The Mazda MX-5 Miata NA and NB generations are close to my heart, and this article is a compilation of sources around the chassis itself. It is a difficult subject to explore given the lack of verified data, but I have done my best to ensure that the information contained here has the best possible sources. Still, take all of this with a pinch of salt, and do conduct your own verification where possible.

Torsional rigidity measures a vehicle's resistance to twisting forces, crucial for improved handling and driving precision. Bending rigidity gauges the ability to withstand bending forces, ensuring consistent suspension alignment and ride comfort. Enhancing these aspects leads to a more responsive vehicle to a point.

A common guideline recommends that the stiffness of a car's chassis against twisting should be ten times more than its resistance to leaning in turns, considering the toughest uneven roads it will face. This is because a car needs to maintain stability and tire contact when one side encounters a bump while the other remains level, a concept known as "warp stiffness." However, increasing the chassis stiffness beyond a certain optimal point yields diminishing returns in performance and might only add unnecessary weight.

While it's essential to have a sufficiently rigid chassis to prevent excessive bending that can impair the suspension, overemphasizing this aspect can miss the role of suspension design and tuning specifically around weight transfter. That said, the Miata chassis can be improved, and we will explore how by first looking at Mazda's evolution of the chassis, and then a path forward which is more contemporary. The image below shows the evolution of the car through different generations. 

Baseline Data:  

Mazda MX-5 (NA, 1990 – 1997)

4,881 nm/deg - 5,152 nm/deg (Unverified)

Source: http://www.roadster.blog/2015/06/mx-5-nd-skyactiv-body.html

Chassis weight: 211kg (Unverified)

Mazda MX-5 (NB, 1999 – 2004)

5,219 nm/deg - 6,367 nm/deg (Unverified)

(+1.3% torsional; +7.6% bending; +35% dynamic torsional)

Source: 949 Racing Bare tub weight and picture

Chassis weight: 224 kg (verified)

The chassis is the backbone of the Miata and the more rigid it is, the more likely the wheels will point and move in the same direction.  For any car to be responsive and predictable, it does not help when the chassis twists slightly and alters the suspension geometry under load.

The NB has several chassis improvements integrated into the body itself.  The cabin of the NB Roadster is 70mm narrower than that of the NA Roadster at 865mm. There is a high hardness steel reinforcement built into the a-pillar to increase rollover protection.

Crucially, the side sill itself is also thickly reinforced, and the semicircular opening of the door also contributes to the overall chassis rigity. Let's dive into frame rails and butterfly braces. 

1. Frame Rails and Butterfly Brace

Various bolt on chassis parts were done by Mazda over its lifetime, but I want to highlight a partocularly well engineered part by Flyin Miata. There is a great article done by who analysed the Flyin Miata frame rails to ascertain it's impact on the torsional and bending rigidity on the car. His simulated frame rails showed to increase torsional rigidity in the chassis by 13% and bending rigidity by 17%.

An analysis was performed by Chris Shieh to assess Flyin’ Miata's frame rail design for improving structural rigidity. The study utilized a finite element model reflecting the Miata's dimensions to test the 17% rigidity increase claimed by Flyin’ Miata. Key variables included the neutral bending axis, affected by modifications like adding a hardtop, and the design and material thickness of the frame rails.

The results showed a 13% boost in torsional rigidity and a 17% in bending rigidity with Flyin’ Miata's frame rails, slightly under the company's assertion but within expected modeling tolerances. Modifying the frame rails, such as eliminating weight-reduction cutouts, had minimal impact on stiffness, indicating Flyin’ Miata’s design effectively balances performance with weight and cost.

The butterfly brace acts as a structural reinforcement for the Mazda Miata, enhancing the chassis's torsional rigidity. Its design, reminiscent of a butterfly's silhouette, spans the car's undercarriage, interlinking with the frame rails at the sides. This integration fortifies the vehicle's resistance to flex and twist, particularly under dynamic driving conditions. By augmenting the frame rails, which bolster the longitudinal edges of the chassis, the butterfly brace contributes to a more rigid and responsive driving platform. It effectively mitigates chassis deformation, leading to improved handling precision and stability during maneuvers. Here is how they tested the new version of the butterfly brace according to Keith Tanner: 

"To test the twisting of the chassis, we supported the car on three jackstands. Two were at the front of the factory frame rails and one was on one of the rear control arm mounting points. The doors on the car were opened to give a worst-case scenario. The distance from the floor to the stock rear jacking point was measured. A jack was then put beside the measured point and the car was lifted until the front jack stand started to unload.

We performed this several times to ensure accurate measurements.The car was then lifted off the jackstands and all measurement and support points were marked. Pre-modification, measurements showed 5.5-6 mm of twist. Post-modification, we found 3.5-4 mm. A 2 mm change in twist over a 6 mm range is a 33% improvement, we rounded that down to 30%."

- Keith Tanner

Based on my research of cars which have both coupe and converible variants, a good rule of thumb is by chopping off the roof of a care reduces torsional stiffness by around 40-60%. Let's explore the effect of adding a hard top to the Miata. 

2. Hard Top

I know the image above is not of a Miata with a hardtop, but rather an uber-rare NB Coupe. In Chris Shieh's analysis, he found that adding a hardtop to a Miata will increase the bending load and effectiveness of floor-mounted chassis bracing. This is because the hardtop will move the neutral bending axis of the Miata up, causing the frame rails to deal with more tensile and compressive stress. However, Bob Hall, one of the Mazda engineers involved in the car's original development shared this insight on the hardtop:

"Like the soft top, the hardtop is an unstressed component. If the hardtop were carrying structural loads, the rear window would pop out of its mountings as the bodystructure flexed by any degree. The top is able to dampen (slightly) some of the lateral movement of the windscreen header, but the primary reason behind this is that with the top up the visual contrast between header and sky is diminished.

With the convertible top raised, the multi-component aspects of the top induce rattles which the single-piece hardtop doesn't have, thereby creating the impression that the hardtop has stiffened the car vis-a-vis the erected convertible top or with the top down. The car's torsional rigidity was measured in all three planes (as a body-in-white as well as a completed car) with the top down, top up and the hardtop on and there were no appreciable differences in the figures. "

- Bob Hall, Miata.net, 20th November 2002, 18:46

3. Door Bars

A Finn with A Miata did some incredible research on door bars and a specific weak point of the car, the point where the front firewall meets the door sills. His measurements have shown that the front of the Miata tends to bend downwards when its front wheels are lifted, identifying a pivot point near the firewall. This observation is coupled with a noticeable twist in the chassis, particularly at the front, which suggests a weak link between the middle section and the front of the vehicle.

Measurements revealed the Miata's front bends downward when lifting its front wheels, indicating a pivot near the firewall and a chassis twist, especially at the front, suggesting a weak link between the vehicle's middle and front. Traditional stiffening methods like Fender Arms and Frame Rails might not fully address this due to their limited connection to the front chassis. Tests on door bars showed they significantly reduce chassis bending; a 70 kg load caused a 0.35 mm deflection, reduced to 0.27 mm with door bars, and a 100 kg load's 0.49-0.51 mm deflection decreased to 0.39-0.40 mm, marking an 18-23% improvement in rigidity. 

The solution involved welding supports across crucial chassis points and adding door bars, enhancing stiffness suitable for non-racing use. Installation of chassis supports and strategic enhancements like triangular plates and robust tubing for door bars, chosen for street car suitability, notably improved structural integrity. 

My Personal Approach

Any modifications to my car could ideally be transfered to another car. Building on this sketch by A Finn with a Miata, I think there are three key areas that provide the best cost benefit chassis modifications to the car that are also bolt in.

1. Flyin Miata Frame Rails (Blue)
2. Hard Dog Motorsport M2 Roll Bar (Red)
3. Door Triangulator (Green)

The door triangular does not exist on the market, and will need to be fabricated. It is different from a door bar, as I hope to design it out of the stiffest possible steel, and create a cardboard aided design steel plate that can be welded to tie the front firewall to the sill.

I owned a ZN6 Toyota 86 that had an unverified torsional stiffness value of about 20,000 nm/degree and I never found it lacking. Assuming the stock NB has a stiffness of 6,000nm/degree I think optimistically, I could get around a 33-40% increase to around 8,000-8,400 nm / degree. Ensuring tires are not too grip oriented and more progressive will keep the car fun to drive. In the end, there are limits to what can be done on older chassis, so being content will be a part of the practice.