Roof Bar Drag Calculations

Tesla

Tesla

Tesla

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Since fitting the roof bars to the Mondeo wind noise has increased a lot, I therefore assume drag has also increased a lot.

Earlier in the week I did a 350mile drive and calculated that I achieved only 33MPG, It should have been nearer 40Mpg than 30MPG from experience. It was mainly the M5 and A30 (main road into Cornwall?). I was driving like a Grandad at 70Mph dead for pretty much the entire journey.

Is it possible to calculate if the roof rails are causing much extra drag and therefore fuel consumption? Is this even a likely cause? The car is running as sweet as a nut.

Cheers,

Chris
 
Even I can manage 33mpg in the Mondeo so I reckon the roof bars might well be adding considerably to the drag and reducing your economy.
 
Drag on a car is MOSTLY caused by the creation of vortices behind the car as it pushes through the air. The car pushes the air out of the way, then the air rushes back in behind creating a swirling bundle of air which sticks the the back of the car. The forced swirling, mixing of the air keeps the pressure low, which pulls back on the car slowing it down. Adding more points where vortices form, like wing mirrors (or your roof bars) will normally make drag worse because it causes more vortices.

However, the amount of energy absorbed by a vortex isn't proportional to its size. If you double the size of a vortex, it causes four times more drag. So, if you deliberately break up a vortex into smaller ones, even if it means you have created more points of drag, can decrease the overall drag force. This is what the spoiler does on most cars, it decreases overall drag by deliberately seeding vortices and breaking up the big one that swirls around your back window.

If positioned right, your roof bars could do the same thing and improve your mpg.... as a general rule though, roof bars should give you a touch more drag. Probably less than driving along the motorway with the window open though. I wouldn't expect that much of a drop from just roof bars though.

Was it a windy day? A headwind of 20mph will be the same as ragging it at 90mph. Also, a side wind will screw the aerodynamics of most cars and have a similar effect. Windy days are bad for mpg.
 
Guigsy said:
Was it a windy day? A headwind of 20mph will be the same as ragging it at 90mph. Also, a side wind will screw the aerodynamics of most cars and have a similar effect. Windy days are bad for mpg.
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I have positioned the roof bars as per the Mondeo owners book supplied with the car, they are the proper Ford bars too.

It was a windy day on Tuesday. I didn't think the effect of wind would be so significant though.
 
Tesla said:
I have positioned the roof bars as per the Mondeo owners book supplied with the car, they are the proper Ford bars too.

I didn't think the effect of wind would be so significant though.
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Will be more than the bars.

A 10mph head wind for example at 70mph will require the same sort of fuel burn rate as 80mph but because you only going 70 ground speed MPG will suffer even more as the journey takes longer.
 
My friend has a fiesta van with a roof rack and got annoyed with the noise so therefore fixed a board sloped to deflect the wind. May also help in reducing drag too.
 
Its something like 10% Ive read in the past. If you do a lot of high speed driving, put up your tyre pressures and again if you carry a lot of weight in the car. Doing that alone could counter any losses
 
Usel said:
My friend has a fiesta van with a roof rack and got annoyed with the noise so therefore fixed a board sloped to deflect the wind. May also help in reducing drag too.
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Unless he's got a board on the back too, there'll be more drag by a large margin. Whistling noise means the vortices are quite small and drag is lowish. When they get big, the noise is much lower pitch which is probably why the board is less irritating. When you open your car window a little, it's high pitch whine, by the time it's all the way down, you can hear the thump-thump noise as each vortex breaks off and the air rushes in to fill the space.

People think that it's the front of stuff that needs to be aerodynamic, but it's the trailing edge that's most important. Look at any jumbo or modern glider, big bluntish nose, but the tail tapers out forever. If you look at the Toyota Prius, Honda Insight, etc that are supposed to be ultra low drag, they all have fairly normal looking fronts, but the roof slopes down to make the back as low as possible. Back is most important!

I think it's fighter jets that have built up the misbelief about drag, but they don't care about the trailing edge so much because the air is a mess anyway after exiting the jet at the back of the body. Come to think of it, fighter jets don't care about efficiency anyway.
 
Drag squares with speed so while driving around town at below 40 ish is fine, I will definately be contributing MPG reduction you are seeing on the motorway.
 
The bar wind noise is a hum as opposed to a whistle.

It can only be heard at about 75mph indicated (70mph real) and above.
 
since work done (energy)=force x distance. You can work out how much fuel and therefore energy you normally use minus how much energy you used with roof bars (not forgetting an engine is only about 10% efficient). You can then divide the ammount more energy used by the distance to give the average force exerted on the bars throughout the journey. You could work out a drag coefficient but you would have to do tests at different speeds and plot a graph to work this out:p
 
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