Moving cart that rotates when acclerating

[travisr34]

Homework Statement

There are 5 forces R1 and R2 which are the vertical reaction forces on the rear and front wheels respectively. F1 and F2 are the friction forces on the wheels. And finally Mg which acts at the center of gravity which is closer to R2 then R1. Only known value is R1 which will be 0 when tipping occurs, because of this the cart will rotate about R2. The rear wheels are powered by gears while the front ones are just stuck to an axel. I would like to know the linear acceleration that the cart must undergo in order for tipping to occur. I don't really want to plug in numbers just solve it leaving the variable terms

Homework Equations

Summ of the forces in the x,y and moments.

The Attempt at a Solution

forces in x =-f1-f2=ma.
forces in y=R1+R2-mg=0
moments about R1 where clockwise is positive=mg(d1)-R2(d2)=I*alpha(angular acceleration)

 

[member 553083]

d1 is the distance from R2 to cg and d2 is the distance from R1 to cg. I is the moment of inertia about the cg. I know R1=0 so I can get a value for R2 but that doesn't seem to help. I'm not sure how to solve the equation since I don't have a value for F1, F2 or Mg.