Mk3 Strange Vibration, an Iso question

Hello,
A while ago I asked about a strange vibration that appeared through the foot peg at 4000rpm and came on suddenly like a light switch.
Thank you all for your suggestions as to what was the cause.
The vibe has disapeared after I pulled the rear wheel apart and found no problems and tightened every thing else. I still thing it had something to do with the primary or drive chain, but I don't know.

Anyway it's gone.

The bike runs well and is quite smooth apart from a handle bar vibration at 3800 rpm and higher. It's not uncomfortable on rides around a couple of hundred kms or miles but does get tiring on longer rides.

I've tried loostening and tightening the front and rear isolastics and thightening and loosening the head steady but it seems the best I can get it is to have the isos 1/2 to 2 holes backed off from tight.
And the head steady about 3/8 inch off tightest.

The bars are fairly thin wall UK style bars which are a lot thinner wall that the Western bars that were on the bike when I bought it, but they were bent and I didn't ride with them.
I've fitted Throttlemiester cruise control which are like bar end weights which I thought may help dull the vibes a bit but they had little effect.

Any suggestions?

Regards Graeme

I s there anyting to be gained by adjusting the front and rear at different clearences ?

I've set the headsteady at the recomended specs, then adjusted it tighter with little noticeable difference, I assume the springs have streched after all these years ?

Graeme, I have the same higher frequency vibes at 4000 +. Never got to the bottom of it and like you checked everything for touching where shouldn't. It feels like the buzz you get on a modern 4 cyl. I have clip-ons and don't feel it through the bars for some reason but I do through the rearsets and especially the gear lever when it's depressed for imaginary 5th when in 4th. Do you notice that? A very coarse vibe through lever? ISO's are set at 010" minimum, anything less and it's far worse. That's with squared off ISO tubes etc. for best attempt at even gaps all round.

Keith, no vibes through the gear selector as you described. I would think your bike would have a different frequency of vibration than mine as your bars are different and your wieght is in different places.
I know from riding singles that a vibration can be moved by changing something.
How do I move the vibration from the bars to the rear pilion pegs.
It's not a high frequency vibe but maybe the longer (than your clip ons) bars are moving, or transfering, the same vibration to the bars ???
Does the head steady adjustment change this ?
Please keep in mind that I have not had years of experience with Commandos, this is my first one so, to me the iso and head steady adjustment is all a new learning thing.
graeme

I don't think isolastics can transmit high freq vibes up/dn. They can not and do not tolerate or absorb any side/side vibration. i detest the slight buzz on rigid mounted twins or 4's after 20 min. Even if primary and secondary balance perfect the valve train buzz gets through. yukky and weary making.

On my Combat the hi buzz in bars and sorta in pegs, was always by a horizontal imbalance in engine components or firing issues, fuel or electrical.
ANY Contact physically between frame and engine/cradle/mount, will feel instantly like whole bike coming apart by broke rod or crankshaft, tail bone to teeth roots. Rarely do I hear a report like that and I only had em because my skid plate could trap tiny pebbles jammed between frame and mount.

I've never had much luck adjusting iso's for comfort and handling if there was deeper issue playing on them. i got rather depressed at times, adjusting iso's so loose the handling got sloppy, yet if near .01" it vibed to annoy.
You have already altered bar mass and felt no change, so my guess in more engine related. That could play out in the primary chain resonance noticed. Hope we learn something by yo'al's reports once its ID's or solved. Likely more than one cause, creating similar symptoms though.

The absolute most pleasing thing on balanced Norton with full rods is the uncanny massive inertial smoothness sense. It can get so good that w/o the wind and noise you feel like a naked baby held in palm of God's shoving hand.

Does the head steady adjustment change this ?

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Not certain but I made a rigid, rod end steady and it improved things to a point where vibes are not so bad. Though the Norvil type ISO head steady is reputed to increase vibes but I have no experience of one.

A while ago I asked about a strange vibration that appeared through the foot peg at 4000rpm and came on suddenly like a light switch.

Click to expand...

Graeme,
I had the exact same problem and in my case the speedo cable was nipped between the front ISO and frame. It disapeared when I re-routed it to the inside.

The vibe has disapeared after I pulled the rear wheel apart and found no problems and tightened every thing else

Click to expand...

Perhaps you fixed when you removed the rear wheel.

I reckon some speedo cables are a little short, mine has pulled apart before.

Cash

Speedo Cable trapped is a new one on me, thanks for the heads up on anything can cause anything in Isolastic Commandos. I've fixed a few things w/o knowing what I'd fixed by taking apart and back together. Once was stuck down kick start lever.
Pulled apart outter cover, but failed to notice the kick lever freed up on removing the foot peg support, put back together, no finding anything to fix, in mystery pensive state I'd hit again, then It hit me duh, and looked to see the slight kicker fouling the support, a slight support tweak solved till next time it hold bike off paint sliding a ways.

I think I found Trixie Combat hi freq cycles of buzz, the under spine tube is bent to the LH 3/8". I know when/how that happened. When deer hit head light in pure side ways impact- bike was lifted off tires to fly sideways as deer pelvis hit helmet to shove me down at ground to ricochet back in air like superman hands out, while bike hit back on tire to hi side way up and land on my extened L knee to crush is and tear-twist apart to put foot behind me back with bike on top.
The engine fins were what hit my knee and bent the under tube.

This is second time similar engine impact on knee bent head steady area.
Last was in June on Wes's '71 drum braking 30ish mph ever so slightly leaned hit invisible grit, front locked to slip out and land me on grass with bike on top. We saw no injury to the tank at the time but later Wes said he smelled gas and found head steady plate bent almost 90' and punctured the inside of fiber Hy Ryder tank.
Top of R knee cap had a dent in it the matched up with brunt in grease mark on blue jeans. His '71 vibed worse I've ever felt prior but no change at all riding back. It got smooth as normal by 45 mph in 3rd and 50+ in 4th. His did not transmit hi freq vibes, just the harsher engine rpm vibes at least up to 3000 rpm but his has no tach. I gave him Trixies' top kit and he was riding it next week.

Nothing pleases me more than a good quickly smoothing out Commando.
One of my favorite metaphors on Cdo's is
They feel like cable and fabric biplane on a grass field till lift off and flying.

These Commandos are all different it would seem to a numpty like me.
Mine, with isos set at 2 holes out from tight and the head steady with 3/8 inch gap before tightest, has a low frequency vibe to a little over 2000rpm then is silky smooth till 3800 and then there is a vibe through the bars as I mentioned.
I gave up measuring the isos as it is much easier just counting holes and suck and see.
I too have fixed things without finding what was wrong, this is very anoying not finding what the cause was.

I can't find anything rubbing or touching the frame,

Maybe I should try having the demons exorsised !

graeme

There can be demons infesting, but they can't stay infected when owner keeps trying this and that new ritual or scared object on it.

Hmm, most noticed thing with my head rod was the elimination of last bit of fork - bar vibes sense getting through. Front link tamed the jostle sense of road texture following. I'd play with the top steady some more and maybe trim rubber donuts rims beveled some. Something is either stiffing up to transmit hi freq or something is resonating a hi freq side to side. Check if can pick up chain links any out of rear sprocket valleys and inspect the teeth and chain links are free. Maybe a rod shell or bolt getting loose to blow out cases, sorry but its possible and part of breath holding while Commando riding. Might check in dual carbs set up for needle upset or even a chip out the slide. Could still be electrical grimlin. Chants and prayers you get relief.

Mr Hobot, I've been exceptionally nice to my Command of late in the hope of making it feel contented. (this works well with old Ducatis)
I'm planning a long ride in 2 weeks to Phillip Island to the Moto GP, so I've been checking and oiling and greasing and tightening and even polishing.
I removed the tank yesterday to check the wiring was happy in its installed position and not getting ready to rub through anywhere.
I lifted of the 3/4 full Interstate tank and as I walked away with it I felt fuel running down my leg. One of the front mount bolts had cracked. Luckly now instead of half way down the east coast somewhere.
So, empty the fuel, fill with water and braze it up. (Gremlins????)

I have checked the chains and sprockets for a tight spot on one of the chains, or a badly machined sprocket and all are good.

Carb is single 34 Mikuni, so no out of balance there.

Not sure I want to think about the rod bolt cinario.

I will play some more with the head steady, that standard Mk3 type, and report next week.

Thanks to all for your thoughts

graeme

Based on my experience with the protoypes, ANY vibration above 1200 rpm is a sign of a problem. It's possible that the various fixes for the frame weakness shown up by modern frornt brakes have compromised the smoothness of the original Isolastic concept.

I'm not kidding, guys, there was no detectable vibration at engine rpms above 1200 on the prototypes. After all the miles I rode to and from work on a 650SS, the Commando was bliss.

graeme, left me smiling at your loving attention and self preservation efforts.
Hope the other tank bolt holds up it share too. I tend to space out on what to do next when feeling vibration, as so many things transmit it. I was in denial for 5 yr now that my knee could have bent under tube, but found that it was last pm. Now hope I can whack er back in line enough to eliminate the buzz, and less strain to fit factory head steady.

Frank, enlightening revelation of 1200 rpm smoothness. I don't mind a bit the rattle and shake below 2000 rpm, just petting to the soul frequency, after that it sucks.

No one else has mentioned such low rpm iso functioning, not even the history of iso development where they kept cutting rubber area down in three stages from 6000 rpm nullification to 2200. Wonder if prototypes had softer cushions. Insights invited.

frankdamp said:

Based on my experience with the protoypes, ANY vibration above 1200 rpm is a sign of a problem. It's possible that the various fixes for the frame weakness shown up by modern frornt brakes have compromised the smoothness of the original Isolastic concept.

I'm not kidding, guys, there was no detectable vibration at engine rpms above 1200 on the prototypes. After all the miles I rode to and from work on a 650SS, the Commando was bliss.

Click to expand...

So why were the prototypes so smooth and how were they set up?

Maybe I'm chasing something I read about Commandos' "silky smooth" and "magic carpet ride" that doesn't exist ???
I'm used to Ducati twins (and singles) and the twins are smooth "ish"
The Commando is a 360 degree twin that (to me) makes sense that it would vibrate like a big single ???
Mine is exceptionally smooth from about 2k revs to 3800 revs, then the bars buzz at a low to mid frequency. I can push on the bars and stop the vibe, and no the bars aren't loose.

Now Frank says that they should be vibration less.

I have loosened the Isos and the shake is terrible. So I have settled at 1 1/2 to 2 holes out from tight for the best handling and vibration balance.

These are very interesting things indeed.

graeme

I've had my Combat smoothing out in low 2000's. Heck yours even demo's how Cdo's should feel, in its window of null resonance.

I can push on the bars and stop the vibe, and no the bars aren't loose.

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Now there's a real clue. In the events I got vibes or buzz the harder I held bars the more I felt it, not less and for sure didn't stop the annoyance. Tire balance, tire wear, stem bearings slack or fork bushes? Lose fender brace or head light?
Sherlock Homes where are ya.

Hmm, I will get Inspector Clouseau on it !

New, tyres, wheel bearings, head stem bearings, fork bushes, etc.

The head light was loose, the fender was ok.

I'll be pissed off if all this is caused by a loose headlight !

Things that make you go Mmmmm !

When I said the vibe goes away by pushing on the bars, I don't mean by pushing lightly, but pushing hard. Can't do that for long.

graeme

GRM 450 said:

I lifted of the 3/4 full Interstate tank and as I walked away with it I felt fuel running down my leg. One of the front mount bolts had cracked. Luckly now instead of half way down the east coast somewhere.
So, empty the fuel, fill with water and braze it up. (Gremlins????)

Click to expand...

Is there any indication that the stud or bolt was hitting the ring on the frame mount where the rubber washers go? It is possible for the tank to shift fore and aft allowing contact that you might feel as vibration in the bars. Is there any sign that the tank is hitting anywhere at all? It may be a coincidence, but things hardly ever crack by themselves.

Mr bpatton, The tank and frame were painted and all the paint is still there. I did look for that.
The bolt appears to have a flat head and is poked through from the inside of the tank, there is a washer over the bolt from the outside and then it appears to be spot welded in place.
The tank was cracked around the washer for 1/2 its' circumfrence.
The other side bolt has been brazed or welded before my ownership.
I have all (and more) of the rubbers so the tank doesn't contact the frame directly.

This leaked as soon as I lifted the tank up, so the rubbers must have been pressed hard enough to stop the leak.
Could have been a real bummer if the bolt had let go on a ride with a full Interstate tank, almost directly over the header pipe.

Keep in mind, from what I've found in rebuilding this bike that it has had a very hard and neglected life in the past.

graeme

Bob may still have a point. if the tank bolt was only held by flexable tab of thin tank metal, that may have allowed it to vibrate into the over-compressed to almost solid rubber which may transmit to chassis. Maybe you got lucky and traded fire of fuel spill crisis for merely annoying list educating mid range buzzes.
Assume nothing, examine check everything, throw nothing away till a few gas tanks proves a new item ain't worse than the old.

I have had a buzzy-rumble occur when cargo straps or floppy item got to flapping
in the breeze against frame. I'd had it with primary chain flop tugging on drive chain tugging on isolastics, d/t worn sleeve shaft bushes play for clutch wobble.