Chain resonating at high speed.

2015 WR250R. 6K miles.
Running 14T front 47T rear.

I've had a chain bounce or resonating issue for the past thousand miles or so. I cant seem to make it any better.
Here is what happens, under certain load ( about 50-70% throttle) and when cruising around 40-55mph the chain gets into this high speed bounce where the taught side, top, of the chain just slaps and bounces vertically up and down at high speed.

Doesn't happen when decelerating, doesn't happen when at WOT.

Figured it was just caused by wear on my sprockets so I replaced both. It did not help so I replaced the chain even though it had less than 3000 miles on it and showed no signs of serious wear.
Still happening.


Video of the issue. Skip first 4 min if you read the description above.
https://youtu.be/HXNRC0zsnOw

I'd appreciate any input, I searched this forum and others and could not find anyone experiencing the same issue.
Friend of mine suggested to check the rear wheel bearing for runout, or check if the sprocket has vertical runout. It could but I would be surprised how the sprocket would be bent or somehow vertically not centered.

Ok. I just watched the video on youtube and it only looks correct when you watch it at 720P 60fps. If you watch it on standard definition it looks like the chain is wobbling up and down slowly. That's not the case, just the frame rate is not fast enough to show the bouncing.

Did you try the obvious remedy, adjusting chain slack?

Yeah in the video the chain slack was at 30mm. I tried tighter down to 20-25mm. I have not tried looser with all the new components, but did try adjusting the chain slack as the first thing I did and it made no impact.

This link describes what I'm seeing.

Lateral vibration


https://knowledge.autodesk.com/support/inventor-products/learn-explore/caas/CloudHelp/cloudhelp/2016/ENU/Inventor-Help/files/GUID-381564A0-CE77-4B21-ADC5-054F585B5CD8-htm.html

You can try the following:

• Adjusting sag
  
• Adjusting compression and rebound damping
  
• Playing with the chain adjusters in case they are not perfectly calibrated between them
  

All the above will have some effect on the resonance of the chain, but the most probable outcome is that they are simply going to shift the resonance, not eliminate it.

Maybe the chain does resonate at some particular load on all bikes? How did you discover this? Is it so obvious that is clearly audible?

Ok. Thanks for those tips.

I have not played with the comp or rebound yet as an option to mitigate this issue.

I discovered this just because I could feel an unusual vibration in the bike. It doesnt feel right. Also others I rode with would comment on how much my chain was bouncing.
I feel like other bike probably do this and it isn't a WR specific issue. Just want to know how to eliminate it. Changing the sprockets made bouncing occur at lower speeds.

Such a resonance requires a continuous excitation, so there's a particular combination of speed, RPMs, suspension tuning, chain alignment, and potentially other geometric factors (e.g. non-concentric sprocket) that drive this resonance.

This just appeared out of the blue and you cannot associate it with any changes you might have made to the bike?

johnkol wrote:

Such a resonance requires a continuous excitation, so there's a particular combination of speed, RPMs, suspension tuning, chain alignment, and potentially other geometric factors (e.g. non-concentric sprocket) that drive this resonance.

This just appeared out of the blue and you cannot associate it with any changes you might have made to the bike?

There isn't anything I can identify that started this issue. I noticed it one day while riding and thought it was being caused by my rear sprocket. It was getting worn but did not look critically worn at that point. I adjusted my slack and rode it that way another 800-1000 miles. The bounce got worse during this time. I replaced the rear sprocket and issue did not go away, now I was much more vigilant towards it.

Since I have done the following
replaced the chain and front sprocket
adjusted slack
Replaced rear tire
checked vertical and horizontal runout on both sprockets ( they are concentric )

Other suggestions I've received but not tried yet, in order of when I plan to attempt them.

Rear suspension adjustment
Swap an entire rear wheel assembly.
Try another chain.
investigate clutch dampening springs and see if something is wrong in the drive assembly internal to the motor. ( Really hope I dont have to go this far )

Is your rear tire out of balance? I've seen that cause chain "bounce"

I don't want to be unhelpful, but I can't recollect any dirt bike I've owned not suffering from this chain-flogging thing under certain conditions. I believe my various Yamahas were/are the worst, but they all seem to do it. My WR-R certainly does it, and I just live with it.

Even my Tracer 900, which obviously is not a dirt bike, does it as well. Bloody annoying.

I found that running my WR-R chain slack pretty loose helped, believe it or not. I see your chain guard is busted off, but on my bike, if I lift the chain and it doesn't touch the front edge of the guard, I'm happy. No doubt there will be plenty of you who don't agree, but meh. My front chain slippers seem to last very well - say around 12-15k - and this bike gets excellent chain and rear sprocket life. It has lived all but 2k of its 37k on the dirt. I run 13/48 usually, but will be going 13/51 at the next new chain.

Your comment is actually quite helpful. It's good to get another data point and hear that others experience something similar without long term wear occurring or other negative issues arise.
However, I'm still concerned about how fast my last sprocket wore out. Only about 5000 miles out of the rear.

I also have a tendency to run my chain on the loose side of spec, but have tightened it up a few times and I see less bounce when the chain is tighter.

I'm still riding the bike most days and still trying to sort out this issue.

Sprockets will wear heavily for several reasons, one being a too-tight chain. Is it worth bearing in mind that when the chain starts flogging, its under a fair amount of tension from the enginengine anyway?
Hope you get it sorted. Might be worth checking your wheel alignment, too.

Tried a 13 tooth sprocket up front. So now running 13/47 and 112 link chain.

Quadruple checked my chain tension and wheel alignment after the sprocket was installed.

Chain still bouncing.


Also tried adjusting rear shock dampening, 4 clicks stiffer (only compression ). No change. Then 4 clicks softer, worse. Returned to stock.
Issue still occurring.

Well, that’s not very satisfying for you.
As I said, I think it’s common to lots of bikes, and kind of goes with the territory - my bike does it and I just live with it.
Not much help to you. Maybe there’s a cure. I’ve never found one and would be happy if someone has!
The noise and vibration is one thing, but the fact that it robs the bike of some rear wheel power is another entirely, and is the main reason I don’t like it. The bike use up energy when the chain slaps around, as it must. Laws of physics and all that. Let’s not go there. Deep water and I can’t swim very well.