How much lateral rear suspension movement is acceptable?

I believe the "Mk3 suspensory spring device" can be just an addon to what is on the standard 1974 850 head steady after a couple holes are drilled in the front of the "box".

I would have thought it should not be possible to move the rear wheel sideways if the ISOs are correctly adjusted. Featherbed frames use to have Silentbloc bushes, and when replaced by brass bushes, there was a difference in the feel. If the ISOs are adjusted correctly, with the Commando crank balance factor, there should only be minimal vibration. If you know the history of the bike, you might know if it has ever been raced - the balance factor might have been raised. If the vibration only occurs while you a riding the bike, I would check the wheels and the chain.

"However, I have been surprised by the amount of vibration, (primarily through the foot pegs) from about 2500 to 3000 rpm. Above 3000 rpm she is very smooth."

Vbration in that range is normal. Road tests from the period say the same thing. Your play at the rear wheel is much too large; adjust ISOs to factory settings and, if you feel experimental, you can try a bit tighter. Don't adjust for more clearance than Norton spec and don't reduce clearance to zero.

Mine had vibration below 3,000rpm, which I fixed by going over the engine mounts. A couple had very slightly loosened after it's restoration. The difference was immediately glorious

Britstuff said:

Hi Grandpaul:

Ah, I see - I think. Hmmm, might have to plan to do the clamp upgrade over the winter.

Click to expand...

Brit,
You can install the clamps with the bike in situ.Get in touch with me as I have the complete instructions on how to do it. I did mine years ago and everything has stayed good.A definitely worthwhile option.
Mike

Hi all:

Sorry for not updating this post for a while. I have been distracted by a, (now fixed) self inflicted electronic ignition issue.

I have implemented a few changes that have made a massive difference to vibration levels! The bike is now pretty smooth throughout the rev range, with just a hint of vibes between 2500-3000 and with minimal rear wheel lateral movement. I might play a little bit more at some point, but for now both vibes and handling are more than acceptable.

Firstly, as suggested by "Cash", I added the "Mk3 suspensory spring device" to my stock, box shaped 850 MK2 Head steady. Secondly, I took the bike off the center stand and loosened the front and rear isolastic through bolts while I adjusted iso clearances and the "Mk3 suspensory spring device" spring tension. Then torqued up the front and rear isolastic through bolts and checked iso clearances, before putting the bike back on the center stand.

I wonder if part of my problem was that I, contrary to what the manual says, previously adjusted the isos with the bike on the bike center stand. Perhaps doing so caused my isos to be incorrectly compressed / off center? It seems that the "Mk3 suspensory spring device" helps by supporting the weight of the engine, making it easier to center the isos. The thought being that if the isos are incorrectly compressed, they would perhaps act like the rubber material is "too hard", which I hear can cause vibration issues?

I settled on a "Mk3 suspensory spring device" tensioned spring (length) measurement of 1 3/8", instead of the suggested 1 1/2". The distance of available thread was the suggested 1/2". I'm thinking that 1 3/8" makes sense given that the the MK II lump is obviously quite a bit lighter than the starter equipped MK III.

Regardless, thanks to the very helpful comments received on this forum, I now have a much better of grasp of the issues and a much more usable
Commando!

Thanks again,

James

I use the AN recommeded 1 and 1/2 holes front and rear for new vernier iso and have found no reason to change. If the spindle is loose in the cradle, I either replace the cradle or add the clamps. There is no appreciable movement of the swingarm when setup that way.

I use the box head steady and the MK3 springs. If your box head steady does not have the holes to mount the spring, just get the parts and drill the holes using the part that is bolted to the box as the guide - align the bottom of the bracket to near the bottom of the box - just be sure to leave enough room for bolt heads inside the box.

Britstuff said:

Hi Cash:

Thank you for your suggestions. I think I am o.k. on the timing and carb balance. I have already spent a fair bit of time on both. Is the "Mk3 850 suspensory spring device" an updated headsteady, the one orginally designed for the MK3? I have thought about upgrading my headsteady, but up till now have now not been able to work out what design to go with and where to get it from. Sounds like some headsteady designs help handling, but can increase vibes. I am definitely open to suggestions, my existing factory "box" type headsteady is a most unwieldy device!

Click to expand...

I have a Dave Taylor style head steady with mk3 springs. Fitting this made a significant improvement to stability especially on sections have road that have longitudinal ridges and there was no increase to vibration. I sort of guessed how much tension to apply to the springs and it worked out ok, I’m sure there is a more scientific method of setting the spring. My head steady came from RGM https://www.rgmnorton.co.uk/buy/dav...ith-spring-device-fits-all-commandos_2776.htm

Dave

Dellis said:

I’m sure there is a more scientific method of setting the spring.

Click to expand...

If you agree with the basis of the spring being to compensate for the weight of the engine then set the spring tension so that the front iso bolt slides in and out easily. Then it pre tension should be equal to the weight of the engine pressing down onto the front iso. Not strictly the full weight but at least the front iso is relieved of the engine weight.

kommando said:

If you agree with the basis of the spring being to compensate for the weight of the engine then set the spring tension so that the front iso bolt slides in and out easily. Then it pre tension should be equal to the weight of the engine pressing down onto the front iso. Not strictly the full weight but at least the front iso is relieved of the engine weight.

Click to expand...

On the side stand . Not the center stand . Or support the bike well straight up and down somehow , without using stands .

I have a three vernier iso setup and very little vibration at slightly tighter than standard clearances. I would use the david taylor at the head if I had known about it. In principle so long as the iso clearances are enough to not restrict the radial motion of the bushing the iso vibration should be defined by the rubber donuts in the iso tube. Some of these may have been harder and I suspect that new donuts will take a bit of break-in although mine seemed to work fine on the first few rides.