If possible try to reduce the vertical size of the bikes if they are higher than the back of the car by removing the wheels and strapping them to the frame.
Quick question about fuel efficency (for the tin top) .
Going on holiday soon and what with the new addition to the family space is tight, and I will not be able to put the mountain bike in the car as I
normally do.
So the options are 1)bike on a rear mounted rack or 2)a roof box and bike still in the car.
What will affect the fuel ecomomy more?
i think the roof box...
Bikes on the back will be in the cars wake (almost stationary air) and will have no effect on the fuel economy unless they are in the air stream (if
it’s a saloon car for example). The roof box will be produce far more drag behind it and probably extend the cars own wake upwards, unless it was
shaped like a drop tank
If possible try to reduce the vertical size of the bikes if they are higher than the back of the car by removing the wheels and strapping them to the
frame.
good question to which I don't know the answer but I would say the bike on the outside.
my guess is that although the roof box is bigger, they are usually streamlined whereas a bike across the back will disrupt air flow more.
you can get rear racks that the bike sits in line with the car as opposed to across - I have one you could borrow.
Ooooh quick replys.
I asked the question because I'd heard that a bike rack will cause a lot of drag, and my logic was that a rook box is much more aerodynamicaly
shaped. A roof box is also a lot easier to manage because once the bike rack is straped to the rear I can't get into the boot.
FYI my car is a BMW 5 series touring.
If you have the touring, my bet would be the rear rack is better. Remove wheels and bars, zip tie them to the frame.
Edit: ...because I buy the clean air/dirty air explanation above.
quote:
Originally posted by antimony
Ooooh quick replys.
I asked the question because I'd heard that a bike rack will cause a lot of drag, and my logic was that a rook box is much more aerodynamicaly shaped. A roof box is also a lot easier to manage because once the bike rack is straped to the rear I can't get into the boot.
FYI my car is a BMW 5 series touring.
It’s all about shape and size. If your bike doesn’t stick out past the sides of the car or up above the roof , rear screen then it will have little
effect on drag. The more it sticks out, the worse it will be. The same goes for roof boxes.
Measure the roof box area and multiply it by 0.3, say.
Measure the area of the bike sticking out beyond the edges of the car. This will be more tricky as there will be spokes and wheel rims and things.
Which ever is bigger will be worse. I’d have thought the box would be worse, purely based on its big size.
A friend of mine used to tow a trailer to carry stuff. He remarked that his MPG improved a lot when towing, mainly because he drove more slowly.
If you take the wheels off, and hang them behind the bike (racks are usally good for 2-3bikes, so if you only have one, there will be room) so they
dont stick into the airstream i wouldnt think you would really notice it at all.
Dont forget you wont be able to open the boot with the bike on the tail.
Daniel
quote:
Originally posted by Mr Whippy
Bikes on the back will be in the cars wake (almost stationary air) and will have no effect on the fuel economy unless they are in the air stream (if it’s a saloon car for example). The roof box will be produce far more drag behind it and probably extend the cars own wake upwards, unless it was shaped like a drop tank
If possible try to reduce the vertical size of the bikes if they are higher than the back of the car by removing the wheels and strapping them to the frame.
quote:
Originally posted by Rob Palin
Roof boxes are supposed to be streamlined but are often bigger at the back than at the front, which is wrong.
Again, remove wheels and take the bar out of the stem and duct tape or zip tie to the top tube. If you have more than one bike, loosen stem clamp
bolts so the fork legs and the stem can lie parallel. You should be able to get the size down considerably.
quote:
Originally posted by dhutch
If you take the wheels off, and hang them behind the bike (racks are usally good for 2-3bikes, so if you only have one, there will be room) so they dont stick into the airstream i wouldnt think you would really notice it at all.
Dont forget you wont be able to open the boot with the bike on the tail.
Daniel
If your going a long way , it might be cheaper to leave the bike at home , and buy one when you get there with the money you have saved on fuel 
quote:
Originally posted by smart51
Not neccesarily. So long as there is a smooth curve from the front of the box to the rear and nice sharp edges at the rear, the rear being the biggest bit is not neccesarily a handicap. A mondeo estate is more aerodynamic than the saloon because the rear screen angle on the saloon is too steep.