Sources of Resonant Vibes

[ozarquebus]

Georgia Guzzi's thread on his unfortunate Victory wheel bearing failure got me thinking about the sources of resonant vibrations felt while riding at highway speeds.

While cruising down the hiway, the SP2 has what could be describe as: a trans-subsonic slowly throbbing hum cycling at about every 3/4 second that disappears when I let off the gas, unchanged by pavement types. I do not notice it on my G5. Its similar to the sound when two bikes with similar engines are resonating side by side while at speed.

What is that sound, but from one bike alone?

If from engine/drivetrain:
a. warning of a bearing failure?
b. indication of some bearing wear?
c. not abnormal for a Guzzi

If from wheel
a. warning of bearing failure?
b. indication of some bearing wear?
c. just your tires (SP2 quirk)?

[wirespokes]

I could be way off base, but I'd be concerned it's the u-joint.

[oldbike54]

I could be way off base, but I'd be concerned it's the u-joint.

 That was my first thought also . It might be a tire slightly out whack or some weird resonance created by airflow .

 Dusty

[John A]

when a Guzzi is in a good state of tune, on a long ride, I can sense two single cylinder engines. I don't know how else to describe it...

[fotoguzzi]

when a Guzzi is in a good state of tune, on a long ride, I can sense two single cylinder engines. I don't know how else to describe it...

me too and they kind of harmonize.

[wirespokes]

me too and they kind of harmonize.

And there's the trick - and one of the objects of tuning. Besides getting the settings correct, it's having them balancing each other. There's kind of a magic that happens when they're perfectly in tune - you can be 99% there and the magic doesn't happen. I've gotten a lot of used bikes I'm sure wouldn't have been sold if they had just been tuned properly. They felt like crap to begin with, but when dialed in were real sweethearts, and all it took was a little tuning and balancing the carbs.