Balance Factor

[Big Sid]

Sid here . In your description this vibration came on suddenly as a new condition , earlier having been smoother , now peaks at 70 mph and then fades away at 80 . And the tank is seen to be moving in concert with the shakes .
My first guess is that some hardware beneath the tank , ie head brackets , through bolts or related , tank bolt etc has backed off and is being excited . Remove the tank and check everything uniting the top member to the heads front to rear . Ck. also the swing arm pivot nuts and foot rest HW . Check the steering head for looseness .
What balance factor was used in the bottom end when built up ? This affects where the shakes are most perceptible , at low or higher speeds . But my guess is that something has come loose which is intimate with mass and is allowing a harmonic to excite it .
Particularly as its recently chimed in where before it wasn't noticeable .

 

[willabilla998]

Thanks for the reply Sid. Apologies, forgot to mention, Bill Jean built this motor for my father. I am not aware what balance factor was used but you probably are. I'll check your recommendations in the next week or so and see what I find. I plan to take this bike on an extensive long distance tour this summer so I'm trying to get a jump on things now.

 

[riptragle1953]

My question would be: how suddenly? Meaning, was there no previous hint of a change. Or was it like all of a sudden, there it was.

 

[willabilla998]

Its hard to say rip. Best I could tell you is within a span of no more than 500 miles. I ride the bike several times per month mostly short days. then the occasional 500-700 mile weekend will get thrown in. On the interstates here 70-80 mph is not uncommon at all. It seemed to creep in around the same time of some minor top end work (replacing seals under the valve caps) and a tank repair. I think this might lend some possibility to some hardware loosening up.

 

[riptragle1953]

This is rather an important point....could it be the the vibration was there all along, or,at some point did it become quite noticeable. The same result at all approximate engine gearing speeds is a big clue too.
The reason I ask is generally a part becoming loose increases the effect as it loosens..... if all of a sudden something has gone outtawhack... this indicates a component failure.

 

[willabilla998]

It is possible that it was there and just not noticed. The tank had been removed to repair a crack at the tail end. I have heard this can be caused by vibration which would indicate that it had been there before. Is it normal for "component failure" based vibration to disappear completely by pushing passed it by 10mph. I don't know what RPM we are talking about in fourth gear at these speeds.

 

[riptragle1953]

It is impossible to make a Vin dead smooth at all RPMs as this factor is a compromise to give you the smoothness at the most used RPM range. So, it stands to reason that there are going to be other RPMs not so smooth. Oddly, the machines I built also had
80 mph in the sweet spot. To me it does not sound like you have trouble. Check out what bothers you vibrating in tune with the frequency of that exact range and do a little adjusting or remounting of those components. Crazy, but handlebars and vibration frequency
can modified with Gorilla glue which weighs nothing.... plug on end.... wet the inside... and pour in a bunch of Gorilla Glue. it will expand a lot as it turns into foam..... let it do it's thing and cut off the mess at the open upper end when dry. Amazing.

 

[willabilla998]

Thanks for all the help rip. I wonder if that "sweet spot means it is better for the motor to run at 80mph than it is at 70mph. Hmmm.
interesting about the gorilla glue! Wonder if Great Stuff would have the same effect.
Thanks Again!

 

[riptragle1953]

I don't know what great stuff is...but if it fills the bars it will certainly change their frequency response. I believe, if you give the engine what it wants... it has to be better. Ever notice on a Shadow clock that the needle points dead ahead ?

 

[timetraveller]

In posting 34 above you tell us that you have replaced some valve cap seals. Some of these are thinner than the originals and it is possible that there is a small collision taking place between the inside of the valve cap and the top of the valve spring. If that is the case then one of the valves will not be seating properly so have you checked the compression on both cylinders?