Is this your ticket to lots and lots of laps? This 2007 Mazda MX-5 has already received the popular 2.5-liter swap plus aero, track suspension and weight reduction–total weight, wet with driver, is listed as 2074 pounds.
Also included: MX-5 Cup cage, Ultrashield bucket seat and fresh belts. Wheels are the ever-popular Enkei RPF1 in 17x9 inches.
We found the …
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.
If the aero analysis were true of it being better with no windshield, one would think you would then only build a prod car cage, i.e. only over the driver side rather than full width in order to reduce drag through the full cage. This full cage may have been done in order to allow a second seat and passenger(?) which would be reasonable for a street/auto-x/time trial car. If used for those purposes, it should still be a fun car.
World MX5 Cup cars use a full cage with full windshield, but no roof, primarily because there isn't one readily available, at least not for the ND, and the rules don't allow it anyway. They're all spec'ed and homologated as raced.
Personally, for an NC (I have one) I would use the NC aftermarket racing hardtop and build a full cage under it, then put a wing as far back as legal, up in the clean air. But that's just me. To each they're own.
In reply to Keith Tanner :
A quick internet search and my memory of watching races says Miata Cup cars run roofless but Spec Miata cars all have hardtops. If was building a race car based on a convertible, and the windshield had to stay I would definitely run the hard top. Drag would be definitely be less plus I like having a roof over my head, especially in the rain. However, if I was building in a class where removing the windshield was allowed, I would think long and hard about it and do a lot of research to see if it's really worth it. There must be people that have data on running a car with and without windshield without any other significant changes? That would be a reasonably good way to establish what's really important - a reduction in lap times. It should probably also be mentioned that drag reduction isn't the only reason to get rid of the windshield and hardtop. Obviously that's also a reduction in weight and a lowering of the CG. Both benefit lap times.
In reply to ShinnyGroove (Forum Supporter) :
I think EP Miatas have close to double the horsepower of Spec Miatas.
In reply to BA5 :
"I'm pretty confident in my 'no-shadowing' assumption on the cage members, "
I do disagree with this part, and it is significant. You have forward, diagonal, and aft bars that triple the drag over the 'no-shadowing' assumption. You would have to be moving well hypersonic for the air not to hit the middle and aft members. You also now need to consider the drag from the driver, the seats, and the rear shelf. Factoring those areas in will probably sway your numbers heavily.
Now, the bottom line - is it more fun without a roof? For this, I'd say it is a no-brainer. For performance though, a cockpit is going to be the winner.
Whoever buys this car should do a quick and dirty test. Fix a plexiglass windshield to the cage and do some runs down the straights. Exit the turn at the same speed for each run. Higher top speed should equal less drag.
If I were to buy it though I'd consider a 3rd bar in the center of the windshield to prevent a tire from coming thru the cage at least.
In reply to BA5 :
This math looks good to me. The assumptions here seem reasonable.
In reply to cyow5 :
I think you got it backwards. The no-shadowing assumption made above is tripling the cage area. The drag due to interior pieces is a good point. They're definitely no longer shadowed by a windshield. Driving a Miata with the top down (and windshield in place), I don't feel significant drag inside the car, it's all turbulent, so without a windshield, all of those parts will be producing drag.
In reply to ShinnyGroove :
I would be surprised if there's any measurable difference in lift between these configurations.
In reply to tyronejk :
Sorry, you're right. I had read that wrong.
tyronejk said:Driving a Miata with the top down (and windshield in place), I don't feel significant drag inside the car, it's all turbulent, so without a windshield, all of those parts will be producing drag.
Isn't turbulence a fairly strong indicator of drag? The Yamaguchi NB book gives the following drag coeffecients for an NA Miata:
Soft top up: 0.38 - note that based on an observed increase in highway fuel economy, the hardtop is slightly less draggy
Soft top down: 0.44
I know my Lotus 7 replica was clearly draggier with the windshield, but 1) that was a large, very upright windshield and 2) it only had a roll bar with no cage. With the windshield on, acceleration would drop off dramatically above 80 mph.
When I got into track days years ago, I was running an NA Miata with a roll bar. I got a hard top and it gained me over a second on lap time and 2-3 MPH on the straight at Road Atlanta. There's a lot of "yeah buts" here... it's a top and not a windshield, underpowered car, unusually long straightaway, etc. It left a big impression on me though.
My other useful data point is my buddy's Exocet. I rode along with him for a few laps at AMP, the aero was just hilariously bad above 100mph. It was like being in a hurricane with a parachute out.
None of this may solve the riddle of the windshield in the NC above, but I suspect that exposed cage is far from ideal from an aero standpoint.
In reply to Keith Tanner :
Oh yeah, I agree. My logic is not well worded. My reasoning is that with a windshield and top down, everything behind the windshield feels turbulent, which suggests that all of the interior parts are "shadowed" by the windshield. This is based on the hand wavy assumption that draggy parts on cars (at typical car speeds and typical car dimensions) produce high pressure, laminar flow in front and low pressure, turbulent flow behind.
In reply to tyronejk :
Ah yes. I agree that without the windshield, the parts immediately behind it will become much more of a factor for drag. I'd love to see some streamlines, I suspect a roll bar behind the seats is starting to see less turbulent air at the top. It would be easy to test. Heck, do Miatas gather bugs on the leading edge of a roll bar? I'm actually not sure.
This thread reminded me of this site I saw a few years ago.
https://robrobinette.com/S2000Aerodynamics.htm
The guy who posted it is a hobbyist but a very smart one. If you read it, he did a bunch of CFD simulations on an NA Miata. With relatively minor changes like ride height or a front lip under the bumper, the drag coefficient varied from 0.29-0.41. The lift coefficient varied from 0.27 to -0.49.
The point I always took from it is that relatively small changes have relatively large effects on drag and lift, and things don't always behave as you would think.
In reply to ShinnyGroove (Forum Supporter) :
It has been mentioned in numerous places that aerodynamics are not an easy thing to paper napkin well. You need to understand how every surface is interacting with every other surface (Including internal flow) to really get it right. Sure you can get some improvement with a basic understanding of aero but after that you need to start quantifying and visualizing what's going on.
In reply to ShinnyGroove (Forum Supporter) :
That study (it wasn't done by that blogger, but is a repost of work done by Hansha Group) is excellent work for sure. I've used it as a reference many times. I'd love to see it repeated with the top down. 9Lives may have the chops but they'd have to have a reason to invest the time.
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