Much of the fanfare surrounding eco cars is devoted to a raft of sexy technologies, like ultra lean-burn engines that operate at fantastic rates of efficiency, sophisticated electric-drive systems, high-capacity battery packs, and space-age materials. There is, however, one car component that, though rather uninteresting compared to these advanced approaches, plays a key role in producing today’s amazingly fuel-efficient hybrids and long-distance electric cars.
The lowly tyre has gone through a development process that is in its own way nearly as impressive as all the advanced gismos, power-regeneration systems and fast-charge technologies used to give us the ultimate Earth-loving city cars and environmentally-friendly performance vehicles. Tyres have gone from flimsy rubber-and-canvas contraptions to incredible devices that produce minimum drag while maintaining safety and ride quality.
There was a time when we might be happy just to make it 20 miles down the road without having to jump out of our horseless buggy to replace a blown inner tube. Today, however, some tyres will happily last for 30,000 miles or more.
Virtually all of today’s environmentally-friendly cars, be they all-electric city cars, (i.e the Reva G-Whizz), hybrid saloons, or super-efficient diesel hatchbacks that can manage 70mpg, depend on tyres specially developed to minimise the amount of energy lost to drag and rolling resistance.
Rolling resistance essentially refers to the ability of the tyre’s ‘stickiness’ to impede smooth rolling motion. To get braking distances down and increase cornering speeds requires the rubber used in the tyre treads to be as sticky as possible.
Unfortunately, this same ‘stiction’ that we want when we need to stop quickly or when we want to go around a corner without falling off the road, works against our cars ability to travel farther, on less energy, and with less noise. This mechanical grip is great when you want it and a real frustration when you don’t.
As a result, automakers have worked with tyre companies over recent decades to develop special super-efficient tyres with minimal rolling resistance. This is not so easy to achieve, of course.
You could simply carve tyres out of wood or steel and you’d have virtually no rolling resistance. You would, however, simply skate off the road at the first corner.
What’s more, if you needed to hit the brakes, you would more than likely find yourself simply sailing straight ahead into whatever it was you were trying to avoid in the first place. And when it comes time to accelerate, of course, the driven wheels would simply scrabble for grip.
The one good thing might be that as the car would just sit there with the tyres spinning furiously, you might be able to avoid going off the road or crashing into anything. But that would be a rather useless car.
So, tyre companies have quite the puzzle on their hands when it comes to engineering these green tyres. There’s another element, too.
Every time the tyre sidewall or tread flexes, energy that could have been used to propel the car forwards, is lost bending the rubber around. A stiff tyre is more efficient than a soft tyre – up to a point (as we kind of demonstrated with our previous steel-tyre scenario).
A stiff tyre also contributes to better control, as turning the wheel turns the tyre and the stiff tyre responds more faithfully to the commands provided by the driver at the helm. Hit some bumpy road, however, and you’ll soon discover the next problem.
Though a stiff tyre is more efficient and generally provides a more crisp driving experience, it can transmit a lot more information from the road surface, leading to a bumpier ride. Fortunately, tyre companies (and the automotive engineers who have to tune the car suspensions to suit these tyres) have plenty of experience under their belts these days and the overall compromise seems to be working very well.
A list of all Eco Cars can be found here.
A list of all Electric Cars can be found here.