Theoretical question of course but what will cause less drag. A bike mounted upright on a car or a bike box mounted on its side on roof bars? I am guessing that the bike box has less drag as it might be blunt nosed but would cause less turbulence as the upright bike has less flat edges.

Answers on a postcard

ND

You would need to know the frontal area and co-efficient of drag to be able to answer the question;
http://en.wikipedia.org/wiki/Drag_equation

Why is this theoretical?
Go out and test it!

i would say almost certainly the bike box will cause less drag, the turbulance around a frame would shock people if they could see it

Yep, smoother surfaces should win that one.

Perhaps you could add an aero front end to the bike box and then rely on the Kamm tail to sort out the rest.

Interesting question. I’m going to guess the roof box will generate more drag. There is a lot more surface area on a roof box, also you get a lot more wetted area (i.e. surface area exposed to the airflow), so the coefficient of drag, I would think, would be worse, so combined with the higher frontal area, so I’m guessing overall drag would be worse.

The turbulence generated around the frame of a bike would be taken into account in its coefficient of drag. But the effects of the turbulence might not be as detrimental as you’d think. Look at F1 cars. A following car drafting a leading car in its turbulence generates significantly less drag that the leading car. So in a similar way the turbulence generated by the handlebars, front wheel and down tube washing out in front of the seat tube, seat stay and saddle might actually reduce the drag generated by the seat tube, seat stay and saddle, and so on with the rear seat stays and rear wheel.

Difficult to say if you don’t know the numbers.

Complicated question this. There’s drag from turbulence and as wobbliscott says there’s also drag from surface friction. To eliminate a tail vortex, the ideal car would taper to a point, except that would introduce enough surface friction to counteract the lack of vortex at the back. The ideal point to cut off the taper is where the vertical section area is half the maximum. This is why Priuses look the way they do.

I’ve also noticed something odd – we know that having a bike on the roof of the car reduces fuel economy a lot, but I’ve found that when towing a caravan it doesn’t seem to reduce it at all. In fact, it may actually improve fuel economy, based on a small number of trips so far.

To answer the question of why this is theoretical is because I was daydreaming about sticking the bike on the roof on our family road trip to Italy. I don’t have a box as yet so there are no real world tests to conduct. And as I don’t have a wind tunnel the test would be done under unscientific conditions.

Interesting debate though

ND

You’d think the much bigger frontal area and far larger overall surface area would slaughter the roof box. Yeah, the frame would be turbulent, but any surface area will ultimately cause thlae air flowing over it to go from smooth, laminar flow to turbulent, and the longer and broader the laminer flow, the bigger the turbulence when it breaks up. I think.

I often thought of adding some kind of fairings to the bike to help it’s aerodynamic properties.

I’ve found that when towing a caravan it doesn’t seem to reduce it at all.

Really? Are you driving at the same speeds? I certainly find that towing a boat trailer has a big impact on fuel economy, and I’d expect that to be more aero than a caravan.

No, I mean that when towing, having the bike on the roof doesn’t reduce economy further. The caravan itself roughly halves economy.

Probably lost in the noise then, though it’s also feasible that the caravan creates a high pressure area in front of it which helps fill in the hole behind the bike.

I know that I’d never put a good bike on a roof rack having seen so many on motorways oscillating wildly. I just assume most people who do this completely forget about them as soon as they turn the ignition key and have no interest in what the bike is going through up there.

My theory was that the bike could break up the air flow enough to reduce the high pressure in front of the van.

I have confidence in my roof bike rack. It’s pretty secure.

If you get a decent roof rack, like the Thule 591 and use it properly there is no way that bike is coming off no matter how you drive……

Actually, can you answer something I’ve always wondered, molgrips. If you’ve got a caravan, why put bikes on the roof?