Strange Vibration

Bonwit · Sep 23, 2009

Okay, I know "strange vibration" and "Norton" are often used in the same sentence, but with my inexperience with this bike I'm not sure if what I'm feeling is an engine vibe. or perhaps an out of balance wheel. Here's the deal, this bike is remarkably smooth through out the power band, but when I reach about 70 MPH and up, I experience a harmonic vibration that repeats steadily ever few seconds. At first I thought it was mis-adjusted (or old) iso's, but today I noticed that the front wheel and fork are shuttering in tune with this vibe. Does this sound like an out of balance wheel to you?

RennieK · Sep 23, 2009

Sounds like it could be your problem. Also the age of the tires. Tire shops have a device that they put a car wheel on and rotate it at regular road speeds. Then they apply a roller to simulate the road surface pressure. This can show up hard spots in the rubber tire that basicallt render the tire useless if they exist.

bpatton · Sep 24, 2009

John, Does the vibration hop up and down or is it shuddering like it's going to twist the handlebars out of your hands? The first might be balance or out of true wheel/tire. Commandos seem to have a wobble mode at around 35mph or so that's pretty common and will yank the handlebars back and forth. Probably too fast for that though.
http://www.dinamoto.it/DINAMOTO/on-line ... bmode.html
Interesting winter reading.

Bonwit · Sep 24, 2009

bpatton said:
John, Does the vibration hop up and down or is it shuddering like it's going to twist the handlebars out of your hands? The first might be balance or out of true wheel/tire. Commandos seem to have a wobble mode at around 35mph or so that's pretty common and will yank the handlebars back and forth. Probably too fast for that though.
http://www.dinamoto.it/DINAMOTO/on-line ... bmode.html
Interesting winter reading.

The fork movement appears to be up and down. The bars seem pretty steady.

What's odd to me is that the vibration is not constant. It occurs on about a 3 second cycle. Three on, three off. I would have guessed that a wheel imbalance would shudder all the time.

bpatton · Sep 24, 2009

Bonwit said:
The fork movement appears to be up and down. The bars seem pretty steady.

What's odd to me is that the vibration is not constant. It occurs on about a 3 second cycle. Three on, three off. I would have guessed that a wheel imbalance would shudder all the time.

That's interesting. When you get two sources of vibration that have differing frequencies they can go in and out of resonance with a given period, or cycle. It goes from null to excited, it's not constant, but it is regular. If you eliminate one of the sources of vibration it goes away. Roadholders aint the stiffest forks in the world, but I'd look at the wheel first. It could be a combination of the two.

Bonwit · Sep 24, 2009

Thanks for the tip. I'll start with wheel balance and let you know how it turns out.

Anglophile · May 2, 2011

Bonwit said:
...and let you know how it turns out.

So, how did it turn out? I'm getting an occasional hop as well.

Thanks,

Hobot · May 2, 2011

Yes when is other shoe going to hit??

I'd check under tank and around headsteady for rubbing. Inspect cradle
around bolt holes. Check tach drive. Fork fluid levels. LIft front on center stand
and very carefully shift it side/side, fro/aft/ and twisting. Be sensitive here
to the merest slackness as many indicate slop in the bushes or stem bearings.
Also tug on drive chain so it flaps abit and see if it tags on swing arm to front iso mount slack. Check not sticks or pebble grit in the iso gaps. Puck spokes for solid tones. Play with tire pressure, especially the difference front to rear. Front should be lower 2-3 lbs normal, but you ain't got normal symptoms and who knows what can buzz one Cdo vs another or even the same Cdo one time or another, ugh.

Its not a full fettered Cdo if its not a smooth flyer. I think wooden piano's might be more straight forward to tune out buzz as it shifts on each string tensioned.

Its possible to have a electrical grimlin cycle like this too, check after dark as ya never know and can't forget brake light wires have done this to me and others. Is tag and fender nailed down good? Chain guard >>>