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Steel or alloy: The wheel story


Showroom new or well and truly used, cars sit on either steel or alloy wheels.

So what’s the difference between the two types of wheels, and which one is best?

Rims pressed from steel used to be universal. They were cheap, easy to make, durable and could be dressed up with plastic wheel covers – or in the old days, chromed hubcaps.

Today, steels are often standard on entry-level cars and light commercials.

An alloy rim is a homogeneous hybrid cast from two or more elements, at least one of which is a metal.

The vast majority are aluminium alloy, whose characteristics of strength and low weight make them desirable in high performance applications. They also tend to dissipate brake heat better and offer superior brake ventilation – good for those quick runs along winding, hilly roads.

But over the years, alloy wheels’ popularity has spread far wider than the high performance market because they give a vehicle a “premium” look that lifts it above one with a set of steels and plastic covers. That’s why they’re so widely used today, when even higher-spec versions of “mum and dad” cars roll off the factory floor on alloys.
A few wheels are also made of magnesium alloy, which has a density only two-thirds that of aluminium. Its advantage is an even lighter weight; but it’s an expensive wheel and somewhat brittle.

Magnesium wheels are usually fitted to uber-exotics like the Pagani Zonda, or racing vehicles where light weight is an over-riding requirement.

Alloy wheels are sometimes referred to generically as “mags”, even though they have no magnesium in them.

Alloys are hot aftermarket items, for several reasons.

Perhaps the car owner didn’t like the look of the old rims; possibly he or she wanted the car to stand out in the crowd.

More likely, they’re going to fit lower-profile rubber and need “taller” rims. There’s relatively little choice in steel rims for people wanting to go “plus-one” or “plus-two”, except in the light commercial and 4WD segments.

If you’re going shopping for alloys, remember:
• Good quality alloys aren’t cheap. If you’re on a budget, try to find top brands that are being genuinely discounted rather than poor quality cheap brands where you might run into such problems as air leaks.
• Kerbs show no mercy to alloy rims. A moment of inattention while parking can cause unsightly and even severe damage. Some rim designs have spokes, or the edge of the rim itself, that stick out beyond the tyre, making them particularly vulnerable. Unless you and anyone else likely to drive the car are perfect parkers, bear this in mind.
• There are several important factors to consider when buying larger-sized “plus” rims; so unless you really know your stuff, always seek advice from a competent specialist.
• One of alloys’ virtues is lightness. This lessens unsprung weight, improving steering feel and braking response. At the upper end of the performance envelope, the strength of a quality alloy can lessen wheel/tyre deflection in cornering. The lower weight also helps maintain the status quo when fitting heavier tyres. However, the weight thing can be a bit of a red herring. Some alloys are only slightly lighter than a roughly equivalent steel rim and a few are even heavier.
• Alloys will show up dirt a lot more, one reason for the big range of specialist wheel cleaners at auto parts shops. Unfortunately, though, the key to a great finish is still hard work.

Alloys off-road
Most 4WDs sold in New Zealand are fitted with alloy rims, yet drivers who use their vehicles off-road in rough terrain often prefer steel wheels.

They like the resilience of a steel rim. If damaged by, say, a rogue rock, and a dent can usually be bashed back near enough to keep going. An alloy in similar circumstances might crack or break, offering no opportunity for the bush mechanic to salvage the situation.

Split steel rims used to be popular on some of the “tougher” 4WDs – and are still widely used in Australia’s Outback. These were effectively two “plates” that bolted together to form the wheel and were particularly suitable for bush repairs and tyre changing.

Auto Trader New Zealand