engine / frame vibration harmonics at ~50mph?

so I was reading the thread about "how fast" and one guy mentioned some odd vibration at certain speeds..

Got me thinking about an odd harmonic vibration that occurs on my bike around 50mph in 5th gear on my bike.

Its similar to the normal engine vibration but between about 48 & 52 mph the vibration seems amplified.. if i go above or below that speed then it quickly subsides


Since this is my first motorbike and i dont know any other riders i dont always know what normal is -- its up to you folks to fill me in

Thinking the harmonic vibrations im feeling at this speed/rpm range are normal, and are being felt when the engine rpms match the rate at which the frame itself would vibrate if hung from a sting and struck with a piece of metal like a gong (aka resonance frequency), thereby amplifying the vibrations felt in the saddle and grips

I'm no scientist but this is what my logic and knowledge allow me to deduce..

Does anyone else get this odd harmonic at certain speeds? Assuming this is normal, but maybe not? Other input?


Thanks

 

It's called a "rocking couple" and all inline 4 CYL (without counterbalancers) have this flaw.

The solution is to ride much faster 8)

 

ALL XJs have a certain RPM "buzz range" (actually there are two, but the other one is way up by redline somewhere) where everything does tend to harmonize through a few hundred RPM.

That being said, this harmonic effect can be greatly reduced to the point of being barely noticeable by making sure the bike is in an ultimate state of tune. By that I mean valves in spec, carbs properly sync'ed, etc., etc.

It really can and does make a difference.

 

Interesting!

If anyone else is interested I just found a good writeup of the "rocking couple" effect in regard to engine balance here:

http://www.tonyfoale.com/Articles/Engin ... alance.pdf



Bigfitz, you may be right that proper valve clearances / engine tune matter here.. just installed new shims yesterday and havent had time to run full paces, but i dont recall noticing the harmonic on the ride to work this morning...

will run through the gears and rpms on the way home and pay closer attention

 

The better the state of tune the less noticeable the "buzzy" RPM range, plus it narrows considerably.

I don't make stuff up, I have two operational XJs and I learned this one from experience. When a 550 buzzes, it BUZZES. I quickly learned they don't have to.

 

welp got rained on on the way home -- hooray for no electrical shorts!

so still didnt have a chance to run full paces, but didnt notice any buzzing at all which is maybe a good sign that im approaching 'tuned'

...getting closer