isolastic replacement suggestions for a 72 interstate

EstuaryBoy said:

According to Mick Duckworth's book it was introduced because of the 'extra weight' of the Mk3 engine...
So would 'earlier' benefit?
You pays yer money.....

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Fwiw, my 74 850 had it (bought used that way) but i only see it in the 75mk3 parts book

84ok said:

Fwiw, my 74 850 had it (bought used that way) but i only see it in the 75mk3 parts book

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The '74 JPN I had, had the MkIII style box also (but no spring)

Well, I installed the spring today - went quite well.
Did all of the installation first - leaving the spring-tensioning nyloc nut loose.
Then loosened the front iso bolt and checked and reset the vernier - it was bang-on 1.5 holes.
Next started tightening the spring - didn't stop until I got to 33mm.
Then, holding the bike vertical on wheels, bounced the forks up and down and checked if I could move the iso bolt.
Had to use a small mallet at first, but got easer (I checked every half mm) to the point, at 39mm, I could push it back and forth with my thumb.
At 40mm it was too hard to move without the mallet again.
So - backed it of to the "easiest" point - 39mm
Finally, retensioned the front iso bolt.
Photos below
Unless there's something dramatically different I won't give ride reports until I have tried a few settings.
Will take a little while as, on top of the test rides, the tank has to come off for every change in spring tension in order to measure spring length.
Cheers
Rob

grandpaul said:

The '74 JPN I had, had the MkIII style box also (but no spring)

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robs ss said:

Is anyone aware of work being done to validate the upward force of these springs?
My 650 engine weighs 43kg - not sure what a commando engine weighs?
If you knew the "spring constant" (k) for that spring and the amount of deflection of the spring required for it to fit over the spring saddle and under the trunnion (x) you could work out how much upward force it generates.
I wonder how close it comes to neutralising the engine weight?
Then, of course, there's the fact that the spring isn't acting vertically (looks to be about 20 degrees) which would mean that if it were to magically have 43kg equivalent force, only 40kg would act vertically and 15kg would act horizontally forward.
So, maybe like statically balancing these engines, there is a compromise to be made.

Someone (was it comnoz?) put a compression spring under the engine, acting off a cross-member.

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Saw that compression spring under the engine deal, tried to find with a quick search but no dice.

interesting maybe from another thread

On a Triumph I have that also has an isolastic mounting system, I have the weight of the engine held up by a valve spring under the front of the engine. There is only one isolastic mount, a Norton one above the head. I had to drill six holes in the mount to get it to be soft enough to work. It smooths the engine out at lower speeds than the Norton.

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Also, 8 posts in on this one

Mk.lll head steady​

Speaking of Mk.lll head steadies, I've been using one for years and when I got it, Brian Slark of British Marketing told me over the phone not to bother with the spring contraption, since that was a bodge designed to hold up the heavier 850 engine in the Isolastics.​

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84ok said:

Saw that compression spring under the engine deal, tried to find with a quick search but no dice.

interesting maybe from another thread

Head steady spring, useful or junk?

I'm going to be fitting a Comnoz / CNW head steady. My question is: is the mk3 spring a good idea to use with these? Mine is a mk2a, so there's no heavy starter, plus my clutch and alloy pulley belt primary is around 8lbs+ lighter than stock. The iso's are new. So, spring or no spring???

www.accessnorton.com

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Also, 8 posts in on this one

Head Steadies | Norton Owners Club

www.nortonownersclub.org

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Mk3 Head Steady Adjustment

Hello, and the best to you all for Christmas and the New Year. Mk3 headsteady had the spring hook over the rear barb and was adjusted to .5 to .2 of an inch gap between the mount and the barrel as per the book. (section C1, fig C1(a) But I didn't measure the spring stretched length. C1...

www.accessnorton.com

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Thanks - but, for me, I'd rather have a go at this relatively easy mod myself than rely on, what appear to be, pre-biased opinions.
Not having a go at those blokes, but the details of how they reached their conclusions appears to be missing.

Did the 5500 mile service today - all went well.
As part of that I decided to move the MkIII head steady spring to the "front hook" (was previously on the rear).
Did that then loosened the front iso bolt and increased the spring tension to the 39mm (coil length) and again, with a bit of adjustment, I found I could slide (more easily this time) the front iso bolt back and forth.
Nipped everything up and Bob's yer uncle, Bernard's yer aunt!
Thanks @grandpaul for your earlier insights on this.
As to the naysayers, who say the spring is a bodge and band-aid, I say "go buy yourself a steering damper!".
What works, works!
Cheers

robs ss said:

As to the naysayers, who say the spring is a bodge and band-aid, I say "go buy yourself a steering damper!".
What works, works!
Cheers

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What benefit did you get from it?

maylar said:

What benefit did you get from it?

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To be honest, I can't say I can feel a quantifiable improvement. I think it is better, but that could just be my mind playing tricks.
I am happy that this set-up gives the front iso the best chance of doing its job properly - and, after all, isn't riding and owning these things all about making us (as in, individual owners) happy. Bugger what anyone else thinks - although I'll listen to other's views as it may alter mine.
Cheers

robs ss said:

... it is also evident (at least to me) that it cannot do anything to reduce the load on the rear isolastic. It has the load it has.

The only way to reduce that is to go on a diet, fat-arse!

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Well - I think I have found a way (others may have too) to reduce the load on the rear isolastics - and it seems to have benefits.
I have started using a "scissor-lift" under the frame just forward of the centre-stand so that both wheels are off the ground.
This actually reverses the load on the rear iso (does nothing for the front) so IMHO gives the rear a bit of a break.
Since doing this (2 weeks ago) I think I have noticed an improvement in smoothness, particularly at mid-revs (say 2500 to 3500).
Has anyone else done this and seen a change?
Chers

Stephen Hill said:

Some isolastic rubber is too hard, and will vibrate at all rpms.
Find a source that sells rubber with the correct hardness/softness/durometer #.

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I agree.

But please, tell us how. I searched for the DaVinci code, NOBODY could offer ANY durometer data, either OEM, or current.
Nobody.
I have a durometer and not afraid to use it.....

concours said:

I agree.

But please, tell us how. I searched for the DaVinci code, NOBODY could offer ANY durometer data, either OEM, or current.
Nobody.
I have a durometer and not afraid to use it.....

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