Roof boxes - do they alter the towing dynamics

I can't comment on your particular car but for some years I towed with two Kia Sorento's on which I used a roof box on our longer trips to Europe and certainly didn't notice any effect on the driving of the cars. There was a slight advantage (other than providing a lot of extra stowage) in those countries with lower speed limits like Germany and Austria I actually got better economy. However that benefit was soon wiped out at the higher speeds allowed in France.

David 

The roof box will undoubtedly have some effect on fuel economy, but it could go either way depending on its shape, size and for/aft location relative to the roof of the car, so it is impossible to pass comment which would in any way tell you very much.

We find that towing with our Mondeo estate we get slightly more MPG with the top box on, but as soon as we go solo that changes and we get less!  Add to that the technical issues of physical size, and shape as Lutz mentions, means the only way to find out for yourselves is to try it and see.  However, you'll have to make sure you check on the same journey, with the same wind conditions.............. because these will also affect the results!!!

We now take a top box if we absolutely need to - rather than on every journey as we used to, because there is a problem storing the top box whilst at the caravan so removing it to drive without it whilst on site isn't really an option!

Thank you for the replies. It's not so much the mpg that concerns me , but the change in the tiguans  profile would affect the air flow over the caravan.  Would the fitting of a roof box affect the caravan ie would it help to induce snaking 

Wouldn't have thought so.  If the box is central as it will act as a 'deflector'.  Snaking is as a result of other factors - nose weight, speeding, poor loading, bad match car/caravan etc.   Wouldn't worry about it.

As others have said when we had a full complement we towed with a rigid roofbox and there was no apparent change in anything. However I didn't like the fact that when not towing it was empty and the car didn't look as good (vanity). I've now got a semi rigid one which I use when necessary. Its made of fabric but that means its easily fitted in literally minutes and you can take it off when you've arrived on site and its fold away to be stored in your car boot (which does seem strange). Google Thule Ranger 90 or see here for a video:

 https://www.thule.com/en-gb/gb/cargo-carrier/car-top-carrier/thule-ranger-90-_-601100

There is a cheaper version from a company called Handiholdall which my brother uses and its seems just as good, wished I'd seen it before I got the thule one 

Caravans, being big boxy things towed at moderate speeds, are relatively unaffected by any form of aerodynamic change, so don't expect any noticeable difference in respect of stability.

We didn't notice any change at all in the aerodynamics and the caravan towed no differently with or without the roofbox.  It certainly didn't 'induce snaking' with a roofbox on.  The only thing we noticed was that we got less squashed insects on the caravan front window with the roofbox fitted on the car!!!!

Our caravan has shock absorbers fitted anyway, so is very stable when towing.

It would raise your centre of gravity but increase the weight of your vehicle favouring the towing ratio.....position is essential to prevent whistling too!

It doesn't affect the weight ratio because the weight ratio is based on the kerbweight of the towcar, not its actual weight.