I need to go on a trip carrying 2 slim roof boxes on top of my car. Unfortunately both boxes open on the right hand side so I am thinking of fitting one backwards so I can open the boxes on either side of the car.
Anybody knw of any reason why I shouldn't do this? Both boxes are Thules if it makes any difference.
Thinking back to my old physics days, surely a blunt front and a pointy back is actually more aerodynamic anyway.....
Thoughts most welcome.
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>> Thinking back to my old physics days, surely a blunt front and a pointy back
>> is actually more aerodynamic anyway.....
Nah, a pointy front and a pointy back is best.
a pointy front and a blunt back is not as good
and a blunt front and pointy back is worse.
What will happen? more noise rise in fuel consumption
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It is really down to how often you are going to load / unload and the hassle factor, against wind noise / extra fuel consumption / journey length. If you are shifting skis it should not be difficult to load them with the boxes the "right way round", a small step ladder / box / milk crate to stand on may help if you have the room to carry it.
Last edited by: Old Navy on Thu 22 Dec 11 at 19:34
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If you and your front seat passenger both wear your baseball caps the 'correct' way round they will counter the adverse drag factor effects of the wrong roofbox.
It must work, since all the gofaster toerags wear their baseball caps backwards in their attempts to squeeze the ultimate performance from the gutless Corsas.
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>>surely a blunt front and a pointy back is actually more aerodynamic anyway
I think that you probably would get lower drag with the roof box the "wrong" way round.
Styling, and most people's incorrect perceptions of aerodynamic shapes probably explain why the roof boxes are always fitted the inefficient way round to begin with.
Last edited by: Number_Cruncher on Thu 22 Dec 11 at 19:58
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>> >>surely a blunt front and a pointy back is actually more aerodynamic anyway
>>
>> I think that you probably would get lower drag with the roof box the "wrong"
>> way round.
I would like to challenge that. I think that's not right.
If you are right 100 years of fuselage design just went out the window, Lets not get a flat fronted roofbox mixed up with the perfect teardrop shape.
Last edited by: Zero on Thu 22 Dec 11 at 20:07
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Nuclear submarines travel blunt end first, and many ships have a bulbous bow.
Last edited by: Old Navy on Thu 22 Dec 11 at 20:18
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Indeed its the perfect tear drop shape, NOT a flat front.
(well not perfect tear drop shape but as close as they can get it in a sub)
Last edited by: Zero on Thu 22 Dec 11 at 20:29
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The description was "blunt", not "flat"
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Depends on your definition of "blunt"
Anyway, I still haven't seen a roof box with a "rounded bulbous" back end that you could turn round. They all look fairly flat to me.
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>> Depends on your definition of "blunt"
>>
>> Anyway, I still haven't seen a roof box with a "rounded bulbous" back end that
>> you could turn round. They all look fairly flat to me.
>>
The box isn't dead flat at the back. It is slightly curved.
www.amazon.co.uk/Thule-Atlantis-600-Sports-Litre/dp/B001P8245A
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Might need a bit of rudder to keep the ship on course ? :-)
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Given that cars need to be constrained in length, was not Kamm the developer of a compromise design in the 30's? This involved a truncated teardrop, with a flat back end extending down to the ground. Thus, bulbous end first is as good as one get get in this instance.
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The problem is, both with roofboxes and cars, the tear drop shape is a very inefficient package with respect to usable interior space. A prime requirement for both.
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If it is correct that a moving water droplet assumes its characteristic shape in order to minimise air resistance, then it seems obvious that roof boxes (and cars) ought to have broad rounded fronts and sharp tapered rears.
That is however at variance with the current design of cars, which is to have bulbous flattened rears. The lovely E-type, Morgans and traditional sports cars all had something approximating the tear drop shape, but these are now derided as being hopelessly un-aerodynamic.
Is the tear-drop shape perhaps only perfect in a falling droplet - does horizontal travel close to the ground introduce other complications?
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If you put one of the roof boxes on backwards, won't you look a bit of a wally?! And every time you stop some bright spark will tell you that it's on the wrong way...
ISTR the Swiss all put their skis on the car backwards... I didn't once because the tips of the skis would foul the opening hatchback, but all the locals kept cooing and pointing out my "mistake"... They don't like non-conformists!
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>>
>> ISTR the Swiss all put their skis on the car backwards...
As do all skiers, to stop the tips chattering in the airflow.
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It's not unusual for cars to have lower Cd values when run backwards.
The biggest complications are the proximity of the grond, and, the complicated airflow around the wheels.
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The last thing you want, with roof box aerodynamics is to create low pressure on the roof of the car.
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Raindrops are the shape they are because of the gravity pulling said drop to earth. Doesn't apply to a roofbox. Look at the shape of a whale, dolphin, seal. All are generally bigger at the front with a slimmer tail. So fitting it wrong way round is probably more aerodynamic.
But you will still look like a Wally.
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Most dolphins I have seen have got pointy heads.
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>> Most dolphins I have seen have got pointy heads.
>>
Followed by a big body and a gently tapering tail.
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>> But you will still look like a Wally.
>>
Wally I can live with. A ripped off roof box I can't.
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Assuming its a temporary requirement, then just get on and do it, drive with care in case the aerodynamics have compromised the handling, and accept the temporary hit in consumption.
Its not going to get ripped off.
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Buy an estate. They're just better, y'see? No need for all that malarky on the roof.
:-))
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>> Buy an estate. They're just better, y'see? No need for all that malarky on the
>> roof.
>>
>> :-))
>>
The two roof boxes ARE going on an estate!!!! A Legacy filled with the family, family dog plus presents!!!! I blame the wife for caking too much.
Not bothered about the fuel consumption as only travelling about 500 miles (and company fuel card).
Thanks all.
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>> The two roof boxes ARE going on an estate!!!!>>
How many months are you going for? Mrs ON and I have travelled around the world with a small suitcase each!
Last edited by: Old Navy on Thu 22 Dec 11 at 22:20
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Yeah but you are used to hot bunking....
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>> >> The two roof boxes ARE going on an estate!!!!>>
>>
>> How many months are you going for? Mrs ON and I have travelled around the
>> world with a small suitcase each!
>>
3 days!!!!!
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aye! - then you could just slide them inside!
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>> Buy an estate. They're just better, y'see?
>>
Apparently not. Should've bought a Transit
:-))
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>> Raindrops are the shape they are because of the gravity pulling said drop to earth.
>>
I don't see how that can be true. If you were in the raindrop you would be experiencing zero gravity.
If you fired a drop of water across an area of zero gravity, wouldn't it still assume a tear-drop shape, because it would reach terminal velocity and be shaped by air-resistance and surface tension?
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Regardless of the shape of the raindrop, why should we assume it's adopting the shape with the lowest drag? It's irrelevant.
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Option:
A. Stick both boxes on an auction website and buy a fullsize box that opens from either side.
B. Stick one box on an auction website and replace with same width that opens from both sides.
C. Place box 1 on the correct side and load. When loaded, close and slide across the car. Mount box 2, load and lock.
If customs want to see in box 1, ask them for a hand unloading a full box 2.
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Roof box shaped car...ze wind tunnel...he say yes !
tinyurl.com/chxfjva
Ted
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When you mention the "caking" issue, how can one put this, is it the, er, effect of the cakes which drives the space deficit or the need to transport said confections?
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I understood raindrops were spheres, not tear-drop shaped. Molten metal used to be dropped from 'shot towers' as the metal would solidify into a sphere before it hit the ground.
No-one seems to be considering the effect of the airflow over the front of the car before it hits the roof boxes. It's not going to be a smooth lamina flow!
We need a wind tunnel, there's probably a few that aren't in use for a couple of days soon.
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>> I understood raindrops were spheres, not tear-drop shaped. Molten metal used to be dropped from
>> 'shot towers' as the metal would solidify into a sphere before it hit the ground.
Yes - there's an interplay between surface tension and aerodynamic forces. At the scale of raindrops and falling shot, surface tension dominates. Bluntly put, you can't easily scale aerodynamic effects. Having said that, full scale wind tunnels are so expensive, they are not common - even leading F1 teams buy time on full scale facilities owned by others.
>> No-one seems to be considering the effect of the airflow over the front of the
>> car before it hits the roof boxes. It's not going to be a smooth lamina
>> flow!
At speed, the boundary layer will be turbulent, but, should be attached to the vehicle. The roof box, however, stands off the vehicle, and may be back in the laminar flow.
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>> At speed, the boundary layer will be turbulent, but, should be attached to the vehicle.
>> The roof box, however, stands off the vehicle, and may be back in the laminar
>> flow.
I think it will be tight. The car roof will have a boundary layer, the bottom of the roof box will have a boundary layer, depending on distance off roof and airflow speed of the two adjacent surfaces I bet they come close to interference.
Would love to see roofbox designs in the wind tunnel.
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>> I think it will be tight.
BT was asking about the flow upstream of the roof box.
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Ah! sorry misunderpeception.
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>>even leading F1 teams buy time on full scale facilities owned by others.
The last time I drove past this one it had a damn great Red Bull sign on it. I thought they owned it.
binged.it/vspDWt
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>> even leading F1 teams buy time on full scale facilities owned by others.
Williams GP have their own.
www.williamsf1.com/gallery/the-wind-tunnel
Team Lotus did a deal to use it as well
www.f1.co.uk/read-news/2011/3081/team-lotus-confirm-wind-tunnel-deal-with-williams-f1
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The Red Bull tunnel mentioned by BT is one of those tunnels which is used by other F1 teams.
As you can see from the scale of these facilities, the costs involved in setting them up, and operating them are far from trivial. The working section, where the object under test is placed is only a small part of the overall footprint of the facility.
One point of interest about the larger tunnels is that they are almost always formed of a closed circuit, to reduce the power required to operate the tunnel continuously.
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A lot of the teams put a huge leap of trust into computer modelled fluid dynamics. At one point it was thought the wind tunnel would be redundant. Some spectacular design duds has brought the wind tunnel back into play.
A 145 metre long, rectangular-circuit shaped wind tunnel is located at one end of the building. Team McLaren uses it for testing development aerodynamic parts, as well as testing aerodynamic set-ups. The tunnel contains 400 tonnes of steel and the air is propelled by a four metre wide fan that rotates at up to 600 rpm.
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>>A 145 metre long, rectangular-circuit shaped wind tunnel is located at one end of the building...
But, it isn't full sized.
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You can draw your own conclusions that I mentioned computer, dud and McLaren in the same post.
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The thing with CFD, and other computer tools used in modelling physics for engineering use is that it needs to be used very carefully.
Although computer software companies would love you to believe you can design a 3D object in CAD in the morning, and simply port the model across to a finite element or CFD solver in the afternoon, what you'll likely end up with is an inefficient model which doesn't really answer the questions a capable engineer should be asking of it.
I would say that 99 times out of 100, you get better results by stepping away from the computer, and asking questions about what you really want from the design, and then, perhaps building a simple sub-model of that region or part of the structure - or even better obtaining a closed form solution to an idealised situation on paper, then applying it in reality.
I've seen reams of nonsense data produced by CFD and FE programs, which then leaves less competent engineers struggling to correctly interpret - they effectively "drown" in data.
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>> Option:
>> A. Stick both boxes on an auction website and buy a fullsize box that opens
>> from either side.
>> B. Stick one box on an auction website and replace with same width that opens
>> from both sides.
>> C. Place box 1 on the correct side and load. When loaded, close and slide
>> across the car. Mount box 2, load and lock.
>> If customs want to see in box 1, ask them for a hand unloading a
>> full box 2.
>>
A and B - I'm travelling tomorrow
C - you can't slide the box across once it's loaded as the fastenings are on the inside.
The other option is D - have really long arms and a step ladder to load it on the 'wrong' side of the roof.
Happy Christmas all and if you're travelling, I hope you pack lighter than my wife!!!!
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>> The other option is D - have really long arms and a step ladder to
>> load it on the 'wrong' side of the roof.
>>
Or play basketball with the luggage from the other side of the car.
If the boxes were meant to go on 'blunt' end towards the front of the car wouldn't the car makers have shown that in their expensive glossy accessories brochure with shiny, happy people sporting very white teeth ?
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>> Happy Christmas all and if you're travelling, I hope you pack lighter than my wife!!!!
>>
Have a safe journey, don't give yourself an arrival time and you won't be late.
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Put the dog in a kennels and use the boot ?
I know this will anger the dog-lovers on the forum, I toned it down from suggesting that you strap the dog to the roof instead of one of the roof boxes ;-)
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Buy a trailer/ buy/take second car/find wife with neater packing habits/stop at home for Christmas
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>> I don't use a roofbox:
>>
>>
>>
>> tinyurl.com/c6nvx8o
>>
Hey, that was me and the family last year!!!!
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>> I don't use a roofbox:
>>
>>
Just try a search of Google images for overloaded car
You aint seen nuffink yet
or why not let the train take the strain
tritoncove.blogspot.com/2011_02_01_archive.html
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Aerodynamic boxes....Nah, not really needed.
www.youtube.com/watch?v=NsevqkForqo
Ted
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Fitting a roof box on 'backwards'. - Zero
