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The science of vibration.

  • Thread starter Thread starter spyug
  • Start date Start date
S

spyug

Guest
I'll start off by saying I didn't do well in physics but I have a bit of a sense of how vibrations can travel. It seems that I am experiencing just that with the mirrors on my FZ as they vibrate so much all I can see is a blur.

In their stock position, they are mounted on the fairing and are bolted to the metal "birdcage" underneath which is in turn bolted to the frame. They are too inboard to give anything more than a picture of my arms and elbows so I built and installed extensions which push them out about 2.5" each side:
mount3_zpse92998cd.jpg


Now the base of the mirrors are bolted directly to the extensions with nothing between them. I now wonder if a piece of 1/4" dense rubber between them might eliminate or at least cut down some of the vibration?

I know rubber isolators work in a lot of applications to dampen vibes. Anyone have thoughts on using something like that here. Would I also have to do the same to the fairing end do you think?

Its a fair bit of work to bodge up so any and all thoughts would be of interest.

good friend SVSooke suggests bar ends and I do have some I could try but I'd like to see if the stockers can be made to work.

Cheers folks.

spyug
 
Just curious....

Were the mirrors vibrating so much prior to you adding your extensions ? or were they vibration free ?

Does the fairing cage have all of it's rubber grommets in place, are any missing, perhaps put some in where it bolts to the frame ?

.
 
Well I think the mirrors in the stock position were essential vibration free but there were no grommets, just a 1/32" rubber pad under the mirror mount. You can see it still in position under the extension in the pic.

The more I think on it, It might be best to put a thicker pad in this location first.

Its going to look like poo but if it works I'd be happy.

thanks.
 
In this case, I'd suspect that the mirror extension is adding some "floppiness" to the mirror assembly as a whole, meaning the vibrations from the bike are getting amplified by the mirror. You have nothing to lose by trying to add some rubber between the mirror and the mount, but I think that would only make it worse.

This is only a guess, but I think your best bet is to use thicker and/or harder steel for the mirror extension. Or brace it up somehow to make it more rigid.
 
This is only a guess, but I think your best bet is to use thicker and/or harder steel for the mirror extension
Click to expand...

Well the extensions are pretty rigid and bolted tight both at the fairing end and mirror base. I was actually quite shocked to see that they didn't work as I've done similar on my GSs and a couple of Yamahas before. I've never experienced a shake, never mind a chronic vibration. I don't think the extensions vibrate themselves, I figure its got to be at the connection points so I don't think trying to stiffen them would make any difference.

Something to keep in mind anyhoo.

Thanks,
Spyug
 
eil said:
In this case, I'd suspect that the mirror extension is adding some "floppiness" to the mirror assembly as a whole, meaning the vibrations from the bike are getting amplified by the mirror. You have nothing to lose by trying to add some rubber between the mirror and the mount, but I think that would only make it worse.

This is only a guess, but I think your best bet is to use thicker and/or harder steel for the mirror extension. Or brace it up somehow to make it more rigid.
Click to expand...

I think this is right on. That flat mount is just like a flexible spring with no damping and so it is what is known as a "high Q" circuit meaning it amplifies the specific natural mode (i.e frequency).

It is possible to dampen but not by mounting in rubber. Best tip is as described ; stiffen the mount and reduce the amount of flex. If that is not enough the final mount should be a mirror isolator which is still available from Suzuki or I have found cheaper replacements on ebay.
 
As a test, try adding a bunch of weight out on the end of the mirror someplace to alter the resonant frequency it wants to vibrate at. Slap a big tire weight on there with a hose clamp or something. When you find out exactly how much weight you need to can find a more elegant solution...
 
spyug said:
Well the extensions are pretty rigid and bolted tight both at the fairing end and mirror base. I was actually quite shocked to see that they didn't work as I've done similar on my GSs and a couple of Yamahas before. I've never experienced a shake, never mind a chronic vibration. I don't think the extensions vibrate themselves, I figure its got to be at the connection points so I don't think trying to stiffen them would make any difference.

Something to keep in mind anyhoo.

Thanks,
Spyug
Click to expand...

can you pluck the mirror? In other words, pull against the mount and have it vibrate or just go thud. If it vibrates then it will shake. A thud means it is damped and should not be High Q and have as much amplification.

The simple test would be to double up the plates which will triple the stiffness and see how much reduction there is. No need to weld them together the bolts are probably good enough for the test.
 
Agree that the extension is adding to the problem. A thicker bracket would likely help. Some sort of rubber isolator would also likely help. You don't want to pinch the rubber too hard though, you need to allow it to float a little sort of like the Suzuki mirror dampers.
 
You don't want to pinch the rubber too hard though, you need to allow it to float a little sort of like the Suzuki mirror dampers.
Click to expand...

This what I was thinking /hoping.

The idea of adding weight to the mount also makes me think. As I said I was a bit disappointed when the mounts didn't perform. I had intended to use aluminum but I figured steel would be stiffer with less flex. I wonder now if aluminum of a similar thickness would move at a different frequency that might not vibrate as much or slow enough that I don't get the blur?

I think I'll try the rubber isolator first and see if I get any joy.

spyug
 
Just give the bike more throttle, and everything will stay behind you....

Problem solved, you won't even need mirrors......

:D
 
Just give the bike more throttle, and everything will stay behind you....
Click to expand...
Sounds good in theory but in my experience the faster you go the more you attract folks to go fast with you......usually the wrong folks as in "cops".

Seriously though, I am just not comfortable if I can't see behind me. I want to see whose rolling up as I'm coming to stop and I need to know who could be coming up on my quarter as I plan on lane changing.

I've nearly been pranged in both scenarios and it is scary. I'm big on situational awareness and mirrors are important for that in my mind.

I'm going to knock off now and try and fab up some "isolators".

Stay tuned to see how they work.
 
spyug said:
This what I was thinking /hoping.

The idea of adding weight to the mount also makes me think. As I said I was a bit disappointed when the mounts didn't perform. I had intended to use aluminum but I figured steel would be stiffer with less flex. I wonder now if aluminum of a similar thickness would move at a different frequency that might not vibrate as much or slow enough that I don't get the blur?

I think I'll try the rubber isolator first and see if I get any joy.

spyug
Click to expand...

stiffness is proportional to the cube of the thickness (or cross sectional area cross the bending moment), so while steel may be stiffer than aluminum adding some thickness quickly overcomes that disadvantage while still weighing less which only goes up in proportion to the thickness.
 
Regarding the bar ends mentioned in the original post. Mine vibrate...I do however have non stock, and likely improperly weighted bars. I did reduce it by adding a very thin rubber strip at the steel tube of the handlebar/aluminum mount connection, and mounting the mirror so its metal made no direct contact with the bars.
 
Aluminum is not good when repeatedly "flexed". It will crack & fail suddenly.

That's to say if you move to AL make sure your mount is thick enough that it can't flex.

I think you have 2 components going on here. 1 is the "floppiness" of the mount amplifying the vibration & the second is that you are probably hitting a resonant frequency.
The latter will probably be noticeably worse at certain RPM's (but might be masked by the first).

:)
 
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