Vibration

[jp]

Had the divi for quite a while now, and it has always suffered with vibration of the fairing and mirrors around 4250rpm.
I know they are prone to this and remember some discussion on this forum (which I cant find)
I've replaced the fairing mounting screws/rubber bushes but no effect.


What I need to know is if anyone knows why the vibration is there and what can be done about it.

I notice that the vibration is still there with the throttle closed so probably not carb balancing.

and its getting worse, puts me off riding it

cheers

[cam7777]

Is there any cracks or breaks in the fairing?

Sometimes when a bike has been 'over', the fairing frame gets bent slightly and pushes the fairing into a position where it can touch where it shouldnt, the tell tale signs can be a crack or graze on the fairing itself.

I would start by removing the fairing and checking for correct alignment and cracks when reassembling.

[amorti]

They all vibrate like a bastard at those revs. You can alleviate it by balancing the carbs, replacing the wellnuts in the fairing, and even using window insulation strip between the layers of fairing if they are vibrating against each other.

But they all do it, and I don't think there's much you can do about it.

I did those things to my bike then realised the headlamp was hanging by one of its mounts only, and the other one was vibrating the two split ends.

[CD]

If you are considering getting bar risers, Rox Anti Vibration risers make it much smoother to use and the mirrors will be usable. But, they ain't cheap and they wont stop noise from the fairing.

[jp]

I've had the fairing off and replaced the well nuts, and didn't see anything out of place.
The engine runs OK, apart from the vibration, and as the vibration is just as bad on the overrun I'm assuming that its not carb balance. (Even so I'd like to balance the carbs but my tool buying allowance is spent out for a while )

The vibration can be felt through the footrests, but the effect is amplified by the fairing

what I really want to know is
a)is it worse than other divis
b) what aspect of unbalance in the engine is it coming from.

I know there are ways of cancelling out vibration either actively or passively and am interested in exploring this but need to know beforehand that its not a simple fault with just my bike.

Has anyone tried to get to the root cause of the problem or come up with a workable solution?

It would be less annoying if it wasn't at the engine speed that I mostly want to ride at.

[amorti]

The motor is air/oil cooled so tolerances are greater - this adds to vibes.
The motor is rubber-mounted, these rubbers are now 13 years old and probably hardened a bit, so that will add to the vibes.

Really, it's one of those annoying things these bikes all do, that I have seen. And all at the engine speed that's most useful / economical.

[jp]

anybody tried replacing the rubber mountings, did it help

its a bit expensive to try on spec.

where does one get such parts in the UK?

[amorti]

I've read some folks on here have had to remove the engine for whatever reason, and it sounds like mostly the bolts will be seized.

[fazerbill]

I remember back when I had a GT250A if I torqued the engine to frame bolts at different loads I would alter the amount off vibration and where in the rev range it happened.
So it may be worth trying this to get rid of the vibes. Higher than standard torque and its sqeezing the engine mounts more so it gives less movement in them. lesser torque than standard will give more movement in the bush.

[CD]

I balanced the carbs on my 10 years old Divvie 600 and made absolutely no difference to the vibes.

MZ strokers have a soft rubber mount on top of the engine, with the engine pivoting on the rear mounting. The engines bounce all over the place, but not much vibe gets through to the frame.

Newer, softer mounting rubbers might be just the ticket.

[jp]

I've been looking into the mounting replacement route and have found the following (along with more arcane knowledge whilst distracted in the byways of google)

there are 6 rubber mountings on the engine plus 4 on the exhaust system, as far as I can see. This is from the fiche, parts lists, haynes and looking at the bike; alarmingly they all have different part numbers.

replacing all these would cost a considerable amount judging by the prices quoted here (I can't find a UK supplier)

not having removed one (yet, waiting for the WD40 to work) I dont know what form they take, ie are they loose rubber bushes or do they have a bonded metal insert, perhaps someone who's removed an engine could say

looking to find a generic replacement from a rubber things producer, eg
or
refurbish/make replacements from castable elastomer of some kind, eg
or
carve some from the solid (rubber).

as an interim I'll try re-torquing the mountings, assuming I can get them loose.

I also have in mind attacking the inside of the fairing with sheets of lead and adhesive.

I'm still not sure whether the resonance at ~4250rpm is engine based and being amplified by the fairing or is a pretty flat (spectraly) vibration from the engine feeding a resonance in the fairing, I guess taking the fairing off should tell.

[CD]

Chances are the rubbers are hard and worn so vibes are getting through. Some new rubbers could be moulded from 2 pack urethane, but who knows what density it needs.

[amorti]

On a rubber mounted engine, I can't see what changing the torque would do, as you are only torquing the top-hat spacer? proviso: Not taken this bit of an XJ apart myself.

[bobh]

The revs at which the vibration is worst seem to be exactly the same on every XJ600, whether faired or not (4000 - 4500), which tells me that it's a basic characteristic of the engine and not something that can be varied by changing the stiffness of the mounts etc. All that will do is to change the amount of vibration that's passed through into the frame and the rest of the bike. The big unknown is whether the mounts should be more or less stiff to reduce the amount of vibration getting through.

From my experience of XJ600N's, one with a standard exhaust and one with a Nexxus 4-1, they both vibrated to much the same degree, so that implies that the exhaust mountings can be eliminated as a source of amplification.

On the whole, if the main concern is sympathetic vibration of the fairing, I'd attack that rather than the engine mounts. Adding lead sheet is an interesting idea - at least it'll stop you being x-rayed while on the bike!

I'm with Amorti on the torque issue - all the mounts are pulled up tight onto a steel spacer, so unless any are actually loose the actual torque shouldn't have an appreciable effect.

A final aside - I always noticed the vibration through the footrests at the start of a run, and then it seemed to go away - I think it was just a question of me getting used to it rather than it getting better after the first few miles.

[fazerbill]

Well Amorti it used make a difference on the Suzuki. Cant really say why but it did.

[jp]

well, I removed one of the rubber mounts,put it back then torqued all the mounts with the following results. (some pics here)

1. the to LH front mount came out easily enough, its more complicated than expected so a generic part is unlikely, tho' home made would be possible. It looked in good shape, still 'rubbery' if a little loose of fit in the crankcase.

2. the bottom LH front looks to be the same arrangement but I didn't remove it.

3. The RH front and all rear components seem to be accessible only with more drastic disassembly, ie engine out, so I didn't.

4 I noticed that that the large washers on the ends of the LH mountings have a flat so as not to foul the crankcase. On both top and bottom this had been ignored and the washer was tight against the crankcase, obviating the whole point of the bush, ie isolating the crankcase from the frame.

5. I repositioned the washers and tightened all the bolts.

6. On running the engine and after an extensive supervised road test (ride to Camborne in the sunshine with Jude) I found the vibration definitely changed. The really annoying peak buzz narrowly centred around 4125rpm was gone, replaced with a wider range vibration from 4000 to 5000rpm, but definitely of lower amplitude, to such an extent that I'm not now constantly trying to ride either side of 4000 rpm.

The amplitude of the remaining fairing vibration does seem to be solidly connected to that felt on the footrests so the fairing is just acting like a sounding board. I tried to exert pressure on the areas of the fairing that were moving the most but couldn't diminish the vibration, making me think that any application of material to the fairing would be futile.

Whilst doing these examinations I noticed that a couple of the valves were noisy so I'll put re-shimming valves on list.

[m40man]

Jun 11, 2010 21:40:01 GMT jp said:

.....The really annoying peak buzz narrowly centred around 4125rpm was gone, replaced with a wider range vibration from 4000 to 5000rpm, but definitely of lower amplitude, to such an extent that I'm not now constantly trying to ride either side of 4000 rpm..

Well done that man - well worth the time spent, it would appear .

[fazerbill]

Well done JP.

[bobh]

Well done. It just goes to show that you can't beat a bit of good old-fashioned common sense diagnosis.