The obvious is that a 4 wheel is more acceptable because it has more stability than a 3 wheel because of its higher rate of tipping. They banned 3 wheel ATV's from the US in 1988 because of the amount of injuries reported from them. The front end of 3-wheelers obviously has a single wheel, making it lighter, and flipping backwards is a potential hazard, especially when climbing hills. Rollovers may also occur when traveling down a steep incline. The consent decrees expired in 1997, allowing manufacturers to, once again, make and market 3-wheel models, though there are none marketed today. Safety issues with 3-wheel ATVs caused all manufacturers to switch to 4-wheeled models in the late '80s, due to consent decrees between the major manufacturers and the Consumer Product Safety Commission�the result of legal battles over safety issues among consumer groups, the manufacturers and CPSC. Recently the CPSC has succeeded in finally banning three wheeled ATV's with attachments to bill HR4040.