How much lateral rear suspension movement is acceptable?

Been working on getting my somewhat recently acquired 1974 MKII 850 Commando back on the road. She is my first Commando, and to cut a long story short, she needed a lot! Much sweat, pain and tears later, and a lot of help from this forum, she runs great. 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. Vernier isolastics, front and back, from Andover Norton. Standard box type head steady. I did not change the swing arm bushes as they seemed to be tight and correctly lubricated, (oil). In order to reduce the vibes I have been progressively opening out the the rear vernier isolastics. I read that if a commando is rough above 3000 rpm, open up the front isolastics, if below 3000 open the rear isolastics. The vibes are now somewhat tamed, but the front and especially the rear isos clearances are quite a bit looser that specs suggest they should be.

Now, after the iso adjustments I have made, when I move the rear wheel side to side I have approx 1/8" lateral movement. Is this too much?

I'm not the sort that scratches pegs on a regular basis. But it would be nice to feel confident that it I am not likely to induce a death weave. Don't think I have ever had a bike before with a rubber mounted engine etc. Definitely feels a bit different from the solid mounted older british stuff I am used to.

Yes it is too much. Can you tell us where the movement begins ?
Rim to hub ? I know you said you checked swingarm so engine/gearbox cradle to main frame ? Hub to axle?

Hi Richard:

The play is definitely the result of opening up the rear iso's. Before opening them up, there was almost no lateral movement. Current 1/8"movement is observed at the end of the swing arm / rear wheel hub.

Britstuff said:

Hi Richard:

The play is definitely the result of opening up the rear iso's. Before opening them up, there was almost no lateral movement. Current 1/8"movement is observed at the end of the swing arm / rear wheel hub.

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Have you checked your swingarm spindle play at the transmission cradle root tube (where the swingarm pivots from)?

You may need to secure it with clamps, oversize spindle or additional security bolt(s).

VERY typical to get slop there...

Hi Grandpaul:

Yes, I have checked for play in the swing arm. The bike was almost completely stripped back to the frame while I cured various maladies. I do not have the rear swing arm suspension clamps, which sound like they are better, but original spec swing arm bushings. However, they seem to be pretty tight.

I know I can remove the lateral movement buy tightening up the rear isos. The issue is that then I will have to live with increased vibration. Just trying to find that sweet spot of minimal vibration, but also not sabotaging the handling too much.

I know you said vernier isos - so rubber components new ?
If so it will certainly be worthwhile at this point to do one of the clamp systems. On my own bike I had nuts welded to the tube and used Allen head cap screws to bear on the shaft - of course for you this system will require a lot of disassembly so the clamps may prove easier.

Hi Richard:

Yes, new front and rear rubber vernier isos. I definitely appreciate that the clamped system is better, but that is not where my current issue is. The play is purely because I opened up the rear isos. If I close up the iso's the play will return to virtually nil, BUT I will have increased vibes. So, the question I would like to get answered is how much lateral play is acceptable - not what is optimal - but what is acceptable.

I'm certainly used to bikes with little to no lateral play in the rear suspension. What I am not familiar with is the Commando setup. The whole rubber mounting thing is new to me.

I work to 8 thou on the rear and 10 on the front. No vibes above 2500 rpm with original 850MK2a rubbers in the rear and AN replacements front with clamps on the cross tube so the rear spindle is supported.

Hi Kommando:

I reckon I am currently around 12 thou upfront and approx 20 thou at the rear. It may be that the rubbers in the new isolastics are of a harder compound than original. I was able to ride the bike for a short test ride, before I pulled it apart for a rebuild, with the original non vernier (probably factory) iso rubbers front and rear still in place. Massive iso gaps - approx 40 thou front and rear, no vibes!

Decisions , decisions - I get it . I/8 “ slop at the rear wheel will haunt you down the road ( literally as well as figuratively) in terms of tire wear as well as handling. I am at a disadvantage here because my bike is a 750 and I have no experience with the 850 variety. Others here may be able to offer an opinion on whether the 850 vibrates more below 3k than the 750 . I know mine is certainly acceptable below the 3k point with no slop at the rear wheel - seems like that should be obtainable with yours as well .
Maybe the answer lies in snugging up the isos until you have no play at the wheel and riding it . If the vibes are too much back off and try again until you reach a compromise. At some juncture you may want to do a clamp system as it offers a way to take up the inevitable wear on the shaft/ tube relationship .

Britstuff said:

It may be that the rubbers in the new isolastics are of a harder compound than original

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You can use AN ones with no issues, RGM have made some too hard but not sure of them currently. Norvil ones are too hard and the central tube can be oddly sized as they bought the cheapest nearest size metric tube.

Britstuff said:

Hi Grandpaul:

Yes, I have checked for play in the swing arm. The bike was almost completely stripped back to the frame while I cured various maladies. I do not have the rear swing arm suspension clamps, which sound like they are better, but original spec swing arm bushings. However, they seem to be pretty tight.

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I think you are getting two distinct systems slightly mixed up:

1. Swingarm bushings - these play between the swingarm's "eyes" and the swingarm pivot spindle itself. They rarely go bad when correctly OILED (not greased).

2. Swingarm spindle & root tube - the hardened swingarm pivot spindle can become loose within the bore of the root tube that is welded to the transmission cradle. This is very common, and the clamp fix is highly recommended. The alternative is an oversize spindle, but the root tube must be bored out to exactly match the oversize spindle. This should not be done with a hand drill!

Hi Grandpaul:

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

I think he has correctly eliminated the spindle tube as the source of trouble because he can make the play at the wheel go away by tightening the isolastics .

When I move the rear wheel from side to side, I can see the whole engine and cradle move as well. When I pull back the left rubber gaiter on the rear isos and move the wheel from side to side I can see the iso's move. As Richard suggested, I have tightened the rear isos back up, (a bit shy of a 1/4 turn) to the point that the rear wheel now has minimal lateral movement. I think it is at this point that a spindle tube upgrade may well help remove what is left of the lateral play. It is now about the same as my AJS model 31, which is apparently supposed to have just a touch of lateral play. My Dominator's rear wheel has no lateral play whatsoever. Presumably one of the reasons she goes round corners so nicely!

I'm going to go for a ride now to see if I can live with the likely increased vibes.

Thank you everyone for your most helpful comments.

It might be helpful if someone who has set their isos to factory specs with good swingarm sturcture could tell Britstuff what he should expect in terms of rear wheel movement measured at the rear side of the rear wheel.
He is measuring 1/8". That measurement is made a long way from the front and rear isos.
It may be that factory spec isos will produce 1/8" inch rear wheel movement if you push the rear wheel sideways hard enough.

To reduce the vibes between 2500 and 3000 rpm ensure the timing isn't over advancing and carbs are correctly balanced, or you could fit the Mk3 850 suspensory spring device that works remarkably well.

Hi Stephen:

Yes, that is really what I want know.

Just went for a quick ride. Yes, the vibes a a bit more prevalent. But the bike feels a bit more more planted, which is definitely a good thing!

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!

I had a sloppy lateral feel in my rear wheel handling. I went through all the possible isolastic measurements and adjustments. I have a set of kegler swingarm clamps installed and checked that they were tight, but my rear end still felt sloppy to me. Eventually I tracked the sloppy feel down to my tire's sidewall flexing. I had a 110/90/18 on a 2.15 wide rim so the tire shape was more easily distorted at the normal 28-30lbs tire pressure.

Increasing the tire pressure adds rigidity to the tire shape, but it also reduces your contact patch area. So, there is a balance to be found if you have wide tire on a narrower rim, where you increase the air pressure to eliminate the tire sidewall lateral flexing but not so much inflation that you end up with a very small contact patch area.... It's something to play with after you've made all the other checks to see if that is where the sloppy feel is coming from...