Vibes driving me nuts!!

Not open for further replies.
Jan 10, 2005
Country flag
Well I'm back on this old subject again (vibration above 4000 rpms).
As a reminder I get a coarse and increasing vibration through the footrests and seat starting at 4100rpm. It really is spoiling the ride.
Since I asked the original question last year I have checked frame mounts for alignment, machined square both ISO tube ends and even shortened one side of front ISO (-.050") adding equal spacer to other side. ISO's are set at 008/012" (min/max around diameter) which is the best I can achieve. Previous to machining there was up to 008" max difference at 90 deg measurements front & rear.
ISO's are Norvil 'Verniers' which are definitely not linear in adjustment so I have to use feeler guage.
Crank was balanced last year with pistons, rings, pins & rods to 63% on various expert recommendations to give smoothness at higher revs. Runout is 002"/001" D/S - T/S.
Carbs are bang on for lift synchronisation.
Only thing I have noticed is a real roughness through the gear lever if I depress it when already in 4th. Could a bent mainshaft be involved? I know the layshaft is straight as that was checked over the winter before the needle race k/s conversion and I'm fairly sure that's not the source as I tried declutching at 4200 in 4th and I'm sure the vibes subsided.
I also put my gloved hand on the head and barrels at 4100/4200 and subjectively the vibes felt less severe than what was coming through the seat and pegs.
The 63% balance did bother me a bit but the vibes were just the same level before it was done. What seems to have improved is the normal 2500-2900 roughness. It's still there but not so nasty.
Primary is a 30mm Norvil belt and that is aligned and running loose as they recommend at 33mm total belt play - cold.
I've done nearly 1000 miles so far this year and with the PR fairing, clip-ons and rearsets it's a lot nicer. OK it's harder on the wrists under 50mph but worth it at motorway speeds. Well it would be if not for this poxy vibe problem.
Went out Sunday to try a few more things (bigger rear ISO clearances) but the neg batt cable decided to fracture as I pulled away and turned right uphill. The obvious happened resulting in only a bent gear lever and broken indicator. I was so mad I could have torched it there but there were too many witnesses!
Sorry for the long ramble but I need help on this one. Thanks.
Sorry about the lever & indicator.
Going through the logic - declutching at 4k+ - if engine revs & vibes die down then the source looks like Clutch cage/shaft or engine. If declutched & revs kept at 4k+ & vibes die down then would have to be power imbalance L to R.
I'd guess from the work done on the engine that it should be better than stock, so first stop would be run out on clutch cage.
Mind you, I'm very much still learning on commando mechanics, so I hope one of the gurus (guri?) :shock: can help out!
I would be running an indicator on each valve retainer and writing down the results of these tests. A cam lobe going away is just too common. If you take the time to use a degree wheel as a reference when recording the lift of each valve you can also double check the grind there are a few out there that are not made right.
You've more than likely changed the ISO rubbers but;
Have you checked for the daft things like the horn catching the swingarm, or some other bit catching on the engine/transmission when you're sitting/riding on the bike. Have you put the choke/throttle junction boxes between the headsteady side plates etc?
Do the vibes change with throttle position or speed?

All things being equal, the Isolastic system should be able to soak up pretty well all undesirable vibration. If it's getting through then either the rubbers are not doing their job or they're being by-passed.

Is handlebar vibration acceptable ?

The only time that I've had numbing Matchless 650 style vibes was after powder-coating engine plates and going on a long motorway trip. The rear bolts loosened off and the primary chaincase was contacting the footrest hanger ! I'm not suggesting this in your case but as Cash says, If any component is "bridging the gap" then you have a route where the vibration can cross.

Which head-steady are you using ? I had a problem with low rpm low-frequency vibes which was causing the steering to shake. This occurred with both standard and Norvil steadies. The Dave Taylor headsteady seems to have sorted that and made it smoother overall.
Only time I had bad vibrations in the foot area, and not in the hand area, was when I had the rear iso set so tight that they couldn't absorb the vibes and the vibes just got passed on to the feet. All went away, when I gradualy loosened the setting until the iso could do it's job. I had the luxury of a rafter to tie the rear frame loop to, thereby allowing the weight to be taken off the iso...but it must be somehow unloaded so you can see when the iso moves up and down a bit. I wouldn't have any nightmares about an unbalance somewhere until I made SURE the iso and it's innards were in order, and moving. If they don't work, you won't have any real idea if the vibes are normal, or not...
Thanks for all the responses gents.
I ran a test on the clutch and gearbox last year driving the whole transmission and wheel with a high power electric drill on the clutch nut. Got it up to 70 in top and did not feel the onset of any significant vibration.
When the motor was apart December 06 for new mains and bare cases to do the frame alignment I checked the cam. It had 12000 on it and negligible wear. I did run it in carefully (2500/20mins off load) and since ring break-in have used synthetic oil. Lift is 368/371 inlet and 373/374 exh. Pretty good for an RGM but it is out on timing, needing 5 deg advance to get close to spec.
I did think about parts touching where they shouldn't and I can't see any problems with a static check, except the outer case to pipe. With the previous Combat engine and shim ISO's (turbine smooth but explosive!) the LH pipe did contact the case, as did the LH footrest. Both marks are still there but no footrest as I have rearsets now. Clearance to pipe is 1/4" so I don't think it's touching but will tape up the case mark and see if gets rubbed. It shouldn't give the vibration I feel as both parts are engine mounted and I always figured it's just the exhaust waggling on it's rubbers more than the engine in that annoying 2500-2800 range?
No choke but throttle cables are loosely run inside side plates. Vibes change with engine speed only, through the lower 2 gears not so bad as revs are only held for a few seconds but you start to notice it in 3rd and where it matters at 4k and 72 mph. (4.114 gearing with higher 35/72 primary ratio and 21T).
Steady is 850 and that used to touch the 1" frame bracing tube where it's relieved at the rear. But, that was with the old engine and no high rev vibe issues. Bars are clip-ons and vibes are not as bad as what I feel through the seat and pegs.
The Norvil vernier ISO's still bug me. Rubbers feel quite soft and easy to install but the way they refuse to adjust in a linear way annoys me. The temptation to revert to shims was resisted but I may yet try this. The fact that all the rubbers are molded on the same tube confuses me to thinking that they might have slightly altered characteristics to the shim type with it's loose assembled centre buffers?
The one bonus of all the frame and ISO tube machining/aligning is that the bike now steers straight, does not weave at any speed (up to 95 is all I tried) and generally feels nicer.
Rear ISO is now at 010/012", front a bit less. I will try the Hewho method and relieve the load. My usual method is by the book with rails supported.
Thanks again......have a feeling this will not be easily resolved, not after 3 years living with it anyway.
As your in the same part of the world as me, Hampshire, why not consider doing what I intend, which is taking the bike to Norman White at Thruxton. When I was there with my gearbox parts a few weeks ago he offered to set up my iso's for me when I got the bike back on the road, which I intend to have him do soon. He reckoned that most people don't know how to set em up properly and he can do it in about half an hour.
If all else fails I may do that. I sometimes overcomplicate things and maybe imagine 'faults' that are just plain normal but in this case the vibes are real and unlike the I am defeated. Never had a problem with the old shim method so I do wonder if the Norvil conversion is to blame. Can't see how Norman could make them work when I've tried all permutations. The vibration started when I installed these which I originally put down to poor adjustment, then out of square tube ends, possible poorer tolerance to slight frame tab mis-alignment than with the shim type etc.
Good luck with yours but knowing Norman he probably won't be impressed with Norvil's parts. He usually isn't!!
Keith, I think that you're probably getting to the crux of it here. You seem to have tried just about everything and Commando motors can be wildly differing in balance without it getting past the Isos - IF the Isos are compliant.

There has been quite a lot of talk over the years about replacement rubbers being "too" hard. I don't know if it's particularly a Mk 111 thing but they do have a name for not being quite as "sweet" as the earlier models. perhaps some of that is due to the bonded rubber construction.

When I last had contact with RGM (a few months back) and asked how the rubbers were, he replied something along the lines of "A lot of people find them a bit hard" I was referring to bonded rubbers. I don't know if the same compounds are used for the loose rubbers. How are your old rubbers ?

You've really got nothing to loose by substituting (apart from a weekend in the workshop and a bit more maintenance in future :) ) That's a small price to pay if it puts the bike back as it should be.

I can't wait to hear how you get on. Fingers crossed !
Do you check the ISO gap after tightening up the main bolt or before? And, have you left enough clearance in the non-adjuster end to ensure free play once the system is tightened up?
At 4000 my old 850 is turbine smooth.
I've never been too happy with the isos on my 750. One thing I did that smoothed it up a bit was shimming of the stock head steady. I noticed once while tightening the stock bits that the engine was pushed over to one side from where it rested without the headsteady installed, putting a cocking load on the isos. By installing shims/washers between the triangle plate/head steady on one side, and between the Lord mount/triangle plate on the opposite side, I got it to bolt up tight without pushing the engine over. This helped my situation a bunch, but did not yield "turbine" smoothness.

I tried the Norvil front iso too (oem MK III in the rear), and went back to used-but-serviceable shim type isos up front, which helped a bit more.

My sweet spot now is 3200 to 3800rpm. Vibes pick up above that. Not terrible but certainly no turbine.
That's a good point, though you shouldn't need to shim as the mounting holes in headsteady bracket are elongated to allow it to be moved either side.
Yes, I'm favouring the rubbers as the source, thinking the bonded design is better suited to a Mk3 than my 750 although I have little basis for that theory except that the rear loses the centre rubber. That has to be a major difference. While they seem relatively soft are they all made the same way (harder material) to cope with the extra weight and one less rubber. After all, Mk3 had the additional top mount spring device or was that just for tuning low speed shakes?

I guess it's an obvious one but yes I do check clearance after torquing the bolts to 25lbs/ft.... and forcing engine/trans away from checking side. But you made a very good point 'cash', free play in non adjuster end.
I checked the rear and found play AT the adjuster end but slightly loaded at the fixed cap. So does having an even gap both sides make a big difference? Can it be that critical to have 4-5 thou each side? I know the shim style needs equal shims both sides but to get the rubbers centred within a few thou cannot be possible, with luck maybe! The same would go for shim design anyway. I remember that 'Turbine' smoothness even if the old Combat motor never went more than a 1000 miles without some problem. More than once I saw 7k in 1st & 2nd without even realising it was up there. It was just so sweet.

After installing engine, head steady is left loose so it can find it's position after the main ISO's are torqued up.

I'm going to find a way of dropping the power cradle to see if I can remove and replace without another total stripdown.

Thanks for all the advice everyone. Will let you know what results.
Not open for further replies.