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Resonant and Harmonic freq calculation in motorcycle frames

Resonant and Harmonic freq calculation in motorcycle frames

Resonant and Harmonic freq calculation in motorcycle frames

I posted this at autobody then realized this would be a better forum.
Hi All
Not sure if this is the exact forum for this question, but I'll toss it out anyway. If someone can help, I will be forevermore grateful. What I would like to know is if there is a way to measure or calculate the resonant frequency of a specific motorcycle frame and swingarm combo. I.E. at what engine speeds (frequency) will harmonic vibrations be introduced into the frame and thus felt by the rider? I am not an engineer, but it seems obvious that the rear fork assembly will act as a tuning fork at some engine speeds, and quite possibly some of the harmonics will be in a range noticeable to the operator. I ask this question because I have a new Harley Superglide sport, and everyone that I have contacted with the same model has complained about a pronounced vibration that came on at 2400 RPM and was gone by 3000 RPM. Thanks in advance.

RE: Resonant and Harmonic freq calculation in motorcycle frames

I'm sure you had to pay extra for the added shake and rumble.  Adds to the character.

Anyhow, for an analysis, you would need to compile a bit of information on frame material and geometry, etc.
What about contacting the Harley Davidson factory?  I know they spend quite a bit on acoustics, etc. and might be interesting to talk to.

From a measurement standpoint, you would need some specialized equipment (accelerometers, signal analyzer, etc.) and take measurements on the frame, etc. with engine at different speeds.  Or, do "bump tests" at different locations and look at the structural response.  
Try calling some local vibration/condition monitoring companies, you might find an interested volunteer.

However, sometimes you can narrow things down without instrumentation-  is the vibration dependent only on engine speed?  eg: all gears, neutral, etc. Or does it vary with road speed? Remember that resonance is a condition dependent on both the structural response as well as level of source vibration.  Any changes to engine or mounting arrangement?  Are there rubber isolation mounts?  How about the exhaust system?  Often this is can be a problem for resonance.  Where do you feel the vibration strongest?

Pls let us know what you find.  You might also try re-sending your post to the Mechanical Acoustics/vibration engineering forum under Mechanical Engineers.

Good luck....   

RE: Resonant and Harmonic freq calculation in motorcycle frames

If the swingarm worked like a tunning fork each side of the fork would have to vibrate side to side like a harmonic oscillator.  Where the mass would be equal to the mass of the rim, tire, suspention and brakes etc, the restoring force would be equal to strength of the rear suspension bushings, the damping force would be equal to the strength of the rear swingarm all equal to the resonance of the engine as a function or rpm.  Holy cow.

I noticed you had mentioned that it happened at different rpm ranges so I am going to scratch that first idea out.  You had also mentined that it had a swing arm so I believe that your bike has a rubber mounted v-twin and I will bet that you feel it in the handle bars also.  Since the vibration comes on at a rpm range due to the motor and not the rpm due to the rear wheel I don't think the swingarm is acting tuning fork.  Those motors don't fire at 180 degrees from each other so they don't typically balance themselves out hence the large amount of torque(I think you will agree).  I think one cylinder fires at 45 degrees and the other at 135 degrees later.  Because they are not 180degrees off(I can't say this for sure just a theory)I am thinking that even though the firing order is fixed at that timing difference and they both change equally according to rpm there is going to be some destructive and constructive wave activity creating a pulse feeling(or resonation)I know this sounds silly, but does the vibration feel like a sin wave.  Forget that I have been up too long.
Long story short no I don't think your swing arm is working like a tuning fork.

RE: Resonant and Harmonic freq calculation in motorcycle frames

Once tried to eliminate, or at least dampen the vibration in a Harley superglide (older model).  Yikes.  D.A.S,T was right about the uneven firing, however it works like this:
720 degrees of crankshaft rotation are required to complete the 4 stroke cycles in a 4 stroke engine.  On a Harley the vee of the cylinders is 45 degrees and both connecting rods connect to a common crank pin or journal.  For simplicity, consider only the power cycle of each cylinder.  Cylinder #1 fires (forward cylinder), the crankshaft then rotates 315 degrees at which time cylinder #2 fires (rearward cylinder), the crankshaft will then have to rotate 405 degrees from the point at which cyl. #2 fired before cylinder #1 fires again, and around we go, weird huh?
Add to that, all large displacement 2 cylinder engines vibrate due to the large power pulses being separted by too much crankshaft rotation.
Having had a couple of Harleys, my opinion is that the period and amplitude of the vibration in the rpm ranges you mention is more than the elasticity and rebound charateristics of the isolastic engine mounts.  But I do not have the knowledge D.A.S,L and brin do concerning the vibrations themselves.  So just an opinion.  Enjoy the ride.

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