Bike carrier - aerodynamics

[Scooterza]

There was a post some time ago where the aerodynamics of rooftop vs rear bike carriers were debated at some length.  There were some who said that a rooftop, whilst it may have seemed to be less aerodynamic, may actually be MORE aerodynamic. 

 

I was of the opinion that a rooftop would introduce way more drag.  And now I have found an article to prove I was right.  Not being one to shy away from an opportunity to crow from the rooftops when I am right, the article appears in the Ride Magazine - November 09. 

 

The conclusion of a pretty scientific study was that whilst rear mounted bike racks do expose bikes to more dust and grime and the possibility of being "rear-ended", from an aerodynamics point of view, they are superior to rooftop mounted models. 

 

The bike carriers used where Thule's ProRides (Rooftop) vs Euroway (Towbar mounted). 

 

Results:

Rooftop = 26 percent less efficient than the car without any rack

Rear mount = 15 percent less efficient than the car without any rack. 

 

In the subaru Impreza 2.0R car used, this equates to just under 1l per 100km saved if you use a rear mounted rack vs the rooftop. 

 

[Minion]

Did they compare the racks on cars other than the Subaru? Did they see if it makes a difference whether it's a hatchback, sedan or SUV? Did they compare different racks? Did they compare different quantities of bikes?

 

I haven't read the article, but it seems that all they have shown is that a ProRide on a Subaru Impreza is less efficient that a Euroway on the same car.

 

[sias]

Edman - don't come and spoil a good argument with facts!

[Dirtbreath]

Drove to Sabie X with 2 bikes and a Thule roofbox mounted on a full Frontrunner roofrack.

 

Just got back from CT where I went to the trouble of loading a smaller roofrack and bike into the car with a "clean" roof.

 

Absolutely no difference in fuel consumption. Pretty high as I drive at 130-140k/hr in a 3dr Prado. Full rack has a wind deflector so not that noisy although the small racks have no such thing and whilst riding up to Jacobs Baai with 1 bike on the roof the whistling drove me mad.

 

There is a very inetersting thread on the landcruiser forum regarding aerodynamics and cars with accessories fitted. The conclusion from a buff in the industry is that when tampering with a car's normal shape in any way a rear mounted diffused is what is needed to redirect the wind flow back down once the car has passed. These look prono so are seldom used. Think of a TT helmet with the pointed end at the back.

 

[Bikalot]

i had asifnificant difference with 3 bikes mounted on my roof with the holdfast carrier,cant remember the firgure off hand but it was something like a 35% increase in fuel consumption, but bearing in mind this was not totally attributable to the rackes as my car was significantly fuller the 2nd time and i was carrying an extra person

[Minion]

 

cant remember the firgure off hand but it was something like a 35% increase in fuel consumption

Compared to the car without the rack?

 

The problem is that there're too many other variables to draw a definitive conclusion.

As an example, driving to the OFM Classic in my Polo, I averaged around 7.5l/100km last year and around 8 the year before, both times without a rack and my bike on the back seat.

In December 2008 I drove down to the coast and back, with my MTB on the roof rack, a passenger and extra luggage. The consumption was, again, around 8l/100km.

Last year I drove down to the Panorama Tour with my bike in the car and got 7.35l/100km. Driving back with two bikes on the roof rack and a passenger, I got 9.6l/100km.