Vibration

Tell us more about what you have done regarding the isolastics. Shims/vernier/clearances/ Head steady fitted?

On my MK2A there is a lot of vibration at tickover, as the revs rise they lessen but increase in frequency then at 2500 rpm it all goes vibration free and I can finally see behind in the mirrors. If you have the harder iso rubbers that are around this will increase the 2500 but still go completely vibration free. If you have vibration throughout the rev range then either something is touching the frame from the engine/gearbox/swingarm or the iso clearances are too tight.

My vibes go at just over 3000 rpm on mine. It's a 1972 750 with dave Taylor head steady and vernier adjustors on the Isos. Vibs don't really go if Isos are too tight are all bits not put in correct order.

Previously steven_somerfield wrote:

The Isos were put together by Norman White, the Norton Guru. I assume therefore that they are set up correctly.

I will consult the workshop manual as to how to set them up then I guess?

Just coming back to Commandos after a long interval ... but on the one I had years ago I eventually came to the conclusion that one should treat the 0.010" mentioned in the literature as a maximum; setting the iso clearance so that there was very slight resistance to turning the shim carriers by hand seemed to give excellent handling with little or no price in terms of extra vibration.

Previously julian_wells wrote:

Previously steven_somerfield wrote:

The Isos were put together by Norman White, the Norton Guru. I assume therefore that they are set up correctly.

I will consult the workshop manual as to how to set them up then I guess?

Just coming back to Commandos after a long interval ... but on the one I had years ago I eventually came to the conclusion that one should treat the 0.010" mentioned in the literature as a maximum; setting the iso clearance so that there was very slight resistance to turning the shim carriers by hand seemed to give excellent handling with little or no price in terms of extra vibration.

I meant to add: 1971 750 with vernier conversion and standard head-steady ...

There is some information in the INOA tech notes regarding adjustment of the isolastics to chase down vibrations at different revs. £12 to you sir at the NOC shop. A right ripping read

The Mk 111 vernier isolastic adjusters have a thread pitch of 20tpi which is 0.050" per full turn. 1/4 of a turn would be 0.012" so 45 degrees from tight will give you 0.0065"

The guidance in the IONA tech notes says

vibration up to 3000rpm front mount ok, rear mount tight, remove 0.005 shim

vibration from 3000 to 5000 rpm front mount tight remove 0.005" shim rear mount ok

vibration 0 to 5000 rpm front mount too tight, rear mount too tight.

Using a thin wooden lever, move the front engine plates across to the right and measure the gap on the left with feeler gauges.. If you have the Mk111 verniers, undo the engine mounting bolt and rotate the collar with the holes to get the desired setting, tighten it all up (25 ft/lbs) you shouldn't have to use a punch on any of it unless it's corroded in which case remove the whole lot and clean and grease it.

If you have the Mick Hemmings adjuster, same thing applies only you don't have to release the engine bolt.

The rear is the same only difficult to access.

With the Mk111 system always recheck the gap after tightening up. The backlash in the adjuster thread is taken up and the gap can/will be dramatically reduced.

If you do fit a link head steady fit the kit with the Mk111 spring to get the best results.

High frequency vibration has many causes such as over advanced ignition or badly worn final drive etc.

A few years ago I fitted Renolds Synergy chain not knowing Renolds had stopped making motorcycle chain and boy did it put a buzz in the handle bars.

Having owned a Mk4 750 where the vibration variedfrom unpleasant to almost unbearable, I bit the bullet and extricated the crank and took it, the rods and pistons to Basset Down and had the whole lot dynamically balanced. This was successful and transformed it, not completely vibration free but very acceptable. When in the balancing rig with the correct weights applied to the crank for the desired balance factor it wobbled violently, post material removal from the flywheel it was completely smooth. An 850 I am currently building has been treated the same way and the results in the rig were identical, engine not run yet so canât comment. In the 70âs when I owned a Fastback it was relatively vibration free - it either had internals that were better balanced/matched (by good luck?) or I was more resilient to the wobbles.

How many of your bikes have had isolastic engine mounting?

If you have an isolastic mounting on the cylinder head, I would be surprised if it didn't shake badly. They rarely work well, because as soon as you tighten the bolts (any of them) it all jams up. Go back to the standard head steady. If you have a standard head steady then slacken off the rear mount by 0.005"

If it's still bad take it back to Norman White. Both pistons going up and down together is a pain

I've long been puzzled about how the Isolastics work given the weight of the engine, gearbox and transmission. As I understand the engine moves up and down with the vibration suspended on the outer rubbers until the inner 'buffer' rubbers hit the mounting tube causing the vibration felt up to about 3000 rpm. Above 3000 rpm the engine vibration disappears because the engine's movement starts going in the reverse direction before the inner 'buffer' rubbers hit the tube and so the engine etc just 'floats' on the outer rubbers. It seems obvious that the weight of the engine etc would cause the outer rubbers to compress bringing the inner 'buffer' rubbers closer to the bottom of the tube than the top and as the rubbers age this would increase the difference in clearance. I'm sure that what we feel as low rpm vibration is mostly the buffer rubbers hitting the bottom and so is a lower frequency. Perhaps a perfect solution is to have something lifting the engine as in the case of the Mk3 head steady which I remember was only to offset the weight of the starter mechanism.