Mk 3 Vibration at low revs

I found by changing the isolastic adjustment, mostly at the front ,I was able to gain some control over where the smooth range is on my bike. (Around 2,000 rpm upwards)  About 25 years ago I added a Norvil Stainless head steady, this had the effect of slightly increasing the vibes, but not by much.

I decided to leave things where they were. The ride becomes ultra smooth over 3,000 rpm, even with the new head steady arrangement. (There is less capacity for engine movement over the original 'mechano' like fish plate))

The ride stays smooth to 5,000 rpm, after that I don't know. Too many covid cops about!

Bike is a 76 Registered Mk III. (Left the factory in June 75) 

Definitely not right; a Mk3 should be sooo smooth when correctly setup with the sprung head steady set just right, the best of all Commandos. There were complaints a couple of years ago that new Isolastics rubbers are/were far too stiff and made the ride harsher than the originals - I'll try to find a link on that for you.

In any mechanical system the natural frequency is proportional to the square root of the ratio of stiffness to mass. That always makes it difficult to alter. To halve the frequency needs 4 times the mass...which is obviously impossible. Or one quarter the spring stiffness, which is difficult because, although rubbers do vary, they probably don't vary as much as that. So hard rubbers will have some effect but probably not to cause a massive change. Are they running out of space to move? Or are they the wrong dimensions?

If it's only a few hundred rpm, then it could more easily be the rubbers being too stiff.

It has been discussed on here before Bob, so if you search there will be some more information; sorry I cannot point directly to it.

There seems to be agreement that replacement isolastics in the past have been too stiff. But I do recall someone having success by drilling holes in the rubber to weaken them.

If it helps, I will say that I purchased new isolastics from ANIL in early 2020 and they work just fine. No idea about other supply sources. I would expect that they have tried to get the rubber stiffness to match the original specification. I recall reading that Norton had to keep reducing the stiffness during development until the vibration became acceptable.

I’m no Commando expert and have only owned my 1976 MK3 Interstate for a few months and covered 600 miles or so but this is how I’ve found mine to behave. I find my bike runs absolutely smoothly whilst constantly accelerating up to 4k rpm. I have not been past that mark as I don’t want to stress the engine as it hasn’t covered many miles since it was rebuilt. When keeping a constant speed then it needs to be running at 2.5k rpm at which point it is very smooth, any less and it begins to vibrate. I find keeping a constant 30mph is only achievable in 2nd gear as using 3rd gear the revs drop below 2.5k rpm. At around 35mph I can keep the bike running along very smoothly in 3rd. Can’t really remember what speed it begins running smoothly in top gear but I think it’s around 45mph and definitely 2.5k rpm. From what I have read and been told this is exactly how the MK3 with standard gearing and well set-up isolastic’s should behave. As stated, I’m no expert so please feel free to correct me if I’m wrong. 

The factory fitted iso rubbers were not all the same hardness, it varied with each revision with the MK3 being hard and possibly the hardest, no idea for the reason, possibly durability. The aftermarket followed suit and so all replacements from the end of the factory onwards overtime became the same hardness as the MK3 or even harder. ANIL a few years ago finally cottoned on that this was the cause of the complaints of extra vibration after the fitting of new iso rubbers. They went back to the drawings and started to make them to one of the softer earlier versions and included the MK3 vernier rubbers in the exercise. You need to work out from receipts or date of restoration etc which iso rubbers are fitted, if they are not the soft ANIL version then they need changing or you can soften them by drilling holes in the sides. My MK2a with original rubbers shakes worst at idle and the vibrations reduce in amplitude with rising revs until 2500rpm when they disappear completely.

https://atlanticgreen.com/images/frontiso.jpg

Found this discussion on the subject: https://www.accessnorton.com/NortonCommando/hard-isolastic-rubbers.16265/#post-236222

Sounds like it's been sorted out for the current Andover offering.

I would add to John's comment, above, about the Mk3. In 1978 I was able to ride a new Mk3 with 500 miles on the clock, straight after it's first service, and it was sublime. Even the electric start worked! My ride at the time was an ex-police Mk2A Interpol so a new bike was always going to feel good, but that properly setup, and new, Mk3 remains my benchmark for what is possible.

Mk3 the hardest?

Quote: "The factory fitted iso rubbers were not all the same hardness, it varied with each revision with the MK3 being hard and possibly the hardest, ..."

John, an interesting comment, are there facts or evidence for this?

I bought a new Mk3 in 1976 and that engine really shook around at low revs. My new ANIL isolastics are possibly stiffer than the old ones, but impossible to compare over 50 years perhaps.

John, an interesting comment, are there facts or evidence for this?

See the graph from Atlantic Green included in the post.

Had this problem myself, buying replacement mk3 iso from a famous auction site!

Followed advice of  Andres Weingartner posted on 6.7.20. Drill 4 x 5mm holes in each side, only had to do front on mine to stop the mirror shaking and my fingers going numb.

Original post is on site under forums / commando if you search back.

Regards Martin.

I read an article many years ago about the Isolastic system design (written by Tony Foale I think). The idea is that the engine rotates about the rear mount and moves up and down on the front with little fore and aft movement. The article discussed why the rear mount was not in the optimum position and how crankshaft balance factor affected the engine movement. Can't remember the detail but it is clear that any restriction on the front Isolastic mount, whether due to overly stiff bushes, over-tight clearances or a bent frame will have a very noticeable effect on vibration, as will an out-of-balance crank .