A roll bar is tubular bar behind the cabin designed to protect occupants in the event of a rollover.
A roll bar or bars are found most often on convertibles because they typically lack the C-pillar that would protect the occupants of a hardtop vehicle in a rollover.
Some manufacturers provide active roll bars that spring up behind the passengers only when sensors detect an imminent rollover. The Volvo C70, Volkswagen Eos, Audi A4 and BMW 3 series employ this feature. This provides cleaner lines and better rear visibility without compromising rollover protection.
Don’t confuse a roll bar with an anti-roll bar, which is another term for a stabilizer bar.
Information for this was taken from Cars.com’s glossary, written by Joe Wiesenfelder.
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Which cars, vans and Suvs have roll bars---bars that protect your head in the event the vehicle flips over?
Those things that look like a "roll bar" on 2005 and on stock MX5 is not a real roll bar. It is called a style bar(just for the look). It does not offer any protections. More importantly, a reall roll bar should be higher than the top of your head. In case of a roll over accident, the roll bar should hit the ground first, not your head.
To answer Dawn's question: Most people think it's safer to drive a SUV, van or Pick-up truck because they are bigger. However, most of these big vehicles are not built for rollover protections. The roofs of these big vehicles are not built to support the weight of the whole thing in case of an up-side down situation. Most people die in SUVs and trucks rollover accidents because the roof collapses to break their neck or head. In fact, roofs of some of smaller cars hold up better in roll over situations because the vehicle has lighter weight to begin with. So, let me put it this way: as long as your big car don’t roll over, you are safer than the smaller cars. But if they do, you are in a bigger trouble than small cars.
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It can also refer to a similar feature used on pickup trucks and utes. it does the same job, but usually is an aftermarket accessory. Many people use them as a mounting place for off-road lights.