Isolastic vibration or something else?

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fltri1550

I have a 74 commando with the shimmed type isolastics. Both front and back are set to 10 thou as per the manual but I am getting serious vibration up to 3000rpm which deminishes a little at 4000 but then builds to a foot tingling annoyance at 6000. I've replaced the front iso rubbers from different sources and have been through the other potential vibration culprits like valve adjustment, ignition timing, primary chain adjustment and carb balancing. I'm now at a loss as to what to do next.
What else could be causing this vibration?
 
Check for isolated vibrating parts not being isolated eg engine touching the frame etc. Does not sound right at all, even running with 6 thou front and rear my vibration clears by 2500 rpm.
 
With a properly setup Commando you should feel no vibration above 3000 rpm. If isolastics are setup correctly and headsteady is intact (they fracture) I would be inclined to look for a crack in the engine mountings, either frame or cradle. Check the badge of your bike, you may be riding a Triumph.
 
fltri1550 said:
I have a 74 commando with the shimmed type isolastics. Both front and back are set to 10 thou as per the manual but I am getting serious vibration up to 3000rpm which deminishes a little at 4000 but then builds to a foot tingling annoyance at 6000. I've replaced the front iso rubbers from different sources and have been through the other potential vibration culprits like valve adjustment, ignition timing, primary chain adjustment and carb balancing. I'm now at a loss as to what to do next.
What else could be causing this vibration?
Click to expand...
I have had just about every issue that can cause vibration on a Commando at one time or another, excluding problems with the iso's themselves:
Clapped out centerstand bushes, loose sidestand bolthole
Rotted rubber pads under the gastank
Loose swingarm
Stem bearings on the way out
Maybe there are some more. What am I saying? The list is probably a long one. The centerstand was the worst. One thing you could try if you are willing to take a risk. Try riding the bike without a helmet. Empty mall parking lots are good for this. Some of the causes make distinct noises. Does the engine hop around a lot? How old are the rear iso rubbers?
 
Peel back the rubber iso covers. Rotate the shim carriers/thrust plates by hand. If you can rotate them freely, then they are o/k, if not, they are too tight. You can even get double vision if they are too tight.
Has the crankshaft been rebuilt?
 
bpatton said:
fltri1550 said:
I have a 74 commando with the shimmed type isolastics. Both front and back are set to 10 thou as per the manual but I am getting serious vibration up to 3000rpm which deminishes a little at 4000 but then builds to a foot tingling annoyance at 6000. I've replaced the front iso rubbers from different sources and have been through the other potential vibration culprits like valve adjustment, ignition timing, primary chain adjustment and carb balancing. I'm now at a loss as to what to do next.
What else could be causing this vibration?
Click to expand...
I have had just about every issue that can cause vibration on a Commando at one time or another, excluding problems with the iso's themselves:
Clapped out centerstand bushes, loose sidestand bolthole
Rotted rubber pads under the gastank
Loose swingarm
Stem bearings on the way out
Maybe there are some more. What am I saying? The list is probably a long one. The centerstand was the worst. One thing you could try if you are willing to take a risk. Try riding the bike without a helmet. Empty mall parking lots are good for this. Some of the causes make distinct noises. Does the engine hop around a lot? How old are the rear iso rubbers?
Click to expand...



Bob, I've checked most of what you refernece above but have never looked at the center stand. How did your center stand impact vibration and how did you fix it?
 
Flo said:
Peel back the rubber iso covers. Rotate the shim carriers/thrust plates by hand. If you can rotate them freely, then they are o/k, if not, they are too tight. You can even get double vision if they are too tight.
Has the crankshaft been rebuilt?
Click to expand...

Flo, I can move freely the 2 shim carries on the front mount. For the rear mount though I can move the left one OK but the right one can only be turned with effort. If I move the back wheel as per the manual to check shimming it can then be moved easily.

No crank rebuild. Other than this vibration the bike runs like a train.
 
I would love to say I found this out by myself, but I had some work done on my MK III and the mechanic told me about it. The mechanic found spacers behind the "Z" plates were missing. He replaced them, adjusted the Iso's and smoothed everything out quite nicely. May mean nothing, but it helped mine. He said, with the spacers missing, the engine cradle could pass on vibration to the frame.
 
Based on my experience with the prototypes, there should be no vibration that you can feel in the bars, footpegs or seat at anything above 1200 rpm.

This was the BIG selling point of the Commando over the other big parallel twins, even including Norton's own Atlas and P-11. The general feeling was that, on the Atlas, a headlight bulb lasted about 5000 miles and it didn't matter if it was ever switched on. Vibration shook the element off its posts. Certainly my ride-to-work 650SS would delaminate your kidneys in a couple of hundred miles. Fortunately, it was only 45 miles from home to work!

You may not be able to replicate the prototypes at the lower rpm range, but if you can feel anything above 2000 rpm, there's something wrong. Qualifier - I never rode the later models, having left N-V in May of 1968.
 
I had the same problem with the rear right side iso end cap being "stuck" and so could not feel free clearance there.

From the right side rear iso, I tapped the main through bolt to the left enough to be able to pull off everything including the cradle end cap.
I needed to use a file and sandpaper around the right quarter inch or so of the cradle removing burring, put the end cap on and it now rotates freely.
and because it does it also sits properly snug up against the cradle, which creates more clearance, and lessened vibration at the rear, foot pegs etc.

this may be your problem................ since you have difficulty rotating the right side end cap freely etc
 
fltri1550 said:
Bob, I've checked most of what you reference above but have never looked at the center stand. How did your center stand impact vibration and how did you fix it?
Click to expand...
The stepped spacer, the holes in the stand, and the holes in the cradle were really loose. At low and high rpm's the stand would wag side to side. Not that much but it's heavy enough and it was loose enough to to really get going. I took it off and left it off for a long time. I fixed it by over engineering the heck out of it. I turned some discs to fill the holes in the stand with 3/8' hole in the centers, got them welded in. I got a thick walled tube welded between the cradle plates. I machined a shaft that just fit the tube and threaded holes in the ends of the shaft to hold the stand. The reason I did it was because I was in a wreck and had neck and back surgery. I gotta start it on the stand or else.
One thing I've tried is to put my finger on the primary case so it touches the primary and the Z plate at the same time. If something is hitting someplace you can feel it, if not hear it. Phil Radford told me once that the rubber bushes have a tendency to glue themselves to the iso tubes and fix the cradle off to one side. You mentioned that all of your clearance is on one side. Ideally, if the rubbers are lubed with silicone grease they will allow the cradle to center itself eventually. Is the vibration at it's worst rpm the same accelerating through it as when you're on the way down? No swingarm spindle slack?
 
which deminishes a little at 4000 but then builds to a foot tingling annoyance at 6000. I've replaced the front iso rubbers from different sources and have been through the other potential vibration culprits like valve adjustment, ignition timing, primary chain adjustment and carb balancing. I'm now at a loss as to what to do next.
Click to expand...
I've also lived with this for many years and despite having the crank properly balanced, a belt drive, perfectly aligned front mount and cradle, crank and gearbox mainshaft runout checked, new gearbox bearings, new 4th gear set, new chain and sprockets, etc., etc., I have a similar buzz that starts at 4000rpm but never gets much worse. I don't notice it through the gears or revving the engine off the centre stand so for a long time I've put it down to the gearbox as there is a really coarse vibe through the gear lever if it's depressed as if going for '5th' gear. Has anyone noticed that? Maybe it's a red herring but I was just thinking? If I hold a gloved hand on the barrels or head at 4k+ the engine actually feels quite smooth and doesn't have that nasty vibe.
Clapped out centerstand bushes
Click to expand...
Now that I hadn't even considered. I should also be able to tell the difference between 2000 and 4000 rpm so that would include or eliminate the box and it does seem to be at a lower than engine speed but I'm not sure. Even tried driving the clutch and rear wheel with a power drill but it didn't have enough power to get the speed up beyond 1600rpm at the clutch (using speedo to calculate gearbox revs). Driving the clutch like this out of gear or with the chain off and over 2300 rpm didn't show any vibration though. My Iso's are the vernier type and set at 010". Any less and the high speed vibes are worse which is why I've been thinking it's the transmission but seeing this thread I wonder! Probably will never cure it as it's only a tiny annoyance but I will pull that centrestand and try without. I know the holes are knackered as it's now on M14 bolts and overbored spacers!!
 
Thanks for all of the input. I've got a list of things to go over this evening, some I've looked at before but will check again, and others that are new.
As for whether the vibration is worse reving up or slowing down I would say it is slightly worse slowing down and most prominant at 3000rpm. I can even feel it if I just sit on the bike and rev it with one foot on a peg. Between 1000 and 2000rpm the engine does bounce around a bit in the frame but then visually smooths out although I can feel it through the pegs.
The front iso rubbers are new but the rear ones were new in 1996. I took the end caps off this week and they still look new not perished and deformed like the ones I took out 13 years ago. I would replace them but everything was smooth until I decided to service the bike a few months ago. Somehow I must have disturbed things which has me left with this vibratrion. As they say "if it ain't broke don't fix it".
Any other thoughts would be most welcome. If you happen to live near Raleigh NC come take a look.
Thanks
Ross
 
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