In the present study, the brush model is reanalyzed and algebraic equations are derived for use in estimating tire grip margin. Application of this estimation method allows the tire force distribution of a four-wheel independent steering (4WIS) vehicle to be optimized by using the objective function of the maximum value of the μ-rate, which is defined as the tire force normalized by the radius of the friction circle. We later discuss a protection function that was inspired by the limiters used in fly-by-wire aircrafts. This function prevents excessive steering command inputs to the vehicle controller. The numerical simulation results obtained using CarSim prove the basic functionalities of both methods.