Wildgoose Chase Moto Guzzi

General Category => General Discussion => Topic started by: Tolstoy on February 22, 2015, 11:45:59 PM

Title: Harmonic synchronization
Post by: Tolstoy on February 22, 2015, 11:45:59 PM
Hello all, question for you:

In riding home tonight on my recently serviced 2005 Nevada 750, @ 4800rpm and 70 mph I could hear the rhythmic harmonic imbalance of the engine. If you think musically, it would be silence, one, two, three, four, silence, one, two, three, four, and so on. At 5,000 rpm it is the same but quicker.

Has anyone else experienced this? The vibration goes through the whole bike, and if I lift off the seat I cannot feel it anymore.

Is it the throttle positions (butterfly's) out of whack? Something else? I just had it serviced @ 9k, it has 10.3k now. Tires were fitted at that time too (sport demons - love these tires).

Thanks for any help or confirmation.
Title: Re: Harmonic synchronization
Post by: Waterbottle on February 22, 2015, 11:58:26 PM
I can play a sort of tune when I lift off the seat sometimes myself   :D
Someone will be along soon with a sensible answer I'm sure, but I would think the rhythm your feeling is just the song your engine plays ?
Title: Re: Harmonic synchronization
Post by: Testarossa on February 23, 2015, 04:00:35 AM
Sounds like the beat of unsynchronized aircraft or boat engines (two or more needed). If it comes and goes rhythmically it's not likely to be the (single) engine but a pair of somethings that vibrate at slightly different rates -- I can imagine the mirror stalks doing this, or the two halves of the handlebar if they're of unequal length/weight, or panniers, or even mufflers if they're attached differently. What else is attached to the bike in pairs?
Title: Re: Harmonic synchronization
Post by: John A on February 23, 2015, 04:59:27 AM
The cylinders,of course. I think sometimes if a Guzzi is tuned perfectly you can hear/feel two separate engines.maybe it's the mufflers though.....
Title: Re: Harmonic synchronization
Post by: jabberwocky on February 23, 2015, 05:31:03 AM
How could that be? The two sides are always going exactly the same speed, being on a common crankshaft.
Title: Re: Harmonic synchronization
Post by: Chuck in Indiana on February 23, 2015, 06:24:06 AM
Hmmm, the engine goes 1 2 silence 1 2 silence etc. much like the Harley potato potato on steroids.
How about it maybe being the universal joint?
Title: Re: Harmonic synchronization
Post by: Wayne Orwig on February 23, 2015, 09:25:59 AM
I just had it serviced @ 9k, it has 10.3k now.

The valves are just tapping louder then you are used to.
Title: Re: Harmonic synchronization
Post by: rodekyll on February 23, 2015, 05:06:22 PM
How could that be? The two sides are always going exactly the same speed, being on a common crankshaft.

There might be one point where they go the same speed, but with the 90 you've got one piston stopping at the top/bottom of its stroke while the other is still moving through the middle of it's travel.  Even a V8, which should be the least affected by the varying speed of the pistons and such often has a harmonic damper.

Any out of balance condition, either with the physical masses involved or the power pushing them, can cause vibration.  I think every bike I've owned has had some range where the harmonics come together.  I mentioned in a previous topic recently that my EV does it for about 150 rpm @4800. 

I'm not sure if the small blocks have u-joint issues like the BB's do, but have you considered a bad u-joint or carrier bearing?
Title: Re: Harmonic synchronization
Post by: lucian on February 23, 2015, 08:05:46 PM
Have you had your ears synched lately?
Title: Re: Harmonic synchronization
Post by: Tolstoy on February 23, 2015, 10:42:41 PM
Thanks all.

Been riding the bike 10k this past year, and this 'issue' only became apparent after the 9k service. I will ask them tomorrow if they sync'ed the TB's. The only other thing is the tires, which were also replaced at the time.

Not sure how to check the u-joint; can I put it in gear and rock the wheel back and forth...listening for noise or will I have to go deeper?

Also ears work just fine, thanks.
Title: Re: Harmonic synchronization
Post by: lazlokovacs on February 24, 2015, 03:35:54 AM
I had a similar thing on a cali vintage,

a discordant vibration after an early service

The TBs seem fine though, vacuum gauges read nice and even.

mysterious
Title: Re: Harmonic synchronization
Post by: smartin108 on February 24, 2015, 06:44:18 AM
Make sure they did not overfill the oil.
Title: Re: Harmonic synchronization
Post by: mtiberio on February 24, 2015, 07:23:15 AM
There might be one point where they go the same speed, but with the 90 you've got one piston stopping at the top/bottom of its stroke while the other is still moving through the middle of it's travel.  Even a V8, which should be the least affected by the varying speed of the pistons and such often has a harmonic damper.

Any out of balance condition, either with the physical masses involved or the power pushing them, can cause vibration.  I think every bike I've owned has had some range where the harmonics come together.  I mentioned in a previous topic recently that my EV does it for about 150 rpm @4800.  

I'm not sure if the small blocks have u-joint issues like the BB's do, but have you considered a bad u-joint or carrier bearing?

small blocks have no carrier bearing. that are a single knuckle ujoint, not double.

alignment is critical however...
Title: Re: Harmonic synchronization
Post by: Chuck in Indiana on February 24, 2015, 07:56:57 AM
small blocks have no carrier bearing. that are a single knuckle ujoint, not double.

alignment is critical however...

Yes, if that spacer washer on the right side is missing... at least I *assume* they are still the same on the "modern" small blocks.
Title: Re: Harmonic synchronization
Post by: rocker59 on February 24, 2015, 08:03:50 AM
Hmmm, the engine goes 1 2 silence 1 2 silence etc. much like the Harley potato potato on steroids.
How about it maybe being the universal joint?


At a steady 80mph on the highway, my Sport 1100 has a rhythmic vibration.  A slow, even rhythmic wave that can be felt all through the bike.

I've figured it's the driveshaft.
Title: Re: Harmonic synchronization
Post by: Tobit on February 24, 2015, 08:45:00 AM
Everything has a resonant frequenie.  Two or more objects vibrating at different frequencies will cause a "beat."  When the amplitude of vibration peaks and is in sync with others, you feel the "beat."  When the amplitude and phase of the different frequencies cancel each other you get the null, smooth, quiet, etc. 

Your mass and placement affects this, as evidenced by the beat going away when you stand on the pegs.  As your weight is transferred to the pegs, you damp their vibration and the resonant frequency of items near them.  Try just lifting your right leg off the peg and see what happens, then just weight the right peg.  (thinking u-joint)

Then check the exhaust system, crossover, mufflers, any heat shields and peg mounts. 

After doing that, I'd check the u-joint.

Tobit
Title: Re: Harmonic synchronization
Post by: sib on February 24, 2015, 09:11:56 AM
As has been pointed out, harmonic vibrations are caused by two things that are vibrating at almost, but not quite, the same frequency.  When I think of what two things on a motorcycle can vibrate at nearly, but not quite the same frequency, I think wheels.  Are they balanced well?
Title: Re: Harmonic synchronization
Post by: Arizona Wayne on February 24, 2015, 04:40:38 PM
My take...
All machines have harmonics even humans hummmm.

I notice that when I take a trip on my beemer, when I first start out I have to get used to the harmonics particular to the r1150 which vibrates one mirror more than the other at different rpm and the "rolling" vibration described by others in this post. After a while I notice it less and less and get used to it. After a days riding its is hardly noticeable.
There have been many books written about the harmonic vibration between engines and the machinery. Its a black art and very mysterious.

3500 miles later I climb aboard the CX and for the first few minutes Good grief!! It vibrates completely different and feels very odd with completely different harmonic vibrations, almost alarming. U joints going bad? Dead or dying cylinder? But then after a while it all becomes very normal and sorta disappears.

The same happens if I have not ridden anything for a while.

Not suggesting that you ignore the signs, of maybe something amiss but to give it time and like a bear in the woods it might just go away.

mike :-)




Yeah, it's kind of like when you're on the fantail deck(rear) of a Destroyer(DD784) made in the late `40s and you hear/feel the rythmic turning of the propeller shafts rotating right beneath you @ X rpm(knots) and it just so happens your bunk is down there too right next to the twin 5" guns/turret which shoot @ night while you're trying to sleep!  :D  You hear all kinds of sounds and only hope you stay afloat....which you do.  I could even hear the water splashing on the other side of the hull (1/4" thick) of our berthing compartment too whilst off Vietnam doing shore bombardment directed by a spotter plane @ the DMZ.
Title: Re: Harmonic synchronization
Post by: kirby1923 on February 24, 2015, 07:43:30 PM



Yeah, it's kind of like when you're on the fantail deck(rear) of a Destroyer(DD784) made in the late `40s and you hear/feel the rythmic turning of the propeller shafts rotating right beneath you @ X rpm(knots) and it just so happens your bunk is down there too right next to the twin 5" guns/turret which shoot @ night while you're trying to sleep!  :D  You hear all kinds of sounds and only hope you stay afloat....which you do.  I could even hear the water splashing on the other side of the hull (1/4" thick) of our berthing compartment too whilst off Vietnam doing shore bombardment directed by a spotter place @ the DMZ.



Amen brother!
Ships have the scariest, shuttering, heaving, creaking, weird noises so that you really do wonder if the thing is going to stay together!! Amazing that they do.
Stack gas!!!
Of course you DD guys were part submarine!!
My hat is off to you.
mike
Title: Re: Harmonic synchronization
Post by: MotoGoosy on February 24, 2015, 09:08:59 PM
If you look at my post about vibration on my EV11, you'll see you're not alone.  Your analogy is quite accurate.  You wonder if the thing is going to come apart.  Be sure, as I just did, to check for loose bolts all over the bike.  And I found just about every bolt I put a wrench to was not tight.