Today DTW turns didactic and we have a short history lesson about wheel cut-outs on the bodyside. Though we covered this a little in 2015 I thought I might elaborate.
The wheel cut-out is where all the sculptural activity of the body side has to meet a much more rigidly controlled boundary. To think of its form, imagine cutting a circular hole in a vertical plane. Then tilt the plane slightly so it leans away from the centre line. The next step is to cut away all but a small strip of surface around that circle.
You now have a curved band, trimmed to meet the base of the car. Whatever else is going on on the bodyside it must meet that almost-flat arc. In some cases it isn’t a flat arc but a rectangular band (see the Caddie below).
In the old days it was easy to do a wheel cut out. Pre-war cars had separate fenders to enclose the wheels. The Buick Y-job shows how these fenders had a decorative and functional aspect. After the Cisitalia (1947) showed how the fender could be incorporated into the main body, the trend was to reduce the sculpting of the wheel arch to the minimum for reasons aerodynamic efficiency. Such designs also required a shallower draft for simpler, cheaper steel pressing.
Getting back to the geometry, the wheels are located in a vertical plane unless the rear track is narrower than the front, in which case the wheels are located in two planes, Citroën step forward. The uppermost part of the wheel arch must be wide enough to contain water spray and debris inside the wheel arch cavity.
The rest of the body side is sculpted according the styling and packaging requirements but must be bridged to the arcs of the wheel arches. And that transition can be quite vexing. Not everyone manages to force the forms from elongated and voluptuous to simple arcs with equal finesse.
Wheel openings are seldom formed only by curved bands arching in space – the shape may be asymmetrical, flattened and must eventually blend with the form of the sills. The front wheel arch and rear wheel arch also seldom share the same silhouette, the rear usually being a smaller opening due to the fact that the rear wheels require less articulation than the steering front wheels.
Vehicles designed for poor roads or off-road use may have larger openings than vehicles designed for higher-speeds and smoother roads: American cars typically have wider wheel openings than European ones, a reflection of the poorer road surfaces of continental USA. The wheel-arch opening has a semantic association: ruggedness asks for a wider opening, speediness asks for a smaller one.
I did an interesting analysis of an apparently round wheel cut out, on a Volvo S40:
If we take a closer look at the wheel-arch:
I used a CAD programme to analyse the curvature. The “fence” around the wheel naturally shows perfect curvature, having a curvature of constant radius all the way around (obviously!). The computer assigned it a value of 2.50 units.
I then used the same tool to analyse the curvature of the wheel arch cut-out, which to the casual observer might look circular. But it’s not.
Look at the red and yellow “fence”. Notice the red bars are shorter around the vertical, right over the axle. That means the line of the wheel arch cut out is less curved, having a bigger radius value there and more curvature left and right of that. It is, simply put, an arc flattened at the top. It is there to add a subtle sense of length to the bodyside.
And here is an example of what appears to be and probably is a wheel arch conceived of as a pure circular form, cut to length, a Spyker:
I’d almost always be inclined to start with the engineering requirement for the wheel cut out and then work from there to the rest of the bodyside. Doubtless some will do the whole bodyside and then impose the cut-outs on that. I was once told Jaguar like to get the wheel holes cut out of the clay model as soon as possible since the holes will strongly affect the way the bodyside is perceived.