An anti-tip system is provided for stabilizing a vehicle, such as a power wheelchair, about a pitch axis and relative to a ground plane. The anti-tip system includes at least one anti-tip wheel. The mounting assembly for the anti-tip wheel is configured such that it traverses linearly in a direction toward or away from the ground plane and is responsive to an acceleration or deceleration of the wheelchair. As the wheelchair accelerates or decelerates, rotational motion of the drive train assembly is transmitted to a guide subassembly within the mounting to effect translation of the anti-tip wheel. Upward translation of the anti-tip wheel enables the wheelchair to negotiate obstacles, e.g., curbs or steps, while downward translation or force enhances stability when stopping the wheelchair or while moving down sloping terrain or surfaces. The anti-tip wheels may be castors and normally contacting the ground during operation.