Isolastic question

I have put my 73 850 back together and went with the MK 3 type isolastics. I set the gap a little loose. I was told to set it .06 and set it to a tight .08. The vibration is still there up untill 3500 rpms you feel it in pegs and can't see any thing in the mirros. When I first got the bike 14 years ago it seemed like around 2500 rpms you would loose vibration. I rechecked both mounts and it seems like the gap on front on top is looser then on the bottom with the feeler gauge. The gauge seems to glide better threw the top of the front mount and on the bottom it seems to be tighter. Any ideas?

Guy

Guy,
Been through the same on my Mk2 850 some years ago and here's the drill. The gap setting as a reference for the old shim Iso system only should be 0.010". But with MK3 vernier Iso's you may need to experiment with different settings. I first did mine by the book, rotate to firm and back off 1/4 turn. It was rough as rocks in a cement mixer! First, don't do this with the bike up on the main stand. It need to be supported on the frame only, i.e. a crate under the frame or stands under the foot pegs. Mark the Iso collars with a felt pen as a reference mark so you can gauge how far you rotate then.Try winding both front and rear Isos to firm and then back out 1/4 turn. Take the bike for a ride and feel what its vibes are like. If neccesary come back and wind them out some more. You can alter vibration behaviour on different sections of the frame by having front and rears set differently.

Also, are you using the MK3 top mount dual hanger spring? I found that to be absolutely the best supplement to the MK3 Isos. Cheap at $30.

Mick

Yes Cdo's should go silent about 2500 rpm. What allows this w/o sloppy loose isolastics is a mystery I'm always on look out for. My friend's '71 is so bad I don't care to ride it again till its calmed down below 50 mph in 4th. My deer crashed '72 would go smooth ~2500 but in some conditions would still buzz me. Never could find the key condition to reproduce buzz for idea of fault. I took tools to adjust the gap while out on the road. I had to open iso gap over factory to get relief, so I suspect front tabs are skewed and touch engine too easy. I know its engine is tipped to LH from injury but was still pretty pleasant over all.
The mount tabs ideally should be parallel but I don't know how they might be trued w/o making them worse. Skewed tabs would measure as tight on one half and open on the other. But so would the whole power unit if tipped by the rear iso mounts. Commandos are famous for a symptom or mis fit showing up in one area but the source is farther away. There is also uneven pull of drive train to twist front to LH and rear to RH.

I cheated on my Ms Peel special and ground the doughnuts thinner-softer all around so she disappeared about 2300. Maybe should only thin the front set and still get the rear stabilizing beef w/o annoyance getting though close settings.

hobot

My experience was very similar to Mick's. Initial settings per iso vendor instructions were teeth chattering. I backed them off until vibration was tolerable and fussed with them a few times over the first 200 miles or so until things seemed to settle in. Haven't touched them for 3000 miles or so now. My impression is that the verniers don't give the same characteristics as the old shimmed isos I had. The original setup was like a switch at 2500 rpm and at 4000 it was smooth as a multi. What I have now buzzes a bit more over the whole range.

Saw this posted once

" Vibration up to 3000 RPM - Rear tight, Front OK
Vibration 3000-5000 RPM - Front tight, Rear OK
Vibration up to 5000 RPM - Both tight"

bmwbob said:

Saw this posted once

" Vibration up to 3000 RPM - Rear tight, Front OK
Vibration 3000-5000 RPM - Front tight, Rear OK
Vibration up to 5000 RPM - Both tight"

Click to expand...

Interesting, I haven't seen that, but will have to test it. I just recently reset mine and managed to eliminate some of the vibs, but up to around 3K it is somewhat rough

I solved a good deal of the same vibration issue you have by switching to new, '"softest" iso donuts.

There was a batch of hard rubber ones sold years ago, vibration transmitted to the frame did not go away until at least 3500rpm.

Just a thought, especially if your's have been in the bike for more than a few years.

In addition, regardless of how soft they are, further contributing to vibration is the rubber donuts "sticking" to the inside of the iso cradles. The cradles themselves need
to be cleaned out and both them and the donuts lubricated with silicone so they are free to squish and move without binding or galling.

Here's some more on isolastic dynamics to loss sleep on formed in place concepts.
[Broken Link Removed]
From: John Pinkham
Date: Mon, 6 Jun 94 17:35:40 EDT

Diagnosis:
Vibration: up to 3000 RPM
o Rear engine mount tight- remove 0.005 shim
o Front engine mount O.K.

From 3000-5000 RPM
o Front mount tight- remove 0.005" shim

From 0-5000 RPM
o Both Mounts tight.

Don't support the bike on the center stand when servicing the isolastics.

From: Chuck Kichline
Date: Tue, 7 Jun 94 10:52:31 CDT

> Diagnosis:
I believe the best (and classic) method is the "night ride" where the analysis and adjustment requirements of the system is diagnosed thusly:

1) All tests should be made at 1000rpm intervals. Each interval should be tested for ~ 30 seconds.

2) All tests should be made in appropriate gear for use at that rpm.

3) Rider will position himself in front of an automobile who has turned on their headlights and is driving at an appropriate speed.

4) The rider will examine the pattern made in the rearview mirror(s) by the headlights for diagnostic wave patterns.

5) Diagnosis:
Figure 8 below 3000rpm
o Rear engine mount tight- remove 0.005 shim

Figure 8 above 3000rpm
o Loose front engine mount.

Horizontal line 3000-5000rpm
o Front mount tight- remove 0.005" shim

Vertical lines 2000-4000rpm
o Loose rear engine mount.

Squiggles at all rpms
o All mounts loose.

Handlebars squiggle at all rpms
o All mounts tight.

Two rock-solid spots above 5000rpm
o Blew your engine or flying off road d/t eyes off path too long.

I open the front up to .010 and it helped I checked the rear and its a tight .016 took it for a ride tonight and I feel it in vibes in the pegs. Should I go more then .016 can you go to far? The bikes still handels well. I have checked the gaps with the bike on its wheels no stands. I don't feel it in the bars like I do on the foot pegs.

Thanks,
Guy

If you know what a low air tire feel like and can ride that down to a stop, then you likely can not set isolastic gap too loose as even so it takes some pressing to enter The Hinge state and its looser-earlier onset is easy detectable-correctable before its unrecoverable. I'd go with smooth disappearing Cdo over hot rod racer riding.
Having both at once is crazy making wonderful, but who can survive WOT everywhere, so smoothness is top of Commando features over all others.

It might be worth it to take out rubbers and thin their edges narrower to soften the nuance vibes but still solid up for good corner control with tighter clearances.
Some have suggested drilling a ring of hole around donuts but I found drill bits don't like rubber and mostly holes just close right up clogging bit so grinding wheel was my choice and glad of it. I reduce Peels by one half width.

hobot

I was going to fit MkIII isos to my MKIIa until I noticed they were branded 'heavy duty'. A second look showed the front assembly to be moulded as a single piece.
The ones I finally fitted are soft and had similar characteristics to the old ones - discussion on the forum revealed the MkIII rubbers are harder.


My bike is all smoothed out by 3000rpm - not sure if it's good at 2,500 as I've never really checked properly. I also run the MkIII spring device, primarily because I have the DT headsteady fitted.

I have had a couple of commandos and jacked around with the iso settings on both and never did I feel they were a smooth as a multi. I think it's important to remember that they were smooth at the time COMPARED to other vertical twins of that displacement range. They were never even close to a 4 cyl honda - and I had a 550 and a 750 Honda during the period I owned a Commando in the 70's.

I remind you'al that even hobot got two factory Combats isolastics working nice.
Unlinked one under 3000 engine vibes go silent so just feels like a light wt bike easy jossled-jiggled by road and wind effects. The tri-linked one with 5 lb lighter flywheel BF 52%, plus narrowed rubbers and factory manual gaps, flat disappeared
under 2500. Uncanny addictive sensation or rather lack of it. I am not happy till I can hardly feel my Commando's but for smooth thrust forward and easy flowing.

My bike would not smooth out until at least 3500rpm, despite putting in new soft rubbers, DT head stead along with the Mark111 spring.

I recently did a complete overhaul on my motor, valves, guides, springs, pistons, rings, new cam, etc, etc.

Now, it is as smooth as silk at 2500rpm.

Perhaps our motors themselves are getting worn out, loose, and this in itself is presenting vibration that cannot be cancelled out under the often mentioned 3500rpm level?

Maybe this does not make sense in theory, but it absolutely made all the difference for me after the motor rebuild.

OH Ugh highdesert. you mean to say the extra vibes can come form a worn loose engine, oh Brother. Hope that's not what's up with my mate's '71, that does not smooth up till 50 mph in 4th. No tach to know rpms but made annoying to have to stay in 3rd to tolerate just flowing with non freeway traffic.

Let me ask you guys this. The main reason I switched my bike over to mk3 type was that I was having a vibration problem I felt it in the foot pegs my feet would become numb with in a 2 mile ride. When I first got the bike 10 years back it would smooth right out. When took the rear isolastics out they where deformed.
With the mk3 type in there it is better but not by much. Like I said my rear gap is .016 right now. Could there be somthing else giving me this problem. I don't feel the vibration in the bars.

Thanks,
Guy

Even setting things up properly with new soft donuts in the cradles, the vibration may never be as less as it was ten years ago.

Your motor may be, like mine was, simply worn out with inherently more vibration.

It was only when I went through a complete rebuild that now now it is as smooth as when I bought my Commandos brand new in the seventies.

Isn't .016 quite a big gap? I think I set mine to .010 or there about and mine smooths out around 3K. According to bmwbob's chart I have an adjustment out in the rear.