Taylor Made: Headstands with Miatas
Something we don’t often think about is how much structural loss convertibles have to make up for by virtue of not having a hardtop. If you think of a car’s cabin as a basic box, the bottom corners are connected in lateral pairs by the axles and in pairs along the AP (anterior-posterior) axis by the frame rails. The box’s top corners are similarly connected by the hardtop. When the car is in motion, bumps and uneven ground can cause the anterior weight of the car to move or shift independently of the posterior—basically creating a moment about the box. If the car has enough structure, it will resist rotation that would manifest in the form of a flex or bend in the chassis. While this type of deformation falls well into the elastic region, cyclic loading over time can still fatigue the car’s frame and increase the potential for failures. While the structural role of the hardtop may not be noticeable to us, it nonetheless makes a significant difference.
When you switch out for a soft top, the majority of structure along the AP axis falls on the doors and frame rails. As a result, you can expect an increase of flex over bumps, etc. You also lose a major safety factor because the driver and passenger(s) are no longer contained in a protective cage. Adding reinforcements in the form of braces, roll bars, or stronger components can help make up for compromises in structure and safety. Some convertibles come off the assembly line pre-reinforced or otherwise designed with this in mind, while for others it falls on the driver to modify the car.
I like to think of the Mazda MX-5 Miata as the little sister of the Honda S2000. Among other design elements, the S comes off the assembly line with roll bar hoops and a reinforced windshield frame designed to support the weight of the car in a rollover. Its manual states that the combination of these components “help form a protective space for the driver’s and passenger’s heads in the event of a rollover.” The added strength and stiffness from these components also cause behavior similar to that of a hardtop, providing more structure for the chassis. This is not standard for all convertibles, and in the event of a rollover in a NA Miata, the weight of the car would crumple the A pillars and fall on the heads of whoever’s in the car. Want to do a pseudo headstand while holding up a Miata? I think not.
This past fall, I installed a roll bar on a white NA from ‘93. The specific bar I used was a four-point with no diagonals—i.e. not approved for track events. In its time trial rules, the Sports Car Club of America defines a track spec roll bar as a “main hoop and diagonal placed behind the driver and supplemented by two braces.” The main hoop is the part of the bar that actually extends above the driver’s head and contacts the pavement in the event of a rollover. A minimum single diagonal is required, although some bars offer double diagonal and/or seat cross-member configurations for additional support and reinforcement.
Each point of the bar was bolted to the body of the Miata using three bolts per point. Additionally, the pillar loop for each seat belt was rerouted to connect to the car through a gusset on the roll bar. For ease of installation, the seats were taken out and the carpet removed from the “hat rack,” as some like to call the storage space above the gas tank. The tank sits right behind the two seats, separated by a couple protective metal plates. The particular NA I worked on required one plate to be temporarily removed and the other modified to accommodate the bar’s rear vertical struts. For the same reason, slots were cut in the hat rack carpeting. ABS and sensor wires had to be shifted—I bundled and relocated them with reusable zip ties. Other modifications to the interior included removing the quarter trim panels to make room for the front vertical struts.
In NAs, the placement of roll bars like the one I installed prohibits the seat belt reels from being removed (unless you remove the bar itself), making this project a good time to upgrade seatbelts if desired. As installation also requires removing sections of carpet, the project is also an opportune time to mod the cabin liner. It’s important to note that the feel of the car under power changed with the addition of the bar—the increased stiffness reduced the amount of flex and shimmy felt over bumps by a large margin, as well as increased the aggressive characteristic of the handling, but consequently accentuated the car’s old and shot suspension. For best performance, projects that significantly stiffen the car are best done in conjunction with any necessary changes to the suspension and sub frame systems. Somehow, for that Miata, I suspect another project is on the horizon.
Special thanks to Jackson Tucker for his help with this project.
Photo at top, from Taylor Tucker: Christening the roll bar after installation.