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Vibration at 8K driving me crazy

3K views 16 replies 4 participants last post by  Plasmablaster 
#1 ·
Kind of silly this but... I just got to ask.

After returning from a 300 mile trip and going at about 100-110 mph the bike suddenly started making a very felt vibration. It was mostly felt through the footpegs but also through clip-ons. The vibration would subside to normal when RPMs dropped below 7500. I pulled the clutch and revved the bike to see if it was drivetrain related but it was still there. During the next days I did some calm rides through town.. everything smooth below 7500RPM. Today I went out again. I couldn't do 100mph due to heavy traffic so I had to go at 70 mph, 3rd gear to raise RPMs to 8000... the vibration was there and even stronger - now I could also feel it through my bum.

The bike in general pulls normally, no loss of power. It's just the damn vibe that wasn't there before and I'm kind of iffed. I could live with it (after all I came from a V2 which was a thug of a vibemachine) but it makes me think something's wrong with it and I might end up with major damage if I just keep riding. The bike has 23K miles on it.

Is this perhaps an indication of any known problems or do these bikes just develop vibes as they rack up the miles and get older?
 
#2 ·
Kind of silly this but... I just got to ask.

After returning from a 300 mile trip and going at about 100-110 mph the bike suddenly started making a very felt vibration. It was mostly felt through the footpegs but also through clip-ons. The vibration would subside to normal when RPMs dropped below 7500. I pulled the clutch and revved the bike to see if it was drivetrain related but it was still there. During the next days I did some calm rides through town.. everything smooth below 7500RPM. Today I went out again. I couldn't do 100mph due to heavy traffic so I had to go at 70 mph, 3rd gear to raise RPMs to 8000... the vibration was there and even stronger - now I could also feel it through my bum.

The bike in general pulls normally, no loss of power. It's just the damn vibe that wasn't there before and I'm kind of iffed. I could live with it (after all I came from a V2 which was a thug of a vibemachine) but it makes me think something's wrong with it and I might end up with major damage if I just keep riding. The bike has 23K miles on it.

Is this perhaps an indication of any known problems or do these bikes just develop vibes as they rack up the miles and get older?
This is an early sign of a coil pack going bad. I just changed the coil pack on my thruxton for doing the exact same thing. Does the bike act this way when the engine is still cold, or only after the engine has warmed up?

the inline 3 engine has a perfectly balanced crankshaft, so the ride should be incredibly smooth. A ride which is not this way is an indication that you have a problem. I wouldn't ride with the bike like this only because you don't want to have uneven pressure being put on the crank in the event that it is a cylinder misfire.
 
#3 ·
Thanks a lot for your answer.

Just to make sure I understand you correctly, by "coil pack" you mean the stator/rotor assembly (basically the rotor which rotates) where electricity is produced?

Apart from that, your reply intrigued me and taking advantage of the quiet night I just took her out on a now deserted 6Km straight road for a test. My findings are:

Finding No1: When relatively cold (five lines on the dash as normal but bike not having worked for long enough to really get hot throughout) vibrations are felt but not very much. I would definitely say that they could even be within what the bike always did, maybe just a small tad higher. However, when really hot, meaning after a long time (returning from a trip for example) the vibes get much worse.

Finding no2: The lower the gear at 8K the worse the vibes. Perhaps this has something to do with how much effort the engine puts at defeating air drag? 3rd at 8K is about 60Mph and 6th at 8K is about 100Mph, a LOT more work for the engine. Lower gears feel like the engine just tries to keep itself running instead of pushing the bike forward... and this causes more vibes.

(In all the above cases when I say "8K" I mean going steady at 8000RPM, not accelerating or decelerating.)

Is there a way to diagnose possible problem with the coil pack? An electrical reading perhaps?

As regards the engine misfiring shouldn't there be some loss of power or strange noises? The engine pulls normally and runs smoothly apart from the vibes.

Any further input welcome.
 
#6 ·
It may be hard to tell at speed, especially early on. You actually would be losing power because a bad coil in essence causes a misfire, which would thus cause power loss, whether you feel it or not.

Ride the bike HARD. See if you can feel power loss that way. Otherwise we can continue to diagnose.
 
#10 ·
Might be worth checking the chain's rub guard on the swingarm. Mine was exhibiting similar behavior and once replaced, went away. Also, check the chain itself for both kinks and driveline angle. Sometimes resonances only manifest at certain rpm.

One last thing, check your clutch and clutch cable. Sounds weird but sometimes lines get pinched around the triple tree and resonances sneak in and can be felt through the bars.

Just thinking out loud here....

Good luck.
 
#11 ·
Might be worth checking the chain's rub guard on the swingarm. Mine was exhibiting similar behavior and once replaced, went away. Also, check the chain itself for both kinks and driveline angle. Sometimes resonances only manifest at certain rpm.

One last thing, check your clutch and clutch cable. Sounds weird but sometimes lines get pinched around the triple tree and resonances sneak in and can be felt through the bars.

Just thinking out loud here....

Good luck.
Thanks a lot for your input... I'll definitely check chain tension and condition of the "rub guard" slider. The vibes are mostly felt through the footpegs so it shouldn't be the cable.

Regarding testing the bike for bad ignition coils as per adrianm413's suggestion, I went out and did the deed. It's certainly not power-related. The bike pulls as normal everywhere.

What I noticed is that the vibration appears at 3900RPM under acceleration (load), disappears 300RPM higher and then reappears (much stronger) at about 7800RPM.... which is exactly double that of 3900.

So it must be some kind of mechanical resonance happening in there. It's either the chain which I'm going to check first thing tomorrow or something rotates not perfectly around its center of gravity and as I said above although I could live with it, I fear it might create some problem.
 
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