Convertible Drag

[nissynis]

Last week, it was over 100 degrees every afternoon here in N.C. Windows up with no A/C was simply not an option. I decided to drop the top on the convertible in lieu of using the A/C, since my commute is about 70% 45 mph or less (especially now that I'm HM).

This got me thinking about drag:

1) When you ride fast with the windows down, I understand it creates additional drag. Does this increase in drag occur because the rear windshield and seats act as a kind of parachute?

2) I ask #1 because I was thinking that maybe having the top back would actually be preferable to just having the windows down, since it would eliminate the parachute-effect. Does that sound reasonable?

3) What about top-back, windows-up? It would seem that that would help improve flow around the car, and reduce the drag effect of the tiny back seat catching wind.

I know a ScanGauge would answer these questions in a flash, but aren’t they about $150?

[Zukiru]

I'd build a spoiler for the behind the front seats.... but that's just me..

I think top down would be preferable to window down top up.

the also make covers for the convertible top to make it more aero while folded down.

[Ophbalance]

Meh, I drove from Johnston County to Durham on Friday and Tuesday of this past week without the A/C... just need a dew rag, is all, to keep the sweat out of the eyes . I'd be willing to bet that no matter what, having the top down is going to net the worst MPG, regardless of window position. I've not owned a convertible for a number of years now, so I can't test it out.

[some_other_dave]

In general:

Top up, windows up: Lowest aero drag
Top up, windows down: Second lowest aero drag
Top down, windows up: Second highest aero drag
Top down, windows down: Highest aero drag

Not having the top in place creates all sorts of turbulence over the rear half of the car. Not at all good for drag! It's even worse than just having the windows down, because the windows are relatively small areas and will create two smaller turbulent areas rather than one very very large one.

-soD

[MaxxMPG]

Originally Posted by some_other_dave

In general:
Top up, windows up: Lowest aero drag
Top up, windows down: Second lowest aero drag
Top down, windows up: Second highest aero drag
Top down, windows down: Highest aero drag
Not having the top in place creates all sorts of turbulence over the rear half of the car. Not at all good for drag! It's even worse than just having the windows down, because the windows are relatively small areas and will create two smaller turbulent areas rather than one very very large one.
-soD

I have a convertible (older LeBaron) and my sister has a Solara convertible. Our non-technical measurements show that at 60mph, you lose about 2mpg while top down/windows down, and about 1mpg top down windows up (or above half way). Much of the top-down drag is a function of the overall shape of the car. My sister's Solara has a much steeper windshield angle and much more shaping to the upper surfaces of the beltline and trunk, so her car doesn't lose as much mpg when the roof folds. My LeBaron only moves when the top and windows are down, as this is the only reason we keep the ragtop since the wife and I have other cars for top-up days. Even in this configuration, both cars (Toyota and Chrysler) easily exceed the EPA numbers. But the trick is to keep your speed down. The top-down drag just means more resistance at higher speeds, so you can offset the difference with lower road speed.

In general, if you're getting buffeted while driving top down, and the wind noise is objectionable, you are experiencing the turbulence that's cutting fuel economy. On easy cruises, you can barely feel the breeze, and that is an indication that the turbulence is not severe and you're not cutting the mpg number by very much.

With the windows up, the air inside the car becomes a bubble that moves along with the car. Air rushing over the top of the windshield slides over this stagnant air and reattaches back near the trunk. How do I know this? Because my wife drives top-down through rain squalls. Keeping the speed at or above 45mph, the falling rain never enters the car. You get a little drip from the wipers pushing water on the windshield and some of it runs over the header and onto your knee. And the folded top gets just a few drops on it. Way out back, the trunk lit gets soaked. But even the upper edge of the back seat inside the car stays dry.

So enjoy the top-down driving, and use the lower speed as a way to give other drivers the chance to get a second glance at the cool car!

-Chris

[Ophbalance]

Mmmm.. the GTC. We had an 88 LeBaron with I4 2.2l turbo convertible. That thing was such a pig though, and far slower than you'd expect out of a turbo charged car. They stuck the same engine in the Laser/Eclipse/Talon... way more fun .

[MaxxMPG]

Originally Posted by Ophbalance

Mmmm.. the GTC. We had an 88 LeBaron with I4 2.2l turbo convertible. That thing was such a pig though, and far slower than you'd expect out of a turbo charged car. They stuck the same engine in the Laser/Eclipse/Talon... way more fun .

The 2.2 turbo is a strong engine, but the 88 LeBaron's automatic transmission was the old 3 speed from the original Aries/Reliant K cars. Geared for economy. The turbo engines need some revs for the turbo to build up some boost, so the car was anemic off the line. But let it kick down while above 15mph and it really hauls. I also have a '90 Daytona with the 2.5 turbo and a 5 speed and that thing really hauls. The GTC conv I have is equipped with the 3 liter Mitsu V6 and 4 speed automatic. It'll lay rubber if you're too heavy on the stiff accelerator. Most Chryslers have that touchy throttle tip in, and you need to learn just how much pressure to apply to the pedal for a smooth and economical takeoff. The V6 is willing to rev, but makes its power low in the rev band, so it never needs to get beyond 2500rpm before upshifting.

And keeping within the topic of convertible drag, the old LeBaron convertibles are a fun and economical way to go 'topless'. Drive them right and their K-car econobox underpinnings let them achieve econobox fuel economy. Set the tires at 44psi, drive slow and easy with the familar DWL and DWB techniques, and you'll see MPG numbers that most people see only with the little Civics and Corollas.

But drive them wrong and you'll never see more than mid 20s for mpg. My wife and I gave my father in law the '93 LeBaron convertible we had. It has a 2.5 4 cyl and 3 speed automatic (EPA 21/27), that my wife routinely got 29-31mpg with. My father in law, though, is a brake tapper. Accelerate to 40mph, brake to 30, accelerate to 40 and so on. He doesn't admit to doing it, but you get seasick going anywhere with him. We suspect its his eyesight, since he had cataract surgery some years ago. But he's getting about 25mpg with it and can't believe my wife would top 30 on most tanks. It's all in how you drive. We gave him the spare convertible when we found out he wasn't going anywhere - even to doctor appointments - because the gas cost was too high. He has a 4x4 F150 FSP that gets 15mpg, so a round trip down the mountain and into town is good for over $20 in the tank. At least with the little ragtop, he's spending 6 dollars for every 10 he'd spend with the truck...

-Chris