“Wheels and Tyres (Part I)”

Motoring corner With CarloverWheels and tyres are one of the most important aspects of any motor vehicle. Apart from their appealing looks, they contribute greatly to the movement and safety of the vehicle and its occupants. For that reason, they must be looked on with great attention. If you want to change the wheels on your car, you need to take some things into consideration. Number of bolts or studs - It goes without saying that you can’t fit a 4-bolt wheel onto a 5-bolt wheel hub. Sounds obvious, but people have been known to fork out for an expensive set of alloy wheels only to find they’ve got the wrong number of mounting holes. Pitch Circle Diameter – Right; so you know how many holes there are. Now you need to know the PCD, or Pitch Circle Diameter. This is the diameter of the invisible circle formed by scribing a circle that passes through the centre point of each mounting; if you’ve got the right number of holes, but they’re the wrong spacing, again the wheel just won’t fit. PCD notation - Stud patterns and PCD values are typically listed in this notation: 5x114.42. This means a 5-bolt pattern on an imaginary circle of 114.42mm diameter. Centre spigot size - This is a tricky one. There are two types of axle/wheel design: hub-centric and lug-centric.For hub-centric designs, the wheel is centred using the spigot before the wheel nuts or lug bolts are tightened. In this design the spigot normally sticks out from the axle of the car which is why it can be used to centre the wheel beforehand. With aftermarket alloy wheels, the spigot hole is often larger than the spigot on the car which is why you need a spigot-locating or hub-centric ring. This is a plastic or metal doughnut that matches the outside diameter of the wheel’s spigot hole and the inside diameter of the axle spigot, and is used to ensure a snug fit between the two whilst centring the wheel. For lug-centric designs, the wheel is centred during the process of tightening the lug nuts, often because the axle spigot does not stick out (either at all, or far enough) to be any use in centring the wheel. Inset or outset - This is very important. Ignore this and you can end up with all manner of nasty problems. This is the distance in mm between the centre line of the wheel rim, and the line through the fixing face. You can have inset, outset or neither. This determines how the suspension and self-centring steering behave. The most obvious problem that will occur if you get it wrong is that the steering will either become so heavy that you can’t turn the car, or so light that you need to spend all your time keeping the bugger in a straight line. More mundane problems through ignoring this measurement can range from wheels that foul parts of the bodywork or suspension, to high-speed judder in the steering because the suspension setup can’t handle that particular type of wheel. Inset and outset are subsets of offset and the relationship is this: positive offset = inset. Negative offset = outset. Typically you can get away with 5mm-7mm difference from the vehicle manufacturer specification before you’ll run into trouble with the wheels fouling the suspension or bodywork. So for example if your stock wheels have an offset of 42mm and you can only find replacements with a 40mm offset, that 2mm difference ought to OK.More inset = closer to the suspension? It may sound counterintuitive, but when you increase the inset of a wheel, you decrease the clearance between the inner edge of the wheel and the suspension components. Conversely, decreasing the inset moves the wheel and tyre closer to the outside of the vehicle where it might scrub and rub against the bodywork and wheel arches. It might help to think of this more in terms of overall offset rather than inset and outset. The most positive the offset, the more the wheel is tucked into the car. The more negative the offset, the more the wheel sticks out. (to be continued)

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“Wheels and Tyres (Part I)”