Clunking Driveline, Oh My Gad!

[Rapid Dog]

To continue from "Terrible Vibrations" and "Ocillating Vibes" thread(s)...
I don't want to beat this into the ground, but I'm worried now...

I checked out my exhaust mounts etc., With the the Vanderlinde the are not really centered well, kinda squashed in the slots. Not sure what to do there.

After that I took Dave Hopkin's advice and set the bike up on the stand to check the run-out on the rear tire. I started it and put it in first gear at idle.
Holy Krap!
The driveline sounded like a 40's military vehicle or sumpin'!
CLUNk, clack, clunk, CLUNkaClUnk, clank!...
At idle, the back wheel is turning albeit not constantly, but at spacing intervels as the driveshaft catches up with the rest of the driveline.
Second gear, CLUNK a Clunk clunk, YIKES I sez! This can;t be right!
Can it?
I mean to say that it's like, loud, I put my hand on the driveshaft housing and can feel the clunking, almost like it's hitting the housing... (but it's probably not).
The driceshaft sounds like it's missing a couple bolts and is just flopping around in there.
What should I do?

[roger albert]

With UJoints with significant misalignment, the jacking effect you are observing on an essentially unloaded arm is a given. I.e. in a perfectly set up bike, that will happen too. What is excessive or not is far harder to judge.

Damn, normally only the winter is an angst fest. Are people still not riding yet for spring/summer ???

[markjenn]

Lots of bikes wil clunk like this, and twins are worse. It MIGHT be an indicator of significant TB sync problems - an engine that is really out of sync will have a lumpy idle which tends to make then clunkier when doing this "test." But I suspect yours does what most of them do. And I doubt it has anything to do with your vibration problems.

My suggestion is to go down to the BMW dealer and ride another S to calibrate whether your vibration is better or worse. Most of us find them acceptably smooth, but few of us would argue that it is close to a Duc or 51 in smoothness. The S is about the worst of the breed. Mine buzzs pretty good around 4K and above 6K.

- Mark

[Rapid Dog]

Really...not to worry then?
Remember my thread on measuring wheel play a while back , Roger.
All that about this, i.e., the slack in the drive train....but this being the first I've had it off the floor and in gear. I don't ever recall my airhead making that much noise tho'.
Really seemed archaic to me that this modern boxer would make such an erradic and noisy sound, like rocks in a metal pipe.
If there's something amiss I want it done on warranty fershure.
Oh, and I've been averaging 150 miles a Sunday all winter by the way...

[Dave Hopkins]

Rapid, I think what you are hearing is lash in the transmission as there is no load, if you could drag the brake very lightly I think you would feel it smooth out.

[roger albert]

Yes, but much of that jerking about has _nothing_ to do with play.
It's that old pesky law of equal and opposite reactions. Angly Ujoints exert side forces. When not controlled (much) by loaded suspension, they make that supension move about. As DH said, worse in twins, and worst in 180 twins (with a regular, but very large interval in firing pulses which causes larger variations in angular acceleration and thus larger angular reactions. A bunch of driveline lash will make it worse. So will any bearings that allow extra play, or a weak shock, or poorly synch bodies. It's normal. If I go push a child down on the playground, we may only infer that I'm a jerk, not that the child was sick because it fell It was only reacting to the force applied (if it was already on the ground, it would have fallen/moved, despite the force applied - because it was already supported/loaded

A mediocre analogy, but quite applicable - and perhaps amusing to the twisted and childless amongst us

[Rapid Dog]

So, the variations in the wheel egagement (and variable clunking) I observed were due to the inconsistency of the motor out put.
Is that the idea?
Gee I guess I could call a good BMW mechanic...if I knew one...

[Flatbutt1]

Just ride the facker till it won't go anymore. All this obsessing is taking up room in your head where the fun should be.

[Rapid Dog]

'jes kool your jets New Jersey, we're having a discussion here.
This is an information forum.
How many miles on your Red of course?

[Flatbutt1]

Mine is low mileage, only 10K. But I have the same clunking you described. I find that the wind screaming in my Schuberth does a great job of quieting scary noises.

[Rapid Dog]

Have you had it on the stand running in gear and heard the clunkity clunk?

[Flatbutt1]

Yup! Thats where I first detected it. My wrench had the same explanation as offered up here by Dr Albert , minus the child of course. Hence my comfort in being a wiseass!

[Rapid Dog]

Well said, er, that's what I look for from youse guys.
I wanna here what your wrench said.
We'ins out here on the Left Coast (at least in the northern L.A. County area) have a severe lack of BMW shops that are readily accessable, as in, "Can I speak personally with the wrench?".
I like to watch...

[roger albert]

Inconsistency in the engine is kind of what I said (and part of)

1: we talk about a motor running at x-thousand rpms.
in reality it averages x-thousand, with each revolution
seeing a variation of speed as a give piston accelerates
the crank and then the crank coasts down.
On a twin, that only happens 1/rev. On a four, twice
per rev, with only about half the amplitude variation.

So, that's one factor.

2: Even if you had a motor w/o that (say, a turbine or electric)
there would have to be some reaction effect from spinning
the U-joints through an angle. (they are not CV joints and
therefore not Constant Velocity and therefore are undergoing
changes in angular acceleration continually. There HAS to be
a reaction.


Let's look at it another way entirely. Picture snapping the throttle
open and closed as you tool down the highway. Does not the rear
end of the bike rise and fall? Even when loaded with around 350lb
of you and bike?

Picture then how much it might want to move w/o the weight on it.
This is what you see on the centerstand. To be honest, I had the
same concern the first time I saw one run on the centerstand. I was
just lucky enough to have had the physics and engineering and practical
experience to work through it. My initial worry was just as bad though.

Going further, picture the friction that must exist when those reaction forces are resisted by loading, and add in the inefficiency of a ring and pinion gear, and then it will be clear why shaft drive loses about twice
as much power as even a marginal chain setup.

Neato

Roger

[Rapid Dog]

Whew!
O.K. Rog, I guess that means I get to ride this weekend...

Well, all in all it's been a good thread, don't you think?

"Safe" to say I won't worry when doin' th' Ton and more.

I rarely do anyways...

[RBMann]

Careful with that running unload in gear stuff. I remember blowing out the instructor's Dodge truck differential in shop class. Higher gears generate some major momentum that really shocks the gear set.

Maybe airheads seem better because they have more flywheel to smooth out the engine speed.

RB

[Rapid Dog]

I just let it idle on the stand.
clunkity clunk ca-clunk...

[roger albert]

I'm sure the heavy flywheel smooths things somewhat. Can't help to torque reactions, but it reduces the crank velocity variations, and thus the part of the play that is normal in the driveline. Yeah, interesting thread. ALMOST made me brake out an old physics book and try to measure the reactions.

Part of it may be the differing design of old airheads, vs later airheads and oilheads (very similar drivetrain) The former only have a front Ujoint.

regards
roger