Another vibration thread.

[Josh Cox]

Have read the other vibration thread, still have a question:

At 2700 RPM ( plus and minus 100 RPM ), there is the standard parallel twin vibration on my Commando, how much of this vibration is the iscolastic expected to take up, it is less than my BSA, I have MKIII iscolastics fitted, does it take a while for them to soften up ?.

The vibration is at exactly 2700 in any gear, so it is the harmonic for this engine.

Cheers,

Josh

P.S. Jordan is a donkey raper.

 

[GRM 450]

Does the viberation go away after 2700? Or get worse? What sort of vibration is there below 2700?

 

[Diablouph]

by GRM 450 » Sun Jul 31, 2011 10:34 pm

Does the viberation go away after 2700? Or get worse? What sort of vibration is there below 2700?

Lord, look what it's done to his cajones.

 

[Josh Cox]

Only at 2700 rpm, otherwise smooth.

A vertical vibration.

I am suspecting that this is normal for the Commando, as this is the only Commando I have ridden, I do not know for sure.

 

[john robert bould]

Only at 2700 rpm, otherwise smooth.

A vertical vibration.

I am suspecting that this is normal for the Commando, as this is the only Commando I have ridden, I do not know for sure.

I have heard the EXTRA soft Iso's available ,give little support , Allowing the motor to jump about...

 

[frankdamp]

On the prototypes, there was a slight vibration at 800 rpms and it was completely vibration free from there up to red-line. My gut feel is that something is tightened up too hard or there's something on the engine/transmission cradle that touches the frame. That cradle is free to move vertically within the flexibility of the isolastics, restrained hoizontally by the polyurethane spacers. IIRC, there's about 5/8" vertical freedom about the iso's cetner bolt that gets progressively stiffer as you compress the iso's.

There really shouldn't be any vibration felt by the rider at any engine speed above 1000 rpm. Must admit, I haven't ridden a Commando since those first two.

 

[Hobot]

Frank my Ms Peel with 3 rod links and trimmed down front iso doughnuts flat disappeared before 2000 rpm and felt blunted nice throbs below that. Are you and I the only ones to have experienced the uncanny disappearing act a Commando is capable of?

I now ridden 6 750's and two 850's to get a sense of their wiggle jiggles and jossle tingles I don't like in them compared to Ms Peel. My fear is with 920 engine vibes may get through so have a crank with changeable balance factor to experiment with. I've got factory '72 Trixie with adjustable iso's to play with getting her to best smooth out for baseline comparison and some experimenting w/o rods.

 

[blacklav]

Hi Josh, you have the same problem as my mk11a has. The vibes come in at around 2600 - 2900. Above and below that there is no problem. I had commando 25 years ago without the vernier type isolastics, and though this was smooth, the engine was moving around in the frame a good half an inch. This one has no visible movement.
I have adjusted the isolastics to varying levels but they seem to make no difference.
If you found the solution please let me know.
Many thanks
Chris r

 

[DogT]

Josh, What kind of isos did you fit? I put on the original style Old Britts ones (supposedly the soft ones) and used the original shimmed collars set at .01". I get plenty of vibration up to 3k and then it starts smoothing out, at 4k gone, but at 45-50mph in 4th gear (19 tooth) it puts my hands to sleep in about 15 minutes and my feet off the pegs in about half an hour, 3rd gear in that speed is much better. Going down my gravel drive at 25 in 2nd gear, my eyballs are shaking in their sockets. My plan is to get the Hemmings adjusters this winter and see if I can loosen it up a bit.

I remember my original iso's were rather smooth throughout the range, but I never adjusted the Isos in 13K miles and when I took them apart, the PTFE or whatever it was washer was dust so there was plenty of clearance, I didn't compare the softness of the old ones with the new ones, they seemed so oblong as not to matter. Maybe it just needs a lot more than .01" play.

I get maybe 3mm of motion with the motor in the iso mounts at worst.

I do think it is smoothing out a bit with usage, or I'm getting used to it, I rather think the former.

Dave
69S

 

[rpatton]

The vibration is at exactly 2700 in any gear, so it is the harmonic for this engine.

I found that you can tell a lot about what's going on with vibration by putting you left hand across the gap between the primary cover and the side plate.
While you are tootling along and increasing the rpm's through that peak, you can feel whether the magnitude of engine vibe's increase and that's what you're feeling at the 2700rpm peak; or it might be that the magnitude of the shaking of the engine isn't increasing so much as the frame has come to some excitation frequency, or harmonic of its own and it is what's going ape-sh!t. The frame is pretty spindly, as frames go.

 

[GRM 450]

I've given up chasing the vibe through the foot pegs at just below 4000rpm. I changed the gearing so it now cruises below 4 and doesn't vibrate. Mk3, it doesn't vibrate at all until 4 and goes away at 5. It doesn't need to be up there for cruising. Good luck

 

[bmwbob]

" Vibration up to 3000 RPM - Rear tight, Front OK
Vibration 3000-5000 RPM - Front tight, Rear OK
Vibration up to 5000 RPM - Both tight"

 

[Hobot]

Gosh these things are like tuning an old wood frame piano! Not much to add but say thanks for helping me place Trixie in isolation pecking order. Trixie is 19T and behaved somewhat like DogT's, shaked to go slow enough to cross drive way Gravel and buzzed pegs and bars to annoy up to 50's, until I brutally got her tweaked head steady set to neutral strain on rubbers at rest. Now she isolated nicely in upper 2000 rpm zone, and what does get through is on similar level as road texture and very blunted. i know there's more to gain as Trixie before a few blow ups was about as good as any factory C'do. It it don't isolate for most riding conditions might as well get a Turnip.

I guess you've gone around kicking and bumping items to hunt up clatters. Can't forget primary chain going bad might hum engine back into you. One sources of some of Trixie's slight tingles is I've not nailed down head light rim with full amount of clips. I suspect Trixie has a bit tight front iso but brake hose now swells too much to even stop walking speed in garage so delayed to experiment again.

 

[frankdamp]

Of course, the prototypes only had the single top tube. It was long before the front brake got srong enough to cause the structural cracking failure. It might be that the strengthened frames have completely different harmonics. Maybe I should keep quiet on this topic?

 

[aceaceca]

Mine has no vibration to speak of at any rpm. I am going to pay a little more attention next time out but what some of you are talking about does not exist on my bike. Nor did it of 3 others I have owned. You have too many widgets on the thing or everything is too tight.

 

[Hobot]

Mine has no vibration to speak of at any rpm. I am going to pay a little more attention next time out but what some of you are talking about does not exist on my bike. Nor did it of 3 others I have owned. You have too many widgets on the thing or everything is too tight.

That is the normal expected state and I've had it on two of my Combats prior so know what I'm missing out at I creep back to it on Trixie Combat. Its just more common that annoyance gets through to frame and pilot. My bench mark is tri-rod Peel I swear flat disappeared from my sensation but for road texture and shove by turbo fan smooth thrust.

 

[DogT]

aceaceca,

I was really surprised at the vibration after rebuilding my 69. However, I have to say the iso's were very old, even the last time I rode it and the washers were dust, so the clearance was probably well over .03 or approaching .1". Granted everything now is tight, the engine is not vibrating in the iso's enough to let the exhaust beat against the rear iso nut and wear a flat on it like before, plus the header to silencer bracket wearing a hole in the left side f/g panel (S type exhaust).

I'm thinking there's a trade off between tight tolerances with tight handling and loose tolerances with little vibration and a loose rear end/front end iso tolerance. But who am I to know? I do know that even when I didn't feel the vibration on the old bike, things were shaking like crazy and beating things up, the shields on the S exhaust rattled off in about 4K miles even if I didn't feel the vibrations.

Dave
69S

 

[Hobot]

I'm thinking there's a trade off between tight tolerances with tight handling and loose tolerances with little vibration and a loose rear end/front end iso tolerance. But who am I to know? I do know that even when I didn't feel the vibration on the old bike, things were shaking like crazy and beating things up, the shields on the S exhaust rattled off in about 4K miles even if I didn't feel the vibrations.

Duh yep and the oil decayed isolastics I ran into gave delightful isolation but hinged in sweepers too much to enjoy staying on power, ugh. When ya get tired of incremental improvements try a rump rod and its two lessor vibration and handling helpers. Also higher tire pressure-firmer tires can be felt as earlier and more isolation I've found recently on Trixie. I remind the that they had to pay police and racers to run iso gaps half of factory specs. ugh.

 

[blacklav]

Hi Josh , have you sorted the vibes out yet.
Its just a thought but have you checked your compression on both cylinders. Mine has the same vibe problem and i am about to remove the top end this winter as the left side is only 75 compared to 97 on the right.
I dont know what the correct values of compression should be, there is no reference to them in the manual.
This imbalance may be the cause.
Will let you know when its done.
Chris r

 

[Josh Cox]

Chris, that sounds really low, to you have the throttle full open when doing the compression check ?.

Believe it or not it makes a massive difference.

Vibration, funny enough I will be readjusting the top isolastic today, which is what the problem is believed to be.