It seems most people are using these the wrong way round, ie. with the pointy end forward. Apparently a strong vacuum is formed behind the flat back end if the wedge is pointing forward. However flipping the box so that the thin end is at the back, allows the slipstreamed air to taper off nicely, creating less drag overall.
The opposite of what you might expect at first glance.
Of course you may prefer to keep the thin end forward to avoid people pointing and laughing "look Daddy, they've got their top box on back-to-front."
Anyone else heard this? (or been brave enough to try it!).
Stu.
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You're quite right - basic aerodynamics, really (where a long teardrop is the ideal shape), although it would help if the blunt end were a bit rounder.
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As I thought two Js.
So is HJ going to start a campaign in the Telegraph to let the 99.99999999999% of users know?
To what degree is this effective, say on MPG?
Stu.
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A sound idea, but if the flat end is too flat, then the air will not flow around the box as you describe. Good aerodynamics relies upon not directing the airflow through sudden changes of direction. A more torpedo-like shape is the optimum for drag reduction, but probably not for cramming your camping stuff in!
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I wonder what the instructions say..Is it possible that everyone has not read them...
Must ask the son in law as he fitted one a few weeks ago.
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Errr, how do you load it if it's the other way around? Stand on the bonnet?
There is an aerodynamic idea called the kamm tail which is sharply cut off, when done correctly the air behind comes off well.
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"Errr, how do you load it if it's the other way around? Stand on the bonnet?"
Um, don't they open from the side?
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"Errr, how do you load it if it's the other way around? Stand on the bonnet?" Um, don't they open from the side?
Ah, you obviously didn't understand me because my foot was in my mouth. My mistake! My mitigating circumstances are that I've never had one. Whatever car you have, it's always going to be full so there's no point in getting extra luggage space.
We've had several trips abroad in everything from a 2 seater Suzuki Cappuccino to a biggish Passat estate. The only thing in common was they were all full. If our next car was a 7 ton Luton van it would be full. I'm sure I'm not alone in this?
Gareth
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My Thule roofbox is meant to be mounted taper-forward. Thule test their models in the wind-tunnel to improve aerodynamics, which probably explains why a number of their models have a pronounced whale-tail lip at the rear.
The box I have has recesses in the base, allowing it to be mounted within 1.5" of the roof. As the box broadly mirrors the shape of the roof of the car it would seem to be aerodynamically sound.
Possibly.
Dunno really....
:o)
No Dosh
mailto:Alan_moderator@honestjohn.co.uk
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Kamm tails are a useful workaround, but still inferior to a 'proper' aerofoil. This should be at least 6:1 in aspect and have its widest point about 1/3 of the way back. Not the ideal shape for most storage, so a square tail may still be the most practical solution.
BTW, I think the gap between the bottom of the box and the roof may be a significant factor. Having the box on the roof and therefore effectively an extension of the bodywork would improve matters.
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Hmmmmm, spoilers on top boxes.
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If this was the case then we are all driving our cars the wrong way round. So the manufacturers should have the blunt bit at the front like a Camper Van.!! I think not !!. Regards Peter
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IIRC Kamm said that if practical considerations prevent you having a "proper" teardrop aerofoil, you shouldn't mess around trying to steepen the curve to the pointy tail, you should just slice the end off at right-angles to the airflow. For those who remember the Alfasud and Citroen GS, they were good examples and pretty slippery as well.
Have a look at this www.lightauto.com/gray1.html
Hawkeye
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Stranger in a strange land
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IIRC, if you take a typical "teardrop" shaped streamlined object, and cut off the front half, you increase drag by 50% whereas if you cut off the rear hlaf, it only increases by 25%. So it would seem taper to the rear might be better.
Ignores the effect of the car's aerodynamics, though
V
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Quite right, HE. Interesting link, too - thanks.
"For those who remember the Alfasud and Citroen GS.."
I do indeed, having owned both. I had the estate GS which was regularly used to ferry daughter (and all her worldly goods) to and from University, and it went just as fast fully laden as when empty. The suspension helped, of course, but it was amazingly slippery for a relatively small car.
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