Fastbacks are not just for Concours points!

I've been crunching some numbers from the Challenge.

It turns out that the handmade fastback I built for The Mumpkin wasn't just a pretty face... our average top speed was 3.6 mph higher WITH the top than without.

It's particularly interesting because it wasn't closed- there were no windows, and the edge at the windscreen was open enough I could almost fit my finger in it. I was concerned that it was acting more like a parachute than an aero advantage.

Apparently, roll bars, open cockpits, etc. add quite a bit of drag.

Food for thought...

In reply to SVreX:

That's a bigger effect in trap speed than I would have expected. Reminds me how, back in the day of black-and-white photos, Porsche used to fit streamlined hardtops to 550s just for the high speed tracks. I suppose it would have helped on the slower tracks as well, but the drivers might have passed out from the heat.

Nice creativity and fabrication on your hardtop. Good to see your Miata featured on the GRM home page!

sailboat masts are no longer round for a reason. All those tubes hurt.

In reply to Danny Shields:

It's certainly bigger than I would have expected too.

Granted, that's based on only 12 or 15 drag passes. Not a lot of science.

But, we took the top off LATER in the evening, when our times should have been a little better. Our ET times were better, but our top speeds dropped when we removed the hard top.

At the (now-defunct) Phoenix International Raceway roadcourse, a Spec Miata would go about ten MPH slower into turn 1 without a hardtop. That's a huuuge hit in a low-horsepower car.

Will need to see the testing between the soft top, hard top, open and fastback. Maybe it isn't worth it to lose the top on a miata for the challenge.

Paul, what does that top weigh? The only reason your ET was less without it has to be the weight difference/bias I would think. Or, did the cooler air offset the increased drag without the top?

Not a big surprise. Even back in the day, think of Leyland taking the Spitfire and grafting the fastback roof onto it for LeMans (which eventually became the GT6).

In reply to DeadSkunk:

I don't think the weight had anything to do with the ET (or at least very little).

We got MUCH better at launches as the night wore on (better driving, better setup, different tires). The air got cooler as the night got later (although it also got more moist). We also cranked up the boost.

There are several explanations for the reduced ETs. Not too many good ones for the difference in top speed.

60' is kind of a measure of launch ability. Weight reduction shows most here (which is why I could jump off the line in my Yugo faster than a Corvette). Top speed is more a measure of HP vs CoD. ET is a combination of both.

In reply to DeadSkunk:

I have a correction...

The ET's were faster with the top, not slower. My bad.

Both the ET's AND the top speeds were faster with the top on.

The top weighs 39 lbs. Car weighed 1888 lbs. with top, fuel, and nitrous on board.

I can just see it next year when teams start taping up body lines and radiator openings. Then there's going to be that one team with different body configurations for autocross, drags, and concours.