Yes, my particular kit uses a centrifigal clutch and no, the rollers do not come away from the tire. You can order this kit from Dimension EDGE or you can call them with any more questions.
There are two mounting styles... one(like Zip's style) that is always engaged and is used mostly for chopper style bikes. And then there is the "Encore" which has a engagement handle on the handle bars to raise and lower the motor.