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Old 08-27-2012, 02:11 AM   #1
fiwi OP
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Pulsing/vibration.

Hello all. I have an issue i woiuld like your opinion on. I posted it here a few months back but seeing as I havent riding the bike alot lately I let it slip. Time for me to get it sorted out.

Roughly around the 15000km mark, my bike started displaying what can only be described as a pulsing/vibration, peaking at around 3200rpm, and only in 5th gear. Every other gear is fine. I took it into the local BMW dealer and they gave me the usual line, feels normal. Whatever. I was notified of a chain recall when the bike was at 18000kms. Got the chain done and to my surprise the pulsing/vibration was no more, great.

Recently I took the bike away on a long trip. Everything was going fine till the bike hit around 30000kms, when the pulsing vibration came back. Once again peaking at around 3200rpm, and only in 5th gear. All up i guess I notice it to some degree for a range of about 500rpm. It is very noticable, to the point where I dont like using 5th gear in that range at all. I feel it in the hand grips and through my feet. It is very constant, very rhythmic. I tested it on the way home from work last night. It pulses at a rate of about 160 pulses/vibrations per minute @3200rpm, which is about 73kph, or close to it. If I clutch in and coast at that speed the vibe is gone, so it is when the bike is under load only, so I assume that it isnt the chain or sprockets. I have tested the chain for stretch, checked it right the way around the length of the chain and it appears to be fine. I could be wrong.

I dont have a huge amount of mechanical knowledge, but I have a couple of queries about this. I originally thought it may be chain or sprockets, due to when it happened the first time, and i had the chain recall done, it seemed to fix the problem. What i cant figure is that if it was a problem with a certain point on the chain, or a sprocket, then at 72 kmh the pulses would be alot faster that 160 per minute, given the length of a chain or the circumference of a sprocket, you get me?. I dont understand what would cause a constant 160 vibes per minute at that speed. I dont understand how the internal gearing works but once again, Im sure nothing has a cycle rate? that slow.

I dont just want to go out and buy a chain, sprockets and chain tool straight away, a few hundred dollars worth. It could be wasted money. Any thoughts?.
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Old 08-27-2012, 05:42 AM   #2
MikeMike
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Check that you haven't lost a balance weight from a wheel.

Have you done a thorough cleaning of the chain? I mean really degrease it, scrub it, and check it link by link?
You might find "fronzen" links or a specific tight spot, a location on the chain that is way tighter than other points.
Check your sprocket teeth for "hooking" where the sprocket teeth take on a distinct "wave crest" type look where the point is ahead of the base. Front and rear sprockets will be, of course, hooked in different directions due to loading.

Check that your motor mounts are torqued correctly, however I doubt it is that.

The last thing, when you do your oil change, check for metal debris, more than the usual fine amount, on the magnet in the drain plug. The internal bearing on the clutch side could be going south. But you'll likely hear that easy enough.

Just a few ideas for you.
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Old 08-27-2012, 05:48 AM   #3
Snowy
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Harmonic.

Tyres, chain, sprocket sizes/wear, gearbox output bearing (check chain tension - make suer it's not over tensioned) and the vibrations from the parallel twin are all coming together to produce a harmonic.

Ride with a half helmet (skull cap, pudding bowl - what ever you guys call them over there) and it'll sound like the chain is slapping against the swingarm as the surging starts.

It's a motor that's prone to harsh vibration at very specific points in the rev range, which is why it has a dirty big counter weight in the sump. Combine that with secondary vibration of any kind and you get a harmonic when the vibration frequencies line up. So it appears as a much lower frequency "pulsing".

If you have ever worked on an in line 4 with a counter balance shaft, and assembled it one tooth out of sync, you'll know what I mean.

I did a lot of Mitsubishi engines 20 years ago and they often didn't have clear marks on the timing drive gears. So you'd end up one tooth off with the counter balance shaft. Hard to know for sure as they still felt remarkably smooth when revving them. But they'd have one point in the rev range where the out of balance caused a harmonic.

My method for testing was to put a glass of water on top of the engine and gradually bring the revs up to redline. You hit the point where the harmonic occurs and the water just disappears. The glass a split second later. That's how bad a harmonic can be. A very narrow rev range of maybe 50 rpm where the engine just tries to shake itself to bits.

A combination of factors is causing a similar issue in the BMW drive train. Mine has it, as did the other 2 I've ridden. I figure it must be normal. It hasn't broken....that bit....yet.
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Old 08-27-2012, 05:53 AM   #4
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Two things I have noticed in the 3 years with my F650GS is that 5th gear at 100kmh is the very smoothest and it is overall, way smoother between 140kmh and 160kmh on the highway than between 110kmh and 140kmh.
Oddball bike.
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Old 08-27-2012, 06:53 AM   #5
Lokey
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My F8 does exactly the same thing (fifth gear pulsing) with about 14K miles on it. I feel it is a product of riding for two weeks in the rain in Canada and Alaska and not being able to lube the chain properly. My can of chain lube emptied itself and it took me a few days to find another can. Shortly after that I noticed the pulsing and it has done it since then.

If you look at my chain it has links that look slightly kinked even though it has been lubed. I feel it is a product of the harmonic vibrations that Snowy mentioned and sympathetic vibrations that develop at a particular RPM + tire speed + chain wear. I adjusted the chain and have largely eliminated the pulse, but it is still there.

I am going to take a five day trip this weekend to Northern California and am planning on replacing the chain and rear tire when I get home. While the chain shows no real sign of being worn out outside of the kinks, I feel that and a new tire ( old one is a Heidenau K60 that was installed in Fairbanks) should fix the problem.
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Old 08-27-2012, 09:54 AM   #6
fiwi OP
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Ok, I hear where you are coming from. One thing I don't understand though. As I said, the vibe/ pulse is at the rate of about 160 pulses per minute at 72 kmh. I would have assumed that any pulse/ vibe in relation to the chain would happen on every rotation. 160 pulses per minute is way too slow for that. In saying that, I don't really understand this harmonics thing very well.
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Old 08-28-2012, 05:33 AM   #7
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Lokey, did your vibes start when you fitted the K60?
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Old 08-28-2012, 05:59 AM   #8
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Fiwi:

It would be an interesting (and fairly easy to perform) test to swap out your front sprocket for a sprocket with one more or one less tooth just to see what happens. Stock on the F650GS-twin is 17T.I know they make an 18T and the F800GS comes with 16T maybe a pal has one you could borrow?

If any pals have a chain with a master-link in it perhaps you could do a chain swap for grins as well?
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Old 08-28-2012, 09:44 AM   #9
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My guess is your front tire may have gone a bit out of round. Spin both the wheels with the bike on the center stand. If there is a bit of a wobble you may have found your culprit. If the tire is starting to cup a bit that can cause the same problem. As someone else pointed out you can get similar symptoms if you threw a wheel weight. Dave
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Old 08-28-2012, 09:47 AM   #10
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If it was a tire problem wouldn't it be just related to speed and not which gear the bike is in?
e.g. 60mph in 5th gear or 60mph in 6th gear would have same vibration?
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Old 08-28-2012, 09:59 AM   #11
fiwi OP
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That's right JR. A stuffed tire or rim wouldn't just affect 5 th gear at 3200rpm. Also when I pull the clutch in and coast the pulse/ vibe is gone. I don't understand how the gears work as such, but is there anyway 5th gear could be stuffed?. Do gears/ cogs become problematic?. Sorry, my knowledge and terminology on this topic is lacking.
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Old 08-28-2012, 11:24 AM   #12
fiwi OP
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JR, can you please tell me the reason for trying a front sprocket with more teeth?. I'm willing to try anything. I don't know anyone I can borrow a sprocket from but for $50 I'm quite happy to buy one. Not so keen on doing the experiment with chain yet as it becomes expensive if the. Bain isn't the problem.
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Old 08-28-2012, 12:48 PM   #13
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Quote:
Originally Posted by fiwi View Post
That's right JR. A stuffed tire or rim wouldn't just affect 5 th gear at 3200rpm. Also when I pull the clutch in and coast the pulse/ vibe is gone. I don't understand how the gears work as such, but is there anyway 5th gear could be stuffed?. Do gears/ cogs become problematic?. Sorry, my knowledge and terminology on this topic is lacking.

I guess 5th gear could be stuffed, except it is not one of those things being reported at a problem. What you are describing can occur if you have a frozen link, which is not uncommon. My experience is, buy a new chain.

Pulling in the clutch is not a test for a frozen link, there is no torque being applied to it. Look at your chain again.
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itsatdm screwed with this post 08-28-2012 at 09:43 PM
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Old 08-28-2012, 01:00 PM   #14
fiwi OP
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I am trying to steer away from new chain and sprockets for now if I can, but possibly that is the only way to eliminate them as the culprit. Call me slow, but I still don't get it. If there was a kink in the chain then surely at 72kmh the pulse rate would be a lot faster than the 160 per minute I counted. And 5th gear only. Tomorrow I will do a
Bit more testing on the chain. Thanks all.
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Old 08-28-2012, 08:20 PM   #15
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The "only in 5th gear" thing really has me

Just to confirm - you are saying that if you:

1) drop to 4th gear while maintaining constant SPEED it goes away (because the RPM goes up?)
2) shift up to 6th gear while maintaining constant SPEED it goes away (because the RPM goes down?)
3) hold 3200 RPM in either 4th or 6th gear there is no problem?

It makes me want to ...

I suggested a different chain and countershaft sprocket because it would be interesting (and if you didn't have to buy the parts ... not too expensive) to see if the problem went away with a new/different chain, or it the problem moved to a different RPM range if you changed the front sprocket.
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