I had a very bad vibration on my bike. It's a Panama Jack, 66cc Grubee, no shift kit or nothing (since it's a single speed, right?). Springer front end, front brakes, banana seat, speedo and rear mirror. So, when I go past 15-17, I get a really bad vibration in the bike. I can not pin point where it is. It was so bad that the mirror was unusable. It got so bad at one point, I couldn't ride on account of the vibrations making my sphincter numb (lol). I looked up how to balance the crank. I was going to tear the motor apart, and just bite the bullet and balance it. I had to do what I had to do. I decided to order the Manic Mechanic rear sprocket adapter and a 40 tooth sprocket. MAN, what a difference it makes. Really. I barely feel anything anymore. Partly because my sphincter is probably dead by now. But mostly because the sprocket is so centered, it feels like a new bike now. I have to say to anyone, if you have a bike, and it has any type of vibration, get a Manic Mechanic sprocket adapter. I went down 4 teeth. The adapter was very very easy, very well made and incredibly easy to install. I wondered why they had instructions along with the kit. It even came with Allen wrenches to install it onto the bike. Plus, I ordered it Monday, was at my house on Thursday. It was the best 75$ I spent on this bike thus far. Also, my chain doesn't rub the rear tire anymore (which was another problem I had). Hope you guys learn from me. Don't be cheap, buy good quality products (from the guys who made them, not cheap knock offs), and your problems will diminish.