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HOW WHEELS AND TYRES WORK
CarPoint looks beyond the jargon to understand why your car needs wheels and tyres.
DOES SIZE MATTER?
A wheel's size dictates what sort of tyre can be fitted, and usually the wider the rubber, the more grip and traction are afforded for acceleration, braking and cornering. Usually, the bigger and more powerful a car, the more rubber is needed to hold its weight and deliver what the engine produces to the road.
So wheel sizes are expressed in two basic dimensions and, for some reason, in good old imperial inches instead of metric. First there's the diameter, which might range from 14 inches on a Hyundai Getz to 20 inches on a BMW X5 4.8is or Jaguar XJ-R. Similarly, the width might range from five inches on the Getz, to something like 10.5 inches on the back of the X5.
Aftermarket wheels can go a lot bigger than this, but they are usually bought by drivers wanting to make a fashion statement, and don't mind paying exorbitant amounts for tyres that will fit.
The other measurement that's critical but perhaps less well known is wheel offset. This is basically the distance between the wheels fitment point to the hub (the centre bit where attaches to the car) and the outer face of the wheel itself. Fit a set of wider aftermarket wheels with the wrong offset, and you might find them rubbing on the car's guards because the track has also been widened. If you see a vehicle with the tyres sticking (illegally) outside of its bodywork, you can bet that is what's happened.
While we're talking wheel sizes, it's worth explaining tyre sizes as well, even if tyres themselves can be a whole different subject. The three dimensions you need to know with tyres are the diameter of the hole in the middle (which will match the outer diameter of the wheel) the width of the tread, and the aspect ratio, or height of the sidewall.
Let's start with width. It is measured in millimetres, which is a bit unusual as the wheel's width is in inches, although in this case we're talking about two different things. One width of wheel, for instance, can have tyres of slightly different width squeezed onto them, or vice-versa. A Holden Commodore, for instance, comes with the same size tyres, even if the 15-inch (diameter) wheels are six or seven inches wide. In this case they are 205mm wide tyres, although that big BMW comes with whopping 315mm-wide rears.
As mentioned, the tyre's inner height matches the wheel's outer diameter, so a 15, 16, 17 or upwards-inch tyre is pretty easy to comprehend. But it's critical when you come to talk about aspect ratio. This is the height of the sidewall, and can affect the tyre's stiffness and handling characteristics, as well as the radius of the whole wheel and tyre combo, and therefore the vehicle's gearing.
Aspect ratio is expressed as a percentage of the tyre's inner diameter to the tread width. So that Commodore with 205mm wide rubber and 15-inch wheels, has a tyre with a sidewall ratio of 65. The X5 with 20-inch rims and 315mm rubber has a much more narrow band of rubber with an aspect ratio of 35.
Throw the whole lot together, and you might have something like 205/65R15 94H appearing on a passenger car tyre denoting, in order: width, aspect ratio, construction ('R" stands for radial) and diameter. The numerals and letters following the size information denote load index and speed rating.
IN THE GROOVE
What else? Tyre construction and the pattern of grooves on the tread is a whole other subject and deserves its own explanation. But its worth noting a couple of other things. Inner tubes, which used to hold the air so important for the tyre's cushioning effect have fallen out of favour now that the beading that attaches the tyre to the wheel's rim is good enough to form an effective seal. Hence the term tubeless tyres.
And even though they might look perfectly round and perhaps even symmetrical, wheels and tyres spin at high speeds and so need to be balanced, because any hint of wobbliness is transformed into a shaking steering wheel and stability issues for the car and its suspension. That's why tyre-fitters carefully balance the wheel, and attach little weights inside the rim to compensate for any movement that's out of whack.
Finally, there's the car's fifth wheel, or spare. Full-sized spare wheel and tyres are falling out of favour with car manufacturers as they look to cut vehicle weight and find extra space for passengers and luggage. So a spare might be a narrow, spindly looking thing made out of mild steel that is definitely for temporary use.
Or there might not be one at all if the tyres are made of a special run-flat construction or there's a compressor and can of goopy sealant that (hopefully) will repair a puncture, but again only temporarily. If you cover kilometres well away from a tyre repair shop, it's worth carrying a full-size spare wheel and tyre.
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