chain upper roller bearing

2013 YZ450F:  I am hoping someone can tell me why my upper chain roller keeps breaking. The rear wheel bottomed out and hit the underside of the fender for first time in 9 yrs  The sag is set to 4 inches.  I am a Motocross rider and the tracks have been a little rougher lately than usual.  The muffler is an aftermarket Yoshimura.  This is the third sub-frame on the bike and it has already been rewelded once.  Twice it has broken at the rear muffler mounting bolt and twice where the upper chain roller mounts.  Could it be the rear shock/suspension?

Edited by rwebmx

While the rear shock is likely at fault for the bottoming, whether from need of oil, a simple servicing, a crushed bumper, or a higher rate spring to deal with either your increase in riding speed or your weight gain, You shouldn't be breaking the upper roller off unless your chain is too tight.

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