Advice with engine vibration

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Fast Eddie said:
When Norman White saw the iso's (of unknown origin), fitted by the PO to my '74 mk2a he was really annoyed, he ranted on about how the factory spent time and effort establishing the correct shaw hardness (?) and that folk with less than a fraction of that resource or knowledge are selling various "upgrades" some harder, some softer, but all "unproven".

I fitted new AN parts and they seem fine to me, although I changed many things at once so cannot make any 'back to back' claims.

But Norman's rant made me realise the rubber isn't just rubber, that the whole concept of the bike is designed around these rubber parts, and that they were designed and specified as such for a reason. So it seems to me that it just ain't worth the risk in not buying genuine parts given their importance and the fact that telling good from bad by visual inspection alone is impossible to any of us lay folk.

Just IMHO of course.
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well put :D
 
I have a 74 Mk11a with unknown rubbers in the isolastics. Euro bars and a stock motor. Mirrors on stems.
No vibes above 2500, clear view out of the mirrors.
Its a joy to ride.
Yes it vibrates a bit below 2500 and walks across the yard at ticker but its a Commando, no problems whatsoever!!
 
The smoothest Commando I've ever owned was a 70 model. Soft rubbers. The motor would visually hop up and down at idle ! Around 2700 RPM that would end and a beautiful smoothness would ensue. Plenty of grease (synthetic) to the washers and a clearance of about 8 thou. Oil based greases hurt (swell) the rubbers. Hope that helps. I have 2 MK 111 's and they are not as smooth. My kicking days are over. :)
 
Biscuit said:
I think handlebar width and shape is a factor too. Too wide and things start moving. Add bar end mirrors and you've really got some parts out there whipping. I'd like to keep the bar ends so I plan to cut down the bars as short as possible. Problem with the MK3 is the stock right hand switchgear/brake master cylinder can foul the tacho at certain angles if the bar is too short.
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I put bar end mirrors on my European Roadster style bars earlier this year as at lower revs in town heavy traffic or at rapid fast road speed either engine vibs or wind vibs made original style useless. I can see far more and they (have a pair) don't ever blur or vibrate the only problem I find is as i have never had this style and they are out of direct line of sight i think you may miss something that may have seen before in the corner of your eye so to speak (in the period you can see admitted) Plus i am not really a fan of the look but they are an improvement so i will leave them an hope my personal taste changes the more i see them. But i don't think they have had any effect on vibration characteristics of my bike in negative way the added weight has if anything helped smooth the bars though I may be just imaging it or subconsciously riding around the bad spot. This is of course just my bike Biscuit your bike may well be different.
 
I don't know where I got it, but it was important and I put it on a post-it note on my tool cabinet years ago

Vibration up to 3000 RPM:
rear tight, front ok
Vibration from 3000 to 5000 RPM
front tight rear ok
Vibration 0-5000 RPM
both tight

I would attempt to loosen the rear vernier a little and see if things improve.

I forget what the rule of thumb on vernier adjustment is, but 4 thou per hole seems to float around in my head.

Start at about 1 -1/2 holes from tight (6 thou) then loosen about 1/2 to 1 hole at a time until the vibration level is comfortable. The tighter the verniers, the sharper the handling (and higher vibration) The looser the verniers, the smoother the engine, but handling may suffer

The advantage of MkIII verniers over the earlier shim system is you can play with the gaps relatively easily without having to buy an assortment of shims. This is why vernier kits are very popular with pre-MkIII Commandos (I have a kit myself)
 
When Norman White saw the iso's (of unknown origin), fitted by the PO to my '74 mk2a he was really annoyed, he ranted on about how the factory spent time and effort establishing the correct shaw hardness (?) and that folk with less than a fraction of that resource or knowledge are selling various "upgrades" some harder, some softer, but all "unproven".
Click to expand...

But even the factory changed the hardness over the years, so who ends up being right. I have the originals in my MK2A and get no vibration over 2500rpm, they are covered in silicone grease and I am sticking to them until they disintegrate, but the current OEM are supposed to be now harder so will be looking for a softer alternative or drilling holes.
 
kommando said:
But even the factory changed the hardness over the years.
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Click to expand...

I didn't know that, is it documented by part number changes or similar?

Are you sure that your originals aren't simply softer than new ones due to age??
 
Don't know if it was because I had so much play, the rubbers were worn out, the PTFE washers disintegrated or what, but there was a noticeable increase in vibration when I put the 71 part numbers (OB) in my 69. I don't find much variation with different settings of the iso clearance. I too am not convinced that the new rubbers are stiffer than the old ones but by now how would one know without having a set of new 68-70 rubbers, which of course is impossible. Per the sagging of the rubbers, I removed my front iso to do some work after only about 3 months from renewing, and the rubbers had already taken a 1/4" set to the bottom. That's when I got the head spring. I never was able to pull the front iso up to center even after turning over the set rubbers. It always has a droop to the bottom even with the spring tightened up as far as I can get it. It does make a noticeable reduction in the vibration at idle, but that's about it. I haven't figured out if there's a difference with the frame or cradle center stands and iso sag either. But like I say, I can see out of my mirror fine at any speed and at road speed I only have a slight buzz in the pegs.
 
Of my three Commando's, the worst vibration was my 1968. There was so much sag in the rubbers that the engine plates were bouncing off the cross tube on the frame! It was bloody horrible. And the higher the rpm, the worse it got!

Stephen Hill
 
If you tap the hole in the commando bob-weight and thread in a steel plug, the balance factor is about 72%. If the motor is rigidly mounted, it will be very smooth at 6000 RPM. Isolastics cannot stop what the loads arising from the crankshaft do to the bearings and crankcases at high revs, if the crank is balanced as it is in a normal commando. Isolastics are simply a cosmetic answer to the problem which the 750 Atlas had when it tried to compete with the CB750 Honda.
The simple fact is that not all bikes are used in the same way - the crankshaft balance factor has to suit the bike's main purpose. If you have isolastics and the bike does not vibrate when you use it in it's main application - BE HAPPY ! - Otherwise start changing the rubbers ?
 
There is a solution for the low RPM shaking and its already been done. It also eliminates isolastic maintenance for good. But its a two part process. First you eliminate the isolastics and replace them with solid Urathane by pouring liquid mix into the isolastic housings. 2nd you must eliminate the vibration at its source by reducing the reciprocating weight - and not just a little but by at least 30% or more. What you end up with is a Commando that handles better with a firmer chassis and feels smooth enough at all RPMS (no rear view mirror blurring).

Thats the simplistic version because this happened years ago and I lost track of the person who did it and don't know the details such as the durometer of the Urathane that was used (there are many choices) or exactly how he held the isolastic parts in place while he did the casting. I do remember him being pleased with the improvement in handling.

Its not rocket science. There are plenty of Commandos out there already with the lightweight internals and there are Ultralight options to make things even smoother. You would have to compromise and guess at the durometer hardness (check out urathane samples, don't go too hard) and use a temporary means to align the parts and hold the liquid urathane in place while it hardened. You could always remove the urathane with heat and try again with a different hard/softness or go back to original isolastics if you missed or it just didn't work out for some reason. Would be an interesting project if someone did it right, worked out the details and shared the info.
 
Jim, One of these days when I am rich again, I will buy a set of you long rods and light pistons. I'm certain that is the way to go.
 
I know this is a never ending topic, but my MK3 is very smooth between 3000-3300, above and below it has vibration, not extreme, just annoying. Has new iso and Dave Taylor head steady
 
With any vertical twin or single four-stroke engine, there is always a balance factor which will give smooth running in a particular application. You cannot expect versatility from these old bikes. Either you ride them fast or slow - and set the balance factor to suit your choice. The gearing you use and the types of road you usually travel are relevant to this. The isolastics probably only give you a bit of leeway.
In effect you probably need two bikes - one for track days and one for touring.
 
One of my friends suggest I should turn my Seeley into a café racer and use it on public roads. Trying to ride it slowly would be horrendous. The motor is set up to be smooth at about 6000 RPM. With any other lower balance factor the top end would destroy the frame.
 
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