsic variables, we'll probably never experience precisely reproducible results every 720 degrees.

I think what bothers me the most about these types of situations is that there exists at least anecdotal reports of similar behavior, yet the diagnosis is so damned elusive!
And, had I not previously owned another 2V example which exhibited no traits even remotely similar to these, I'd almost be completely happy if someone of repute said "Oh hell yeah, they ALL do that!" and I'd just bugger off down the road.

Moreover (and I'm certain we've all been at this point) I'm not certain I truly give a rat's ass WHY it's happening, I just want it to stop and stop NOW, thank you very much! [smiley=ok.gif]

Ah well, I guess it's just a case of FIDO and we'll see what comes out the other side.

Run it like I stole it and fix it when it breaks! ;D
 
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