The main difference between aftermarket rear control arms and stock rear control arms is the difference in the amount of adjustability. Many of the later BMW and MINI models come with non-adjustable rear arms that don't allow you to change the camber. Some of the newer models do come with adjustable rear arms, but only allow for a camber adjustment between 0 and 0.5 degrees. This is better than no adjustment at all, but usually is not enough to reach a desirable degree of camber upon suspension alterations. Since stock rear control arms in MINIs and BMWs offer little to no camber adjustability, if you choose to lower your vehicle with a performance spring set, you should strongly consider investing in aftermarket adjustable rear control arms. Anytime you add a drop to your vehicle with performance springs, a considerable amount of negative camber is added to the rear tires. Excessive negative camber in the rear can negatively affect manueverabiliy and control. Additionally, the inside of rear tires wear out quicker. The general consensus on acceptable rear camber ranges from 0 to -1.5 degrees. When aftermarket spring sets are added and used to drop the vehicle, the amount of negative camber increases. Say for instance you decide to drop your BMW or MINI 1.5 inches. This amount of drop equates to about -3 degrees of camber on the rear wheels - 100% more than the aforementioned acceptable amount! Even if your BMW or MINI stock arms are adjustable, they will probably only be able to reduce camber to -2.5 degrees due to limited extension. With aftermarket rear control arms, you can make camber adjustments from anywhere between 0 and 4 degrees. Aftermarket arms for BMWs and MINIs from reputable designers can reduce negative camber and increase handling. If you're thinking about dropping your car a few inches and already have a rear sway bar installed, performance-grade adjustable arms are a must. Too much negative camber in the rear disrupts the benefits of your swaybar. If you've installed a swaybar, especially on your MINI, you've installed it because you want to reduce understeering. Negative camber in the rear will increase understeering and, consequently, decrease the effectiveness of your swaybar. When extenable rear arms are taken advantage of, you're able to retain the benefits of your swaybar and lower your vehicle to a desirable level. Lastly, BMWs and MINIs have four rear control arms - two upper and two lower. It's only necessary to replace lower ones with performance-grade extendable arms to effectively decrease camber. Upper arms may remain stock, adjustable or not. After replacing your stock rear arms with aftermarket ones, expect a huge difference in handling and easier camber adjustments.