- The roll bar stiffened the hell out of my chassis. Night and day. - As ENV said, you could try welding (or even bolting) a bar between the sides just behind the seats. There is a factory bar there on some later miatas, and an aftermarket company or two sell some as well. I haven't knowingly driven one with that bar, so I can't tell you if it really makes a difference or not. I would imagine it would do at least something - Companies make braces that run the length of the frame rails. They are like secondary rails actually (cover the factory rails). They stiffen the chassis a ton as well. Then they went even further and made a brace to go between those secondary rails, forming a huge x. That extra X is still questionable as to whether it is worth it or not. My boy tells me it didn't do anything and he's doing very well in local auto-xs, but all the loyal FM sheep say it's the greatest thing ever. But I would question them since, well...nevermind. link to pic
There's all kinds of little braces you could put places on most cars. I'd suggest looknig at other convertible sites and getting ideas from them
