Motoring Discussion > Roof-racks and mpg. Miscellaneous
Thread Author: Runfer D'Hills Replies: 19
View Threaded 

New Tue 16 Aug 11 21:49 
 Roof-racks and mpg. - Runfer D'Hills
I know there's no real science in this so don't be getting all cross about it but it's at least interesting to those who take note of such things.

Some here will know I do a fair old mileage. More or less a thousand miles a working week when I'm busy, sometimes more.

Anyway, my car doesn't have roof rails and most of the time it has a pair of roof bars on it with bike racks bolted to them. I tend to leave them on because we use them a lot and it's a bit of a faff taking them on and off. Couple that with a full sized glass panoramic roof which fills me with horror when lifting the bike rig on and off and the line of least resistance seems to be to leave it well alone.

Last week though I decided to take it off to give the racks a proper clean up and grease etc. Never got around to putting it back on at the weekend as for once we didn't go mountain biking.

The odd thing though is that just for the curiousity value I re-set the fuel consumption trip thing on the dash display. I expected to see at least a slight mpg improvement. My experience with running with three bikes on top of the car for long runs is that can have up to a 20% negative effect on fuel consumption particularly if much of he journey involves motorways as when heading across France etc.

This though is just a comparison of fuel used with empty bike racks in place versus no roof-rack at all.

My car is a 2.0 diesel 4x4 auto Qashqai and normally returns ( with the racks on ) 36.4 - 36.8 mpg in mixed but mainly long distance with a bit of London driving. Over the past week and a half and some 1200 miles with the racks off it has recorded, yes ladies and gentlemen....36.5 mpg.

Odd eh? Now I know these things aren't 100% accurate but it ought to be consistently innaccurate I'd have thought wouldn't you? Maybe the car is just so brick-shaped it simply doesn't make any difference?

Last edited by: Humph D'Bout on Tue 16 Aug 11 at 21:50
Reply to this message | Report message
       
New Tue 16 Aug 11 21:57 
 Roof-racks and mpg. - Zero
I think the last line sums it up Humph, it was much the same with the Touran. Another brick shaped mobile.

Question is tho, is it quieter without the masts & rigging?

Reply to this message | Report message
       
New Tue 16 Aug 11 21:59 
 Roof-racks and mpg. - Runfer D'Hills
Above 80 you can hear them a bit as windrush ( I should think....) Not that....

:-)
Reply to this message | Report message
       
New Tue 16 Aug 11 22:09 
 Roof-racks and mpg. - Zero
how wonder how much better for economy a rear clip on bike rack is....
Reply to this message | Report message
       
New Tue 16 Aug 11 22:11 
 Roof-racks and mpg. - Runfer D'Hills
Wouldn't know, don't hold with them. Scratch your car and bikes you know....

:-)
Reply to this message | Report message
       
New Tue 16 Aug 11 22:15 
 Roof-racks and mpg. - Zero
really? I was thinking of buying one, but I shall give the idea a miss then.
Reply to this message | Report message
       
New Wed 17 Aug 11 10:28 
 Roof-racks and mpg. - TeeCee
>> Wouldn't know, don't hold with them. Scratch your car and bikes you know....
>>
>> :-)
>>

They do have one major advantage. A mate was following a vehicle that had bikes on roof bars onto a ferry recently. One small moment of forgetfulness cost the driver two bikes and a severely chewed-up roof....
Reply to this message | Report message
       
New Tue 16 Aug 11 22:01 
 Roof-racks and mpg. - Dave_
A few years ago I had a regular run from Bedford to Bradford, a 330 mile round trip starting after the evening rush hour had cleared. I'd heard that using my car's aircon was bad for fuel economy, so I used the journey as a test to see if it was true. The car was a 406 HDi90 estate, which was pretty poor on fuel all the time - about the same figures as Humph's QQ IIRC. The economy (on the computer) was identical to the tenth of an mpg whether aircon was in use for the whole journey or not all.
Reply to this message | Report message
       
New Tue 16 Aug 11 22:08 
 Roof-racks and mpg. - Runfer D'Hills
The other thought which occurs to me is that despite many motorway miles each week my car usually tells me that the average speed achieved is just about 44 mph. A reflection of traffic density no doubt. Based on that albeit punctuated with short blasts at er slightly higher speeds it probably means on average the drag factor is negligible.
Last edited by: Humph D'Bout on Tue 16 Aug 11 at 22:09
Reply to this message | Report message
       
New Wed 17 Aug 11 06:32 
 Roof-racks and mpg. - madf
A Kashcow is just a mobile brick on wheels so roof racks probably give some turbulence which improves the airflow.

(seriously)
Last edited by: madf on Wed 17 Aug 11 at 06:33
Reply to this message | Report message
       
New Wed 17 Aug 11 07:51 
 Roof-racks and mpg. - Iffy
...probably give some turbulence which improves the airflow...

I've not done exhaustive tests, but running the CC3 with the roof down doesn't seem to effect fuel consumption one way or the other.
Reply to this message | Report message
       
New Wed 17 Aug 11 08:01 
 Roof-racks and mpg. - Zero
you put a roof rack on the CC3?
Reply to this message | Report message
       
New Wed 17 Aug 11 08:18 
 Roof-racks and mpg. - Cliff Pope
Guess what - it makes absolutely no difference with a Volvo 240!

I leave the roof rack on all the time because I am always carrying stuff and picking up finds from skips. It's annoying to have to pass up a nice sofa or a stack of fence panels or a telegraph pole just because one has left the roof rack at home.

I once found several bales of hay that had, er, fallen off the back of a lorry. Experience told me never to carry hay or straw inside the car, as the bits are almost impossible ever to get out of the upholstery.
Reply to this message | Report message
       
New Wed 17 Aug 11 08:21 
 Roof-racks and mpg. - Iffy
...a telegraph pole...

Good, sturdy cars them old Volvos. :)

Reply to this message | Report message
       
New Wed 17 Aug 11 08:44 
 Roof-racks and mpg. - sherlock47
or a telegraph pole

I have this visual image of the pole extending 10' in front of the car with a sharpened point (or better still an antique finial) just waiting to accelerate at an errant pedestrian......................

tinyurl.com/4437lg6
Reply to this message | Report message
       
New Wed 17 Aug 11 08:50 
 Roof-racks and mpg. - Zero
>> or a telegraph pole
>>
>> I have this visual image of the pole extending 10' in front of the car
>> with a sharpened point (or better still an antique finial) just waiting to accelerate at
>> an errant pedestrian......................

Instant picture of Dick Dastardly and the mean machine. Do you have a snickering canine sidekick?



>> tinyurl.com/4437lg6

What a magnificent telegraph pole!
Reply to this message | Report message
       
New Wed 17 Aug 11 09:03 
 Roof-racks and mpg. - madf
>> I once found several bales of hay that had, er, fallen off the back of
>> a lorry. Experience told me never to carry hay or straw inside the car, as
>> the bits are almost impossible ever to get out of the upholstery.
>>

Hay with embedded thistles is worst... I discovered some once in a moment of passion....
Reply to this message | Report message
       
New Wed 17 Aug 11 09:46 
 Roof-racks and mpg. - Fenlander
My C5 Tourer has rails as standard and usually runs with nothing on.... currently around 54mpg.

The Halfords (Thule) roof bars with 4 bike carriers on will drop it to 50mpg and make a fair bit of noise compared to its normal silence.

Put 3/4 bikes on and it drags the consumption further down to 44mpg.

So I guess a slippery shape is a big part of its excellent fuel consumption and that's easily ruined.

We're very shortly off on hols to Scottish west coast taking the inflatable boat which was planned to travel deflated on the roof. I hate the noise and extra fuel so much (plus the possibility of the locals nicking it while overnighting at a Glasgow Premier Inn!) I've put huge effort getting it inside the car. I do have the advantage of no spare wheel so managed to pack a huge amount into that space which just made the difference.

But then Mrs F says if the roof's free we can take the bikes as well... can't ever win.
Reply to this message | Report message
       
New Wed 17 Aug 11 12:29 
 Roof-racks and mpg. - WillDeBeest
In contrast to Cliff's Volvo experience, my S60 suffers badly from having bikes on top - 48mpg drops to more like 40. On the other hand, our wide but fairly streamlined Thule roofbox had much less effect - 45mpg in France when it was still our holiday car.

The Verso struggles to better 43mpg nude, but still gave us 41 last time it had both the Thule on top and four bikes on the towbar, which fits with Zero's 'already broken' observation on the Touran's aerodynamics.
Reply to this message | Report message
       
New Wed 17 Aug 11 13:48 
 Roof-racks and mpg. - -
People who travel with bikes on top of the car, you're a peculiar lot here i thought bikes were for riding, or is it the equivalent of the must-carry bottle of water for correctly attired female runners.

Anyway for the few normal ones, i wonder if the only slightly worse fuel consumptions given the turbulance created could be subconcious more careful driving.

When we've got the dog with us neither of us cause excessive G forces if we can help it, i wonder if you might be doing the same with the bikes aboard...for different reasons...mind you..;)
Reply to this message | Report message
       
Latest Forum Posts