By Norman H. Garret II in the book “Mazda Miata – Performance Handbook – 1998”
First, let's talk about the structure that holds it all together. The Mazda engineers who dedicated themselves to designing a new-from-the-ground-up convertible back in the mid1980s were among the first to use high-powered computer modeling in developing an automobile's entire structure. As a result, the Miata has one of the most rigid chassis of any convertible ever produced. This is particularly important in a sports car, in which the chassis is asked to resist cornering forces and provide a firm base for the suspension. Adding greatly to the Miata's overall rigidity is a structural beam that runs from the engine-transmission assembly to the rear differential. The purpose of this beam (the Powerplant Frame, or PPF in Mazda-speak) is to tie all the major drivetrain components together. This eliminates the "rubbery" feel that a drivetrain can produce when you're hard on and off the throttle, something a sports car might be 3' doing in a set of twisty curves. The PPF also isolates the points at which the drivetrain components attach to the chassis. In a Miata, the entire drivetrain is supported by just four points-two on the front subframe and two on the rear subframe. The front and rear subframes, in turn, are mounted to the body in specifically strengthened and isolated locations. This is much more stable than having separate mounts for each of the drivetrain components, and lends a lot to the Miata's overall stability on the road. Beginning in 1992, Mazda added a rear subframe brace as an evolutionary improvement to the rear subframe. This brace had some effect on reducing the Miata's sensitivity to that infamous 65-mile-per-hour vibration induced by imperfect tires. Miatas are very sensitive to tire pressure. If your car has a 65-mile-per-hour vibration, the first thing to check is that you're running 28 psi in each tire. Twenty-six to 28 psi is right where the Miata likes to have it; above and below this sweet spot you're almost guaranteed to create vibration on some cars. The pre-1994 Miatas are particularly sensitive to this. In 1994 Mazda stiffened the chassis further, greatly reducing (but not eliminating) the vibration problem More importantly, the brace made the rear subframe more resistant to deflection under cornering loads, which keeps the rear wheel-tire combination better aligned with the pavement. Brainstorm Products sells this simple subframe brace that does the trick. The bend in the bar allows fitting var¬ious aftermarket exhausts-an important consideration. In 1994, along with a minor facelift to the interior, Mazda made more advanced changes to the chassis of the Miata. Front and rear subframes were stiffened and braced, greatly improving rigidity and reducing the flex in the platform. Fortunately for owners of 1990-1993 Miatas, all of these changes can effectively be replicated via the aftermarket. Many aftermarket suppliers sell rear braces that simply bolt onto the existing pre-1994 subframes. Cockpit braces that run between the shoulder-harness pivot points are also available to help stiffen the rear passenger-compartment structure. Also available are front subframe braces that reduce deflection by closing the open tunnel near the control-arm mounting points. All of these updates can be performed at nominal cost on your pre-1994 Miata. Another way to increase the rigidity of a Miata's chassis is to install a "sport-style bar." These used to be called "roll bars" until the lawyers got involved and everyone had to start calling them something else, just in case someone put one in his or her car and got hurt in a rollover crash anyway. These "style bars" have the effect of stiffening the rear bulkhead to a great degree and can help reduce the 65-mile-per-hour vibration sensitivity at the same time. They generally attach to the towers that support the shoulder-harness pivots just behind the seats. Some also come with rearward supports that tie into the frame rails surrounding the fuel tank. The latter type are used by many driving schools, but they are less popular for street use because they are difficult to use in conjunction with the soft rear window. If you want to go for ultimate stiffness, a full roll cage (as used by SCCA racers) will tighten your Miata to the nth degree, creating an exceptionally stable platform for the suspension to work from. But for most of us using our cars, a true roll cage simply isn't practical, since it complicates entry and egress of the car and operation of the soft top. The simplest way to improve the structural rigidity of the Miata chassis is to buy and install a Mazda hardtop. A side benefit of the hardtop is that the highway noise is reduced about 5 decibels, which makes for a quieter car on long-distance trips. Many road-warrior Miata drivers taking long trips install their hardtops for the highway portions, and then take them off and store them in their hotel rooms while they enjoy driving around their final destinations. Strut-tower crossbraces (steel bars that connect the shock tower mounts together across the engine bay) are very popular in today's performance aftermarket. These braces work great on cars with MacPherson struts (thus the term "strut-tower crossbrace") because these suspensions have no upper control arm. The entire lateral load from the upper portion of the wheel hub, plus the shock loads, plus the spring loads, all enter the chassis at the upper strut anchor point. That's a lot of force for the sheet metal of the engine compartment to handle. As good as tower braces are for some cars, they aren't as useful on a Miata. On a Miata, the hub forces are contained by the upper control arm, and the upper shock mounting point only sees spring loads and damping loads. These are markedly less severe than the lateral loads imposed by a strut suspension, and therefore the Miata sees considerably less flexing at the tops of the shock towers. The double A-arm suspension allows each component to do its specific job-the coil-over shocks damp the ride and hold up the weight on the corner, while the control arms are free to take the massive side loads of braking and cornering. Left to their own specific tasks, all these pieces can be made smaller and lighter for less harshness, less suspension inertia, and lower unsprung weight. This is another example of how Mazda spent surprising amounts of money in building the Miata, even though it was making an "affordable" sports car. MacPherson struts are cheaper to build and install than double A-arms, and Mazda already had struts on most of the firm's other cars. It would have been an easy matter to adapt the 323's struts and a trailing-arm rear suspension to the Miata, but the end result would have been a machine that drove much less like a sports car. In fact, Ford built that car-it was called the 1990 Mercury Capri, and it is now out of production. In any case, there just isn't enough force going into the Miata's upper shock mounts to create a lot of deflection in the engine compartment. That isn't to say that these braces are useless on a Miata, just that they should be the last brace you buy. You'll get much more benefit from the various subframe braces or a style bar. The Miata's chassis is very sensitive to shock-absorber quality and condition. You can choose from the original equipment shock absorbers as available from your dealer, or standard shock absorbers from the aftermarket. After a tire upgrade, the best way to instantly improve your Miata's handling and ride quality is to upgrade to performance shock absorbers.