Roof-rack Aerodynamics

Does anyone know how much extra having an empty roof rack attached might cost in mpg due to drag? Is it significant?

strokeslover

http://auto.howstuffworks.com/fuel-efficiency/fuel-economy/roof-rack-affect-mgp1.htm

12.7% less mpg @ 55mph, not a hugely scientific experiment, but i'd guess its not too far off the mark!

paddyp

Its around 1-2 mpg max on our passat over thousands of miles.

It certinly wont be anywhere near 12.7% on something as unaerodynamic as the average camper van with a high torque engine.

It already has high coefficient of drag, high cross sectional area so the rack would have to be a fully loaded monster to have that kind of effect.

Thanks Paddy. It's probably possible to create a deflector outtov somemat...the thick plottens...
Also surely carrying a bicycle has mpg reduction capabilities that aren't factored in given that it is afterall a bicycle.

paddyp

Sir Liamalot wrote: »

Thanks Paddy. It's probably possible to create a deflector outtov somemat...the thick plottens...
Also surely carrying a bicycle has mpg reduction capabilities that aren't factored in given that it is afterall a bicycle.

My brother says about 6mpg - 11% with bikes on the roof of his galaxy so pretty significant, the back is definitely the place for them.

If you need access to back doors there are swingaway bike racks although I don'y know who sells them around here, I couldn't afford one annnyway :-p

Ah decision's made now I'm getting a single tier half size rack fitted tomorrow, looks ideal.
If I lose more than 2mpg I'll probably rig some sortov counterweight pulley system to lift it on/off.
Incidentally I noticed one today make of two bolted planks on risers :-D.

Not so into bicycles anyways I tend to break them within 30mins. I use a downhill skateboard for the short hops.

I'll let you know the results when they're in. Time to fill the diesel tank anyway new budget in two days!

niloc1951

Sir Liamalot wrote: »

Does anyone know how much extra having an empty roof rack attached might cost in mpg due to drag? Is it significant?

Considering the average motorhome is about as aerodynamic as a garden shed I don't think an extra little bit on the roof will make much of a difference

paddyp

Amazingly the new ducato is supposed to have a drag coefficient 0.31 which is the same as our car. You can bet thats the swb l/r version with skinny tyres and solid hub caps mirrors retracted etc. etc though.

Results are in after 250 miles of various conditions; negligible/nil mpg loss at up to 80kmph. The Burner chimney's still probably the greatest offender.

Mwahaha...ladder mountin' time!