Excerpts from MOTORING - Getting the Maximum from your New MINI
by Gary Anderson and Don Racine
In your everyday driving, by now you’ve discovered how much fun your MINI can provide, especially when you can let it out a little bit and experienced its estimable performance and handling capabilities. But if you really want to have some fun with that great MINI, we encourage you to try some of the various kinds of organized events that are available to you. You can head out on the highway for club tours, participate in organized track days, or try your mettle against the clock in autocross events.
With a few days of track time under your belt, or maybe a day or two out on the autocross course, and lots of corners that you have been practicing to go around just right, you may be ready to consider tweaking the suspension a bit for better handling performance.
Once again we’re back to trade-offs. For the average driver, with average passengers, the ability of the car to absorb bumps without rattling the dentures of gran’pa in the back seat is at least as important as how fast the car will get around corners.
As a result, when designing the shocks, springs, and suspension on any car, engineeers are going to err at least slightly on the side of a soft ride. The trade-off is that the car will lean more when going around corners.
In moderate corners, as the car leans, that weight transfer is going to take weight off one of the powered wheels, and push the other wheel more firmly against the pavement, causing it to scrub a little bit. Push that car too hard through a corner and that lean could even turn into a roll. Either way, you’re not going to go around the corner as fast as you would if the car didn’t lean so much.
In the interests of improving your potential to get around corners, you may want to think about changing the trade-off, so that the car may not ride as softly going over bumps, but it will lean less going around corners. To do this, you’ll want to consider replacing the springs, upgrading the shocks, adding a rear sway bar, and changing the rear control arms.
Each of these suspension modifications can be installed separately, if your budget is limited, and they can be installed in the order in which they’re discussed. If you can swing the expense, you can save money and gain maximum improvements in handling, by installing all the components at the same time.
Anti-sway Bars to Reduce Understeer
In a front-wheel drive car, all of the steering and power comes from the front wheels. The usual result is that the typical front-wheel car has a tendency to understeer. If the car is going to fast, or turned too abruptly when entering a corner, the car will push ahead in a straight line, rather than turning to follow the direction of the front wheels.
Understeer is a good thing for the average driver turning the average corner, since the car is less likely to swerve or skid, should the driver turn the steering wheel too far or too fast. In fact, even most modern rear-wheel cars are engineered to have a little understeer.
However, since we want to get around the corners faster than the average driver, and we’re willing to invest some time and practice in learning to drive the car better, reducing that understeer seems like a good idea. Though it seems counterintuitive we can reduce the understeer on the front wheels by altering one component of the rear suspension, the rear “sway bar.”
The MINI Cooper S has a sway bar on the rear for just this purpose, to help tune the suspension. The rear sway bar keeps the rear wheels of the car more level as the car goes into corners. A sway bar—or as it is generally and more accurately referred to, an “anti-sway bar”—works by connecting the wheels on either side of the car to one another and to the chassis.
As the inside corner of the chassis begins to move up when the car rolls toward the outside of the turn, the sway bar transmits some of this motion to the outside rear corner. The net result is that the inside corner doesn’t go up as much, and the outside corner goes up more, than they would without the sway bar.
Again, think of the car as if it is balanced on a pin at its center. If we can keep the back end flatter on the turns so that the inside rear corner of the chassis doesn’t rise, less pressure is put on the outside front wheel and the car doesn’t push, or understeer as much. Instead, the rear end of the car comes around more easily. Instead of resisting the turn, the car will follow the line of the turn more easily.
However, the stock sway bar installed on the MINI represents a compromise between reducing chassis roll and affecting ride comfort in favor of ride comfort. It is also fixed in place, so it doesn’t allow any choice of response regardless of what you’ll be using your MINI for.
To improve on that situation, aftermarket suppliers have developed a stiffer rear sway bar that also has an adjustment range from harder to softer responsiveness. The original sway bar is 13mm thick, while one typical aftermarket sway bar is 16mm thick. The thicker bar is capable of transmitting more force from one side of the car to the other, helping the car stay level and balanced on tighter turns.
This upgraded sway bar also has sliding attachment points at each end, so that the range of movement can be changed. With these adjustments, you can leave it on the soft setting for daily use, so that the car sways a little bit more, but doesn’t ride quite so roughly. When you’re getting the car ready for a track day or an autocross competition, you just release the tension bolts and slide the attachments to the hard setting to increase the car’s cornering capability.
The typical dual-use adjustable rear sway bar kit , including the sway bar and bushings is available for about $250. The installation is straightforward, but does require putting the car on jack stands, and then removing the old bar and getting the new bar to slide in around the rear suspension components and wiring harness, so you may wish to have an experienced professional shop do the job for you.
Even heavier rear anti-sway bars are available for drivers who expect to spend a greater proportion of their time on the track or do serious autocross compeition, might consider one of the heavier rear anti-sway bar that are also available. Some of these heavier bars have a more positive multi-position adjustment mechanism. The heavier rear bar is generally used in conjunction with the substitution of a heavier front roll bar in order to keep the car balanced from front to rear as well as side to side.
Front and Rear Strut Brace Kits
The chassis of the MINI Cooper and Cooper S is well-designed and pretty stiff just as it comes from the factory, which is one of the reasons why your MINI felt satisfying to drive when you first drove it away from the dealer. Several reviewers have remarked that the car felt “as if it had been carved from one block of steel.”
However, as you start to drive the car harder, on track days or around the autocross cones, or just like tossing it around backroads corners, you might want to stiffen up the chassis just a bit more. This upgrade is one of the easiest in this book. It is accomplished simply by bolting a “strut brace ” across the front of the car in the engine compartment, to connect the tops of the two front strut towers to one another.
The benefit of this brace is to stiffen the front of the car to reduce flex in the suspension. Where the stock MINI feels pretty stiff and strong in corners, the added strut brace makes it feel really solid so that the car turns into the corners more easily without hesitation. Of course, since the brace is visible to anyone looking in the engine compartment and is an attractive accessory, it certainly adds to the performance credibility of the car.
A stiff, well-constructed strut brace is desirable on a car that is running with low-profile, high-performance tires and especially desirable when installed in conjunction with a lowered suspension. A typical well-engineered strut brace will cost about $325, and can easily be installed by anyone with the right-sized wrenches in less than an afternoon.
Chassis stiffness becomes increasingly important as the MINI is used more often for serious autocrossing, and is an important element in suspension tuning. For this reason, many tuners also prefer to add a rear strut brace to the MINI before making other changes to the rear suspension of the car. Like the front suspension, this additional component is bolted on to the rear suspension mounts and extends across the car. A good rear strut brace can be purchased for about $200 and is easily installed.
Performance Spring Kit and Shock Upgrades
Two suspension components have the most direct effect on ride quality and handling performance, the springs and the shocks . The length and resilience of the springs determines how easily and how far the body will move when the wheels hit a bump or when weight is transferred
in acceleration, braking, or cornering. The shock absorbers reduce the amount of rebound on the springs, helping the body to return to equilibrium after it bounces.
Let’s start with the springs. Because most car owners put more emphasis on ride comfort than on cornering and acceleration performance, most cars are equipped with fairly soft springs that are designed with a good amount of spring travel. The basis Mini Cooper would be considered in this category.
The Mini Cooper S, with its sport suspension (optional on the Mini Cooper) does use heavier (less-resilient) springs with a bit less travel, but the engineers are still assuming you just want good street handling, and aren’t going to want to go a little fast in the twisty bits or take the car on a track or autocross course.
For these purposes, a stiffer spring and lower ride height will be in order. With a stiffer spring, the car won’t cushion you as much on the bumps, but more important, it won’t sway as much on corners, or shift back and forth as much on acceleration and braking. With a shorter spring, the center of gravity will be a little lower, also reducing the amount of side-to-side or front-to-rear body roll.
Good high-performance spring kits are readily available in the aftermarket. These improved spring kits will help your car maintain its stability when starting, stopping, and turning without wallowing around. These kits won’t make your car ride so rough that your passengers will complain, but they will definitely increase the predictability of the car in the corners and help reduce your lap times. A typical upgraded spring kit sells for less than $250.
While most of the good performance spring kits will work with the original equipment shocks, you might want to consider upgrading your shock absorbers at the same time. The shock absorbers work together with your springs so that the car doesn’t just bounce up and down and up and down every time it sways or hits a bump.
Actually, the term “shock absorber” isn’t quite accurate, since the springs actually absorb the shocks from uneven road surfaces, while the shock absorbers help counter the effect of the springs. The English call them “dampers” which is a more accurate term.
The shock absorbers in the MINI are long tubes that are installed between the wheel and the chassis in parallel with the springs. Inside the outer tube is a piston with a special valve that allows fluid to move from the main tube into the piston as the shock absorber compresses and then move back into the main tube at a slower rate when the shock absorber extends.
The shock absorber works by compressing easily when the spring compresses, but then reducing the rate at which the spring expands. So, instead of continuing to oscillate up and down as it would if only the spring were in between the chassis and wheel, the chassis comes back to a neutral position after only one or two movements.
Like original equipment springs, original equipment shocks are designed to do their job with emphasis on comfort, rather than performance. They damp the spring movement just enough to avoid making passengers seasick, but not enough to give a harder ride. To improve your handling, you’ll want even less oscillation so that the car will return to a neutral position more quickly.
By installing performance shocks , you still get some springing action to absorb the bumps and weight changes, but the car will move less and return to neutral more quickly after acceleration or braking, or in between corners. A set of performance shocks designed specifically for the MINI, such as the one by Koni, is a good complement to shorter, stiffer performance springs. One MINI aftermarket catalog offers the Koni Coil-over kits can certainly be used to improve the handling on street cars. However, they are more likely to be installed by owners who expect to use their MINI frequently on the track or autocross course, since they are available in different spring rates and do offer the means to adjust ride height at each corner.
Different spring rates will be appropriate, depending on the experience of the driver and the frequency with which the car will be used in competition. Springs that are closer to stock firmness will be appropriate for the person who doesn’t compete too often, and also wants to use the car for street use. On the other hand, if the MINI is only going to be used for competition and the driver is quite experienced, the preference will be for a much firmer spring.
By adjusting the ride height at each corner, the owner can balance corner weights to compensate for other changes that have been made in the car, since balance is very important in tuning the car’s handling for the race track.