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Note: Before reading this article it should be noted that manufacturing differences and tolerances dictate that fitting advice regarding a certain car may differ slightly from that given.
Fitment information
The fitment of an alloy wheel is described as follows: 7x15 4/108 ET35
7x15 is the width and diameter of the rim, expressed in inches. As a guide most 14" rims are 6" wide, most 15" are 7" wide and most 16" and 17" rims are 7-7.5" wide. Certain manufacturers may make a wheel that is particularly narrow or wide for it's diameter.
4/108 is the Pitch Circle Diameter (PCD) This is the number of nuts in a circle on the mounting face of the wheel, and the diameter of that circle in mm. The Fiesta has a 108mm PCD.
ET35 is the offset in mm (35mm for ET35). This is the distance from the centre of the wheel to the bolt face. the offset for the Fiesta, as with most Fords is 35mm (ET35). It is important to have the correct offset of wheels to prevent the wheel or tyre catching the arch, or unwelcome handling characteristics such as tramlining. It is possible to decrease offset, but not increase it. Decreasing is done by fitting
spacers. For example a wheel with an offset of 38mm can be decreased to the correct 35mm by fitting 3mm spacers.
Sizes
The standard size wheels on the XR2i 8v are 13" with 175/60 tyres, all other performance models came with 14" and 185/55 tyres. The RST was the only model to some with alloys as standard. The XR2i and Si had steels as standard and alloys as an option, although most XR buyers had the alloys, and the RS 1800's alloys became an option rather than standard fitment towards the end of the production run. An inexpensive and easy upgrade to make is a set of the RS Turbo or XR2i / Si alloys. These can be picked up in the classified ads in the magazines from people who are upgrading. Alternatively specialist breakers often have them in stock.
The most popular size of wheel to upgrade to is 15". This size combined with 195/45 profile tyres will give the same rolling diameter as the original wheel and tyre combination. Maintaining the same rolling diameter will also prevent the tyres from catching on the arch or suspension, even when the car is lowered. Check when purchasing a wheel and tyre package as most retailers list the deals with cheaper 195/50 profile tyres. The 50 profiles will fit the car, but it may be necessary to fit spacers to clear the shocks or trim the arches. They may also rub on the front crossmember on full lock.
The next step up is 16" rims. These usually require a little work to fit without catching. The usual points of contact are the spring platform on the rear, and the plastic arch trim and metal lip on the front. The arches can be trimmed and the suspension worked around as explained in the fitting 17"s article. Most 16s are 7" wide although the Mondeo Si design is available in 6" and 6.5" widths and with a 195/45 16 tyre, may fit straight on. 16s are a good compromise between looks and handling, they are big enough to improve the looks of the car dramatically, but not so big as to cause unwanted handling side effects.
An interesting wheel in this size is the Escort Cosworth. These rims are 8" wide and as a result are extremely difficult to fit. The offset is also much higher than the Fiesta, so the wheels protrude from the arch quite far. It is possible to make them fit, although it involves a lot of arch work and the car cannot be lowered much due to the sheer width of the wheels. The narrowest tyres these rims will take is 215/40 16s.
17"s
17" rims are possible, but will require major work to the arches and also tweaks to the suspension. The modifications required are covered in detail in the Fitting 17"s article here. It is still possible to have the car sitting quite low on this size of rim, drops of 65mm or more are achievable. Speedo readings are not as affected as some people make out. The larger wheels will cause the speedo to under read by around 7%, but as standard it over reads by a similar percentage, so in effect it's actually giving a more accurate reading.
Rims of this size can affect the handling somewhat. Firstly the acceleration of the car will be slower than before. A rough estimate would be adding a second to the 0-60 time, however, the top speed will increase slightly. Torque steer will increase, as will road noise. The ride quality will deteriorate due to the lower tyre profile, and the car will tramline more (act like it's running on rails). To be honest, it's nothing like as bad as most people make out, and anyone concerned with the 'side effects' of fitting 17"s should drive a Fiesta with them on before making a decision one way or the other.
205/40 profile tyres should be used, and will fit both 7" and 7.5" wide rims. Some brands of tyre are narrower for the specified width than others. For example a size quoted at 205/40 should be 205mm wide, but this can sometimes be more or less by up to 5mm. A careful choice of tyre can help when fitting large wheels as a rounder sidewall profile or narrower width will help with arch clearance. It's worth comparing different tyres if clearance is an issue.
Locking Wheel Nuts
As alloys wheels and high performance tyres are expensive, you should always fit locking wheel nuts. These are specially designed nuts that have a unique shaped head and require an adaptor key to be removed, preventing anyone with just a wheel brace removing the wheels. The design of the nut is such that each set is different and that the key for one set will not fit another, even of the same brand. The nuts are supplied in a pack of four with one key and an identifier tag so that a replacement key may be obtained if the original is lost. Each of the four locking nuts replace one standard nut on each wheel. It's advisable to purchase the round type with a recessed head, rather than the other type shown below, as the round heads cannot be removed by hammering a socket over them and using that to unscrew the nut, a common trick.
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