NOTE: The following applies to the Classic Mini Cooper, if you have a BMW MINI (2002 thru present) Sway Bar Information can be found here!.
This is a subject I should never get started on; I will try to give you the short story!
For any of you that don’t know I have been racing Mini Coopers for over 30 years. On this subject, I have some very definite experience. Before my later years in Road Racing I was national Autocross champion in a 1966 Hydrolastic Mini Cooper S. While I have not road raced a classic Mini on anything other than 10” tires, my last couple of years racing the new MINI Cooper on BIG tires does apply to the classic with 13”.
At the very basics of this issue is ‘Balance’ and ‘Compromise!' The Compromise was started by the manufacturer when he decided to make a road car as safe as possible. All road cars are designed to have understeer (push or plow.) It's generally considered that if an inexperienced driver is going to get into trouble it will occur when he goes too fast into a turn, and rather than have the rear end snap around, the car is designed to push. In our front wheel drive cars that was not hard to achieve; with all the real weight and driving force on the front tires, it is easy to exceed their ability to grip.
The second part of the compromise is the manufacturer’s goal to make a car that handles well but is also comfortable to drive. If their intent was to sell cars only to road racers the results would be totally different. A car with very stiff suspension and very low roll center, such as a go-cart, will handle very well up to a point but would be terrible as a street machine; ‘compromise = comfort + handling’! In the very early days it was just a matter of making the car stiffer with something like Koni shocks set on ultra hard. We soon learned that we could go much quicker by controlling body roll while allowing the suspension to actually work. The purpose of a suspension system is to keep the tires in contact with the road surface under all conditions and body roll was the factor that affected it most. Sway bars are designed to control body roll and transfer energy from the body to the suspension.
While ‘Compromise’ continues to be a big issue with the application of sway bars to a Mini Cooper, ‘Balance’ also now becomes a key word. The use of sway bars allows you to control the roll center while maintaining a soft enough suspension to allow the suspension to work while still allowing the car to be driven on the street.
The principle behind a sway bar is to achieve a ‘balance’ to the car, front to rear, such that when we turn at our chosen speed we can maintain control of the car. As a Mini Cooper will naturally understeer in order to get closer to a balance, we either have to decrease the understeer or INCREASE oversteer. In the true sense of the equation we can only decrease understeer (independent from affecting the rear) by increasing the tire grip on the front tires by using wider or stickier tires. Once you have done all you can to minimize understeer at the front, you need to compromise the overall handling in order to achieve the desired balance. This is done by inducing oversteer in the rear by the addition of a sway bar. The key here is to understand that a rear bar does not decrease understeer but rather it increases oversteer in an attempt to get to the balance point!
It is at this point that we need to also talk about other compromises in the equation: Driver skill, driver technique, and typical use of the car. A driver with little or no experience should have a car that basically understeers. As his experience and skill increases, he will be able to control and enjoy a more balanced car. All of us have seen drivers go around sweeping turns with the car going back and forth from under- to oversteer and back to under, etc. This car is said to be perfectly balanced, right on the edge. The driver’s skill is now what gets the car around the fastest. But put another driver in the car and he proclaims it handles like a pig! This is where technique comes into play: some drivers insist on seeing how far into a turn he can drive before applying the brake, while others slow down and accelerate thoughout the turn. This is an entire subject on its own with hundreds of book written on it. My only point here is to have everyone understand that what is the perfect set-up for one car and driver is not necessary the same for the next.
All this just to talk about sway bars on the Mini Cooper. I would suggest that for perhaps 95% of people the use of a rear sway is the best and only option. I would NEVER attempt to drive a Mini Cooper in any kind of performance way without a rear bar. In fact, I recommend it for anybody that asks, even for 100% street application. A rear bar in a Mini Cooper, along with the proper shock and ride height set-up, can achieve what I think is the best way to go fast in the Mini and that is with ‘trailing throttle oversteer’. This experience is like none other in cornering. When done right you can drive the car VERY hard into a turn almost to the point of losing grip on the front tires (understeer) only to find that if you lift you foot off the throttle quickly that the load transfers to the front resulting in less traction at the rear and a ‘tendency’ for oversteer! This increase of load on the front tires then allows them to grip more and you find yourself again able to apply power to accelerate through the turn. Remember a tire has only so much grip, and you can use it up in either turning or accelerating. The stiffer the rear bar the sooner this transfer will occur.
It is almost impossible to put enough rear bar into the Mini Cooper to get it to actually oversteer - not that you can’t get the back to come around, but this is more due to technique than set-up. Remember how all the old rally guys would toss the Mini in one direction only to get the rear end to slide in the opposite direction!
Now comes the question, when do you use or need a front sway bar? Remember I talked about a rear bar being all that is needed with a proper shock/spring ride height setup; well, a front bar is used to again compromise! As body roll has been identified as the major contributor to handling if the car has less than optimum spring/shock and/or ride height, then a front bar is helpful. Most Minis on 13” tires have a higher roll center than those of us on 10”; most drivers want to drive their car on the street in addition to racing of some sort; both of these require compromise. One of the modern issues with the classic Mini has been the recent introduction of coil springs to replace the stock rubber cones. When you cut through all the hype, you find that the justification for the change is not improved handling but improved ride comfort. The car just feels better, and doesn’t pitch as much in daily driving. But guess what; here is that terrible word ‘compromise!' In technical terms you can install a spring set that is ‘stiffer’ than any stock runner cone. But, these springs will feel softer on the street – because you have more body roll! In order to achieve a better feel the spring design has compromised on body roll.
All this said, a front sway bar is most commonly used when springs have replaced the rubber cones. Does this mean the springs are bad? No, it’s just a compromise that we choose to make and we need to understand the end results. With the springs and a front and rear bar you can actually find it easier to achieve the best of both worlds: handling and comfort. But when setting up a Mini Cooper with that many variables you also need to include the driver’s experience and skill as yet another variable. Remember that cornering (ignoring the impact of the driver) requires the application of the entire suspension system, not just sway bars!
P.S. Call me anytime on the phone and I will be more than willing to actually give you the long version of this story!
And a few links that might be helpful: https://www.minimania.com/web/SUBTYPE/SearchText/Search/c%2Dajj400/Search_Inventory.cfm
And another review yields the following options to reduce understeer:
Increase rear tire pressure while decreasing front
Use 145 tires on the rear and 165 on the front (not practical)
Increase negative camber on the front – I would suggest that on street tires you would use 1 ½ degrees negative
Adding a little bit of toe out in the rear will definitely induce oversteer! https://www.minimania.com/web/SUBTYPE/SearchText/Search/ms73/Search_Inventory.cfm