shudder at 4800 RPM

Finished most all of the needy things with the bike today and took it for a 100 long mi evaluation.

The Good: Between rural and 70+ interstate speeds I got exactly 50MPG. The new front brakes & SS braid line was sweet.

No Bad but I have an observation; at 4,800 RPM or thereabouts, there's a vibration I feel in the chassis, not the handlebars. I do not feel it below or above roughly that RPM. I've eliminated the tire balance as the cause and going downhill at that RPM with the clutch in & it goes away. Now that I'm looking for it, I feel it happening in each gear at that RPM. When I get to 5K RPM, it goes completely away & it's not noticeable till 4800. Probably covers a span of 150 or so RPM. Bike feels like an electric motor above & below that.

Any idears?

Thanks!

 

I should add the shudder is not a major thing, it's just reproducible and only there at this area of RPM.

 

Sounds like normal to me. There is a small band at which you will feel vibration. I get mine right around the 54 -58 mph range. I can tell when I am just about to 55 w/o looking at the speedo, cuz the vibe starts and when I am just about 60 it disappears. It's not big, loud, hard, or anything. It's just 'there'. It's done that since the day I got it 9 years ago.
When I'm on the highway, in 5th gear, I'm just around 4800 rpm at 60 mph if I remember correctly. I'll try to remember to write things down and tell you specifics when we talk on the phone again.

Yup, 50mpg is about the right area. I get anywhere from 45 -53 depending on wind. Overall average for me is 47 when I'm good, 45 when I'm horsing it.

ttyl

 

I have the same issue with my 650. Right around 4500 it starts to vibrate and then goes away at 5000. Kind of an inconvenient place for highway riding. I've tried a few things to eliminate it but have come up short. My 750 doesn't exhibit that at all. I have another 650 I'm rebuilding -- it will be interesting to see if it does the same thing (Seca engine though).

 

Sounds to me like the way to eliminate it on the highway is to go faster :roll: My 750 Seca pegs the speedo at 6k in 5th...just try that

 

Yeah, I kind of felt the same, it's right where I'll probably want to drive speed wise so the option is to go a bit faster as slower isn't great when people are going to keep trying to pass you.

It almost seems as if there's a series resonance and whatever causes it happens at that same RPM. That's three of us feeling it on the same bikes so there might be something as a feature. Be interesting to see if more have noticed this.

 

check your rubber engine mount bushings........

 

every material has a resonant frequency. When an external periodic frequency matches this materials natural structural (resonant) frequency, occilation occurs. Say for example an external periodic frequency of 100 hertz (equal to 6000 rpm from an engine) and natural structural frequency of a metal (your frame or casing or a component etc.) being 100 hertz, you get occilation/vibration. This is why the tacoma narrows bridge collapsed, you can tune your guitar through harmonics from another string, and why a tuning fork sounds when its note is sounded.