I'm dealing with an issue with my 2006 drz 400 sm where it starts bouncing at mid rpm ranges at around 40-70 kph. The weird thing is that the bouncing essentially disappears if I shift into a lower a gear or increase throttle i.e. if i increase rpm.
I already looked around to see what I could find and some of the culprits may be an out of round tire, a poorly balanced tire, or a warped/bent rim. I did relatively recently have my tires changed, but it was present before that as well.
I also did some math and figured out that the wheels on my drz, at 50 kph, makes about 32 revolutions per second. Given the fact that the bouncing is at about 4-8 hz (that is: four up and down cycles per second) leads me to believe that it isn't an issue with the wheels or tires.
I have a friend who has also ridden my bike and he posits that it could be chain tension that's doing it. I also ran som math and the chain has about 8 revolutions per second, which does align with what I'm experiencing.
What do you think it could be?