antimony
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| posted on 21/1/09 at 10:17 AM |
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What affects fuel efficiency more?
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?
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A1
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| posted on 21/1/09 at 10:26 AM |
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i think the roof box...
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Mr Whippy
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| posted on 21/1/09 at 10:28 AM |
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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.
Fame is when your old car is plastered all over the internet
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02GF74
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| posted on 21/1/09 at 10:31 AM |
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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.
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antimony
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| posted on 21/1/09 at 10:41 AM |
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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.
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idl1975
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| posted on 21/1/09 at 10:44 AM |
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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.
[Edited on 21/1/09 by idl1975]
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smart51
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| posted on 21/1/09 at 11:11 AM |
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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.
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dhutch
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| posted on 21/1/09 at 12:14 PM |
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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
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Rob Palin
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| posted on 21/1/09 at 01:03 PM |
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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.
^^What he said^^
Roof boxes are supposed to be streamlined but are often bigger at the back than at the front, which is wrong. I know the manufacturers test them in
the wind tunnel but I also know they don't try them the other way around, despite it being (politely) suggested to them.
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smart51
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| posted on 21/1/09 at 01:47 PM |
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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.
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.
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idl1975
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| posted on 21/1/09 at 03:29 PM |
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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
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clairetoo
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| posted on 21/1/09 at 09:05 PM |
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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 
Its cuz I is blond , innit
Claire xx
Will weld for food......
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Rob Palin
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| posted on 21/1/09 at 10:52 PM |
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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.
Well there is more to it, true, but that reasoning is still valid.
The Mondeo example is a bit misleading (and I'm not sure it's correct) because the majority of Estates do have higher drag than their
related saloons, although their rear lifts tend to be lower. Only if a saloon has really excessive vortex drag does it normally get as bad or worse
than the estate.
Minimising base area whilst maintaining attached flow is a prime goal for drag reduction, whether on a car or anything else. It's fundamentally
why tear-drops are the shape they are.
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