Passive Mass Oscillation Damping... - Honda Motorcycles - FireBlades.org
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post #1 of 10 Old 06-04-2003, 8:30 PM Thread Starter
 
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Re: Passive Mass Oscillation Damping...

Someone on the sabmag list today posted about how he got a new case of headshake when he went for a ride without his bar end weights installed. This was on a bike with a much more conservative setup than our machines. Anyways, he'd never seen that before, and hasn't again since putting the weights back on.

So that got me to thinking, since mine has supposedly had the recall done and I've installed the big*ss hvm bar end weights, that might explain why I've never even seen a hint of wiggle out of my front end. With nearly a pound of additional weight on the end of each clip-on, that would certainly change the subsceptibility towards headshaking.

Thoughts?

Dan - still planning on the ohlins someday



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post #2 of 10 Old 06-04-2003, 11:58 PM
 
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Re: Passive Mass Oscillation Damping...

Weights are for vibration not headshake. Racebikes have no weight in the bars. I removed the stock barend (half of the weight in each bar) and installed delrin sliders and i can't get it to shake.
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post #3 of 10 Old 06-05-2003, 12:37 AM Thread Starter
 
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Re: Passive Mass Oscillation Damping...

(Duh&#33

I know the weights are to reduce perceived vibration. My point is that maybe they also have the benefit of helping to reduce headshake.

dan
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post #4 of 10 Old 06-05-2003, 2:05 AM
 
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Re: Passive Mass Oscillation Damping...

They have no effect, whatsoever. Your friend had many variables come together to cause the headshake. It just so happens that it occurred while the bar ends were not installed.

Only when he can absolutely, down to the gnat's hair, duplicate the episode, only now with bar ends installed, and not experience headshake, can he say and prove to others that the bar ends contirbute to a more stable motorcycle.

Until then, they just help attenuate the vibration generated by the engine.
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post #5 of 10 Old 06-05-2003, 8:26 AM
 
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Re: Passive Mass Oscillation Damping...

I've got to disagree on this, scuse the length. The bar end weights are potentially a very real factor in headshake.
I made a long post on headshake many months ago, maybe on Motorush but the situation in essence is this.
Headshake is in effect a resonance phenomenon - it occurs when the driving frequency of the force causing it happens to coincide with the natural frequency of oscillation of the system.
The driving force typically arises as a consequence of deflection of the wheel over a bump or whatever. Castor, tyre grip, gyroscopic and inertial effects then conspire to push it back to centre (they must do to keep the bike tracking staright). This is the driving force.
Normally the gain of the system is by design such that even when the centering force causes the steering to overshoot centre the amplitude of the resulting oscillation will decay to zero in a few cycles due to damping effects. (in the tyre, the headbearings, how you hold the bars etc.)
Life gets interesting if resonance occurs - if the frequency of this force matches the natural frequency the system (the forks, you and the bike) will adopt if left free to oscillate - the amplitude goes off scale. (in practical terms the forks go stop to stop and you get lobbed off the bike)
The point is that damping, tyre and other stiffnesses and MASSES are the big factors determing the natural frequency of the system.
Significantly changing the bar end weights (given that they are so far outboard) will have a significant effect on the moment of inertia of the mass oscillating around the headstock - as would a heavy tyre, a heavy wheel, callipers mounted outboard of the pivot axis, how hard you grip the bars etc.
This means that changing the bar end weights in either direction could potentially retune the system to move it in or out of resonance.
The problem is that modern sports bikes operate in a zone relatively close to resonance - quite small and seemingly insignificant changes can be very significant. The other point to remember is that changes in either direction are just as likely to cause problems - increasing rake, trail or damping could for example worsen a situation.
Fixing headshake involves finding way to move the system out of resonance. A steering damper or tighter head bearings (a la the 954 upgrade) can do it by delivering more or less damping. More or less tyre pressure or a different tyre wall construction can do it by altering the stiffness and damping of the tyre. A lighter or heavier tyre, brake or mudguard can do it. More or less fork sag can damp the oscillation by keeping the tyre longer in contact with the ground - it damps the oscillation through friction. Reducing or increasing the castor can do it by changing the self centering force.
It's very hard to predict, hence trial and error tends to be the way out. The conventional wisdom is not always right - for example it's not necessarily the result of too little rake and trail.
The worst tankslappers I've ever experienced were on an old 70s Yamaha with lots of rake to which I'd fitted a set of early aftermarket and very heavy cast alloy wheels - it went instantly stop to stop at every opportunity under acceleration. (think slapper on the main street in Dublin while accelerating off traffic lights!&#33 I could not stop it happening with all of my strength, only the restricted lock stops I'd installed to clear a fairing made it survivable.
A final point. Mass damping is actually another principle which involes hanging a second system out of the primary one and tuning the two systems so that they cancel out each other's forces at resonance.
The concrete block hanging on a spring in your tumple drier is probably the most common application of this principle, but it's also used in active noise suppression systems, airframes and all sorts of places.
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post #6 of 10 Old 06-05-2003, 9:28 AM
 
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Re: Passive Mass Oscillation Damping...

Ondablade: You can move the resonance, but you may just be moving the oscillation to a different speed range by changing the weight of the vibrating components. By playing with the damping, bar end weights, handlebar length, relative mounting height, wheel/brake weight etc., you can change the point of headshake from say 100 mph down to 80 mph or up to 120 mph (examples only). The solution would be to tune the resonance out of the forseeable speed range (say 250 mph). Unfortunately, there are a few more variables that may induce this oscillation at different speeds regardless of the 'tuned' resonant frequency (relative angle of lean, ground surface, weight transfer under acceleration, lift at the fairing canards etc., etc.

I think perhaps this gentleman's bike was not setup very well and removing the weights potentially added the 'final straw' to his configuration.
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post #7 of 10 Old 06-05-2003, 10:07 AM
 
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Re: Passive Mass Oscillation Damping...

Sorry if my post was a bit theoretical. The bar end weights for sure were probably the last straw which just tipped an already marginal set-up bike into the zone - you'd not normally expect them to have that sort of effect and many are running both heavier and lighter (alloy clip ons) set ups without problems which kind of proves the point.
I'd differ a bit on resonance in that I suspect that a given bike (system) will slap at a given frequency, plus presumably to a lesser degree at harmonics of this if they arise in practice.
In the case of a given bike problems may arise at different speeds and in different situations but I'd imagine this is because maybe more than one circumstance is able to deliver the frequency of driver needed to kick things off.
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post #8 of 10 Old 06-05-2003, 11:01 AM
 
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Re: Passive Mass Oscillation Damping...

I agree that once the fundamental is there (that the steering head will oscillate at), that any number of circumstances can excite that particular frequency or it's related harmonic structure. *However, I believe also that the steering head will oscillate (actually float is a better word) at several non harmonically related frequencies as well due to an entirely different set of criteria.

I have observed this on my 250s' as they would shake at very closely related speeds which couldn't possibly be even or odd related to each other (say 81 and 83 mph, but an entirely different shake frequency (bar oscillation) and under different onset conditions).



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post #9 of 10 Old 06-05-2003, 12:18 PM
 
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Re: Passive Mass Oscillation Damping...

I guess I was drawing on the theory and the fact that the bikes I've had shake always seemed to go at about the same frequency.
Intersting that you've seen different - if so it kind of implies that unless it was picking up other harmonics (and I don't know if this happens either) the whole resonance theory may be for the birds.
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post #10 of 10 Old 06-05-2003, 9:45 PM
 
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Re: Passive Mass Oscillation Damping...

After reading Tony Foale's Motorcycle Handling and Chassis Design, it's easier to understand this phenomena.

In section 6, under Suspension Principles, there are complete descriptions on how the suspension plays a large role in resonance... essentially that the tire, wheel bearings, fork stanchion material's hardness, fork tube material's hardness, oil volume, damping method and material, spring material, triple clamp; material, shape, and hardness, handle bar mount material, and bar material, are all directly responsible for the total resonance in the front end. These variables, combined with weight bias induced by raising the rear, or lowering the front, are responsible for noticeable oscillation.

The effect of weight connected to the resonating materials, i.e. bar end-weights, is directly related to the type of material they are connected to. The more weight at the end of a material that is likely to resonate at the same frequency as other components, the more drastic the effects on the human attached to the machine.

Conversely, the less weight attached to a material, the more likely it is to resonate with the harder components it is attached to.... sort of a damned if you don't do your research, and buy what they sell you, and expect it to always be a wonderful tame little motorcycle scenario.



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