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The Shop
Its not a great pic of the welds but from what I can make out it looks like those welds had been failing for quite some time. if so that should have been caught and corrected...
It’s like I said in my previous thread that I linked, this is a critical area with severe consequences for a failure (as the OP found out), and the fact that none of the Japanese manufactures even make an attempt to do this right is kind of scary honestly. People are all freaked out about their bold new graphics and new frame flex characteristics that amount to jack shit in the grand scheme of things but are evidently perfectly accepting of unbelievably poor design and workmanship in this critical area.
It's just that there used to be a Red 'Edit' sort of below and off to the side of the Red 'Quote' thingo, always on the screen, that seems to have disappeared recently.
It may have something to do with my finally killing my ancient workshop Vista system 'puter (I think all of the high frequency signals finally did for it), and going to a '10' system. I'm Hi Tech, I am.........
That, and I am a complete computer cretin, and, will almost certainly forget your advice, instantly.
For reference I just went and removed the bolt from mine. Can clearly see the mark where the bold was. Not sure how much difference this would have made but can't of helped.
Pit Row
I can recommend the Bulletproof guard. My front bolt was almost ground flat so I had to replace it recently LOL. It's done it's job in the gnar rock sections.
The chain has several rotated pins. Usually this only happens if the pin and bushing surfaces are badly worn and running dry for an extended period. There is so much surface friction between the pin and bushing as it articulates around the sprocket that it can overcome pin's capacity to hold itself on the link plate. The pin to link plate design is a frictional fit to keep the pin stationary in the link plate during normal operation. Once 1 pin in the chain rotates, the neighboring links start to experience accelerated wear since the engine load is no longer being applied evenly to all links as the chain engages with the sprockets. You ended up getting these 3 to 6 link sections of chain that resist articulation, this will 100% induce an oscillation on the slack side of the chain as the stiff links transition from loaded to unloaded. What can happen with the slack side oscillation is the chain can hit and/or drag itself across the bottom of the chain guide, transmitting a cyclical force into the chain guide mount, thru the mounting bolts, and then into the bracket & bracket welds.
Look at the wear on the inner link plates. They are all ground flat on one side. This would be the outward facing chain surface that slides across the chain guide and swingarm. The nylon sliders in the chain guide are worn badly, indicating the chain has been wearing on both the top and bottom of the guide for quite some time. As the nylon material starts to wear thin, the dampening capacity is reduced thus more of the chains force is being passed into the chain guide bracket. Doesn't seem like much but you have to think of the extra force over millions of cycles (review your S-N curves)
The primary drive sprocket is severely worn. The teeth have hooking wear as well as side wear. Hooking wear is a result of running a sprocket for too long, or running it with a chain that is elongated. As the tooth wear increases, the contact pitch line increases as well, which causes a more abrupt collision between the link pin and inner bushing as the outer roller slams into each tooth face upon entry into either the driver or driven sprocket. Again, this is another condition that can be a catalyst for chain oscillation.
The rear axle is running pretty far to the rear position, this increases the span distance between the driver and driven sprockets. With a rough running chain, the slack side oscillation amplitude will increase.
When the chain is oscillating on the slack side under load, as it comes into the chain guide it could be snapped down and forced to align as it transitions across the slider and into the first tooth engagement. There is little doubt in my mind that are some bending forces going on in that scenario. After thousands (if not millions) of cycles, the welds eventually begin to fatigue, and a high force load, even under normal operating condition, can rip the brackets off.
The Op originally stated he hit a large rock and it ripped the bracket off, but then later recanted saying he couldn't verify he actually hit a rock. He said the failure happened just before a jump, so he was likely on the throttle hard and I could see where if the chain oscillated just right under load to hit the chain guide with enough force to finally overcome the last bit of strength the welds had left in them. Maybe the rider did in fact hit a rock or something hard, but I still would argue the welds were fatigued by that point anyways.
Look at this photo of Anderson's bike as he is loading the chain up a jump face. Look at how much the slack side of the chain is oscillating. You can see where it first enters the chain guide, it is starting to pull it back up and into alignment for entry into the driven sprocket.
I know the Op doesn't ride like JA21, but the chain is still going to behave similar. Imagine that same photo but with all the worn components shown above. Does that help explain how much of a beating the chain guides and brackets are taking with worn components?
My diagnosis may not be popular because it shows the true root cause to be lack of chain, sprocket, and slider maintenance, but I feel that I am correct on this.
When people argue against this point, I'll ask...
"What is that 3rd bolt and tab back there for? Why did the engineers put it there?"
Post a reply to: Inherent Yamaha problem