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Old 11-07-2012, 06:23 AM   #1923
gtbensley
Gnarly Adventurer
 
Joined: Jul 2010
Location: Kittery Maine
Oddometer: 251
Quote:
Originally Posted by DFH View Post
Is this wobble through the handlebars when decelerating & when he has one or both hands off the handlebars? Both the front & rear wheels on motorcycles have a natural resonant flutter frequency, like a supermarket trolly wheel that flaps like crazy at certain speeds. Its easy to envisage the front wheel flutter rotating about the axis of the steering head, but also the rear wheel can do the same thing rotating around the steering head, but as the distance from the steering head to the rear wheel contact point is much greater the rear wheels flutter frequency is lower. The tricky part is when the natural flutter frequency of the front wheel becomes a harmonic of the rear wheels frequency. As the mass of the rear wheel & its lever ( basically all the motorcycle from the stem rearwards) has considerably more inertia the rear wheels attempt to flutter excites the front wheel into flutter mode. All bikes do this, but the road speed at which this occurs can be modified by design. The usual option is to design this flutter harmonic point to occur at low road speeds (30 to 45 mph for those that like feudal measures) and use the rider as an organic steering damper. Coast down through this speed range on any brand new, bog stock bike with both hands off the bars & you will spot the build & decay of harmonic steering flutter

As I said before all bikes have a point of harmonic flutter, so why is this a problem for your friend now & not before?

Things that will make the flutter point more obvious include tyres worn flat in the middle from too much slabbing, adding weight to the rear of the bike especially rigid hard panniers & double especially a loaded top box, rear suspension with too much sag, too low tyre pressures.

So start with the obvious. tell him to ride with both hands on the bars... death grip not necessary.
Cast a critical eye over how much crap he is carrying on the bike... or post a piccy so we can all waggle a finger at him.
Set tyre pressures @ 36psi front 42 psi rear from cold
Set preload so the bike sags 35 mm @ the rear with rider & all luggage installed. My guess a 12 year old VFR with stock shock & spring this will be impossible, but let us know how much sag their is now.
Check condition of tyres. Never ridden on Shinkos but any tyre worn flat in the centre from slabbing will cause problems long before its at legal limits.

Good luck & get back to us with how its going.
Thank you for the response.

The wobble is there when he rides with both hands on the bars, it can be while he accelerates, holds a steady speed or starts to slow. Yes there is a lot of crap on the bike and that's what I figured was causing it initially. We removed all the gear though and it's still there.

We have already set the tire pressure to what you suggest as we'll as looking for flat spots on the tire. Sag is all the way up.....but there is still probably more like 42mm of sag.

Lose head bearing maybe? What about raising the foks 1-2mm to see if that changes anything? We will be in San Diego until Friday morning and hope to solve it by then.
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