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I've been looking at the Thule Atlantis 900 roofbox.

http://www.thule-car-roof-boxes.co.uk/thule-atlantis-top-boxes/thule-atlantis-900.htm

It stores up to 650 litres.

Does anyone here know if this will fit on an Audi A4 saloon or similar? Its not an estate so I'm worried that it will be too long!

I've got a 470 litre box at the moment and it fits easily. I'm not going down the trailer route unfortunately!

Thanks for the insight.

Would you say this is too much overhang?

Here is option B, same box, just moved back slightly:

Definitley too much overhang. It will try to lift.

All the air going over the bonnet has nowhere to go when it hits the roofbox.

Remember also a car like that will not be designed to take that kind of loading on the roof so the handling may be awful.

Unstable and MPG through the floor.

Also not sure but I dont think the police would like it much.

In the end it comes down to safety both of you and other road users.

Hows about this one? It hangs over the front to a similar degree.

I'm not too worried about weight luckily, as the car can hold 75kg on the roof, I'm planning on putting less than 40kg in the box (just the bulky light tenting items like sleeping bags and foldup chairs etc). I'm not too concerned about swerving, as I never go over 60-65MPH when riding with a roof box. I really don't race around on the motorway.

My big concern is safety. When you mention safety, do you mean that the box will rip off and go flying? What if the overhang in the above picture is as much as the overhang with the smaller box below? The main overhang will be in the rear, rather than at the front. Would that help?

Here's an interesting read. Because roof boxes are so aerodynammic nowadays, they very marginally affect MPG. Air conditioning and having 3 lads in the back is the real MPG killer!

229776/the_mpg_mythbusters.html

I guess my final question is - if the large roof box overhangs as much as the smaller roofbox ( at the front) would you think I'm safe?

The roof boxes can be shifted forwards and back by about 25cm, so I'll have to slide it all the way back. Is my logic wrong? Shout at me if you think I'm making a big mistake!

Personally I wouldn't use either of those top boxes on that car.

Draw a line along the plane of the windscreen up over the roof of the car, the roofbox should be behind that line. It should not overhang the roof at all.

Remember as you are driving along you are pushing a big block of air out of the way. That air needs to go somewhere. Cars are that shape so the air can be pushed out of the way up over the roof of the car easily. With that roofbox overhang it can't go anywhere, its going to go up over the windscreen and hit the bottom of the box. That is going to slow the car meaning it will have to work harder to do the same speed. That will hurt your mpg. A properly fitted roofbox wont do that. Also all that upwards force could reduce grip at the front of the car which may hurt handling.

You say you arent bothered about swerving but what if the car in front has a badly fitted roof box which falls into the road in front of you? You will have to swerve to avoid it and are you confident the car will handle predictably and effectivly?

Ultimatley it is your decision to drive a vehicle you feel is safe, all anyone else can do is advise you.

Have you tried contacting thule? See what they say.

Thanks for the tips jamen, I however didn't mean that I'm not bothered about swerving or loss of handling. I wouldn't think about mounting a large roof box if my car lost a noticable amount of handling. What I meant was that I'm not worried about my car losing any handling due to a roofbox. A lighter car might suffer, but I'm pretty sure that heavy A4 will be fine.

Would it make sense for me to say that there is an opposite force pushing the roof box down, counteracting a large proportion of the upwards force? Perhaps that is why the above link shows a negligable amount of MPG loss due to a fully laden roofbox.

I phoned up a thule recommended dealer in Crawley that sells roofboxes. Strangely enough, he happened to have owned one of these boxes and also used to own an A4 saloon and A4 avant/estate. His reconning was "The estate swallows up the roof box, the saloon has a bit of overhang, but it doesn't look riiculous. Both drove without a problem, although it feels like there is another car on top of you!" They have one that they rent out so I'm going down there, mount it, test it out and if it works out, I'll throw my cash down! If anyone has had experiences with saloons and big roof boxes, please shout, I'd be most appreciative.

Here is the shortest roofbox in the Atlantis range, it till sticks out quite a bit!

I've just phoned up roofbox.co.uk and they have said that the Thule 900 mounts on an audi saloon quite happily. It is a giant of a box, but there are no safety concerns.

Luckily for me, I have a physics buddy who works in a university :-

His quick analysis was that the aerodynamic qualities of a roof box make it a very safe method of transporting extra luggage. The supposed uplift does not exist in any great quantity, because there is a 10cm gap underneath the roofbox. Instead, the downward force produced by the sloping front of this roofbox (I guess it applies to all current roofboxes in the market) creates added downward force which more than compensates for any minimal uplift experienced. In short, your sticking a big spoiler on the top of your car.

I guess that seals the uplift myth. The only way serious uplift could occur would be under extremely unusual circumstances, such as accelerating quickly downhill against a very strong headwind. In which case you should slow down.

Or if the boot of the car is so heavily laden that the front of the car is significantly higher than the rear, thus disrupting the downforce that the roofbox is creating.

Or if we feel like having a bit of fun...mounting the foorbox upside down.