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Fuel Economy-Wind resistance vs drag resistance

Submitted: Thursday, Nov 02, 2006 at 10:43

Dieseljuice

Hi all,
I was just wanting some thoughts on which would cause the greatest amount of fuel usage. A full length roof rack with a roof bag, that when loaded sits about 35-40cm high or towing a trailer/camper trailer that i guess would weigh around 500-700kgs.
Just interested to know as i am considering both options. Full length rack and bag, 2xoztents and all my gear, or a camper trailer?

Plus there is the functionality issues and practicality....what is best??

Regards
DiesellJuice
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ThreadID: 39067 Replies: 6
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AnswerID: 202508   Submitted: Thursday, Nov 02, 2006 at 10:59

Member - Roachie (SA) replied:

hahahahaha......I have BOTH (aren't I a silly bugga!!!!).

This topic has been mentioned before and there is no right or wrong answer. It all depends on speed and terrain (plus other things too i guess).

If you're in the high country, then dragging a trailer is gunna hurt your economy the most. If you're going flat out (say 110k/h) along a flat highway, then the roof rack with all the gear is gunna be like trying to drive a block of flats. However, I reckon studies have been done that show the effect of a roof rack is negligible under about 60k/h.

Soooooo; if you were prepared to daudle along at 60k/h with a roof rack, you'd probably save some $$$$.

In the end, I believe it's a case of just grinning and bearing the cost at the bowser, to pay for whatever set-up you deem is necessary for your own family's camping and travelling needs. The overall difference is not going to amount to a huge sum of cash in all liklihood.

Cheers

Roachie

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There's only one thing more painful than learning from experience and that is NOT learning from experience!
Reply 1 of 6
AnswerID: 202509   Submitted: Thursday, Nov 02, 2006 at 11:00

Robin replied:

In terms of fuel economy , the roof rack will win over the trailer , as trailer provides rolling resistance as well as its own wind resistance.

In terms of practicality - this is more up to what you wish to do.

We try to avoid trailers because they limit the places you can go to (rough 4wd tracks)

Robin Miller
Reply 2 of 6
AnswerID: 202515   Submitted: Thursday, Nov 02, 2006 at 11:26

Kiwi Kia replied:

Rolling resistance is low and the trailer is sheltered by being lower then the tow vehicle so I would strongly sugest that the trailer is the best option. As has already been noted the faster you go the more resistance builds on the very un-aerodynamic load on the roof. If you often travel a route that has a good long hill you can test it by taking your foot off the acelerator and seeing what speed you coast at with the trailer, try again without and again with a loaded roofrack.
Reply 3 of 6
FollowupID: 462030   Submitted: Thursday, Nov 02, 2006 at 11:39

Robin posted:

Hi Kiwi

Doesn't work that way , trailer will create significant wind resistance simply because of a gap of more than 1/2 meter between it and the car.

It further creates turbulence down the length of the sides and wheel axle assembly , this disregarding the fact that it has many times the surface area of a roof load .

Both can be streamlined to some extent but by applying the same effort , its a lot easy to get the roof rack system right , because of its much smaller dimensions.

Robin Miller
FollowUp 1 of 7
FollowupID: 462060   Submitted: Thursday, Nov 02, 2006 at 13:10

Off-track posted:

Hmm interesting. My uneducated addition would be that the trailer provides more mass therefore more momentum when taking foot off accelerator. This extra mass will also work against you when the accelerator is applied because the engine has to work harder to pull the mass, not to mention the extra rolling resistance and the extra wind resistance.
FollowUp 2 of 7
FollowupID: 462087   Submitted: Thursday, Nov 02, 2006 at 14:08

Kiwi Kia posted:

Hi Robin, As a commercial pilot I am not unfamiliar with what causes drag. With a large caravan or trailer I totally agree with you but a small single axle camper hiding behind the tow vehicle will not cause very much drag at all. The effect of the burbling (buffeting if you like) airflow between the tow vehicle and the trailer will not be great if the trailer is way lower then the tow vehicle. Tie a few streamers on the roofrack at the rear of the tow vehicle and observe them at different speeds
FollowUp 3 of 7
FollowupID: 462097   Submitted: Thursday, Nov 02, 2006 at 14:37

Redback posted:

Well i have roof bars with an awning and pole carrier and tow a camper.

I've just recorded the low fuel economy ever without the camper on 10.6l/100k

With the camper it varies from 11.5 to 15.0/100k depending on how hilly it is, head winds, speed and dirt or tar roads.

To many variables to be excact.

Baz
FollowUp 4 of 7
FollowupID: 462098   Submitted: Thursday, Nov 02, 2006 at 14:40

Sign-man posted:

Kiwi
If the trailing edge of the tow vehicle were a Kamm or NACA shape- then the drag may be minimal. BUT the shape of the back of most 4WD is similar to a block of flats, and the turbulance is amplified. There's no need to tie streamers to the vehicle, just look at the back windows of 4WD (with or without trailer) after an outback trip.
FollowUp 5 of 7
FollowupID: 462124   Submitted: Thursday, Nov 02, 2006 at 16:11

Kiwi Kia posted:

Sign-man,

Dust is dust, you can not measure drag by the amount of dust on a back window!!! Yes it is a back eddy but that does not equate to high drag. Try the streamers - I have! A practical demo is worth more then an opinion anyday.

Try them on a wingtip (below 130 kts) they can be stuck on with simple selotape and demonstrate wingtip vortices very well.
FollowUp 6 of 7
FollowupID: 462138   Submitted: Thursday, Nov 02, 2006 at 16:45

Robin posted:

Hi everyone

Well we haven't really defined the trailer/roof rack sizes but given sensible
typical setups I believe a good first approximation to the wind resistance
of both is given by the frontal surface area, where trailer front sits more than a meter behind the vehicle. (This is important point because resistance of trailer
rapidly increases when its seperation approaches 1/2 the vehicle width).

Even a smaller camper trailer has some 3 times the frontal area of a roof rack
set up even with jerries stacked in it.
Hence a reasonable assumption is that a smaller unit will have 2 to 4 times the
total resistance of roof setup.

Robin Miller
FollowUp 7 of 7
AnswerID: 202555   Submitted: Thursday, Nov 02, 2006 at 13:54

ImEasy replied:

What Roachie said,

"I believe it's a case of just grinning and bearing the cost at the bowser, to pay for whatever set-up you deem is necessary for your own family's camping and travelling needs"!
Reply 4 of 6
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AnswerID: 202572   Submitted: Thursday, Nov 02, 2006 at 14:58

Member - Stephen (WA) replied:

DJ,

At highway speeds, I get the same fuel economy with a loaded full length roof rack as I do when I tow my Kimberley Kamper.

Cheers
Stephen J.

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Reply 5 of 6
FollowupID: 462158   Submitted: Thursday, Nov 02, 2006 at 17:23

Peter 2 posted:

I agree with Stephen, over 25 years with a troopy and many outback trips it doesn't matter whether I have a roof rack or tow the camper (lightweight under 500kgs Supamatic) they both affect fuel economy to the same degree.
In soft sand or mud the trailer will increase the fuel used where the roof rack won't under the same conditions.
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FollowUp 1 of 1
AnswerID: 202649   Submitted: Thursday, Nov 02, 2006 at 19:01

Willem replied:

I did my own little test a while ago

GQ Nissan 4.2 diesel non turbo with 3/4 roof rack, set back.

Without Roof Rack on non windy day 11l/100km

With roofrack on non windy day 12.5l/100km

With roof rack and trailer in tow on non windy day 15l/100km(average)

With roof rack and trailer in tow into the wind 20l+/100km

I have gone the trailer way to stop having to reach up on to the roof for stuff. In varialby one stick heavier things up there. Am hoping to get rid of the rack all together one day :-)

Cheers
Karoo Jackal
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Willem

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Reply 6 of 6
FollowupID: 462241   Submitted: Thursday, Nov 02, 2006 at 21:42

Dieseljuice posted:

Thanx guys for the replies, I really appreciate it. I see it will just come down to personal choice in the end and how I want to go camping. I am thinking that the rack, bag, and 2xoztent maybe the right option for me. I have been wathing the exploroz site for a year now since i bought my 4x4, I think I might cough up for the membership, its a great forum for us who love the road and the bush and our DIESELS!!!

All the best

DieselJuice
FollowUp 1 of 1

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