You need this: Track-ready, NC-chassis Miata

I remember when the Skip Barber school closed and they were selling off their NC school cars for like $12k.  By the time I heard about it they were already gone.  A well-prepped NC is a MUCH faster car than a similarly prepped NA/NB, and that's with a stock motor.  I'm not sure about that price but I bet that thing is great on the track.  I wonder why they cut the windshield though?  Seems like the aero would be worse without it with that cage sticking up like a sail.

In reply to ShinnyGroove (Forum Supporter) :

Good question about the windshield. Because race car? Maybe an aero engineer can weigh in. :) 

In reply to ShinnyGroove (Forum Supporter) :

First and foremost - frontal area. Drag is a combination of frontal area and Cd.  Lower either or both = less drag.  The Cd probably went up a little because of the cage hanging in the wind.  But probably less than overall benefit of losing the frontal area of the windshield.  Also, now the air can get to the wing better, though it may not be very clean air because of the cage.  I'm guessing that a nice aluminum tonneau cover over most of the cockpit would help a lot. 

Most convertible SCCA Production class racers have cut down the windshield, so many others thought this was a good idea too. 

 

In reply to David S. Wallens :

It would be neat to put one in the tunnel before and after chopping the windshield off to quantify the benefits. (Or not). Might be even better to do CFD so you can visualize what's going on. 

Cages are just SO messy, I'd love to see that actually quantified. A hardtop and windshield would probably have less overall drag, but will that cage fit under a top?

In reply to Keith Tanner :

Moot point with the windshield gone.  Although the shape of the cage seems odd and complicated for something not meant to fit inside a car with a roof. 

In reply to msterbeau :

Well, it's a moot point for THIS car. But generally speaking, would it have been better off with the windshield?

If it's a MX-5 Cup cage, I believe those race with hardtops at times. 

This car is the (previously yellow) goodwin project light car, right?

We can actually do a pretty good estimation of which is better aerodynamically with some basic math.

Someone already noted the equation for drag:

Fd=1/2*pv^2*Cd*A

p and v are going to be constant for our little example here because we're driving the car in two different configurations through the same air at the same speed.

Now we need to fill in for some of the variables:

Cd of NC miatas is .38 for the NC1 and .36 for the NC2 according to "the internet".  I'm not real up on my miata chassis, but again according to the internet a 2007 miata is an NC1, so we'll go with .38 for this example.

Some more internet sleuthing gives me a width of 67.7" and an overall height of 49".  Let's subtract 5" of ground clearance to give the actual frontal area of:

AregularNCmiata = 67.7in*(49in-5in) = 2980in^2

Although the windshield certainly factors into the Cd, we're going to assume you keep roughly the same Cd when you chop the windshield, so we need to calculate the area without the windshield.  Some quick MS Paint CAD showed that the windshield frontal area is ~1/3 of the overall height from the ground (49/3 = 16").  So the area without a windshield is 

Anowindshield = 67.7in*(49in-5in-16in) = 1896in^2

You might already be seeing that the cage is going to need a LOT of area or a REALLY high drag coefficient to overcome that area reduction....

Luck for us, the drag coefficient of a circular shape is readily available (why, on the internet of course!).  The Cd of a circular rod perpendicular to the wind is 1.2 in laminar flow (worst case scenario, it's .3 in turbulent flow).

We're going to assume no shadowing of the rear cage members, and the cage is roughly the dimensions of a windshield frame (two vertical supports the height of the windshield and one across).  As a quick and dirty estimate, I'm going to say the cage has roughly 3 'windshield frames' worth of cage area.  So I'm estimating the cage area as roughly:

Acage = ((2*16in)+67.7in)*1.75*3 = 523 in^2 (2 16 verticals plus the 67.7 width times a tube diameter of 1.75 time the roughly 3 'windshield frames' worth of cage)

So now to bring it all together:

Fd (regular miata) = .38 * 2980 = 1132 (not adding any units here because this is kind of a weird 'coefficient')

Fd (caged miata, no windshield) = .38 * 1896 + 1.2 * 523 = 1348

Huh, I wasn't expecting that. The caged one looks like it's about 20% worse than the windshielded one.  That's actually pretty bad.

So let's talk for a minute about some of my assumptions.  The areas I calculated for the car are probably fine. The Cd for the windshieldless car is probably less than the standard car, but I can't find any good numbers on that in a 20 second Google search. I'm pretty confident in my 'no-shadowing' assumption on the cage members, but as a general rule I'm guessing the car is going fast enough that the flow over the bars isn't laminar.  In that case:

Fd (caged miata, no windshield, turbulent) = .38 * 1896 + .3 * 523 = 877

So if you're going fast enough to have turbulent flow over the cage (seems likely) the caged car is ~20% better.

After typing up the whole thing, I'm going to go with at track speeds the caged miata probably has less drag than the stock one.

But your gas mileage driving around the paddock is really going to suffer.

^ this is a nice attempt but I think the necessary assumptions make the work mostly invalid.  I'm not an aerodynamics expert but I know that the difference between laminar and turbulent flow has a huge effect on.... everything.  Smooth flow over a larger surface will usually be more efficient than turbulent flow over a smaller surface.  Relatively small geometric changes could make big differences to the drag coefficient.  As an example, I race an open cockpit car and I was a little shocked at how big of a difference a helmet spoiler made that is probably 30 square cm of surface area.

There's also the question of how you define "better" or "worse".  Less overall drag might make you faster in the straights, but additional lift is going to give you less grip.  A CFD simulation could probably give you some meaningful insight, but ultimately lap times would be the final judge.

I know that generally EP and FP Miatas without windshields turn faster lap times than SM's, but they're also significantly lighter and have a lot more in the way of modifications.