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Aerodynamic impact of going topless?

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gregd

Active Member
Dec 31, 2014
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CM98
I was scanning the Model 3 thread More efficient to open a window or use the A/C ? about whether it's better to use the AC or open the windows, and started wondering what the impact is of driving a Roadster with the top off.

I understand that around town, there is no aerodynamic effect. But what about at freeway speeds? I presume there will be an impact, but how much? Would it matter if the windows were up or down, with the top off?
 
i expected so much more given the title of this thread
Why, were you hoping for something like this?

FatGuyInCar.jpg~c200
 
In a thread a few years back, I wrote the following. The speeds weren't very high and there were several variables so this is not definitive, but the difference is likely small as we couldn't see any difference at all:

A couple of years ago 6 Roadster owners took a ~55mph trip from Bellevue WA to Mt Rainier. Tomsax kept track of statistics (they are in a thread here somewhere). As far as we could tell, there was no difference in energy usage between the cars with a hard top, with a soft top, with a mesh top, and with no top. No real difference between 1.5 and 2.0 cars either. The cars with more and/or heavier passengers did use a small amount more energy. The only big difference was that one car had a bike on a rack - that car used about 15% more energy.
 
Not a direct comparison, but my Lotus variants over the years have been noticeably more thirsty when driving at 70 mph+ without the roof off. Given the windshield roll hoop and top of the window glass are common I'd expect the same relative differences.

Again in my experience, lower speeds, it makes little difference. Once you start getting uncomfortable buffeting in the cabin (and any thing paper gets sucked out the roof) that's what corresponds to lousy gas mileage. My thinking is there is a point where the aerodynamics breaks down.

I haven't done enough mileage using the Visium hardtop to do a comparison between the soft top / no top but I'd be surprised if the Visium wasn't the winner of the three options.

(The ironic thing is my current one probably has less range than the Tesla, so I do sometimes get range anxiety in an ICE, hence taking more of an interest in MPG than I should for such a car ;) )
 
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Thanks, all, both for the info and the smiles :)

I kind of guessed the effect might be small, given the urban legend (or was it Myth Busters?) that one can drive in the rain above about 40 mph without getting wet. And I am betting that taking the top off would win out over the A/C on a moderately hot day, given what the A/C does to my range.
 
I kind of guessed the effect might be small, given the urban legend (or was it Myth Busters?) that one can drive in the rain above about 40 mph without getting wet.

Not a myth. I've tried it in a Tesla Roadster (not intentionally). For the amount of rain that time, the speed threshold seemed to be about 50kph; Above that I stayed dry. You can also experiment, top off, by pushing your hand up near the rear roll bar. I get an inch or so before I hit full force air resistance (vs immediate at the front near the windshield).

So, I reckon there is not much resistance from that part of the car. At speed, the air is being pushed up and over by the windshield.
 
The Roadster gets so much range that I don't care about small differences. I'd rather be comfortable. That means cloth top on in cold or rainy weather, mesh top in warm or hot sunny weather. No top is fine in town, when the weather is nice, but on the freeway it's too much wind for comfort.
 
The Roadster gets so much range that I don't care about small differences. I'd rather be comfortable. That means cloth top on in cold or rainy weather, mesh top in warm or hot sunny weather. No top is fine in town, when the weather is nice, but on the freeway it's too much wind for comfort.
Sure, and most of the time, comfort and enjoyment is the first priority. But I've got some trips coming up within the next month, and if, for example, taking the top off cost me 20%, I probably wouldn't do it even if it would be more pleasant, because it would impact my ability to make the trip.
 
Topless?

It depends.

Are you planning to go the "full Maurice"?

Big hair, chest wig, medallions.

It makes quite a difference, aerodynamically speaking.

It gives the car a Hee Bee Bee Gee Cd.
 
Sure, and most of the time, comfort and enjoyment is the first priority. But I've got some trips coming up within the next month, and if, for example, taking the top off cost me 20%, I probably wouldn't do it even if it would be more pleasant, because it would impact my ability to make the trip.

I'd highly recommend the Visium top if you can get one. The thing I hate most about having the top on is the cabin feels dark/claustrophobic.

The Visium makes it feel _much_ airier, almost like going topless, but on freeway driving trips it's a godsend for wind/noise.

I haven't got a mesh top, but I'd love to know from those that have both the mesh and Visium how they compare.
 
I had never heard of the Visium top before reading the above and doing a Google search. The Visium appears to be a solid tinted glass hard top. So it would be suitable when you need a fully-enclosed top. The mesh top is for when you want the free flow of air, but not the full-on hurricane of topless on the freeway.

I love the sun when I'm hiking or kayaking, but I don't like full sun when sitting. I like the cloth top in chilly or rainy weather, and the mesh top in warm, sunny weather. In fact, I love the mesh top. I suppose the glass top could be good in cloudy weather when it might rain but you want a bit more light, but it also must be heavy. Both the cloth top and the mesh top are lightweight and easy to put on and take off. I can imagine any hard top being heavy and awkward.

The mesh top is also great when you are parked in the sun for a couple of hours. With any enclosed top the car would be an oven. With the mesh top, no problem.

Sure, and most of the time, comfort and enjoyment is the first priority. But I've got some trips coming up within the next month, and if, for example, taking the top off cost me 20%, I probably wouldn't do it even if it would be more pleasant, because it would impact my ability to make the trip.

I hear ya.