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? Here’s two solutions: open the other rear window so air pressures are equalised, or tell the person in the back seat to shut the window and use the air conditioning instead.
Are sunroofs useful or a waste of time?
Aaah – sunroofs. They can be wonderful on a beautiful sunny day or on a starry summer night – you roll back the glass and experience all the smells, sounds and skyward views on offer as your car cruises through the city or the countryside. And they look great, too.
Oooh – sunroofs. They’re nothing but a pain in the neck – literally. When they are open you have to crane your head backwards to get any view, when you should in fact be concentrating on driving. And not only can they add to the purchase and maintenance cost of any car, but they can take up valuable headroom.
The head would tell you that they’re pretty darned useless. Why would you want to chop a hole in the roof of any car when you’re already surrounded by holes that are filled with glass, most of which you can wind down and up anyway?
Not only that, but you can only open a sunroof when it’s not raining or cold. It all plays havoc with your car’s aerodynamics and your fuel cost will rise. Let’s not even begin to think about the expense of repairing a sunroof if its electrics break down.
But the heart would tell you that sunroofs are cool in more ways than one. They’re cool because the most times they are used is on hot sunny days when there’s nothing better than fresh air and sunshine pouring in from that cavity above your head. They look cool too, giving any car a more sophisticated and expensive look.
And here’s some useless information. Some people say that mobile phone reception is superior in a vehicle with a sunroof, because a car with a completely enclosed roof gets in the way of the signal.
We’re not saying if that is right or wrong. We’re just saying...
But back to the silly question. Sunroofs are a bit useless in that they don’t add to the efficiency of any car.
They don’t control interior temperatures like air conditioning does, they don’t contribute to overall visibility like windows do, they impact on available headroom, they are potentially expensive to maintain, and when open they can play havoc with fuel consumption.
But then again items such as alloy wheels, tinted glass and leather upholstery are useless too.
Can you drive in jandals?
Aaah – summer holidays. The days when we give our minds and bodies a rest from the daily grind. We give our feet a rest too, getting rid of the shoes and either wearing jandals or go barefoot.
But hang on: is it legal to drive while wearing jandals or with no footwear at all?
The quick answer is that it is not illegal. You are allowed to drive a vehicle wearing just about anything – which is just as well, otherwise motorists would probably be banned from driving in footwear ranging from gumboots to high heels.
But driving in jandals is frowned on by the likes of the police and the Automobile Association, because it can be dangerous, particularly if the weather is wet, which increases the potential for your foot to slip off a vehicle’s pedals – especially the brake pedal.
The law says it is illegal to drive a vehicle in an unsafe manner, and the proverbial jury remains out as to whether wearing a pair of jandals or being barefoot could constitute ‘‘unsafe’’ in the event of an accident.
Road safety experts say if you are wearing jandals, it’s safer to slip them off and drive barefoot. But they add that it is much safer to not drive this way at all, and to wear a decent pair of shoes with treaded soles while operating your vehicle.
And what about motorcycles? The same thing applies. While you must wear a helmet while riding, it’s not illegal to not wear shoes. But the mind does boggle as to what could happen to a bare pair of feet in a crash.