Isolastic Rubber Hardness

A little while back I bought a cheap durometer to check the hardness of the tyres on my old bikes. If truth be told, I don’t ride enough and most of my tyres harden with age before they wear out. The durometer has done its job well identifying some old tyres with a hardness of 80 that were starting to be skittish.

I am presently rebuilding a 1972 Commando and I thought that it would be interesting to compare the hardness of modern isolastics against the originals. My Commando was laid up in 1977, so I am reasonably confident that the isolastic rubbers I have taken out are original.

My durometer is not calibrated, but it measured a hardness of 58 on a new tyre that I recently fitted that has a specified hardness of 60. So I have a reasonable confidence in this instruments measurements. The durometer measures type A Shore hardness.

So, I measured the dimensions and hardness of my original Isolastics and mark 3 vernier type isolastics from AN and RGM. In each case I took 3 Durometer readings and took the average.

I know that this does not match up to Dyno Dave’s work, but I wanted to know how current products matched up to the originals.

Results

1. Hardness

Isolastic rubber Original AN RGM

Front Outer 48 44 57
Front Inner 80 44 57
Rear Outer 49 65
Rear Inner 70 65


2. Dimensions

Diameter of Rubbers(mm) Original AN RGM

Front Outer 49.0 49.9 53.3
Front Inner 42.4 42.7 42.5
Rear Outer 39.0 40.7
Rear Inner 33.9 33.9

Observations.

1. The AN front isolastic is remarkably close to the original. RGM’s are harder
2. The original inner rubbers are harder than the outers, while the modern ones are the same throughout.
3. AN rear isolastics are harder than the originals.
4. AN rubbers are probably the same dimensionally as the originals, allowing for compression of the originals.
5. The RGM front has a larger diameter and coupled with the harder composition are likely to be more difficult to install

Note. For budgetary reasons, I only bought a front RGM, not anticipating there to be a difference between front and rear.

Conclusions

I thought that the AN’s were closest to original. This backs up the anecdotal evidence you see on the forum. I ended up fitting these and they were easy to install with silicone grease lubrication.

Very interesting Steve.

Out of interest, how old was the tyre that tested at 80? And was it the same type that was supposed to be 60?

Old tyre was 12 year tt100. New Avon road riders measure 59.
The tyres I checked against were actually dirt track race tyres that my mate knew the hardness of. Couldnt find published hardness for motorbike tyres.

I searched for info on this when shopping for new rubbers. Found damn little, and NOTHING from the sellers.

Norman White told me that AN were closest to factory spec.
He also became quite animated (with disgust) talking about companies who were making harder / softer ones given the extensive work the factory did in R&D and testing.

Using the old rubber for a base line Hardness ? after many years in service the rubber will harden and also dimensions will conform to the tube diameter it is fitted to

the correct comparison would be to have original factory specs for both dimensions and hardness Does any one have access tot he extensive testing records ?

Obviously, I could only compare what I had.
The original rubbers were in remarkably good condition, pliable with hardly any cracking. They were compressed in the direction of the load, so I measured side to side.
The originals were probably real rubber and may degrade less than today’s synthetic equivalent. Inside the tubes there is also no air or uv light to degrade the rubber.

I do agree that the rubber may have been softer but it is hard to believe that it could have been that much softer.

I guess that Andover Norton have the specs, but wouldn’t expect them to publish them.

Anyway, just a bit of fun that I thought some may find interesting.

I just purchased the same type of durometer for AU$36 on eBay and I look forward to doing some testing myself. I've got new AN MK3 isolastics to test, along with a couple of other new sets that came with a project I purchased a few years ago. Given it's not a five minute job to remove and replace isolastics it makes sense to me to be able to install parts that are as close to the correct specs as possible. I realise that using a cheap testing device won't be perfect, but I think the comparative figures will be useful.

I got caught out. I rode my bike for over a year with the occasional metal to metal feel on the overrun. It never had any vibration worth thinking about. I finally bought a set of heavy duty isolastics from RGM and fitted the front ones.. The vibrations were terrible. I thought about the fact that with no vibrations previously and almost no rubbers left, they must be less if I have less rubber. I removed the H D rubbers again and trimmed down the outsides. After that, the vibrations were a hell of a lot less. I think I will take them out again and cut more off the bottom of those rubbers to see what happens. Shit, its only a wee bit of money, and doing the front mount is quick and easy.

Dereck

Just did it. 45 mins from the time I looked at the clock in the kitchen, went down to the garage, found my tolls and went for it. Now we are in level 2 of lockdown, I think I can sneek a ride.

Fast Eddie said:

Norman White told me that AN were closest to factory spec.
He also became quite animated (with disgust) talking about companies who were making harder / softer ones given the extensive work the factory did in R&D and testing.

Click to expand...

How did he know they were making them harder/softer than OEM spec?
Did he know what the OEM spec was?
This point appears to be the central issue here.

robs ss said:

How did he know they were making them harder/softer than OEM spec?
Did he know what the OEM spec was?
This point appears to be the central issue here.

Click to expand...

Yes he did. So do AN. But I’ve forgotten.

If my (rapidly) fading memory serves me correctly, both RGM and Norvil used to advertise the fact their isolastics were harder/softer than OE spec....
RGM listed: 'Heavy duty', whilst Norvil's blurb reckoned theirs lowered the rev limit at which they became effective.

I DEFINITELY DON'T RECOMMEND BUYING THE HEAVY DUTY ISO RUBBERS. No difference noticed with my ride today. Vibrations tapering down to [ up to ] 3000 and gone from 3250 upwards.

The factory changed the hardness themselves over the years, they added hardness for some reason until the last MK3 version which were the hardest. AN were making the hard version but some years ago after some research they changed to the current soft version which is the one to get. If you have hard ones you can soften them by drilling holes into the sides.

Old thread resurrection....

Seems I have a set of hard rubbers in my rear mount. I'm at 40 thou clearance on the rear adjuster (Hemmings unit) and still have pretty harsh vibrations. Mirrors basically useless for most of rev band around town (1100-3500). I've had these rear iso's in place about 14 months....6000 miles (since a major strip down to bare frame). In that time I've broken one oil tank support tang (rear one) and three tail light/plate brackets. The front iso's seem to give plenty of vertical motion to the engine while blipping the throttle and at at 10 thou adjustment. They were sourced from Norvil, with "New Type" in the description. The rears were from a different vendor (can't find my order so unknown right now). Norman White in his restoration book mentions how he checks iso's...with bike on CS, pry up between a frame member and against the SA spindle (or other easy to lever engine/cradle/SA assembly) to feel for how much movement occurs at the iso collars. My rears barely move vertically, while front have quite a bit more.

Dreading having to pull the rears out....

.

concours said:

I'd be interested to read what the numbers are. Do you have a durometer?

Click to expand...

Nope. But if I ever get them out I can send you them for autopsy....

Here are two of the three failed tail light brackets:





Second failed at lower attachment ears. This newly purchased from canadian AN distributor, had poor build issues, such as turn signal mount bracket hole ears not square to each other; also some dents/bends in the plate "flat" area. It failed within a week of installing and seller sent me a replacement. It is what is on bike most of last seaon and just suffered the same failure as my original one in first picture.

When I first built my 850 I fitted rubbers from Norvil. The bike was unpleasant to ride due to the vibes. I spoke to Norman about this who suggested I tried a set of the AN type. This I did & the problem disappeared.