Bikes on car roofs - good idea?

[Mick F]

We saw a few cars on our way home from Gatwick with bikes on their roofs. Most of the cars like that were foreign - German mainly. It got me thinking about the fuel efficiency and "drag" that a pair of bikes produce.

So, a car drives from Berlin to Cornwall - 1,000miles(?) and back again with two bikes on the roof. How much extra petrol would be consumed with bikes versus no bikes? Say 33% more?

2,000miles at 40mpg equals 50gals @ £7.00 per gallon equals £350 without bikes on the roof.
2,000miles at 27mpg equals 74gals @ £7.00 per gallon equals £518 with bikes on the roof.

Therefore it costs £168 to carry a couple of bikes on the roof for 2,000miles.

Also, considering the Germans may want to tour England and perhaps Scotland too, their mileage could easily be double that - making the fuel costs double too.

Why do people do this? What's wrong with an aerodynamic box on the roof to lay the bikes down inside? It would pay for itself with just one trip.

[Vorpal]

No one can see what cool bikes you have if they're inside a box.

[Si]

What's wrong with an aerodynamic box on the roof to lay the bikes down inside? It would pay for itself with just one trip.

I would guess that a) no one makes one, or at least advertises it widely, that will take an average bike, and b) people are losing the skill of home-manufacture - if you can't buy it then it can't exist.

[Mick F]

Roof boxes exist now, and big ones too.

Wheels off, turn the 'bars, a bit of padding, and two bikes would fit in easily.

Maybe people just can't be bothered ...........

[meic]

We bought my wife's bike back from Berlin on the roof of our old Fiesta, along with a lot of other things and it did make a very noticeable difference to the economy of the drive but nowhere near 33%.
It was probably nearer 10 or 15% which was shocking enough.

Did you also notice that most of the cars were doing 85mph+ instead of 60mph, which is so commonplace that most people dont even think about the fact it will possibly be costing them £100 for a journey that length. With or without cycles.
Or driving brick shaped 4x4s which always use more fuel at speed.

We put all of our bikes in a trailer and the fuel economy was possibly a bit better than normal (lower speed limit with a trailer). Then had to pay the amount saved for a more expensive ferry ticket.

What all of these observations show is that fuel is still too cheap.

I was given a roof rack bike carrier and almost never use it. They are useful for small slow journeys but then it isnt worth all the hard work of getting them up there.

[Mick F]

We bought my wife's bike back from Berlin on the roof of our old Fiesta, along with a lot of other things and it did make a very noticeable difference to the economy of the drive but nowhere near 33%.
It was probably nearer 10 or 15% which was shocking enough.

The thing is, an "old Fiesta" wasn't the best aerodynamic car in the first place. Modern cars get much of their high-speed efficiency from having a smooth slippy shape. Our Clio is old enough, but it slips through the air very well - I can tell that by the lack of flies/bugs stuck to it in the summer!

This smooth/slippy surface effect is completely ruined by having windows open, let alone having a roof rack. We came back from Gatwick with the windows shut in 25degree sunshine, but we had the air conditioning on - FAR more efficient than opening a window. We returned 50mpg for the 230miles.

[SilverBadge]

Roof boxes exist now, and big ones too.

Wheels off, turn the 'bars, a bit of padding, and two bikes would fit in easily.

Maybe people just can't be bothered ...........

Boxes aren't that aerodynamic either.
Bike on its own at 25mph consumes somewhere in the region of 100W. At 75mph that's still less than a kilowatt per bike, so I'm guessing that's well under 20% more fuel for the pair. Bike box for two bikes is going to be what, 50cm tall and over a metre wide, so that's a big extra chunk of frontal area to add, presumably with a worse drag coefficient than the car on its own too, so hardly negligible. Never seen any figures for the relative aero merits of roof vs tailgate mounting.

[Mick F]

I can't argue any figures, it was just a hunch from what I've read in the past about people leaving empty roof racks on permanently and how much extra fuel they consume.

Roof boxes are designed to aerodynamic, so they must be better than a "bare" bike.

[essexman]

...and thats why i have a Corsa with a pop pout bike rack. Always on but not always on

[karloman]

I've never transported a bike on my car roof, just the idea scares me.
I've got a roof box and with it on my car uses 15% more fuel at motorway maximum speed. And it's noisy. But it's also very practical.
I also own a trailer for hauling heavy/bulky/dirty stuf. I tried it for carrying bikes on holidays and it's OK on motorway until you limit your speed and don't get a puncture. It happened to me once and changing a wheel with cars/lorries passing by you is a great experience. Never again (Statistics say your life expectancy on the hard shoulder is significantly shortened).
Now I use a Thulé rack that mounts on a trailer hitch. It's an expensive option but it's safe, doesn't increase much fuel consumption and can be stored easily when not in use (even possible inside my car).
My car is on the left :

[Malaconotus]

I had a Thule rack for the back of my VW Passat and transported the bike up and down the M1 a few times about three years ago. I calculated that it increased fuel consumption by 20%, and quickly realised it would be cheaper to own a bike up north and a bike down south than to transport one regularly between the two.

[eileithyia]

Big estate, chuck it all in the back, safe from; weather, tea leaves, and overhead height restrictions.
Failing that next car will be a van.....

[thirdcrank]

I've recently been researching roof boxes for a different reason (son has a new dog which occupies the luggage space in his car) and carrying a bike in one does not seem to be expected by the manufacturers. Although internal mountings are made for ski's, none seems to make anything to attach a bike. On the contrary, narrow boxes are advertised as leaving enough space on the roof bars to carry a bike next to the box. Perhaps there's a marketing opportunity here, but don't test it with too many concrete blocks

[Mick F]

.......Perhaps there's a marketing opportunity here, but don't test it with too many concrete blocks

Seems daft to me that any rooftop box can't be customised with fittings/padding.

You can buy all sorts of bike transport boxes, so it's not much of a design leap to fit it on a car.
http://ctcshop.org.uk/cycling/p/CTC/22/ ... 5300002583

Please note: This Bike Box is not a car roof box and is not designed to be fitted to the top of a car. It does not contain any fixings, brackets or roof bars allowing this.

[Neil Mc Ivor]

I have never liked or used a rear carrier for bikes on a car. Preferring the roofrack option, mine (Thule) held the bike upside down with the handlebars and saddle clamped to the roofbars.
I never noticed any appreciable increase in fuel consumption.

Other factors such as weight carried by the car and driving speed should also to be taken into consideration.
For example:
If someone is used to driving a car with one other passenger then fills it with as many persons are there are seats, their attendant luggage, one bike for each on a roofrack (or rear carrier) then drives at the maximum legal speed on a motorway is going to notice an obvious effect on fuel consumption not to mention handling.