Vibration above 3200 rpm

Hello, I think this is iso related but I'll ask anyway. Mk3, 1977. Stainless isolastics and rebuilt everything.

Up to 3000 rpm it's very smooth, "after 10 to 20 kms" above 3200 rpm a vibration starts which I can feel through the bars.
Drop to 3000 rpm and it's silky smooth.
A straight road at 3500 kms and the vivration is there, a slow left bend and the vibration changes, straighten up and the vibe doesn't go back to where it was before the bend in the road. Turn to the right or slow to 3000 rpm, then speed up again and it vibrates as it did before the bend. (hope this makes sense)
Any rev above 3000 there is a vibration I can feel in the bars, it's bearable but it isn't silky smooth like it is at 3k or below.

Isolastics too loose?
Front? or rear?
Head steady?

The vibration feels like a rumble, a bit like worn main bearings feel, but I'm sure it isn't mains.
It handles very well when pushed on mountain roads, no weaving or wallowing, very surefooted.

I noticed the vibration after doing 600 miles on a rebuild, on a highway travelling to the hills. (Never done this before.)

Regards Graeme

While I don't claim to be an expert here, here's a couple of things to check;

check to make sure that nothing rigid is touching or very near the motor/ gearbox / isolated unit. Could be some simple bolt or hose ( clamp ) touching between the frame and engine unit at higher revs.

What clearance are you running ? Start at 10 thou and fine tune from there. My method of setting clearances is::
Tighten front and rear mounts up with no clearance - this centres the unit in the frame. Shim headsteady so that it is perfectly balanced / even on both sides. Set the front clearance and then the rear clearance. I prefer to use the holes to ascertain the correct clearance - always had trouble getting a feeler gauge in there ! And make sure you dont lose the clearance when tightening up the nuts - I mark them with a paint pen first.

Don't forget the spring on the MK111 head stready - this can really make a difference. Your manual will suggest the clearance range to start with. ( can't remember off hand )

Best thing I ever did was to get my crank balanced by greg fitzpatrick of triton motorcycles in melbourne. I run at 5 thou smooth as....

I had the crank balanced during the rebuild, and it's pretty much the same vibration through the bars at anything over 3500.
It was when the bike was gently leaned into a curve that the vibe changed. Which made me suspect iso adjustment.
It has only just started being noticeable after 600 miles, so I thought the clearence has changed a bit. Maybe some paint has worn?????
But I thought loose isos made less vibes but added handling issues ?
The handeling is good, no wollows or weaves.
It's not terrible, but it is noticable, and it didn't do it for the first 600 miles.
I'll read up on the head steady bit.

Thanks petejohno

It was when the bike was gently leaned into a curve that the vibe changed. Which made me suspect iso adjustment.

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That would make me suspect something has worked loose and is contacting the frame and/or engine unit. Also check that the exhaust system is clear of everything. Finding these things can be a real pain. If your happy nothing is contacting, redo the iso's as described. And don't forget to suspend the bike by the frame rails only. Sorry if this is stating the obvious mate.

If you powder coated, could that have now chipped away causing loose bolts and other loosening connections? Hoping you both used balance factors around 50-60% for the Commando.

Yep, sounds like loose stuff to me.

You might have something loose but what you experience is what I've lived with for 13 years. New vernier ISO's made it worse, regardless of ISO settings. Anything over 4000 and it buzzes through the footpegs, not the bars now since I have clip-ons. I get a coarse buzz through the gear lever when depressed in top even though the engine feels smooth (gloved hand on head at 75+). Crank balanced to 63% wet though that made no difference. Frame aligned and straight so no excessive tension on ISO's. ISO ends squared off etc. Clutch assy checked for balance, crank and mainshaft within .002" runout. I'd love to get to the bottom of it but since it gets no worse after 4500 I can tolerate it though I have to run .010" on front and rear for it to be acceptable.
I blamed the new ISO rubbers but since they are quite soft I don't think it's an issue. I have a belt drive but it was the same with the chain.
Since it changed for you after the rebuild and changes on the road after turns and straights will be interested what you find. Mine's the same under all riding conditions. Hope you find something.

Since it changes with different stresses, it points to variability; variability is due to looseness.

One thing a Commando should be is smooth. I agree with Paul. Loose or broken. Once I'd checked all the fasteners I'd start looking for cracks. Headsteady, iso mounts, cradle, frame, etc.

An ugly thought. Are you sure about the bottom end?

norbsa48503 said:

If you powder coated, could that have now chipped away causing loose bolts and other loosening connections? Hoping you both used balance factors around 50-60% for the Commando.

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Not just chipped but if you bolted up parts with the powdercoat between the powdercoat will compress and make things loose. Go through and check tightness on everything.

After the rebuild was your last job to center the head steady ? If not it might be loading up the iso's.

Cash

What some folks blame on the vernier Isos may have more to do with increased displacement (745cc vs. 828cc)

Weird, you say your Commando is real smooth UP to 3000rpm?

Does not vibrate at 1500rpm while standing still, the front wheel does not dance up and down.

Does not vibrate at 2500?

How is this even possible?

I have backed off my isos all the way, and it still vibrates under 3000.

A smooth commando under 3000, the reverse of my experiences! Good luck!

Keith 1069 you might want check that having less than .002 run out on your crank and mainshaft is acceptable.

I thought the runout would have to be zero?

Last first,
High Desert, It does vibrate at 1500 rpm and shakes the wheel up and down etc. normal. At 3000 rpm it is silky smooth everywhere. Above 3k there is a vibration in the bars that stays there as the revs rise, it's not horrible but my question was why does it vibrate more as the revs rise above 3k ? For the first 600 miles of it's life I didn't notice this vibration. That's why I assumed the isos had worn some paint I missed and settled in ????

Mr Cash, I did't centre the head steady, thanks for the tip, I will check.

Shwoosh Dave, Jim C and GP, not powder coat, paint. I did remove (or thought I did) paint around the fixing points.
I see your point about something loose or cracked. I will check this weekend.
Jim, the vibration feels like a main bearing growl, but it changes as the bike is leaned slightly. I know what knackered mains feel and sound like in Ducati bevels, it's like that but the vibe can be felt in the handle bars, and goes away under 3000 rpm.

Keith and Norsba, balance factor was 65. It didn't vibrate pre 600 miles, so I would think balance isn't an issue ?
I never rode the bike before I rebuilt it, so I dodn't have any experience with it or any other Commando.

Thank you all, I will check your recomendations and report back.

Regards Graeme.

Check the top head steady. Side plates and box mount. Bike leans over vibration changes does a rumble? Been there felt that.

Mick

Mick, yes a rumble that changes when leaned over, felt throught the bars.
What am I looking for ? too tight, too loose, or loose box section where it's fixed to the head, and side plates loose? Do you mean that the mounting bolts may have come loose ?
This vibration is something that I could live with, but it didn't do it before.

graeme.

Jim, the vibration feels like a main bearing growl, but it changes as the bike is leaned slightly. I know what knackered mains feel and sound like in Ducati bevels, it's like that but the vibe can be felt in the handle bars, and goes away under 3000 rpm.

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My thoughts with mine. Changed the bearings which were near perfect but did not alter vibe.
Popular places for unintentional contact are the LH pipe to outer chaincase at the rear and inner case to swingarm end cap. Mine has marks from both (mostly from a previous life)! There is good static clearance on mine to the pipe (0.25") but I still wonder if it touches under some conditions as I know it has also done in my time with the bike.

GRM450

All the above advice is correct. I suggest that you take the following steps
1. Make sure nothing is contacting the frame. The exhaust is a commom problem - check rear rubber mounts as well. Look for signs as previous post.
2. check the clearances as I previously advised. You have'nt said what clearance you originally used . Don't forget the engine.gearbox and swing arm are independant of the frame. Isolastics are a compromise to keep the movement " manageable " and minimise vibration at the same time. Too much clearance and the unit will move too much and not only effect handling but let the whole unit flex and possibly contact the frame.
3. If all seems OK then it is most probably a mechanical problem of some sort.

And remember Nortons are not completely smooth throughout rev ranges. It is normal to have the front wheel bounce at low revs - it is normal for the bike to shake under 2500rpm then smooth out above that. You will still get some handlebar tingles on a high speed cruise although nothing annoying - certainly no worse that many 4 cylinder bikes. ( the only completely smooth bike I have ever ridden is a Norton F1 rotary ).

Hmmmmm....

Has anyone mentioned putting the bike up on it's centerstand and attempting to wiggle the rear wheel from side to side, to see if the swingarm spindle is a bit loose in it's tranny cradle bore?