Isolastics useless little doughnuts

The isolastics I've seen all have the large cushions standing proud of the smaller ones by over a 1/4" yet the wildest witness marks on the front iso mount never gets over 1/8" in any direction, so small cushions never engaged and always look pristine in the sets I've seen new and old. What was Norton thinking and why not replace em with some the would engage some at max engine motions from pistons and road loads? I've ground 45' bevel around the large cushion rims so only ~1/4" flat left which definitely isolates sooner but still didn't allow small cushions to be touched in my rough riding. Bob Patton stuffs 6 large cushions in the rear to stifle the eventual sag but don't know what's be tried like this in the front iso.

Wondered the same thing when I pulled/replaced the isos on my '73. It looked to me at the time as if two of the 4 iso rubbers could never make contact. Maybe it was some sort of union work right thing back in the day...they had an installer they had to pay so they gave him two isolators to install that actually were never needed!

Questioned that many times, I was told that if the engine became over loaded then the smaller rubbers prevented excessive movement.
thats from the man who makes them!

Hm Ok then no one knows why the small doughnuts are so small to be useless then. I will remove them in next Peel and stuff in some extra large ones- ground down just a bit so they don't engage until 1/8" motion occurs, which takes some wild riding conditions on chassis and swingarm - as just engine orbitals alone can't compress the large cushions that much on its own.

Would 1/8 engine movement be to much for the rear chain .if the swing arm was fixed to the frame? or is more to this than meets the eye? I am guessing that the rear chain would'nt take kindly to be put under load ..then jack hammered :!:

Good questions I've thought about to come to current conclusion - that the suspension travel puts way more tension changes in chain than most extreme isolastic motion could, so yes I can see no reason not to mount swing arm to chassis but maybe tansmits engine vibes through chain into spindle into pilot. This assumes proper adjusted chain of course. Peel had magic handling combo and as far as I could test-tell, swing arm flex or twist was not a hindrance at all. But I'm putting a lot more power on for harsher maneuvers that flip bike off rear hi sides so backing up the boxed swing arm on the Z plates. For sense of load in situation I hope to do on purpose someday, imagine a flat tracker spinning rear so bike would fall over if not held up by sliding boot around a turn, but doing it on the curving ditch slope walls layered in marble to fist sized rocks, ugh. The dangerous issue of swing arm flex to me is not its max loads like above but the held sweepers with low long held angles the oscillations can build up. I press that on purpose in good places till breaks loose then have to change steering style to avoid flat tracker slides out of lane or off the edge. The snap backs of swing arm and forks can spike loads that can be used to help ricochet sweepers [but tends to crash others] if you can predict them and their recoil reactions enough to catch the crash to carry on.

Rear iso mounts can hold 6 large cushions and it mostly pivots not bounce like front and moves fro-aft resisting/dampening tire power pulses so would be way I'd try if fixing swing arm to chassis. More cushions in rear help delay the sag than can change rear link angles to possibly bind and vibe at some point it might not otherwise.

It is possible, isn't it? that the smaller doughnuts do in fact come into contact but rarely and only when an extreme load is imposed, snubbing any overload that the big doughnuts would otherwise suffer? Perhaps some kind of alternative for dye-chem that would "witness" whether this happens.

Maybe it was some sort of union work right thing back in the day...they had an installer they had to pay so they gave him two isolators to install that actually were never needed!

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interesting comment, but it is my understanding that in the 1970s Norton was not a "union shop"

anyone out there clarify this, were the rank and file Norton workers unionized back then?

I do not believe the small duoughnuts can ever touch unless such harsh loaded the frame crumples and pilot broken. Peel is made to take apart so will pack with clay or melt a wax ring or something around the small one then pull wheelies, fly across roots and ruts plus jumps and landings and dragster take offs then have a look see to guide me and settle this mystery. Similar to the mystery function of the thin paper washers shown in fork diagram.

Hi , could be the big belt and small braces solution....................

Peel is set up to investigate this isolastic mystery. I don't think the small donuts do a thing and should be replaced by large cushions in the rear but not sure what to do with the front iso. With the beveled two cushions in tri linked Peel it worked a smooth and secure treat. But even with the links I could not tighten up gap w/o getting vibes nor open up gap over ~.010" or got some hinging sloppiness. Links had to be set in most slack adjustment or I got vibed by the links binding as frame and iso rubbers distorted.

Peel's front mount left witness marks on total motions and didn't move nearly enough to touch the small cushions and doubt any one has crashed or hi sided on purpose as harsh as I have on Peel. The crashes never ever happened in testing cornering only d/t objects in path while going straight ahead in pastures or THE Gravel paths.

Interesting discussion. Note that on the factory Commando Production Racers, the front and rear isos were modified by the addition of another of the large diameter rubber cushions to each, so that the front iso now had 3 of them, and the rear had 4. If you think the smaller cushions will never be of any use in a standard Commando, then they must have been really useless in the PR. Note that they didn't remove the smaller cushions, just added one more of the larger ones.

Ken

Ken, you add more evidence towards fact small cushions are some mystery pay off item to a suppler and useless as tits on bore hogs or nubbin scars left of horse's 5th toes. The factory isolastic Commando makes a great touring bike and ok handler for its era but needs help from dangerous frame twist spring backs - only solution I know that stopped me looking for anything better is > one robust rump link and two compliant helper links. My obnoxious joys on Ms Peel were done with half the rubber mass contact in front isolastics not an extra 3rd more vibration transmitters, so maybe someone is missing out on something special. So at least 3 useless items in Commandos, paper washer in forks, bend tabs on Combat cam thrust washers and small dia. cushions.

I guess I should also have mentioned that the balance factor spec for the PR crankshaft was 62%, and that might have had something to do with the changes to the isolastics. Then again, it might not.

Ken

lcrken said:

Interesting discussion. Note that on the factory Commando Production Racers, the front and rear isos were modified by the addition of another of the large diameter rubber cushions to each, so that the front iso now had 3 of them, and the rear had 4. If you think the smaller cushions will never be of any use in a standard Commando, then they must have been really useless in the PR. Note that they didn't remove the smaller cushions, just added one more of the larger ones.

Ken

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Herb Becker modifies the isolastics in his Commando racers in a similar manner; I don't recall the exact formula but he adds one or more larger rubbers. His bikes are remarkably smooth; smoother than any Commando I have ridden which goes a long way in improving rider performance. Holmeslice and Doug McRae did a swap ride once and one or more of them can chime in if they wish.

As for the smaller doughnuts, I am only speculating here but I suspect when you launch the bike hard or hard shifting will cause the larger doughnuts to get "used up" so smaller ones are included for those periodic excursions. Adding more of the larger doughnuts would make the system less compliant so I suspect they went with more compliance and a set of bumpers for excursions.

PR raced with an extra donut in front and 2 more in rear. I do not understand nor agree with DWS logic, ie: PR had more compliant cushions - as adding more rubber area decreases compliance and as stiffening up handling seemed to be PR's goal then they sure wouldn't of softened their cushions to nullify their reason for extra rubbers. I suspect like the police paid to ride with tighter gaps PR pilots just grinned and tingled.

If someone really wants to test if small cushions touched in any riding condition they can endure, just drill a small hole in iso mount over a small cushion and put a pin in there sticking out a bit and tape a lump of clay on and then remove to see if pin made a deeper impression than at rest. Hm, now that would be informative on Peel. If any one can advise how to do so please pipe up.

Not to worry all this BF stuff and handling compromises has been solved a decade ago while enjoying rather more compliant front cushions isolation. I bevel off about half the front's thickness on my 2 isolastic Commandos. I added two extra rubbers in rear. Bot Cdo's allow rides dawn to dark and not want to get off, though the factory one I am very careful with.

I can't believe small cushions are ever engaged in conditions a pilot can stay seated. I'll toss the small guys and try a couple more big ones in the front but cut down 1/16" all around, or beveled bluntly to a complete peak just short of contact so only most severe loads engage them but then they may help resist too much cradle in frame distortion while still allowing what i want and crave. This ain't academic on Peel side loads tripping out with power for the abrupt rear side out braking or allow full side hook up for the hi side leaps.

What matters most to me is my spine-brain stem becoming one with the rear tire interface. If ya pay close attention - tire howls and sqeals and chirps over lap with sensations of valve train and engine vibes resonating thru frame and grips and seat. I can't keep many things in mind at once so only focus on 3 and not feeling a bike between me and surface is insanely fun. If the chassis can take it there's ways to crash just ahead of time so the rebound crash prevents ever hitting the ground. Feels like the ground has a repelling force field that gets stronger the harder and closer ya throw down on it.

I'm enjoying this topic.....I hadn't thought of modifying the isolastics this way.

cjandme said:

I'm enjoying this topic.....I hadn't thought of modifying the isolastics this way.

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If you're interested, this is the section from the PR catalog describing the mods.



Ken

Awesome, thanks Icrken. Cj