Hard/Tricky Problem on Energy Conservation

[ShamTheCandle]

Homework Statement

If the car is given an initial speed of 4.0 m/s, what is the largest value that the radius R can have if the car is to remain in contact with the circular tract at all times?
See the attached file (named: car.pdf) for the drawing.

Homework Equations

PE=mgh ----> Formula for Potential Energy
KE=(1/2)mv² ----> Formula for Kinetic Energy
Fc=mv²/R ----> Formula for Centripetal Force

The Attempt at a Solution

My initial attempt was to assume that the car has zero velocity at the top of the circle. However this is not true, since the car should "remain in contact with the circular tract at all times". This is an even problem from my textbook, so no answer is given in the back. By the way, I am referring to "Physics 6th Edition" by Cutnell & Johnson.

Thank you for helping!

 

[ShamTheCandle]

I am sorry if the attachment didn't show up. Here is it.