Tech briefing: Wheels and Rims

Of the countless modifications one can make to any 4×4, changing the wheels is often the first port of call. This may be purely for looks, or to accommodate a bigger tyre − but there are some things to consider before you splash out…

The factory fitment of 19-inch (and above) wheels on many of today’s crop of premium SUVs and 4x4s is not going to fit well with a lifetime of hard 4×4 use. These massive rims fill the wheel arches, and are shod with low-profile tyres which are great for prancing about the city and make for stable cornering at speed (and a crashy ride) − but they don’t handle bumpy gravel, and a rocky trail represents a very real puncture threat.

You need compliance (from a tyre with a higher profile), and you might well want more grip (from a wider tyre). But if you want to change your tyre, you will most probably have to change the wheels supplied by the manufacturer.

There is a staggering choice of designs, sizes, widths and price points, from those that will set you back little more than R1000 a corner, to pricier works of art that might require a visit to your finance officer and some skelm accounting. And, of course, there are also some technical issues to consider.

Just as tyres affect handling, fuel consumption, braking, traction and steering, your choice of wheel has an effect on the same set of parameters, with various benefits and drawbacks.

In general, a smaller wheel shod with a higher-profile tyre will allow more flex off-road, be safer to deflate, and less prone to punctures; but it won’t offer the on-road cornering traction of a low-profile tyre. This is due to the sidewall flexing, and thereby lifting the inside of the tyre’s contact patch. How small a wheel you can select is subject to some obvious limitations.

Diameter

Of the various parameters, including offset, spacing, centre spigot size and bolt patterns, you’ll want to choose your diameter first. As a rule of thumb for off-road use, we don’t like to use a wheel diameter which is more than half the size of the tyre diameter. So, that would mean a 17-inch wheel for 37-inch tyres, a 15-inch wheel on 30-inch tyres, and so on. This rule of thumb must be adapted to a vehicle’s most frequent driving

Then you need to make sure that the new set of stylish 17- inch rims will fit over the brake callipers, which on today’s SUVs and 4×4 bakkies have grown to match stopping ability to power. This area is well-researched and much talked about on internet forums, so you will know that (for example) the Discovery 4’s standard 19-inch wheels and low-profile tyres are not an ideal combination: low-profile tyres are puncture-prone and so can’t be deflated for terrain such as sand, but fitting bigger profile tyres on wider rims has the potential to foul the wheel arches. It’s a problem being addressed by both wheel makers and tyre suppliers in an attempt to find a happy medium for the typical buyer’s usage profile.

Stud patterns

After choosing your wheel size, you’ll need to make sure that the wheel has the correct number of studs, as well as a stud pattern to match your vehicle. Most 4x4s have 5 or 6 studs, but the industry standard is to express a stud pattern as two numbers.

For example, a stud pattern of ‘6 x 125’ would mean that you have 6 studs and the distance between opposing studs is 125mm. However, the measurements between even bolt patterns (4 or 6) and odd bolt patterns (like a 5-bolt system) will be different. With a 6-stud pattern, you measure the pattern from the centre of two opposing bolts. With a 5-stud pattern, you measure the distance from the centre of one nut to the back of the furthest opposing one.

Steel vs Alloy

This is an age-old debate that is slowly being outdated by manufacturers who now release almost all modern 4x4s with alloy wheels, barring the commercial base models. But why?

Apart from the way alloys look, and the endless design possibilities they offer, they have various benefits that steel wheels don’t – including the ability to shed heat buildup faster. They can be just as strong as steel wheels, yet are around 30% lighter. The weight-saving reduces unsprung mass, which reduces wear on shock absorbers, and also improves ride quality. Less rotational mass also means improved acceleration and fuel consumption, making the standard fitment of alloys a no-brainer for OEMs.

Unfortunately, alloys do come with a few drawbacks, including a high price and the fact that these wheels are more difficult to repair. A bent steel wheel in the bush can be knocked straight with a sledge hammer, but an alloy will more likely just crack. Alloys are also generally unsuitable for use as jacking points. For these reasons, you are more likely to see military vehicles, competition off-road vehicles and overlanders – in which reliability trumps all the other attributes − fitted with steel wheels.

Types of wheel

Bead-locker rims are a subject on their own. These use a bolt-through collar to lock a tyre to the rim to ensure that it does not move or de-bead under low-pressure off-roading conditions. Most low-end steel wheels are single-piece, used primarily for radial tyres, although some vehicles (like the Land Cruiser 70 Series and the SFA Hilux before that) were supplied with cross-ply tyres with inner tubes, held in place with a split-ring collar.

Most alloys are produced using one of three methods, each directly affecting strength, and thus durability. You will want to select a wheel that is as strong as possible, and the choices are as follows:

• Forged – a solid billet of aluminium is forced between forging dies to create a finished product that is extremely dense, strong and light.

• Low pressure casting – molten alloy is poured into a mould, creating a wheel that is not as strong as a forged one.

• Counter pressure casting – a vacuum is used to suck molten alloy into a cast, helping to remove impurities and create a stronger wheel.

‘Offset’ or ‘inset’ are terms used to describe the point at which the wheel’s mounting flange sits within the width of the wheel. Inset and offset dramatically influence handling, and can potentially make steering very heavy or very light depending on which way you go.

• When the surface of the mounting flange is in the middle of the wheel, then the wheel has zero offset.

• If the mounting flange is on the inside of the centre of the wheel (closer to the vehicle), offset is negative. (This is also called inset).

• If the mounting flange is outside the centre of the wheel (closer to the kerb) then there is a positive offset. Most standard vehicles are equipped with wheels that have a slight positive offset.

When buying aftermarket wheels, people generally opt for a more extreme positive offset, as this widens the track of the vehicle, thus improving stability and rollover angles.

Note that increasing the inset of the wheel will decrease clearance with the inner edge of the suspension and various suspension components, like the tie rod and wishbones. Monitor this carefully, and seek advice.

Increasing the offset of the wheel will, in turn, push the wheel rim and tyre further outboard, which may cause the tyre to foul and potentially damage the wheel arches of the vehicle while it is turning or flexing over obstacles.