isolastic question

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MexicoMike

MexicoMike

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Having sorted out the engine stuff, I disassembled the front isolastic mount. I shimmed it a while back to get the .010 clearance but I had never taken it completely apart until today. SO... I''m trying to understand the setup.

There are two large rubber bushings at the outside which press fit into the mount and two smaller ones at the inside. The two inner bushings are too small to actually bear on any part of the mount itself. I can't see what possible use these inner bushings provide. They just sort of "float" there with at least 1/4 inch or more of clearance between them and the mount. Why are they there? There is NO WAY the system can flex enough to bring those bushings in contact with the inner tube surface. I don't understand what purpose they serve. ???
 
Are you SURE the large donuts will not compress enough to bring the smaller ones into play?

Are you SURE Norton put them in both front and rear mounts just because they liked spending some more money on 60,000 Commandos?
 
Mike,

I don't know what year your bike is, but my 69 had 4 donuts in it. Two of them were pretty much attached to the motor mount tube and 2 smaller ones were just loose in the assembly. I bought new rubbers, a new spacer with circlips and new smaller rubbers. So far have not installed them, but here is a picture of the new ones on top and the old ones on bottom. Sounds pretty typical what you have. If I were you I would just replace everything, but there is a thread recently about this, there are also soft and hard rubbers depending on 750 or 850. Do a search on the isos and you will find a lot of info.

isolastic question


Dave
69S
 
Haven't seen mine in a while but the pic looks familiar and those smaller central ones aren't so much smaller than the outers that I can't conceive them lending a bit of support when the outers (which are no doubt subjected to more extreme stresses) compress somewhat....Mike raises an interesting question I'd never thought about but HighDesert is surely right in suggesting that the factory had something in mind with this design.
 
The small ones are basically "bump stops", the large ones are doing the work isolating the engine vibes.
 
Yeah the lower pics look just like mine though the inner tube, inner surface of the mount and the circlips were heavily rusted. Maybe the inner donuts could actually hit the tube but it would take a lot of motion and the outer donuts are pretty stiff. Frankly, I can't see how the headsteady could allow enough movement for those inners to hit but I guess it must happen. As noted, Norton must have had a reason for setting it up that way. But I was surprised, I expected to see 4 donuts the same size.

I'll order new internal parts from Old Britts; I had contemplated going to a vernier adjuster but I had a '71 Commando that I sold in '78 and the whole time I owned it, it only needed adjustment it once. It was also a race bike as well as a daily rider so it got a fair bit of use.
 
I had a possible "AHA" moment in the middle of the night...

I don't believe that the small donuts could actually hit anything because the headsteady wouldn't allow sufficient movement to do so. BUT, if the headsteady failed, THEN the smaller donuts would provide the necessary cushion to prevent serious movement of the engine. SO I think the small donuts serve no purpose in normal operation; they are there as a safety device.

Of course, I have no idea if my AHA is correct or not but it woke me up so I decided to burden others with it! :)
 
I guess you could assemble your bike without them and report back. :wink:

The isolastics are there to absorb vibration, but there has to be some real flexing going on while you are going down the road on uneven surfaces, twisting and accelerating out of corners. Not to mention that most of us where probably a lot lighter when these bikes where manufactured...I hate to think what I do to those pour rubber donuts now that I have the weight of my experiences riding with me. They may limit fore and aft movement as much as up and down so the headsteady, assuming it wont allow that much flex, may be the pivot point. I am just guessing, but my guess says you better put 'em in.

Russ
 
Well, I wouldn't consider NOT putting them in, regardless of how they actually work. ;) Just speculating because I like to know how things work and why they were designed the way they were. OTOH, I don't think it's necessarily correct to say that since Norton installed it, it's right since it's common to read complaints on these forums about how poor Norton engineering was. The old saying re the Commando is a good one:

"When they designed the Commando, Norton looked at their great frame and their crappy engine and threw away the frame!" :)
 
The big doughnuts are called 'bushes' and the little one are 'buffers'. They wanted a soft one for the isolation of the vibes but couldn't risk metal on metal contact at the extremes. So 99% of time the big ones are doing what they're designed to do. The buffers are kind of like the rubber stops on a shock.
Make sure you apply lots of silicone lubricant, Sil-Glyde is one, to the whole lot. The bushes will weld themselves in the tube and lock the iso's up so you will have a hell of a time trying to get equal spacing on both sides with the iso's.
 
I have full faith that had Norton goofed up the design on the isolastics (head steady not withstanding) that Ludwig would have sorted it out by now.

It never fails to amuse me how much verbal abuse the Norton engineers get and how the bikes still have such a devoted fan base regardless. I guess they must have done something right, or since there is a fool born every minute, most of them must be Norton owners. :shock:
 
=m in the midst of replacing my iso with hemmings vernier and new rubbers. i was also scratching my head on the buffers not filling the bore. it seems that the bush rubber is softer than the buffer rubber. it would seem the softer bush rubber cushions the lower amplitude vibes and the buffer plays no role. however when the amplitude increase the soft bush rubber is displaced enough to allow the buffer to have an effect. i suspect the buffers are located where they are to maximize their effect. all supposition on my part
 
"It never fails to amuse me how much verbal abuse the Norton engineers get and how the bikes still have such a devoted fan base regardless."

Totally agree and frankly, despite quoting the bit about the COmmando, which I have always felt amusing, though not really correct, I think the bike is the best of the Brittbikes and I consider it the "affordable" Black Shadow. The fact that when we moved from NY to Mexico City I sold my 996 Duck and kept the Commando pretty much shows my opinion of the bike! :)

I think the isos work very well and have no complaints about them at all. It would have been nice had the vernier adjusters been on Commandos from the beginning but, OTOH, the need to actually adjust the isos is rather seldom and I don't consider it much of an issue to do it "manually."
 
I'm sure that a fully loaded Commando hitting a pothole at 70 will bottom out the small diameter buffers.

I'm also starting to wonder whether using all large diameter "donuts" would be a step in the right direction for my monoshock projects...
 
If all large donuts were used and no small ones, wouldn't there be less absorption, and therefore more felt vibration?
 
A bit more vibration, yes.

Better handling with managable vibes, I believe, is obtainable.
 
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