Many organizations who run track events do not allow convertibles to participate in them.
NASA Allows the S2000 due to the integrated roll over protection that the car comes with from factory. If you check the club's CCR's (
http://www.nasanorcal.com/roll_bar_allowed_cars.htm) you can see a list of convertibles that NASA allows on track without a rollbar;
<TABLE style="WIDTH: 315px; BORDER-COLLAPSE: collapse" border=0 cellSpacing=0 cellPadding=0><TBODY><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; WIDTH: 313px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21>
Aston Martin DB9, DBS</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Audi A4</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
BMW 1 Series</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
BMW 6 Series</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
BMW E36 (Optional)</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
BMW E46</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
BMW E90</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
BMW Z3 (Optional)</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
BMW Z4</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Chrysler Crossfire</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Dodge Viper</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Ferrari 355</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Ferrari 360</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Honda S2000</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Infiniti G37</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Jaguar XK8/XKR</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Lamborghini Murcielago</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Mercedes C Class</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Mercedes CLK</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Mercedes E Class</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Mercedes SL (1990+)</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
MINI Cooper (2009+)</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Porsche 996</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Porsche 997</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Porsche Boxster</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Saab 9-3 (2004+)</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
Volvo C70</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
VW Eos</TD></TR><TR style="HEIGHT: 15.75pt" height=21><TD style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; FONT-STYLE: normal; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; FONT-FAMILY: 'Times New Roman', serif; WHITE-SPACE: nowrap; HEIGHT: 15.75pt; COLOR: black; FONT-SIZE: 12pt; VERTICAL-ALIGN: bottom; BORDER-TOP: medium none; FONT-WEIGHT: 400; BORDER-RIGHT: medium none; TEXT-DECORATION: none; PADDING-TOP: 1px" height=21 width=313>
VW New Beetle</TD></TR></TBODY></TABLE>
Convertibles not listed (such as the Miata) don't offer roll over protection adequate for NASA and will need to have an SCCA/NASA approved rollbar installed (correctly) to be let on track. This means you will have to comply with all racecar cage bar diameters/wall thickness/mounting as well as having High density padding anywhere the driver is able to hit their head. Bolt in cages are allowed, just make sure they are SCCA/NASA approved.
I am a NASA safety tech, I tell drivers, "Anywhere you/your passenger can touch the bar with your hands while strapped into your seat needs to be padded." The padding can usually be bought at track during NASA events in case you need to add some in a spot I see that you may have missed
HTH