Drag

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Wed 20 Sep 2006 15:51
Drag - tyro

(Subject of thread changed from "The Unbelievable HJ Column" to "Drag")

Some of you will have read the HJ column in the Telegraph tinyurl.com/hjyzx in which a reader tells of the improvement in fuel consumption achieved by putting mountain bikes inside the car rather than on a bike rack.

A physics teacher at the local school read this to a class, as a practical example of the importance of reducing the coefficient of drag.

One 16 year old boy absolutely refused to believe that this was true, on the grounds that a) no one would write to a newspaper on such a topic, and b) no motorist would ever think of doing such a comparison.
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Wed 20 Sep 2006 16:00
Drag - Statistical outlier
I have never checked specific figures, but if anything, my experience would be that the differences even larger.

However, I still put the bikes on the roof quite frequently, because it's much safer than having them inside, and it stops the inside of the car being coated in 3 gallons of mud.

Point to note tho - -put the bikes as far apart as possible on the roof - it prevents the vortices mixing, and reduces drag very dramatically.
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Wed 20 Sep 2006 16:06
Drag - Altea Ego
The vortices however act as a very good cleaning agent in getting the mud off.........
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TourVanMan TM < Ex RF >
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Wed 20 Sep 2006 16:11
Drag - cheddar
Post deleted - reason why?

www.honestjohn.co.uk/forum/post/index.htm?f=2&t=33...5

DD
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Wed 20 Sep 2006 16:11
Drag - Statistical outlier
True? I assumed most interaction would happen downstream of the bikes? No idea which is correct.

It's a real effect tho. Years ago, in my irresponsible youth, my Mum's Metro GTi topped out at 75 with two bikes next to each other, and more like 95 with them on opposite sides of the roof. Saying that, I'm ashamed to say I verified it would do 120 without bikes.
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Wed 20 Sep 2006 16:16
Drag - Lud
Perhaps persistent speeders could have a variable number of bikes clamped to their cars to slow them down. The state would benefit from the extra fuel tax paid.

We would enjoy seeing £700 chav Corsas and the like with several grand's worth of mountain bikes on their roofs.
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Wed 20 Sep 2006 16:19
Drag - Statistical outlier
As a teenage Mountain Bike racer, I managed to get about £7k of mountain bikes on the roof of my Mk I 1.1 VW Jetta shed. The car was optimistically worth £200, would do about 60 without any bikes, and a lot less once the bikes were up there. It required patience when on the way to meets!!

Saying that, you could four wheel slide it at about 10 mph into my parents road (just a normal 90 degree turn in). Great fun, although terrifyingly little grip now I come to think of it.
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Wed 20 Sep 2006 16:31
Drag - SjB {P}
With my Mountain bike mounted vertically atop my V70 2.4T, instantaneous consumption at 70MPH increased from the 30 MPG or thereabouts that I typically expect to get on level ground in still ambient air to around 26 MPG. I was amazed it was as much - I thought may be 1 or 1.5MPG - but since repeated several times I always get the same result.

I too now take the bike in the boot unless I have rear seat passengers.
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Wed 20 Sep 2006 16:41
Drag - SjB {P}
70MPH:

Actual (GPS confirmed)
75 MPH displays on the speedometer.
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