Rear Isolastic Adjuster incorrectly fitted

Richard,

Rear iso adjustment is normally carried out on the timing side of the bike.

Front iso adjustment is usually done on the opposite side.

So it's the front that's on the wrong side, but it won't matter as I think the adjustment gap probably centralizes when in use.

Unless somebody else knows differently? 

Front is adjusted on the timing side and rear on the primary side. I wouldn't bother changing what you have though as it's unlikely to be a major problem. 

Hi Richard

Can I just ask how easy or Not easy is it to change out the rear ones, and how to adjust them ? as my boots are all split and I have had a pair of them for a while but never got round to replace them on my Mk3 did Not know how bigger job it was to do them so never started it.

Cheers 

Mo

see access norton thread “mk3 rear isolastic replacement “ and read down several replies to LAB’s one.

I swapped the rear ISO on my Mk3 850 in 1994 without any issues at all. I did it because I thought Norton had got it wrong. My theory is quite simple, when setting the ISOs I'd rather move the whole power train and rear wheel over to one side by say 0.003" than to move the front and rear adjusters in opposite directions by 0.003" as that would put the rear wheel out of alignment by a lot more than 0.003". Do the maths.

Richard,

You can replace the rear isolastic without removing the engine, but it is a bit of work. I tried this without removing the primary but could not get enough movement to fit the vernier type isolastic. With the primary on, there is just about enough movement to remove and replace the original type iso rubbers. So, remove the primary chaincase, loosen front iso, remove head steady, remove z-plates,remove air filter box and then hinge the engine down.

I have attached a couple of photos showing how I did it. You don’t need to remove the battery tray and oil filter and the other bits I removed. I only took these off because they needed smartening up.

Hope this helps.

@Dave Taylor

Your analysis is predicated on the hypothesis that all the wear in the Isolastic occurs on one side only. While the wear is unlikely to be exactly the same on both sides, I wouldn't like to guess the difference. And front to rear wear ratios are unlikely to match either. In short, fitting both adjusters on the same side of the bike is just as likely to result in the problem you are trying to avoid as the standard fitment.

The adjustment is on one side only. I would have thought wear take up difference would  be reduced by roughly 50% by putting the adjustment on the same side?

Dave,

Let's suppose that you initially fit the isolastics with exactly 5 thou clearance on each side. Now there's no certainty that this will result in the rear wheel being aligned and central with the bike, either when it left the factory or now, but let's suppose it is.

After riding it awhile you find that there's 16 thou of play in the front isolastic so you adjust back to 10 thou. Let's assume that wear is equal on both sides - again, not something we can be sure of - so that the clearances are both at 8 thou. And yes, adjustment will take up 6 thou on one side and nothing on the other. So the isolastic clearances will now be 8 thou and 2 thou which, initially at least, appears to have skewed the engine/swinging arm across the frame.

But in reality you have reduced the width of the isolastic unit. The centre position, as you grab the rear wheel and move it side to side is exactly as it was originally. The forces on the rear wheel when riding far exceed anything you can apply by hand so the wheel will move around the centre position as it always has - no change.

And it is only the rubber buffers in the outer tube that hold the isolastic in any rest position anyway and it is reasonable to assume that being constantly pushed from side to side while being vibrated at 120Hz will allow the buffers to reposition themselves to some 'preferred' position. Of course, the preferred position might not be in the middle anyway but the point is that the isolastic is unlikely to remain in the same position immediately after adjustment for any significant time.

If the wear in the isolastics is not equal in both sides then this will cause the engine/swinging arm the be skewed across the frame. But I don't think it's possible to predict which side will wear more than the other. It might depend on thigs like your riding style, which side of the road you ride on and how distorted your frame is (see below). More relevant to this discussion is that the side the adjuster is fitted doesn't affect this.

This all rather academic anyway given that no Commando is as Bernard Hooper and John Favill drew it: isolastic end caps being true the tube; the engine plate being perfectly square and bolted up exactly in line; frame being millimetre accurate with perfect welds. It has been tried of course : https://nortonclub.com/docs/Straightest_Commando_Frame.pdf . Also, when you adjust the Mk3 isolastic you're bending the frame, and who knows what this does to the alignment.

Which is a rather lond-winded way of saying you should put the rear isolastic adjuster where you find it easiest to get at (the front must be fitted as per the manual).

I have just replaced my front and rear Isolastics and found them to be exactly as yours are, that is to say incorrect as per the book. I have ridden it that like for 35 years. If you do decide to pull them - as I did because mine were as slack as a welly top - I didn't have much difficulty. I borrowed an engine crane to lift the frame from the loop behind the seat and used a small floor jack to move the engine as required. Basically you are rotating the frame around the front wheel spindle and the engine gearbox around the rear wheel spindle. I thought about leaving the primary in place but came to the conclusion that it isn't a big job to remove and it makes the Isolastic job that much easier. I also made up a couple of pullers using 8mm stud bar and plenty of washers and spacers - and the old units just slide out. Also useful for getting them back in.