Isolastics Mk3 vs Mk2

Good luck trying to drill these rubbers as mostly just dodges the bits cutting edges to close back up with swarf inside and just three holes won't soften all the way around so likely won't affect their vibe threshold. Iso rubber can age two way, soften up by solvents or harden up by oxidation. Ride w/o a head steady to get base line of how much and when it comes into handling play and vibration transmission. Just don't get complacent and forget it ain't there because it don't come into play until riding a bit beyond what police would notice, then suddenly its Very Noticed. Just a mild leaned pass across a double yellow line in a wide-ish curve should do it.

Another way to sense its effect is w/o head steady straddle bike and oscillate forks back and forth while looking down to see the waging working its way up and down whole bike. Pay attention to the natural resonance and be ready to back off if ya ever get a hint of that powering up through sweepers. Another bit of insight can be gained with a mount out in a vise whack the through bolt with a mallet.

If I had time and money and all the energy in the world I'd further refine the bevel angles for optimal progressive dampening tuning. Beveling can equalize or customize all the elastic grades and states of rubber condition. Could also slice some shallow slots in the rim or groove its center, but drilling, get back with your reports on success with that. I just held mine against bench grinder at desired best guess angle.

O.K. bretheren ,will back off on drilling out rubbers idea. So much work to make a potential mistake. Best was my 70 S model sitting beside me in boxes.Soft rubbers. Sweetness.

moorejm said:

I stuck new MkIII vernier isolastics in my 1972 Combat a couple of years ago and have been generally underwhelmed by them. The bike vibrates, not terribly, but I expected better. I have fiddled around with the gap settings and found that there was little change in the amount of vibration. I have a low mileage 73 850 with its original (I believe) isolastics. It is much.... much smoother than the Combat. I'm not going to mess with them.
Mick

Click to expand...

I did the same on my '74 and was also disappointed in the results. No matter what I set it to it vibrates more than it did with the stock isos.

It seems that there have been some unhappy upgrades. This certainly shouldn't have been due to the vernier adjustment vs shims? Would it have been the rubbers?

My orignal question was along the lines of: are two rubbers (Mk3) better than three (Mk2) in the rear isolastic. It seems we have migrated back to the problem of finding suitably supple rubber.

Russ

rvich said:

My orignal question was along the lines of: are two rubbers (Mk3) better than three (Mk2) in the rear isolastic. It seems we have migrated back to the problem of finding suitably supple rubber.
Russ

Click to expand...

I have five rubber bushes in the rear iso. From idle to 3000rpm it has a mild vibration, then it goes to glass smooth at 3000rpm. The same engine with MkIII iso's shook like crazy at idle, diminished slowly through the rev's up to 3000rpm, then went glass smooth. I think that five is better than two, so maybe three is better than two. A possible theory being that the low balance factor of the engine has it vibrating more in line with the cylinders and pivoting on the rear iso. The rubber bushes I'm using now are pre MkIII 1995 vintage in the rear and 2008 (soft?) in the front, w/ Hemmings adjusters.

RIder Patton left out his findings of how quickly the rear doughnuts take a sag set and lower the rear to load the front iso more and alter the suspension geometry from how it was first set. I only put 4 big cushions in rear as stop gap to Bob's 6. Again I remind thee that a good bit of what feels like engine is actually road texture and wind eddies, till that's gone - then its a uncanny stable smooth sense of huge loaded down Goldwing's mass inertia but still a light flickable narrow tire bicycle - with a torque punch. Stiffen up the rear and soften up the front rubbers seems a key for improvement. By chemistry or mechanical your call.

Frank the endurance Norton tester said his Commando's would go smooth below 2000 rpm, hm.

I am in the process of fitting Mk3 isolastics to a 1968 commando. The front seem ok using a new mk3 engine mount and the adjustable Mk3 adjustable isolastics. The problem is with the rear. When fitted with both adjusters turned all the way in and using the ptfe washers (which seem quite thick) there is no free play in the frame. The adjuster is tight up against the frame mount.
Anyone have any advice, is it ok to use skims instead of the ptfe washers or will that present a problem.
Cannot see that I have done anything wrong but clearly there must be some free play 10 thou ?
Pete

I think some machining is needed..Lab will chime in soon.

fastbackpete said:

When fitted with both adjusters turned all the way in and using the ptfe washers (which seem quite thick) there is no free play in the frame. The adjuster is tight up against the frame mount.

Click to expand...

But did you try to adjust it?

fastbackpete said:

Anyone have any advice, is it ok to use skims instead of the ptfe washers or will that present a problem.

Click to expand...

Personally, I can't see that metal rubbing against metal would be a good idea, no matter how well it was lubricated.

Try measuring the differance between the "nipped up" iso assembly ptfe washers fitted! and the gap between the frame mounts, if it's 1/2 mm or so then the frame may have tweaked in a bit... skim off the ADJUSTABLE side, make the differance 10 thou less and fit the assembly.

john robert bould said:

Try measuring the differance between the "nipped up" iso assembly ptfe washers fitted! and the gap between the frame mounts, if it's 1/2 mm or so then the frame may have tweaked in a bit... skim off the ADJUSTABLE side, make the differance 10 thou less and fit the assembly.

Click to expand...

If the assembly is too tight, then I would think that unscrewing the adjuster should "spring" the frame apart by the necessary 5 to 10 thou' or so.

How much would a frame force apart,before becomming "over stressed"?

L.A.B. said:

john robert bould said:

Try measuring the differance between the "nipped up" iso assembly ptfe washers fitted! and the gap between the frame mounts, if it's 1/2 mm or so then the frame may have tweaked in a bit... skim off the ADJUSTABLE side, make the differance 10 thou less and fit the assembly.

Click to expand...

If the assembly is too tight, then I would think that unscrewing the adjuster should "spring" the frame apart by the necessary 5 to 10 thou' or so.

Click to expand...

john robert bould said:

How much would a frame force apart,before becomming "over stressed"?

Click to expand...

Personally, I don't think I'd be overly concerned if I found it necessary to spread the frame by 10-20 thou at that point. Perhaps others would be, I don't know?

Agree, infact if the frame needs to be lightly "jacked apart" would be good, as this action will take out the clearance in the threaded adjuster/nut. allowing simpler adjustment
All treads have some slack/play in the design..unless its a ball thread.
One thing i have noticed is the lack of squareness ,this is very common..its the way the dies /taps are set up. the nuts "wobble" on the treaded tube, this produces a un-even gap between the assembly.
The best approach is to skim the adjuster nut face on the centre tube,[between centres] not a easy job...but well worth the effort. as this will ensure the nuts /ptfe washers will be square....any mis-alignment between the parts will cause rapide wear.
Once skimmed the adjuster nut face should be polished, with a 1mm rad on the edge.
Its far to easy to take replacement parts as a "Ready to fit " replacment!

L.A.B. said:

john robert bould said:

How much would a frame force apart,before becomming "over stressed"?

Click to expand...

Personally, I don't think I'd be overly concerned if I found it necessary to spread the frame by 10-20 thou at that point. Perhaps others would be, I don't know?

Click to expand...

grandpaul said:

I have found that the Taylor top steady with Mark III spring type isolastic and SS vernier Mark III isolastics provide the ultimate solution.

The Taylor unit is pre-drilled to accept the Mk III spring kit.

Click to expand...

Hi Grandpaul,
Have you always run with the dave Taylor head steady or did you swap from the original set up?
I have the original still fitted on my 73 750 but was considering buying a Dave Taylor item to replace it, as you seem to be singing it's praises did you notice a big difference when it was swapped out?

Cheers
Mac.

I made a tool with a bit of leverage and managed to turn the adjuster enough to give clearance. I didn't realise it was that tight in the frame. I can get the 10 thou required now so thanks for all the help.
Pete

Its just luck of the draw or 3 decades efforts of elite refiners whether tabs allow long life or not. My 1st Combat didn't need tab spreading to slip mounts in/out w/o hassle but had completely rubbed away-dissolved any low friction spacers remains. My 2nd Combat had only half of one side of composite spacers left and the tabs sprung together when mount removed so needed front iso mount tube machined shorter to fit the adj. iso, because no matter how much threaded spreader and sledge hammered in wedges, it all sprung back exactly where it was prior. Ugh, though did impress me on the thin tubes torsion bar spring back memory characteristics. If a rubber Lord's mount type factory head steady is made stiff enough to resist the isolastic binding then it annoyingly transmits buzz on lean and side wind forces. I liked the adjustables for fine tuning on the road side front to back tighter and looser till nice compromise that differed in my un-linked vs tri-linked Combats. Most consistent pleasant thing I did on both those was bevealing the front cushions

The one thing I did not see mentioned is the ease at which you can remove the motor with early isos vs later type.

In early 1973 I bought a brand new Combat 750 Fastback. When on the centre stand you could physically pull the back wheel sideways both ways and could feel the resistance against the isolastic rubbers. I am not talking about slack as felt with worn isolastics. At idle the engine would jump around in the frame and the front end would pulse in an odd rhythm.

On the road below 2500rpm the vibration was obvious, then smooth until 3000rpm where there was another vibration event. Beyond that was glass smooth. Where I lived was a particular bend in the road with a dip in it and pushing hard when two up into this bend was a disconcerting "bending" of the frame as one hit the dip. I didn't like it. My Dominator had never done this. In every other sense though, the handling was typical Norton.

I currently have a rebuilt 1975 Mk III with new isolastics. You cannot pull the back wheel sideways when on the centre stand. The engine does not jump around at idle nor is there front end pulsing. On the road there are various periods of vibration including at higher rpm, but not offensive. There is no feeling that the frame might "bend" when pushed in corners. My thoughts were that the stiffer isolastic rubbers were a deliberate attempt to return the handling to that expected of the unapproachable Norton.

I recently rode another Mk III and the vibration experienced was the same. Not offensive but apparent. To me, the Mk III in every way is a better machine than any other Commando I have owned, and there have been a few.

Yes, the MKIII is a different bike in "too" many ways.
I was really starting to like you, Phill, until you started with all this Pro MKIII crap.
I have advocated for the MKIII to have its own section in the forum so toi not confuse predominate membership.
Again, they are just too darn different.