Convertibles: top up or top down?

If you have a convertible, should you drive it on track with the soft-top up or down? There are a number of things to consider.

• Visibility – top down is safer because you can see behind and to the sides better
• Disaster – top up is safer because your arms are less like to flop outside the vehicle if your car rolls
• Top speed – top up is faster because there is less drag
• Corner speed – top down is faster because there is less lift

Well, that’s not very well resolved. There are reasons to have your top up and top down. If you look at the Spec Miata rules, they allow you to run with a factory hard top, which most people do, or run completely naked.

• NASA – 8.1.2 Hardtop may be used, and if used, must be securely bolted in place.
• SCCA – 6e. Convertible tops and attaching hardware shall be completely removed. Cars may compete with the Mazda factory detachable hard top in place (latches shall be replaced with positive fasteners), but it is not mandatory.
• Specmiata.com – 7.e. (but the exact same wording as SCCA).

Is a Spec Miata actually faster with the top off? I don’t know. Given that nobody drives a top-down SM, it’s probably faster with the top on. On the other hand, when Best Motoring did an all-convertible battle, they found that the cars were faster with the tops down (the exception was the Boxster S, which was nearly identical). Maybe it’s because racing isn’t time attack (i.e. there is drafting).

Here’s a screen shot of the results from the video.

The video is definitely worth watching.

One of these days I need to do some back-to-back testing with top down and up to see what is faster in my car.