Shake/thumb felt through the footpegs, only in 3rd gear

[Squishy]

Hello,

Since my 4000km trip last summer (where my 400 got hammered  ) I've noticed a shake in the rear, felt through the footpegs. I feel it about 4 times a second at 50km/h.

I figured it was something like the chain, rear wheel bearings or output gearbox shaft bearing..
After I checked all of these but found nothing (chain is only 1.5 years old and well maintained, no play on rear wheel/output shaft), I did some test rides to locate the problem better, and noticed it was only happening in 3rd gear. It's worst at around 4500rpm, 50km/h. 
In 2nd gear or 4th gear at 4500rpm or 50km/h there is nothing.

Does anyone have any experience with this kind of symptom? Is a symptom of a worn gear sprocket?
There's no play on the output axle, and even put in 3rd gear at 4500rpm on the center stand, but axle looked perfectly stable.

Thanks

[Gouraami]

Is it only on constant throttle? Or is it acceleration/ deceleration?

[Squishy]

Is it only on constant throttle? Or is it acceleration/ deceleration?

It happens on constant throttle, and also when I accelerate slowly.
It's hard to tell whether it happens when I go full throttle because of all the noise and of course the tremendous g-forces the 400 produces.

[Gouraami]

and of course the tremendous g-forces the 400 produces.

 

Have you checked for play on the swing arm?

[Squishy]

and of course the tremendous g-forces the 400 produces.

 

Have you checked for play on the swing arm?

Not specificly, but there's no play on the rear wheel and on the center stand when turning the wheel at 4500rpm in 3rd gear the wheel is rock solid.
Also a play in the swing arm should also be felt in 2nd of 4th gear...

[Gouraami]

You are right, the chain and rear wheel will be moving the same speed whether in 2nd or 4th at that speed.

But the frequency  the motor creates at 4500rpm combined with the frequency of whatever is not correct could combine to create the resonance issue, that is why it is only in 3rd gear.

[Squishy]

You are right, the chain and rear wheel will be moving the same speed whether in 2nd or 4th at that speed.

But the frequency  the motor creates at 4500rpm combined with the frequency of whatever is not correct could combine to create the resonance issue, that is why it is only in 3rd gear.

It could... theoretically. However I tried 4500rpm in 2nd gear and 4th.
I also tried 50km/h in 2nd gear and 4th... so that resonance has to be VERY specific if it only happens in 3rd gear.

Also it happens at about 30-60 km/h.. so not that specific.

[ventYl]

Similar thing happened to my other bike - 1965 Jawa 250. I went to ride for maybe 20 km s, there have been short photosession with another similar machine and then we went home. Bike started to shake in 4th gear about 2 to 4 times per second. With my bike it was rather violent shaking and for sake of safety I finished my trip in 3rd gear (it has no 5th gear or above). Result was bent shifting fork for 3rd & 4th gear resulting in partially engaging two gears at the same time. When that happened the bike shaked resulting in the partially engaged gear to slip off and then again. At this place I have to make a note that this particular bike is notorious for problems with bent or worn shifting forks causing variety of shifting problems.

If your sprocket was worn I think you would hear *LOUD* whining from gearbox each time you engage 3rd gear. Even slightly worn and/or off axis sprocket (e.g. due to worn bearing) would make *A LOT* of noise at low RPM eventually getting less loud as the RPM raises due to rotational forces compensating misalignment.

My 6th gear sounds a little bit like this when it finally reaches some speed: https://www.youtube.com/watch?v=p1uchm4dOSg in low RPM. As the RPM raises above 6k there's is virtually any unusual sound. I've learned that it is caused by worn or out of limit bearing of 6th gear sprocket and the sound is caused by misaligned gearing contact patch. Some bikes have this condition but it looks like it does not tend to make any intermittent damage. I am checking oil for metalic parts and during last 12 000 km it is clean of any metal particles.

[Squishy]

If your sprocket was worn I think you would hear *LOUD* whining from gearbox each time you engage 3rd gear. Even slightly worn and/or off axis sprocket (e.g. due to worn bearing) would make *A LOT* of noise at low RPM eventually getting less loud as the RPM raises due to rotational forces compensating misalignment.

My 6th gear sounds a little bit like this when it finally reaches some speed: https://www.youtube.com/watch?v=p1uchm4dOSg in low RPM. As the RPM raises above 6k there's is virtually any unusual sound.

My bike has also started making a whining noise... but not in 3rd gear.. I think it's in 5th and/or 6th gear...
The shaking itself is not very violent. I can easily ride the bike without any problems or being afraid of it shaking apart... But at low speed it's definitely noticeable. It's like there's a small stone in my tire, or a VERY small dent in the rim like on bicycles.

[ventYl]

Check how 6gear motorcycle gearbox shifts. If sprocket pair engaged in 5th and 6th gear is at the same end of gearbox I would suspect one of axle bearings (but it is only quick tip and I am glad I didn't have to dismantle my bandit's gearbox yet).

[Squishy]

It's why I suspected the outgoing axle bearing.. But I can't find any play in it.. and it looks very solid when turning on the center stand.

Doesn't mean it can't be worn though... 

Perhaps it's the other axle...

edit: i just looked at the diagram.. obviously both axles are in use for every gear.. so I don't see how a bearing problem could only show in 3rd gear.. I'm suspecting the 3rd gear sprocket or the 3rd/4th drive gear.

[ventYl]

Worn bearing IMHO wouldn't cause shaking. Shake means something has irregular shape and/or gets slightly blocked periodically. If sprocket was so much off-axis the whine heared when in 3rd gear would be hearable even on Moon.

[Gouraami]

Mine also has a bit of a whine in 6th gear...

I think you posted the incorrect youtube link ventYl...

[Squishy]

I think you posted the incorrect youtube link ventYl...

Why? It makes exactly that noise at 0:35 

[Gouraami]

Oh! hahhaha

I watched the first 10 seconds of the video and was confused 

Ok cool, mine has a lower pitch than the one at 0:35