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They don’t just make your classic Ford look the part, the right set of alloy wheels will actually give numerous performance advantages, too.

The right wheels can make a car. Likewise, fit the wrong ones and you will undo all the previous hard work that’s gone into making the car look and perform better. But with the right rims in place you will not only improve the overall appearance of your classic Ford, but you also make significant improvements to the way it handles, accelerates, and brakes too. Wheel design has much more of an impact on performance than many people give it credit for; everything from the car’s unsprung weight, to the cooling efficiency of your brakes, to the geometry of your suspension set-up, is affected by the wheels you fit. Therefore, fitting the right ones is essential for peak performance. Let’s take a closer look at exactly what’s involved.

Width

Width is fairly self-explanatory, although the rim width is actually measured from inside lip to inside lip, not the overall outside measurement. Therefore it’s quite common to see people saying they have 8 inch wide wheels, when in fact they are only 7 inch wide. Most wheels will have a kerb lip of between 10-12 mm (on both the inside and outside edges), so the overall measurement from outer lip to outer lip may well be somewhere near 8 inches, but that doesn’t mean it’s an 8 inch rim. The important factor is where the tyre mounts, so the rim width is measure from inside lip to inside lip to take the thickness of kerb lip out of the equation.

The general consensus states that wider wheels are better, because it means you can fit wider tyres, and wider tyres mean more grip. However, there are exceptions. Firstly, it is possible to fit a tyre that’s too wide for the application. This may not be overly applicable to road cars, but it’s certainly true for race cars where getting the tyres up to temperature is an important factor. On the back of a Mk2 Escort, for example, there isn’t as much weight and spreading the load across a huge tyre means that it doesn’t generate enough heat to work properly. It also depends on the surface the car will be driven on. A silky smooth race track offers lots of contact area with the tyre so a wider tyre works well, but on loose gravel or snowy/icy conditions a narrower tyre and therefore narrower rim actually help dig into the surface to help gain grip. 

Diameter

Diameter is fairly self-explanatory and refers to the outside diameter of the rim. Fitting the biggest set of wheels you can find is necessarily the best idea in terms of performance. Bigger wheels require lower profile tyres to keep the rolling radius as close to the original as possible (in order to keep the gearing correct, and not to affect the suspension geometry too much). Also bigger wheels weigh more than smaller ones, so you will increase the unsprung weight, which is a bad thing in terms of performance.

PCD

PCD refers to the bolt pattern of the wheel, and ultimately which cars it will actually fit. PCD stands for ‘pitch circle diameter’ and is the diameter of the circumference of the centre line of the drilling holes. Imagine drawing a perfect circle that went the centre of all the boltholes, then measure the diameter of the circle you’ve just drawn – that’s the PCD. It’s a standard unit of measurement used to identify the bolt pattern of a specific wheel. It is always quoted as the number of boltholes followed by the diameter of the imaginary circle we just mentioned. Ford’s 4×108 PCD indicates the wheel has four boltholes, and the diameter of the circle is 108 mm, whereas the 5×108 PCD has five bolt holes, again on a 108 mm diameter circle. 

 

Centre Bore

The centrebore of a wheel is the diameter of the hole in the back that fits onto the flange on the hub. Most modern wheels are hub-centric, which means that the centrebore will fit tightly onto the hub, transferring the load onto that component. With this set-up, vibration is massively reduced and the studs or nuts do nothing other than hold the wheels on. The other type of wheel is the lug-centric version where it’s the studs or bolts that take the load of the vehicle, because the centre bore doesn’t always fit snugly onto the hub. It’s not uncommon for mass-produced aftermarket wheels to feature a larger-than-required centre bore size, which is then shimmed back to the required size using plastic spigot rings.

Offset

The offset of a wheel not only dictates whether it will fit a particular application or not, it is also crucial to the handling properties of a car. Although it can sometimes confuse people, offset is actually quite straightforward. The easiest way to understand it is to draw an imaginary line directly down the centreline of the wheel. If the hub mounting face is directly on this line the wheel as an ET of 0. If the mounting face is nearer the outside face of the wheel the offset is positive, whereas if the mounting face is nearer the inside edge of the wheel the offset is negative. The offset is measured in mm, as the distance between the centreline and the mounting face. So an ET45 wheel has the mounting face offset from the centreline by 45 mm, towards the outside face of the wheel. An ET-15 wheel has the mounting face offset from the centreline by 15 mm, this time nearer to the inside edge of the wheel. 

Changing the offset of your wheels can have a huge affect on the way the car looks and handles. However, many wheel manufacturers will suggest you keep the offset as close to the OE wheels as possible, which will have been designed to work with the car’s suspension geometry and tested to dial out unwanted characteristics such as bump steer, understeer, or torque steer issues. By changing the offset and therefore altering the geometry, you can literally bolt some of these issues back on the car. 

Rim profile

The rim profile is the shape of the wheel between the inner and outer kerb edges. It is often viewed as a cross sectional drawing to make it easier to understand. The rim profile has to include a tyre well to allow the tyre to be fitted to the rim. This is basically a cut-out which gives the inside edge of the tyre somewhere to go when the outside edge is being fitted over the rim. There are standardised measurements for different rim profiles to ensure they are compatible with specific size tyres. The most common automotive rim profile is branded ‘J’ (hence the term 7J wheel, and so on).

Alloy composition: Cast

The vast majority of alloy wheels available will have been produced using a casting method. This process is relatively inexpensive compared to other processes, is easy to control and quality-check, and allows for quick production — all of which help keep the overall cost of the finished wheel down. There are two main casting methods used in alloy wheel production; gravity casting, and negative pressure casting. 

Gravity casting is the easiest to understand, it basically involves pouring the molten alloy into a mould and allowing it to cool. Once cooled, the mould is opened and the cast wheel removed.  Negative pressure casting works in a similar way, but rather then pour the molten alloy into the mould as with a gravity cast system, it is drawn up into the mould using a high pressure vacuum. 

This helps eliminate the trapped gas that can sometimes occur as a result on the gravity casting process, resulting in a stronger wheel that is much less porous.

Alloy composition: forged

Unlike casting, forging an alloy wheel doesn’t involve melting the alloy to a liquid state. Instead the forging process uses intense heat and enormous pressures to push the alloy ingot into the shape of a wheel. One of the main advantages of the forging process is the way it affects the alloy material’s grain structure. The starting ingot would have been cast, and as such the grain structure will be non-directional, effectively the grain structure will be as it landed when it was poured during the casting process. 

The huge pressures of the forging process means that the grain structure can be forced to run from the centre of the wheel outwards, along the length of the wheel’s spokes and not all in the same direction. This is part of the reason why a forged alloy wheel can be as much as 300 per cent stronger than a regular cast alloy wheel. 

alloy wheels

Alloy composition: split-rims

Most cast alloy wheels are a one-piece design, but split-rim alloy wheels are made up of two or three pieces. The most common is a three-piece split rim, comprising an inner rim, a centre, and an outer rim. 

These designs originate from motorsport back in the ’60s, when limitations to the casting process meant it was only possible to produce a wheel that was around 6 inch wide. To overcome the problem wheel manufactures used a cast alloy centre (modern designs from companies like Image Wheels use a centre CNC-machined from billet alloy) and then bolted on spun-aluminium inner and outer rims to form a three-piece wheel. And by using different size inner and outer rims it was easy to alter the width and offset of the wheel, just by bolting on a different spun-aluminium section.

It’s also possible to have a two-piece split rim, where the centre also includes the inner or outer lip as part of the same piece. A spun aluminium outer or inner (sometimes cast) is then bolted to the centre to form the completed wheel.

alloy wheels

Inserts

The centrebore of a wheel is the diameter of the hole in the back that fits onto the flange on the hub. Most modern wheels are hub-centric, which means that the centrebore will fit tightly onto the hub, transferring the load onto that component. With this set-up, vibration is massively reduced and the studs or nuts do nothing other than hold the wheels on. The other type of wheel is the lug-centric version where it’s the studs or bolts that take the load of the vehicle, because the centre bore doesn’t always fit snugly onto the hub. It’s not uncommon for mass-produced aftermarket wheels to feature a larger-than-required centre bore size, which is then shimmed back to the required size using plastic spigot rings.

alloy wheels

Centre-lock wheels

Unlike conventional wheels, centre lock rims don’t take their drive from the wheel studs. Instead they feature either a splined centre that mates to the wheel hub, or have drive pegs in the hub that slot into the back of the rim. A centre-locking nut then holds the wheel to the hub. This allows for a very precise and very quick wheel change, making them ideal for motorsport applications. However, centre lock wheels do require a completely revised, one-piece hub flange, or unique adapters to make them work.

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