Some summer silly car questions
Left-of-centre queries about holiday motoring are answered here, writes Rob Maetzig.
Here’s hoping for a long, hot summer – and some quality driving time. Over the course of the year we run a series called Silly Car Questions, focusing on things that we’d all like to know... but might seem too strange to ask. A number of summer-themed ones have piqued people’s interest, so here are some things you really need to know about holiday motoring.
Windows down or air-con on?
With summer driving, there’s nothing like having the windows down so fresh air can waft around the vehicle’s interior.
And if you are in either of the front seats, open windows also provide the opportunity to rest your elbows on the doors – which for some reason is always a sort of sociable thing to do.
But then again, when the weather is really hot and sticky, it’s a very comfortable experience to have all the windows up and the car’s air conditioning on.
That way, while everyone on the outside is sweltering and sweaty, you can tool around all cool and comfortable. It might not be as sociable as having the windows down and your elbows out but it’s a lot cooler.
Experts say that turning on a car’s air conditioning adds about 5 per cent to a vehicle’s average fuel consumption. But experts also say that driving around with the windows down, particularly at highway speeds, causes air turbulence and drag that also adds to average fuel use.
A few years ago in America, General Motors and the Society of Automotive Engineers tested a sedan and an SUV at various speeds: it showed that running the aircon requires more fuel than driving with the windows open.
The Mythbusters TV series found the same thing. The team drove two SUVS with the same amount of fuel, one with the aircon on and the other with the windows down, around a raceway at 70kmh. The air-conditioned SUV ran out of petrol first.
Many say that if you really want to conserve fuel, the best thing to do is to open the windows while driving in urban areas, and to close the windows and use the air conditioning on the open road.
But even then, the resultant fuel efficiency all depends on the type of vehicle being driven – a lump of an SUV or ute, or a more streamlined sedan or hatch.
What causes that throbbing sound?
You’re cruising along the open road, when somebody slightly opens one of the car’s rear windows. Suddenly your ears are assaulted by a pronounced throbbing sound. The only way you can get rid of the effect is to open another window, or shut the offending window.
This buffeting is known as the Helmholtz Resonance, so named because it was first described in the 19th century by a German physician and physicist called Hermann Ludwig Ferdinand von Helmholtz.
Back then he didn’t have a motor vehicle that he could use to study the effect. So he used a bottle – discovering that that’s the hum that can be created when you blow over the open top. It’s all to do with air resonance in a cavity. Vortexes are created that compress and decompress the air, which causes the hum. Or a throbthrob-throb in the case of a car, which has an interior that is a very big cavity.
As a result of these studies he even created a thing called a Helmholtz Resonator, which he used to identify various frequencies or musical pitches present in music and other complex sounds.
Experts say the problem is particularly bad with modern vehicles because their smooth design and assembly quality is so good. In the old days, lessstreamlined vehicle shapes meant air boiled around their bodyshells, which means it was more difficult for the vortexes to be created. And even if they did, other windows and doors weren’t as air-tight as they are now, which made it difficult for differences in air pressure to occur.
What can you do about it?