One of the things we like best about Miatas is the way they handle. We can say that with absolute conviction, and yet it's entirely possible that any two of us won't agree for a second on what handling actually is. Some say it's all about grip. A car that pulls 1.02 Gs on a skidpad handles better that one that pulls 1.01 Gs. For others it's about lap times. Given two cars with the same engine, the one that turns better lap times handles better. For the more philosophical driver, handling might be that elusive quality of being one with the machine. For the more technically-minded, it might have more to do with steering feedback, camber changes, and roll couples.
Definitions that sound more reasonable to us include any or all of these qualities, and yet we're sure it must be simpler than that. So we stick to our own personal definitions, even if we can't quite articulate what they are. Handling certainly doesn't seem to be anything quantitative, something you can measure, but rather something you experience and either like or dislike. Still, there must be something we can point to, some qualities that everyone agrees are necessary to good handling, because we all agree that Miatas handle great, and that Toyota Corollas (for example) don't.
It should be noted that our definition of handling is the traditional one, as defined by that small segment of the population that likes to drive cars fast, at or near the limit. Toyota may feel their cars handle well, and most of their customers might agree, but they're talking about ride comfort and the lightness of the controls. We're talking about grip and road feel and the kind of driving that you just can't do well in a Corolla. We might therefore define good handling by how the car drives near the limit, the way it takes a turn, from braking to down-shifting to planting the tires and powering out.
What Handling Feels Like
A good-handling car has to have great brake feel. You can modulate the brakes right up to the friction threshold, whether in a straight line or in a turn. You can feel when the tires are gripping, and when they're losing their grip. It's not any kind of special or sixth sense, but rather driving experience combined with sight, sound, and the kinesthetics of decceleration. Obviously that feel of grip is key to handling. More about that later.
As you start into the turn, two things happen quickly. The car starts to lean and the tires start to build lateral grip. You can instantly feel the point where the tires are maximizing their grip, that point where they've planted themselves in the turn. As you squeeze on the throttle, you can feel the rear tires exceeeding their grip and starting to drift. At the correct drift angle you mash the throttle, and the additional power once again plants the rear tires. With a wide grin, you straighten the steering wheel and accelerate quickly out of the turn.
It's a great feeling, and we notice several things that are important to the experience. One is responsiveness of the car to the controls. It has to be a linearly proportional response—any adjustment to the control makes an exactly equal adjustment to the car. We would definitely include responsiveness in any definition of handling. Responsiveness is a quality that's achieved partly through well-designed linkages and partly through chassis stiffness. A soft chassis absorbs motion, slowing down the transmission of control inputs. We can't measure responsiveness directly, and we can't really measure the design of the linkages, but we can measure the stiffness of the car. Springs soften the chassis as much as the lack of torsional stiffness, so both stiff springs and chassis braces improve responsiveness.
Another thing that's important is that feeling of grip, the feeling that the tires are planted in the turn. Like a climber testing a cam, once the tires are planted we can lean on them and know they'll support us until the road straightens out. How well we feel that grip, that planting of the tires, has everything to do with how well the car handles. The mechanism for transmitting that feeling to the driver is called communication, and car designers understand its significance.
Designing a Car to Communicate
Car designers are often in a bad place when it comes to communication. There are many forms of communication, and a lot of drivers don't want most of them. They don't want noise. They don't want feedback in the steering wheel if it means any kind of vibration or harshness coming through. They don't want to feel any bumps in the road. Keep all that stuff out of the drivers seat, and they'll think the car handles just fine. So even though designers understand the importance of communication, they have to engineer it in a way that doesn't offend the typical buyer, and send him to the car dealer down the street.
Unfortunately, most of things you do to increase responsiveness and communication also increase noise, vibration, and harshness (NVH). So car designers look for alternate methods of comunication. One of these is body roll. Most drivers apparently don't feel that excess roll is a problem, but it does communicate something to the driver. Excess body roll can be
achieved by softening the springs, which has the added benefit of isolating bumps in the road. Of course softer springs mean that responsiveness goes out the window, but in most cars (and all Toyotas), elimination of noise, vibration, and harshness (NVH) is top priority. Responsiveness and communication can be engineered back in as long as it can be done cheaply and without compromising NVH.
Fortunately, not all car manufacturers have the same priorities. Companies like Mazda, BMW, and Porsche seem to design in handling first, then try to engineer reduced NVH if it can be done without compromising responsiveness and communication. In Mazda's case cost is an additional factor, less so for BMW and probably not at all for Porsche. Look at your car commercials to see where a company's priorities lie. Mazda ads are all about driving.
A car has several ways to communicate what it's doing. Tires are a big factor. Some tires communicate better than others. Some make a lot of noise at the limit, while some are eerily quiet. Some have stiffer belts or sidewalls than others. Some increase slip angles progressively near the limit, while others are totally linear until they finally give up. We can sense all of this through our eyes, ears, and seat of the pants.
We can also sense what the front tires are doing through the steering wheel. As a tire approaches its limit of adhesion, its slip angle increases and so does the self-centering force acting to straighten the tire. Properly-designed steering, even power steering, transmits that force proportionally to the steering wheel, so that a slight increase in slip angle transmits the exact same slight increase in force to the wheel. An even bigger clue to the front tires reaching their limit of adhesion is that rapid loss of self-centering force as the tires start to slide—the wheel suddenly becomes lighter. Changes in the self-centering force are all we really need to know from the steering wheel. All other communications—bumps, etc.—are less critical.
Of course that's just the front tires. For the rear we rely on vision and kinesthetics—the sense of where we are and how we're moving. We may hear the rear tires working hard and guess that we're reaching the limit, but it's not until we see the car start to yaw and feel the back end start to move sideways that we know for sure we've reached it. As far as communication goes, that's not as good as a steering wheel, but some tires do a better job of it than others, and like everything else this form of communication improves with experience.
The fact that the front tires do a better job of communication than the rears is one of the main reasons an understeering car feels safer than an oversteering one. We often wonder why we should care whether we go off the road nose first or tail first, but that's not the point. We can feel the front tires losing grip better than we can the rear tires, so we're (slightly) more comfortable when the fronts go first.
As noted earlier, body roll is also a form of communication. In addition to the sound of the tires and the forces on the steering wheel, we have a sense of the limit of the tires from the lean angle of the car. But we know that increasing body roll decreases responsiveness, so for our purposes this is a non-starter. Of course we wouldn't want to eliminate roll altogether, but we can get as much a sense of the approcahing limit from one degree of roll as we can from five degrees of roll. G-forces are equally important in estimating how close we are to the limit.
So in a roundabout way we've come to the conclusion that good handling is about responsiveness and communication. The more responsive a car is, and the more direct and linear the communication, the better it handles. Grip and power definitely enhance handling, but don't contribute to it. We can't measure either of these factors—responsiveness and communication—directly, but we might still have a way to evaluate them quantitatively.
When we talked about driving a great-handling car through a turn, we described how it felt to plant the tires—like a climber pulling on a cam. How fast the car delivers that feeling, or whether it delivers it at all, is as good a measure as any of responsiveness and communication. We often describe a good-handling car as "tossable", meaning that we can steer quickly into a turn without first feeling for grip, then "catch" the car once the tires are planted. This takes a certain knowledge, based on having driven this particular car through enough fast turns, that the tires will in fact plant themselves, and communicate it back to us fast enough to keep the car from spinning.
The faster the turn, the less likely we are to toss the car, because the less likely we are to be able to catch it. We know how long it'll take for the tires to plant themselves (responsiveness), and how much longer it'll take to report back to us what they're doing (communication). In a 30 mph turn, no problem. 40 mph, maybe. 60 mph, probably not. But that speed, that "tossable limit", is a very good quantitative measure of how a car handles. In a stock Miata with decent shocks, the tossable limit will of course vary by driver, as everyone's reaction times and experience levels are different. A Miata newbie might find the limit at 30 mph, while Lewis Hamilton might not find it at all.
The good news is, the tossable limit will go up for everyone if we can improve the Miata's already-stellar handling. The two best ways to do this are with better tires and a stiffer chassis. By better tires we don't mean grippier, although that's always a plus, but tires that are more communicative. This is always a tough one to determine in advance. DOT doesn't have a rating for it. The best you can do is read the reviews and hope the reviewer understands what linear communication feels like. Because tires communicate best when they're properly aligned, you not only improve grip with a performance alignment, but handling as well.
A stiffer chassis is achieved through a combination of springs, shocks, and chassis braces. Braces should come first, because torsional flex adversely affects the responsiveness and communication efficiency of just about every component of the car. Roll bars, frame rails, door bars, and your standard array of undercarriage braces are all effective. Not only will the chassis feel more solid, but handling will improve noticeably. The tossibility limit will go up.
Stiffer springs and shocks are the bread-and-butter of improved handling, but this is where the toughest decisions lie. When you buy new springs and shocks, you become the car designer. You have to decide what levels of NVH are an acceptable trade-off for a stiffer chassis. You have to know whether or not these particular springs and shocks will allow the car to respond faster or communicate better. For the most part, any stiffer spring will do that, at least to a point where chassis flex begins to be a problem again. Shocks, on the other hand, may or may not help.
Shocks need to be chosen with a specific spring in mind, and even more importantly with a specific damping curve for the type of driving you do. Shocks that are too weak for the springs will reduce responsiveness, and will adversely affect the ride. Shocks that are too stiff will respond quickly but may communicate the wrong information about what the tires are doing. As far as damping curves go, that's an inexact science based on an aggregate driving style. If there was one "best" curve, all the shock companies would use it. Shock engineers may be able to determine the best curves for themselves. For the rest of us, adjustable-rate shocks are the way to go.
One suspension stiffener with a significantly reduced NVH trade-off, and relatively inexpensive as a side benefit, is bigger sway bars. These are so cheap and so effective, it's a wonder the Miata wasn't designed with bigger bars from the start. In Mazda's defense, sway bars do increase harshness slightly, but nothing offers a better improvement in handling for anything close to the cost. The improvement can be felt immediately, with a tossibility increase of at least 10 mph.
Handling is the Miata's strong suit. It can still be improved, but to do that it's useful to understand exactly what handling is, and what contributes to it. Of course handling will always be subjective. It'll always be a trade-off, and those trade-offs will never be the same for everyone. Even Mazda couldn't pin it down, not for everyone. They got close, though. The rest is up to you.