The transformation of a car with a widebody kit or bolted-on fender flares (spatbordverbreders) is the ultimate step in car tuning and motorsport styling. Inspired by iconic racing classes such as GT3, DTM and Japanese tuner icons, a widebody conversion widens the car by 30 up to more than 100 millimetres per side. This not only creates a brutal, muscular stance, but also physically makes room for extreme wheel sizes: think wheels from 10.5J to 14J wide, deep concave profiles and negative ET values in combination with airride or track coilovers.
Still, a widebody installation is the most invasive modification you can make to a body shell: "How do you install fender flares in a responsible way?", "Why must the original inner and outer lips be cut away and welded watertight?", "How do you prevent the wheel arches from rusting away within two winters?" and "What does the MOT (APK) legislation say about protruding wheels and body widenings?" In this in-depth technical guide the bodywork and chassis specialists at Wheels 'N Customs explain the full metalwork, corrosion protection, wheel-arch sealing and fitment calculations step by step.
1. The Hard Reality: Why Cutting Away the Wings Is Inevitable
A common misconception among novice builders is the assumption that fender flares can simply be bolted over the existing wings. Anyone who fits wide wheels or spacers and lowers the car without cutting away the original wheel-arch lips immediately encounters a catastrophic problem:
Why fender flares fail immediately without cutting
While the fender flare creates optical space on the outside, the original steel wing still sits in exactly the same place. As soon as the car goes over a speed bump, compresses in a corner or drops on air suspension ('aired out'), the wider tyre slams full force against the razor-sharp factory steel lip.
Tyre damage: The steel lip cuts deep grooves in the tyre sidewall, with an immediate risk of a blowout at high speeds.
Shattered flares: The upward impact of the wheel breaks the fender flare from the inside or rips the mounting bolts out of the sheet metal.
Necessity: To use the full suspension travel and to be able to lower the car, the steel outer and inner lip must be cut away 30 to 70 millimetres higher.
2. The 5-Step Installation Protocol: Cutting, Welding & Sealing
Installing fender flares requires precision and skilled metalwork. Our workshop technicians in Nijkerk follow the protocol below to guarantee structural stiffness and absolute rustproofing:
Step 1: Dry-Fit, Height Measurement & Marking
The car is placed on the lift and the wheels are removed. The damper or strut is compressed (or the airbag deflated) to simulate the maximum droop/compression position of the new, wide wheel.
Place the flare 'dry' against the body using clamps and masking tape. Mark the contour of the flare and also mark the cutting line: this cutting line is always at least 30 to 40 mm below the mounting holes of the flare, so there is sufficient material left for the bolt connection.
Step 2: Cutting Back the Inner Wings (Cut & Tab Method)
At the front the wing is usually single-skin (thin sheet steel), which can easily be trimmed with a pneumatic cut-off tool. The rear wheel arch, however, consists of two layers: the outer wing and the inner shell (tub).
Cut away the outer panel. Then cut vertical slots in the inner shell every 4 to 5 centimetres (relief cuts / tabs). Hammer these steel tabs outwards with a plastic mallet until they contact the outer panel again.
Step 3: Tack Welding & Stitch Welding
Because the inner shell of the wheel arch is directly connected to the cage structure and torsional stiffness of the car, it must never be left open. The inner shell and outer skin must be structurally rejoined.
We weld the folded tabs in place with a MIG/MAG welder using stitch welding (intermittent tacks). This prevents the thin body panels from warping due to excessive heat input. Excess weld material is ground flush.
Step 4: Hermetic Rust Prevention & Seam Sealant
This is the most crucial step against corrosion. Once the welds have cooled and been degreased, we treat all bare cut and weld edges immediately with a high-quality 2K epoxy primer or zinc primer.
Next we apply a generous layer of industrial polyurethane body seam sealer over the entire weld seam. This seals the seam 100% airtight. The inside of the wheel arch is finished with a tough bitumen or wax undercoating (stonechip protection).
The way the flare is mechanically anchored to the car determines durability and finish quality. There is a world of difference between a professional installation and amateur DIY work:
Professional Standard: M5/M6 Rivnuts (Threaded Inserts)
In the outer skin we drill holes into which stainless steel or zinc-plated rivnuts (threaded inserts) are pressed with a special rivnut tool. The holes are pre-treated with zinc primer.
This creates a solid piece of internal thread in the body. The widebody kit is then secured with stainless steel M5 or M6 Allen or Torx bolts with sealing washers. Advantage: the fenders can be removed at any time without damage for maintenance, paintwork or inspection.
The Big Mistake: Parkers & Self-tappers
Screwing ordinary self-tapping parkers directly into the sheet metal is a recipe for disaster. Due to engine vibrations and driving wind the holes will elongate within a few thousand kilometres.
Moreover, the sharp self-tapper damages the metal, causing acute rust blisters around each screw hole. For serious projects never use self-tappers!
The Indispensable Rubber Trim (Fender Welt): \r\n Always place a special T-shaped rubber sealing trim (fender welt) between the edge of the ABS or carbon fender flare and the painted bodywork. Without this rubber profile, the hard edge of the flare will, due to microscopic vibrations, rub the car paint down to bare metal within a few months. The trim also accommodates tolerance differences and seals the seam against ingress of sand.
4. Wheels & Fitment: Extreme Negative ET Values & Wide Rims (Up to 14J)
Once the arches have been widened by 50 to 80 mm, standard factory wheels sink deep into the wheel well (the 'sunken wheels' effect). To make the wheels sit perfectly flush with the flare edge (the ultimate 'flush' stance), two wheel-engineering solutions are possible:
1. Extremely Wide Custom Wheels (10.5J up to 14J): Instead of spacers, high-end projects opt for solid wheels. Wheels 'N Customs supplies and fits rims up to an impressive 14.0J wide (as shown in our official Tyre Width per Rim Table). Brands such as Japan Racing (JR-Wheels), Concaver Wheels or our bespoke forged RFC Custom Forged 3-Piece wheels can be built with gigantic polished outer lips and extreme negative ET values (e.g. ET-15, ET-25 to ET-44).
2. High-Quality Bolt-On Spacers / Adapters (System 2): Want to retain or adapt your existing wheels? Then solid bolt-on track extenders or hub adapters of 30 mm to 60 mm per wheel are required. These are fastened to the original hub with special bolts and must feature a fixed centring flange (hub-centric) to eliminate vibrations.
Tire Choice & Tyre Stretch: To prevent the tyre shoulder from contacting the inner side of the flare under maximum compression, tuners often choose a responsible mild stretch (for example a 265 tyre on a 10.5J rim, or a 295 tyre on a 12J rim). Always consult our sizing guidelines for safe tyre pressures.
5. MOT Inspection & Legislation: What Is Allowed and What Is Not?
When building a widebody road car, clear legal frameworks apply in the Netherlands and Belgium:
Inspection Requirement / Parameter
Netherlands (RDW / MOT Guidelines)
Belgium (GOCA / Technical Inspection)
Tread Coverage Shielding
Wheels may protrude a maximum of 30 mm beyond the bodywork. The profiled tread must be covered by the fender flare.
Strictly 0 mm! Tyre and rim must be 100% within the wheel arches.
Sharp Parts & Edges
Per MOT requirement: no sharp protruding parts that pose a hazard in collisions. Bolt heads must be rounded off.
Strict checks on rounding radii in accordance with European type approval.
Increase in Track Width
Up to 2.0% increase compared to the registration certificate is immediately permitted. Above that a formal RDW individual inspection is required.
Change of track width requires official homologation or validation report.
Clearance During Suspension Travel
100% clearance required in all steering and suspension positions. Contact = immediate fail!
Have Your Widebody Conversion Professionally Supervised in Nijkerk
Installing a widebody kit is an irreversible modification that requires millimetre accuracy, expert welding and uncompromising rust protection. In our professional workshop on Riddererf in Nijkerk our specialists advise and guide you in choosing the right wheel sizes, spacers, airride systems and body components. We ensure a 100% correct fitment without your tyres rubbing on the inner edges of the wheel arches.
Personal Fitment Advice for Your Widebody Project?
Unsure which wheel width, negative ET value or tyre size matches your fender extensions? Our experts will happily calculate the ideal fitment for you in advance.
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