head shake over a bump

[[email protected]]

......or natural frequency (critical speed) excitation of the front end, caused by the frequencies generated by the front tire and wheel at 30-50kph. The two collide, and the poorly dampened front end shakes uncontrollably, until the exciting force is removed (slower than 30, or higher than 50 kph)
Which is why the Furbur fix works, in some cases. removal of the rubber washer changes the natural frequency of the front end (mainly the forks and trees), so that it is not excited by the lower-speed revolutions of the front wheel assembly.
Just a guess....
Cheers!

unk:

[/QUOTE]

Interesting perspective. I guess its possible that at a certain MPH the frequency can change to a point that the resonance frequency is reached and the handlebars will oscillate with extreme amplitude. I would think that there are many factors that could introduce a change in frequency. Installing the furber fix could be one, just as much as a new tire, or bearing adjustment.

:ummm:

 

[DanQuin]

In my opinion head shake comes from several different factors, and often people over look the routine in search of the grander issues. On sportbikes the common response to any of it is "get a steering damper." Nice to see actual discussion on it, rather than the general put a band aid on it type response.

Things to consider:

• tire condition - is it unevenly worn or cupped? pressure correct?
• wheel - straight; bearings in good condition; balanced
• brakes - rotors straight; calipers free
• fork oil - reasonable service life; fork levels even
• triple trees - all bolts tight
• steering head bearings - in good condition and properly torqued
• fork air pressure - if you are using air for preload you need to ensure the pressure is even

What I see most often on bikes is oil that has been in use for many years. The oil should be changed periodically with fresh oil and ensure the levels are even.

Hope this helps some. I've noticed that my VMax likes to get shaky when my tire pressure drops low. I'll be installing some stiffer, straight rate springs and valve emulators, and then tuning the whole front end this season, so hopefully I'll be able to wave at other riders soon :biglaugh:

 

[Miles Long]

Interesting perspective. I guess its possible that at a certain MPH the frequency can change to a point that the resonance requency is reached and the handlebars will oscillate with extreme amplitude. I would think that there are many factors that could introduce a change in frequency. Installing the furber fix could be one, just as much as a new tire, or bearing adjustment.

:ummm:[/QUOTE]
My thoughts come about due to my own experience with the low-speed wobble, and the conditions countless other riders have described - that the wobble only happens during a narrow speed zone(in my case 30-50kph), and that is not dependent on gear selection. Therefore the excitation is based on the rotational frequency of the wheel.
Another reason why the Furbur fix works is perhaps not because of a change in the natural frequency of the fork assembly, but perhaps because it merely adds more dampening to the assembly...thereby reducing the oscillation of the forks and handlebars as the wheel goes through it's critical speed.
I'm sure a Vibration Analyst with a few basic instruments - an FFT meter, an impact hammer, a couple tri-axial accelerometers - could prove or disprove this theory. "Ringing" the front end with the hammer to obtain the natural frequency would be the easy part. Obtaining the cpm of the front wheel at 30-50 kph would be a tad more involved, but still possible with the bike itself stationary. The difficult part would be mounting the FFT meter on the bike, hooked to the pickups, and going for a spin to capture and store the data. This person would have to be a hard-core bike enthusiast, or partially drunk. Think any of your colleagues fit this bill, Mike ???

 

[[email protected]]

This person would have to be a hard-core bike enthusiast, or partially drunk. Think any of your colleagues fit this bill, Mike ???[/QUOTE]

I'm sure we have a guy or two that could fit into both of those categories.

We don't have any test equipment that will measure mechanical frequency. We do have quite a few pieces of gear for analyzing spectrum, intermodulation, power transfer efficiency, etc.. But this is all relative to radio frequencies.

 

[VMAX Eddie]

I was getting the deceleration shakes mostly at higher speeds and thought was probably front tire. Tire was actually worn bumpy. But was also seeing a slight wobble if I stood up on the pegs and watched the front wheel.

Changed tires and didn't get any better, maybe worse. Did the Furber Fix and I think I got it too tight, was getting the drifting side to side. Been loosening it slowly but still get some.

Worse yet at around 95 the front end vibrates like an SOB, not horrid but bad enough. Then if I decelerate from anything over 85 I get severe drifting. Scared the shit out of me the other day. Was in a line of 3 cars hauling ass on the interstate, around 115, and decel'd, bout shit my pants when it started drifting really bad left to right and back. Had to hammer the gas to get it to stop.

Makes me wonder if the guys at the bike shop actually put the dyna beads in, I got a bent rim/axel/something, or the Furber Fix screwed something else up. It's starting to piss me off!

 

[Biker Dash]

this

 

[soundguy]

If you want to know more about motorcycle dynamics have a read of this...there will be a test at the end. :confused2:

I suspect your friend was correct - inappropriate right hand jerking is known to cause (amongst other things) wobbling of handlebars and also blindness is done too often.

http://www.chromedoutmag.com/rake-trail-off-set-frame-geometry/tech-tips/

 

[jfeagins]

I was getting the deceleration shakes mostly at higher speeds and thought was probably front tire. Tire was actually worn bumpy. But was also seeing a slight wobble if I stood up on the pegs and watched the front wheel.

Changed tires and didn't get any better, maybe worse. Did the Furber Fix and I think I got it too tight, was getting the drifting side to side. Been loosening it slowly but still get some.

Worse yet at around 95 the front end vibrates like an SOB, not horrid but bad enough. Then if I decelerate from anything over 85 I get severe drifting. Scared the shit out of me the other day. Was in a line of 3 cars hauling ass on the interstate, around 115, and decel'd, bout shit my pants when it started drifting really bad left to right and back. Had to hammer the gas to get it to stop.

Makes me wonder if the guys at the bike shop actually put the dyna beads in, I got a bent rim/axel/something, or the Furber Fix screwed something else up. It's starting to piss me off!

Go take a seriously HARD look at your BACK TIRE! Get the bike up on a stand, start it, and put it in gear, so you can observe the runout as the tire is rotating. My bet is you have serious need of fresh, true rubber back there. If it's new, perhaps it isn't seated correctly. Perhaps the tire is bad right out of the factory? (I've had a Shinko like that) Also re-evaluate those front neck bearings for proper tightness & that there is not flat spot from prior over-tightening.
Won't hurt to lift the front end of the bike and double check for runout on the front tire as well. Chit happens. Bad tires happen.

 

[VMAX Eddie]

Go take a seriously HARD look at your BACK TIRE! Get the bike up on a stand, start it, and put it in gear, so you can observe the runout as the tire is rotating. My bet is you have serious need of fresh, true rubber back there. If it's new, perhaps it isn't seated correctly. Perhaps the tire is bad right out of the factory? (I've had a Shinko like that) Also re-evaluate those front neck bearings for proper tightness & that there is not flat spot from prior over-tightening.
Won't hurt to lift the front end of the bike and double check for runout on the front tire as well. Chit happens. Bad tires happen.

Wow wtf! Took your suggestion and there's a significant wobble in the tire. I checked the back rim and it's got a very slight wobble to it as well but not to explain the tire. Yes both front and rear tires are brand new Shinkos. Just had them mounted last week. I had some decel shakes with the Dunlops on it but the rest of the problems came with the new tires and Furber Fix.

Just ran it up to the bike shop that mounted the tires. Guy said it's not that uncommon for a tire to do that, really? At that point we noticed the weights were still on the bike, even though I had the dyna beads put in for the balancing. We pried them off and I went for a quick test ride. Got it up to about 105. The vibration at around 95 seems gone, or hardly noticeable. Slight decel wobble at around 85-90 but nothing at 105. I think I still have the bearing nuts too tight which isn't helping.

With the screw driver test showing a slight warp on the rim probably means I need new rims down the road right? I'm going to undo the Furber back to stock and dial in the bearing spanners and see what happens.

 

[jfeagins]

Wow wtf! Took your suggestion and there's a significant wobble in the tire. I checked the back rim and it's got a very slight wobble to it as well but not to explain the tire. Yes both front and rear tires are brand new Shinkos. Just had them mounted last week. I had some decel shakes with the Dunlops on it but the rest of the problems came with the new tires and Furber Fix.

Just ran it up to the bike shop that mounted the tires. Guy said it's not that uncommon for a tire to do that, really? At that point we noticed the weights were still on the bike, even though I had the dyna beads put in for the balancing. We pried them off and I went for a quick test ride. Got it up to about 105. The vibration at around 95 seems gone, or hardly noticeable. Slight decel wobble at around 85-90 but nothing at 105. I think I still have the bearing nuts too tight which isn't helping.

With the screw driver test showing a slight warp on the rim probably means I need new rims down the road right? I'm going to undo the Furber back to stock and dial in the bearing spanners and see what happens.

Another believer. :biglaugh: unk:

For some strange reason, most people WILL NOT consider a rear tire as potentially causing anything to do with a front end wobble.

If your tire seller has sold you a bad tire, they should be ordering you a new one by now. If not, maybe you can get it dismounted, then remounted to see if it isn't just a problem with how it beaded up. A new tire should have ZERO runout from side to side. A slightly used one, on the other hand, can sustain damage to one or more of it's internal cords, causing it's core to become out of round etc.... I've had Dunlops in years past that would develop internal cord problems due to all the horribly bumpy brick streets where I used to live. No external damage visible, just a side to side run out after a few hundred miles of use and/or abuse by me.

 

[VMAX Eddie]

I ordered the tire online and just took the wheels off to have them mounted. I might just do the whole thing myself next time!

I shot a quick video of the wobble. Also can hear the Vboost solenoid do it's chirp. That started since I replaced the battery with the more powerful one suggested on VMO. Doesn't always do it. Mostly if the bike has been setting. Anyway here's the tire wobble:

https://www.youtube.com/watch?v=chu8JZEFvfc

 

[jfeagins]

Yup. That'll do er!

Your tire shop seen that yet? Tell you it was normal?

If you were to do that again, and with the bike in 3rd gear or so, and give it a few more rpms, you'd probably see that tire try to wobble the entire bike from front to back as it sits there on the stand.

 

[VMAX Eddie]

When I showed the guy he brushed it off saying, "I'm not too concerned about that." He mentioned something about shaft drives are more likely to do that, blah blah. He checked the bead, which seems to be even. So either got a bad tire or the rim is bent. I took a milk crate and held a screwdriver in place sliding it up to the edge of the rim. The wobble is showing up there as well, but not at all like the tire is doing. Obviously any wobble in the rim would be accentuated in the tire.

That said you think I got a bad rim or tire? I can't say it was like that or not before I had the tires done. Can't imagine they bent the rim just swapping out the tire. If they pinched or pried too hard somehow I should be able to see some sort of damage to the rim I would think.

 

[Miles Long]

When I showed the guy he brushed it off saying, "I'm not too concerned about that." He mentioned something about shaft drives are more likely to do that, blah blah. He checked the bead, which seems to be even. So either got a bad tire or the rim is bent. I took a milk crate and held a screwdriver in place sliding it up to the edge of the rim. The wobble is showing up there as well, but not at all like the tire is doing. Obviously any wobble in the rim would be accentuated in the tire.

That said you think I got a bad rim or tire? I can't say it was like that or not before I had the tires done. Can't imagine they bent the rim just swapping out the tire. If they pinched or pried too hard somehow I should be able to see some sort of damage to the rim I would think.

Yamaha's wheel runout service limits ('85-2000 model years)
Axial - 2 mm Radial - 2mm
2001 - 2007
Axial - .5 mm radial - 1 mm
Mother Yamaha really tightened up her specs! Probably as a result of so many poor handling complaints??

Like you state, Eddie, any wheel runout will be accentuated in the tire.
However, you might be looking at a "worse case" scenario here.....perhaps the tire was mounted so that the maximum runout of the tire is in the same location of the max runout of the wheel. Just a thought.
Cheers!

 

[jfeagins]

I think it'd be fairly hard to bend a rim w/o leaving marks. I dunno. From the video, your rim isn't running out nearly enough (not visible in the vid at all) to cause that tire to be as far out as it is. I guess you know where NOT to buy any more tires in the future. Great to see your safety is of the utmost concern to him.

If reseating the tire don't improve it, you can either ride softly until it's used up, or bite the bullet for a better tire from SOME PLACE ELSE.

The Stinko I have on now is the exact same way, but didn't get like that until I had better than 500 miles on it. I figured I must have hit one too many chug holes and damaged a cord. Since it is now primarily just my to/from work bike, I figure to use it up, then replace it. But when I do replace it, I'm going back to the OEM Dunlop - because IMO it looks the best, and I'm nostalgic like that, plus, I've learned my lesson on Stinko's.

You still need to pay close attn. to your neck bearings to be sure to minimize any unwanted wobble that tire may want to cause. Over tighten, and you will have the slow speed weave, which is almost as irritating (to me) as the high speed wobble.

 

[VMAX Eddie]

Yamaha's wheel runout service limits ('85-2000 model years)
Axial - 2 mm Radial - 2mm
2001 - 2007
Axial - .5 mm radial - 1 mm
Mother Yamaha really tightened up her specs! Probably as a result of so many poor handling complaints??

Like you state, Eddie, any wheel runout will be accentuated in the tire.
However, you might be looking at a "worse case" scenario here.....perhaps the tire was mounted so that the maximum runout of the tire is in the same location of the max runout of the wheel. Just a thought.
Cheers!

That's an interesting thought, could very well be the case.

I think it'd be fairly hard to bend a rim w/o leaving marks. I dunno. From the video, your rim isn't running out nearly enough (not visible in the vid at all) to cause that tire to be as far out as it is. I guess you know where NOT to buy any more tires in the future. Great to see your safety is of the utmost concern to him.

If reseating the tire don't improve it, you can either ride softly until it's used up, or bite the bullet for a better tire from SOME PLACE ELSE.

The Stinko I have on now is the exact same way, but didn't get like that until I had better than 500 miles on it. I figured I must have hit one too many chug holes and damaged a cord. Since it is now primarily just my to/from work bike, I figure to use it up, then replace it. But when I do replace it, I'm going back to the OEM Dunlop - because IMO it looks the best, and I'm nostalgic like that, plus, I've learned my lesson on Stinko's.

You still need to pay close attn. to your neck bearings to be sure to minimize any unwanted wobble that tire may want to cause. Over tighten, and you will have the slow speed weave, which is almost as irritating (to me) as the high speed wobble.

Yeah I looked pretty close at the rim before doing the test. If it is out of whack it's nominal. I guess that's the easiest solution. Take the wheel back off and have them reseat it. I don't think it's something they did though, although the reaction was less than desirable. Just a bad tire which won't matter if they reseat unless the alignment is off like Miles suggested.

I like the look of the Qualifier the best as well. I couldn't pass over the price of the Shinko though. Seems like the Dunlop should be somewhat cheaper. Either of you use the alternate size tire for the rear? Like a 170/80 15 or whatever it is.

 

[jfeagins]

Until my most recent Stinko, I had gone 28 years always running the OEM Dunlop.