Car care: Beware risks when upgrading wheels

By Jack Biddle

Fitting a wheel that is not fit for purpose can instantly make a vehicle very unstable on the road.
Fitting a wheel that is not fit for purpose can instantly make a vehicle very unstable on the road.

Let's face it, a plain steel road wheel doesn't really do much to enhance a vehicle's looks especially when the wheel trims are missing or have suffered more than their share of kerb damage.

The steel rims are usually reserved for the cheaper base models these days with the more highly specced models either offering an alloy wheel of the same size, or a larger diameter option complete with a lower profile and wider tyre. As a combination, the larger wheel and vertically challenged tyre tends to fill a vehicle's wheel arch a lot better and can instantly transform a vehicle's overall desirability.

A word of warning however if you're thinking of enhancing your vehicle's looks with a set of new alloys. It's dangerous to assume that apart from size, all road wheels are basically the same.

The reality is road wheel design plays an important role in determining a vehicle's overall steering geometry settings, so fitting a wheel that is not fit for purpose can instantly make a vehicle very unstable on the road and potentially extremely unsafe.

When a wheel size is increased in diameter, the side wall or height of the tyre itself is reduced so the rolling radius pretty much remains the same as the smaller wheel fitted with the bulkier tyre. One of the benefits of a lower side wall profile, apart from looks, is less tyre flex which can help "tighten" a vehicle up especially for those into spirited driving.

For an extreme example, look at some of the exotic sports cars on the road and you will notice hardly any tyre height at all; some of them actually look as if they are running on rubber O-rings!

So when new car manufacturers offer a genuine or approved aftermarket road wheel upgrade for their specific models, they have taken into account, and allowed for, the unwanted changes that can occur such as speedo inaccuracy, unpredictable handling characteristics and impeded suspension travel.

But checking for a compatible increase in wheel size and stud pattern alone is only part of the exercise when considering a road wheel swap. It's where the rim actually positions itself on its mounting surface that is vitally important.

One of the key elements in road wheel design is called "offset". It's the distance from the wheel mounting surface to the centreline of the rim. Fitting a wheel with the wrong offset will mean it will position itself inwards or outwards further (depending on the offset) than intended and can instantly make a car unsafe. For example, there could be minimal clearance between the inner rim surface and brake calliper or top suspension arm plus, when suspension travel is at its extremes or when the vehicle is under hard braking, the tyre and front guard panel may touch and interfere with the vehicle's steering. In addition, the original manufacturer's steering and suspension setting can automatically change and alter the vehicle's road holding and steering capabilities and characteristics.

So while a vehicle may well look the part when parked up and fitted with new alloy wheels, there is the potential for an awful lot to go wrong once it hits the road.

There are a number of businesses who specialise in aftermarket wheels and suspension modifications. They all should have the necessary tools and information to be able to help you make the correct choice.

If your vehicle is a current model and still available new, then it would pay to do a price comparison with a genuine or approved aftermarket alloy.

In theory, the franchise dealer should only sell what the respective new vehicle distributor is willing to stand by in terms of total acceptability.

If you have already made a change and fitted the correct aftermarket alloy wheels (same size), then also check to make sure the wheels nuts are compatible if you have retained the original spare wheel.

To the mums and dads out there who have children that want to change the wheels on their cars a special plea: please be careful and work through all the issues.

- NZ Herald

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