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Old 09-23-2020, 03:34 PM   #3566 (permalink)
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Originally Posted by kach22i View Post
In another recent thread you mentioned no need for rear diffuser if the underbelly behind the rear wheels were at 2.8 degrees or less.

I assume this rule of thumb is for a range of heights normal to road cars, let's say between 4-8 inches?

The template posted through improperly used by the article's author has a rear departure angle of approx. 2.8 degrees, right?

Right answer using wrong method, or does that Miata need a rear diffuser for an un-yet stated reason?
1) the 2.8-degree upsweep IS the diffuser. It begins at the beginning of the rear wheelhouse. It has the lowest drag observable for a diffuser.
2) as to ground clearance, the body must be able to clear 10-degrees ( approach) at the nose, 10-degrees ( breakover ) between the axles, and 16-degrees ( departure ) at the rear. The 'template' respects the three criteria.
3) the Miata has it's own Sports Car Club of America ( SCCA ) race car category. Many owners take them to the 'TRACK' on weekends. Elapsed time on the course is the primary interest. Winners will have balanced downforce and top speed for the winning edge.
Technically, a diffuser will not function if the onset flow coming to it is perturbed. You'll see mid-engined supercars ducting the exhaust through the body to the wake area just so the can create a really clean belly and diffuser.
I'm not sure what the SCCA rulebook allows in competition.
The Miatas I've seen are all roadsters. No roof! Seems like they'd be going after splitters and rear wings as far as the law allowed.
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