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-   Aerodynamics (https://www.miataturbo.net/aerodynamics-119/)
-   -   Hardtops (https://www.miataturbo.net/aerodynamics-119/hardtops-98567/)

Occamís Racer 11-15-2018 01:36 PM

When I look at all the awesome aero on Time Attack cars, I always wonder why they use the stock hard top? Are you required to do that by the rules? Wouldn't a fastback have a lot less drag and make the wing work better?

When I was building my fastback I did some research and found that the ideal rear window angle was around 12 degrees. I used more like 15 degrees for better visibility, but it's still good. The Miata hardtop rear window is really close to 30 degrees, which is apparently the worst angle you can have, worse even than a vertical square back. So is it the rules, laziness, or what that has everyone using the factory hardtop?


ThePass 11-15-2018 02:01 PM

Very few cars are using the actual "stock" hard top, they are all lighter weight aftermarket alternatives, but yes factory shape. For the fastback, the cost is not insignificant so that's probably the primary reason you don't see more cars with them. Cost aside, as it would be for the top cars, the fastback certainly helps both drag and airflow to a wing. When compared to the superlight factory-shape hardtops, the penalty is weight at the highest point of the car .

concealer404 11-15-2018 02:05 PM

Shipping might suck.... but the CCP fastback appears to be the same flimsy featherweight garbage that their factory-shape tops are.

I can't imagine more than a 5-6lb penalty compared to their factory-style.

Scaxx 11-15-2018 02:24 PM

I picked up an AK top last year at MRLS for a stupid deal and although I haven't weighed it, it's actually pretty light. I'd be interested to see how much more it is than the stock top. I'll try to throw it on the scale next time I have it off the car.

Occamís Racer 11-15-2018 02:36 PM

I guess if you're buying a fastback, then cost and shipping is a problem, but there are a lot of DIY folks here doing much more difficult things. I built my fastback from a Treasure Coast half top, some skateboard laminates, fiberglass, and lexan. It probably cost me $400 and it weighs 14 lbs. The DIY effort was about equal to making an airdam and splitter.




ThePass 11-15-2018 05:52 PM

Originally Posted by Scaxx (Post 1511274)
I'd be interested to see how much more it is than the stock top.

This is not apples to apples. The lightest fastback should be compared to the lightest possible stock shape hardtop. My hardtop is 8 lbs. The fastback is significantly heavier (I've weighed some). I'm not saying it's not worth the aero benefit, just saying it is indeed heavier than the alternative.

endura 11-15-2018 05:59 PM

where's that drag vs backlight angle chart from?
I've contemplated buying/building a fastback. Looked up top speeds of various ''true hatchback/fastback cars and their convertible models. Focused on convertibles with long rear decks/sharp rear window angles like the miata. Surprise out of the research was insignificant or no differences in top speeds of various cars between, otherwise identical, coupe vs convertible models. Corvette was 197 vs 199, for example. High vs low wing made a bigger difference (10mph) than coupe vs convertible. Audi R8 is same for both. So is the Boxster/Cayman. At this point, I can concede the ''better flow to wing'' but not the ''significant difference in drag''. Anybody has info. contrary, I'd love to see it as I can't find anything with real world results.

Scaxx 11-15-2018 05:59 PM

Nor did I intend it to be. It would be an apples to apples comparison on my stock hardtop.

endura 11-15-2018 06:23 PM


Blackbird 11-15-2018 07:24 PM

Originally Posted by concealer404 (Post 1511271)
Shipping might suck.... but the CCP fastback appears to be the same flimsy featherweight garbage that their factory-shape tops are.

I can't imagine more than a 5-6lb penalty compared to their factory-style.

I have a car w/ a CCP fastback here, it's actually not half bad for a race top, but it's not as light as you think.
IIRC we scaled it at 24 Lbs for the top and trunk lid that goes with it.

Scaxx 11-15-2018 07:30 PM

Huh, heavier than I would have thought. I'd guess mine is about mid 40s/low 50s but it's supposed to be more of a street top. Is that top just a skin?

Blackbird 11-15-2018 07:38 PM

Yes, skin with a couple strips for reinforcement in key areas and gel coat.
Fit and finish was not OEM, but definitely better than the garbage skin tops that are commonly sold at least around here in socal.

Savington 11-15-2018 08:50 PM

I have a couple of the CCP tops in the shop as well, all OEM shape. They are nice race tops. Fitment is 9/10, if OEM is 10/10. I have seen far, far worse. I have only seen one nicer race-style OEM top, and it was 3x the price, and you cannot buy one today.

Occamís Racer 11-15-2018 08:59 PM

How much does weight matter? If light weight was more important than aero, nobody would do aero. If weight high up mattered, nobody would use a wing. Aero is heavy, but provides clear benefits.

For sure a fastback weighs more than a hardtop, but whatís 6-8 lbs when there might a large reduction in drag and an increase in wing effectiveness?

If you look at my second construction photo, you can see how narrow the fastback is at the rear edge of the side window. Compare this to a standard hardtop - look at the huge gap there. This was a key design element. Not only is there less physical material (lighter weight) but less wind goes into the cockpit from the open window. Air goes right past the window and reattaches (or so I believe).

As for materials, fiberglass on its own is heavy. If you want light weight and rigidity you use surfboard construction - lightweight core with thin skins of fiberglass or carbon on either side. I build boats as a hobby, and so I have those kinds of materials around. The only reason to use solid fiberglass is popping out multiple copies, which is a cheap and easy way to do it, but always going to be heavy.

It would be interesting to see more fastbacks. I donít believe the factory-shaped hardtop has good aero unless youíre simply comparing it to an open top.

Savington 11-15-2018 09:05 PM

What if I told you the drag reduction from a fastback vs. an OEM top and a wing wasn't that dramatic?

apexanimal 11-15-2018 09:29 PM

Not that it would be better than a fastback, but vortex generators would help the factory-shaped hardtops perform better.

jspeed.713 11-15-2018 09:48 PM

You could compare raw power to weight ratio of the different setups ie adjusted power (hp-hp consumed by drag) and additional static weight of fastback vs lighter hardtop but increased drag.

endura 11-16-2018 12:08 AM

718 boxster, 718 cayman, 170mph top speed, both.
R8 199mph, R8 conv. 198mph

2005 mustang. coupe 147mph., conv. 149mph. (conv. 250lbs heavier.)
1999 mustang gt 138mph, convertible 139mph.

Blackbird 11-16-2018 01:44 AM

I think that people confuse OEM development budget and ability to design and test with small aftermarket companies low production volume items.
CCP set out to make a good looking fastback top and sell some of them, they never set out to improve the aero on the car...

rrjwilson 11-16-2018 05:28 AM

@endura I believe you are getting stuck on top speed.
For low power cars they will be drag limited.
The cars you are quoting are not likely to be drag limited making top speed just a function of gearbox and engine RPM.
Stock miata with softop up will do the maximum (128mph maximum) while with the top down only 92mph is possible (at least for a 1.6 litre).

Drag affects the rate at which top speed is achieved.
So for the Mustang (which i would guess is drag limited) achieving the top speed will be time consuming two or three minutes but through fluke of design gets a slight bump in top speed.

What would allow more insight into the drag versus top speed would be a speed vs time plot.
Given a stock car and the various tops you could get some comparative numbers for drag based upon time taken.

You could calculate (net) force from this.

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