Miata question

Jay Lutz

Jay Lutz

Well-known member
Just curious with the bizillions of EP and FP Miatas competing. Forgetting about downforce for a second, which has lower longitudinal drag and therefore superior performance potential?

* An open cockpit car with no windshield and lower crossectional area but a high Cd OR
* A closed hardtop car with a windshield and higher crossectional area but a much lower Cd?

The nicely done grabber blue Blue Bayou FP Miata that just sold on this forum got me thinking. Surely someone has done a simple on track coastdown test???? The answer to this riddle could put you on the podium at a high speed Runoffs track.

Thoughts, how about some real data????
 
I'll bet that MAZDA either has done it or COULD do it. They have a total vehicle model, a suitable CFD analysis program. Almost as good as a real test. Who has the right connections inside Mazda???????????
 
OR..........who has a windshield and hardtop they could duct tape onto their open racer to do a back to back coastdown test. EZ!
 
If I really wanted to know, I'd contact David Blue Bryson. :think: Oh and don't forget, he's a real shoe.
For the Runoffs this year and year 2016 I'd take the open car, for 2015 the closed car.
 
yep..........Bryson was the guy/car I was referencing. Couldn't remember his name but the car looked like a good deal at $20kish
 
I have no data on this, but do have some really nice instrumentation that we could test with! :D

It is something that I've pondered a bit, and my guess is that "open" is probably best.

It will come down to details and may vary car model to car model, but I suspect that the aero drag is similar between the two, and that the tie breaker is then the lower center of gravity derived by losing the relatively heavy and high windshield and roof.

I also suspect that the exact configuration of the add-on "windshield" (aka cockpit front fairing), roll bar and rear cockpit depth/shape will have a lot to do with the Cd of the open version. Does anyone remember the ca 1980 Steve Johnson FP Spitfire w/ very large "Accusump cover" in the cockpit?

Al Seim
HP VW Scirocco 1.6
 
Al,

My gut says just the opposite..........that the windshield car would be better. But only the data can talk with authority!
 
Jay Lutz":287k2ltj said:
Al,

My gut says just the opposite..........that the windshield car would be better. But only the data can talk with authority!
Click to expand...

Car at what speed at what % of lap distance?

1990 Miata frontal area = 18.42 square feet (CD = .38)

1990 Miata windshield frontal area approx. = 4.7 square feet (-24% of total square feet)
1990 Miata without windshield = 14.7 square feet

Wind screen/open top CD?
Roll cage CD?

:think:

A 1993 Mazda RX7 has a frontal area of 19.34 square feet and a CD = .29
 
I suspect that the open windows in the hardtop account for a lot of drag when you use the hardtop. I'd venture that the total area of the two open windows is not much less than the open area of a well built open car. Throw in the extra frontal area and I think you're better without the top (cage design dependent).

I know of one car that has done back to back testing and it's being run without the top. And he posts here ... and I bet he reads this thread ...

-Kyle
 
I think that you should reach out to Jon Goodale. I know that he has run his MX5-bodied GTL car both ways. He currently seems to be running it without the windshield and did so at the fabled Road America track. I think that there may be a hint in there somewhere. :)
 
Rob":37egu5m6 said:
I think that you should reach out to Jon Goodale. I know that he has run his MX5-bodied GTL car both ways. He currently seems to be running it without the windshield and did so at the fabled Road America track. I think that there may be a hint in there somewhere. :)
Click to expand...

Not dissing what your saying about Jon and Road America. IIRC he ran the top most years at Road America. Jon will be along shortly with the true facts.
 
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