Arch modifications are essential if you want to fit wheels bigger than 15", or are going very low. This guide will explain exactly what is involved and the methods explained hear should allow you to fit 7x17" wheels and lower by 65mm or drop 90mm with 15"s.
Offset
If you image looking at the wheel front on (as in if you are looking at the the front or rear of the car). Then imagine that you have X-ray eyes and can see through the wheel and see the face that mates up to the hub. Offset is the distance (in mm) that this face is from the the centre-line of the wheel. The standard Ford Offset is ET35. Obviously if the face is to the right of the centre-line (looking at the left hand wheel on the front of the car), the wheel will stick out further. If it is to the left of the centre-line then the wheel will stick in further. Obviously, the idea is to get a matching offset (+ or - a few mm) to the standard car so that the wheels do not stick out too far (as they will touch the arches on cornering or with heavy loads) or stick in too far (will foul the shock absorbers / springs).
Before You Start
The first point to take into account is that on the Fiestas it is not only the arches that cause problem when fitting large wheels, the suspension cups will rub on the tyres, certainly at the rear and, dependant on rim type and offset, maybe the front. The plastic arch liners on the front will rub the tyres when steering, more so the lower you go. Modders who are running 15" rims can skip this part, but those of you with 16"s or 17"s, read on:
Suspension
There are several ways to avoid the tyres rubbing the shocks. The easiest and most expensive is to fit a 2.25" coilover kit from someone like Spax or AVO. The smaller diameter of the springs means the tyre clears the springs, eliminating any contact. However, coilover kits are expensive and don't always provide the solution. If you run 7.5" wide or higher offset rims you may need spacers, metal plates that sit between the wheel hub and the rim to push the wheel further out of the arch. For example, one of my Fiestas runs 7.5x17" Smith's Infinity alloys with an offset of ET38. This correct ford offset is ET35 (mm), this means that the ET38 rims will sit 3mm further in to the centre of the car than the ET35 rims. This 3mm isn't enough to affect the handling of the car, but it does mean that the wheels are 3mm closer to the shocks, increasing the chance of contact. Another factor that doesn't help is that the rims are 7.5" wide, rather than the more common 7" width, you should, if possible, stick to a 7" width, although some tasty rims are only available in 7.5x17" sizes, such as Momo Arrows. These two factors make the wheel sit very close to the shocks, and to compensate for this, I have fitted 10mm spacers at the rear and 3mm at the front.
Another method I have seen on many Fiestas is to use standard sized aftermarket shocks such as Koni, and "shave" the spring cups where they rub by grinding them down slightly. Spacers are still needed, but 3mm ones should suffice.
Below is a table, designed to help identify what suspension work needs to be done, this is presented as a guide only, and should not be regarded as definitive. You may find that your ET40 8x17's go straight on with your Penkse coilovers...
N.B. 60mm shocks mean 60mm diameter springs (standard size e.g Koni). 205/40 tyres should be used for up to 7.5" rim widths.
| Wheel Size | Example Wheel | Offset | Suspension | Suspension Mods Needed |
| 7x17" | TSW | ET35 | 60mm shocks | Shave spring cups and / or fit 3mm spacers at rear. |
| 7x17" | Kahn | ET38 | 60mm shocks | Shave spring cups and fit 3mm spacers at front and 6-10mm spacers at rear. |
| 7x17" | TSW | ET35 | 2.25" Coilover | AVO - none, Spax - 3mm spacers, dependant on wheel. |
| 7x17" | Kahn | ET38 | 2.25" Coilover | 3mm spacers, dependant on wheel design. |
| 7.5x17" | Momo | ET35 | 60mm shocks | Shave spring cups and fit 3mm spacers at front and 6-10mm spacers at rear. |
| 7.5x17" | Smith's | ET38 | 60mm shocks | Shave spring cups and fit 3mm spacers at front and 6-10mm spacers at rear (tight fit). |
| 7.5x17" | Momo | ET35 | 2.25" Coilover | AVO - none, Spax - 3mm spacers, dependant on wheel. |
| 7.5x17" | Smith's | ET38 | 2.25" Coilover | AVO - possible 3mm, Spax - 3mm spacers front, 10mm rear, dependant on wheel. |
Tools Required
- cross-head and flat-blade screwdrivers
- socket set & ratchet 6-10mm
- hacksaw
- files / sandpaper / glasspaper
- angle grinder
- Haynes manual
Arches
- Remove the wheel for the arch you want to work on, ideally you'll have the car up on ramps but If this is a DIY job just jack up the end you're working on.
- Remove the plastic arch liners, the Haynes manual will tell you where the retaining nuts are.
- If it's an XR or RS model the next step is to remove the plastic arch extensions, they bolt on inside the wings.
- These extensions have a lip of about an inch which needs to be trimmed back, In the photo above the car is running a 6x14" wheel. If you imagine the car with a 7x17" wheel, the extra width of the wheel will cause it to protrude out of the arch further and the lip will come into contact with the tyre when the suspension travels.
- The L shape of the extension needs to be trimmed to an | shape as shown in the second photo, use the hacksaw to trim, then file and paper the edges.
- At the bottom of the extension, where it joins the side skirt, the wheel will never touch this so it is not necessary to cut this back.
- The inside of the arch will now have a metal lip. This needs to be ground down with an angle grinder.
- To avoid corrosion, prime and re-paint areas of bare metal.
- Re-fit the plastic extensions, these may need to be affixed by an alternative method as the nuts that hold them on may affect tyre clearance. Many bodyshops will be able to bond the extensions on, although this may make them difficult to remove in future.