Smoke and Rob Roy Fridays: Orbiting Heaven

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.

1938 Buick Y-Job: source

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.

1947 Cisitalia: wikipedia.org

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.

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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.

1984 Cadillac Coupe De Ville image : ThatHartfordGuy at Wikipedia

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:

2004-2012 Volvo S40

If we take a closer look at the wheel-arch:

Volvo S40 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.

2004 Volvo S40, curvature of wheel

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.

Not circular, flatter at the top.

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:

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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.

Author: richard herriott

I like anchovies. I dislike post-war town planning.

30 thoughts on “Smoke and Rob Roy Fridays: Orbiting Heaven”

  1. Good morning, Richard. An interesting piece, thank you, particularly for someone like me with no design background. That 1947 Cisitalia is extraordinarily modern looking. I would have guessed that it was from a decade later. Only the split windscreen dates it as older.

    Wheelarches are, I would guess, a necessary evil as far as most designers are concerned. Do you design the bodyside, then simply cut out the required arc, or do you make them integral to the design. One car that had a subtle but significant change during its lifetime was the Jaguar E-Type. The Series I and II cars had simple cutouts in the sheer bodyside:

    The Series III cars added a subtle eyebrow that significantly changed the way light played on the bodyside:

    If one can look past the heavy handed front end on the later car, I think the wheelarch treatment (and wider track?) adds muscularity to the body that is absent from the earlier car.

    1. Perhaps you missed a career in the area: I hadn´t spotted that difference. The second car is more finished to my eyes. Quite possible the small flanges add rigidity to the panels as well.
      Whether you start with the arches depends on whether you´re drawing it or making a clay or a digital model. I only suggest they way I´d do it. For a clay I think the bodyside is blocked out so the modeller can drag the tools evenly from one end to the other and up and down. Then I think the holes are cut when the main surface is ready. For a CAD model one can set the structures up any way you prefer but even then the main bodyside is probably done as one set of patches and the wheel arch lips blended to those. They are usually secondary, emergent surfaces.

  2. Looking back through the Jaguar archive, it appears that the E-Type has not been subjected to a DTW design analysis, apart from one Micropost about shutlines. As a heretic who doesn’t regard the E-Type as “one of the most beautiful cars in the World”, I’d welcome such an expert dissection of the design.

    1. They´ll be burning me on the same stake, Daniel. They do have an otherworldly space-ship appearance when moving. Even still, they are a smal bit on the over-rated side. There are at least five Alfa Romeos and Lancias that are far more lovely than the E. Much of the E´s appeal is due to reflected neon light of the swinging 60s.

    2. Jumping on the E-Type pyre whilst returning to wheel arches, the narrow track has always looked wrong to me, giving the car a slightly flabby appearance. I’m not sure whether the location of the wheel on this ‘z-axis’ qualifies as styling exactly, but it does have a significant impact on the appearance of a car.

  3. Then please allow me to take a bullet for you, Richard:

    The biggest problem with the E-Type is the excessive curvature of the windscreen, which forces the A-pillar to be too upright. This is particularly apparent on the coupé, where the B-pillar looks to be at a greater slope than the A, giving the impression that the door glass is narrower at the bottom than the top. It isn’t, of course,* otherwise the window wouldn’t wind down, but it just looks awkward to my eyes. The straight bottom edge to the DLO is at odds with the sinuous curve of the waistline below, a problem highlighted on later models with a really naff chrome fillet. The lower corners of the door and bonnet should be radiused, rather than square, and the rear bumper should stop slightly before the wheel arch opening, rather than interrupt its curvature, in the manner of the front bumper

    I’ll get my tin hat…

    * The door window runners are actually parallel to each other.

    1. You´ve spotted all the items I´d have listed. The mismatch between the glasshouse and top of the door (the sinuous bit) is almost so egregious as to be invisble. The small radii on the door shutlines and bonnet leap out from the sea of curves. That rear bumper and the wheel arch is another stylistic judder.
      It´s not terrible. It´s even quite pleasant in its own way. What it is not is beautiful; parts are but not the whole. It is a fine fish soup with pebbles instead of croutons.

    2. Like the Mark 2 saloon, the E has become something of an automotive cliché. The E-Type shape is such a caricature it really shouldn’t work at all, yet somehow, through a deft distillation of exaggeration and restraint, it does. Flawless however, it is not.

      Daniel’s points are good ones but like all good points they have equally good reasons. Firstly, the E was developed initially as a roadster. It’s basic style was set by Malcolm Sayer according to his aerodynamic principles, then finessed by Bob Blake under Sir William’s instructions. The fixed head happened quite late in the car’s development, and was mocked up by Blake, largely by eye and very much upon his own initiative. Lyons saw it and immediately sanctioned it.

      However, while Blake had mocked up a more raked windscreen for the fixed head, Lyons insisted upon the roadster screen being used, for cost reasons. Similarly, because the wing line was set for the roadster, it was imposed upon the closed car. This became even more of an issue when the 2+2 Coupé was created – a car allegedly nobody but Lyons wanted. Hence the chrome flash below the DLO – employed to distract the eye from the visual discord.

      The other issues as mentioned were most likely cost related. Lyons refused to properly tool-up for the E-Type, convinced it would not be a big seller. So it was made up of a jigsaw of small pressings, created as cheaply as possible. The E-Type was half the price of an Aston Martin, yet offered similar or better performance. But while it was largely hand-built, unlike the Aston, it was costed to the last brass farthing.

      Incidentally, the wheelarch flares for the Series III were a slightly contentious addition, largely owing to cost implications. Initially, tacked-on ‘eyebrows’ were investigated, which mercifully didn’t arise. Not everyone liked the finished items, one Browns Lane insider suggesting they gave the car the appearance that it had been involved in an accident…

    3. It’s intersting that you mention the E Type as having something of a somewhat caricature.
      Many years ago T&CC had an article on the SS100 containing a cartoon from the eatly Thirties (from MotorSport I believe) showing a sports car with an impossibly long bonnet and suggested that this cartoon had been the template for the SS100’s design.

  4. Good evening everyone. Once more, a great article on parts of the car that I’ve obviously seen, thought about, appreciated (or not) but never really understanding as to why so thank you for enlightening my Friday-jaded mind.
    The Cadillac remains wonderful to behold, but I never realised the humble Volvo S40 wheelarch would have me studying hard.
    As for the E-Type, series 1 is poetry in motion but shooting down folk isn’t my style.
    Consider this Shakespeare quote a mild rebuke; “there is nothing good or bad – but thinking makes it so.”

  5. A thought-provoking article, as ever. I find the transitional period, which took place in the early 40’s, when the crossover from separate mudguards to integrated wings took place, interesting. The Morris Minor is an example of this.

    Various artists have imagined cars without wheel openings. I can’t recall whether these images have been posted on this site before, but if not, please see below. The cars from the 50’s and 60’s seem to work best, to me.

    https://www.newlyswissed.com/in-switzerland-where-cars-can-fly/

    https://www.designboom.com/art/sylvain-viau-flying-cars-04-21-2015/

    1. Thanks for those. Yes, the Morris Minor and perhaps the Volvo P-something or other PV 544 which was replaced by the Amazon (sort of) in 1956. The cars with totally fared in wheels tend to look very heavy, don´t you think.

  6. Eóin, thanks for your insights into the E-Type pre-production history. Frankly, I’m amazed that Jaguar could produce something so ground breaking, given the limited financial resources available. It’s nice to understand the background to the (minor) compromises in the design.

    Andrew, your opinion on the Series I is every bit as valid as mine, maybe more so! I’m actually relieved not to have been challenged more forcibly for criticising such a widely revered design.

  7. Great photos of the DS, Richard. It’s hardly surprising that they look so “right” flying without wheels as the DS looked raather more like a 1950’s sci-fi movie spaceship than a car.

    The second photo might have well been taken on Portmerion beach. Anybody remember why?

    1. Hi,

      I don’t know where the 2nd picture was taken. But I think it wasn’t somewhere where there was a tide, putting maybe Portmeirion out of the picture. Plus, they always had Mont St Michel at home if they really wanted a tide ‘thing’ I think.

      Maybe somewhere more like a lake, in the pics below we can see they used stilts buried deep in the mud to hold the car up and then added the ballons between the water and the car.
      Citroën also showed a version with covered wheel arches and no wheels at the Motor show then.

      https://ibb.co/hShgaA

      https://ibb.co/euiXhq
      https://ibb.co/c9nEvA
      https://ibb.co/fNTb8V
      https://ibb.co/gsTXhq

      Going off subject and talking about a different kind of cut out. I was looking at the Fiat Mobi and was wondering why the transparent glass area at the back was so tiny ? It’s incredibly small. Especially for a city car. Is it cost ? even though the rest of the expanse is still glass but somehow blacked out…..

      https://ibb.co/f0btNq

  8. I don’t remember a beach at Portmerion but then it was a typically balmy, summers day in North Wales with hurricane speed winds and horizontal rain. But there must be a Prisoner reference in there somewhere., Daniel?

    1. …is the correct answer!

      Well done, Andrew. The spheres supporting the DS put me in mind of the large white ones that pursued the eponymous prisoner along the beach, hence the Portmerion reference.

  9. NRJ – great pictures of the Citroën.

    Interesting to see the Fiat Mobi – I wasn’t aware of it and I’m a sucker for small cars and cars we don’t get in the UK. I guess the Mobi would overlap with the Panda too much (?). It has shades of the Renault Modus, at the rear. The rear glass is odd and I’ve been trying to work out why. Tall back seat, possibly? This video shows more of the car.

    1. Looking at the video above reminded me that a lot of those small cheap cars now use one-piece front seats, headrest and seat back joined together, to keep the costs down I guess.

  10. Hi Charles,

    I didn’t think of the possibility for tall back seats ! It must be like what was explained for the Talbot Horizon’s dark rear windscreen. That sounds like a reasonable explanation.

    Maybe it’s just too basic for Europe (crash test results ?) even compared to the Panda ? Some of the construction solutions remind me of the 107/C1/Aygo trio. The first time I saw it, it’s the massive rear lights that shocked me 😀

    1. Yes – it certainly doesn’t do well in crash tests, unfortunately – 1 star for adult protection in the less demanding Latin NCAP tests. While I understand the need to tailor vehicles to markets, I think it’s poor that cost cutting extends to safety. Then again, if it’s a choice between no transport or an affordable car, I guess you’d have to pick the latter.

    2. Yes this crash-test question is such a thorny subject, on the one hand manufacturers are not viewed favourably in this light giving the impression they’re serving 2nd class customers but there’s always the other side claiming these people would travel with their whole family on a moped if it wasn’t for these cheap vehicles and this is far more dangerous.

  11. ….In the crash-test video you can see the engine bay disintegrating compared to the equivalent European models !

    1. The shutlines on the Ford Probe’s hood and bumper remind me of what Peugeot has done much later on the new 508. It looks very bad on the 508 I think. Peugeot calls it the “carrossier” hood because, according to them, you can only cut it the way it is cut if you have complete control over the quality process and they imply that it shows that they’re now, at least, semi-premium.

      https://ibb.co/iesMaA

      https://ibb.co/bZMa2q

    2. The Ford Probe´s panel gaps look like the exact places fillets would be placed on a CAD model of that shape. The base of the window doesn´t work too well.
      That image of the 508 shows two nasty panel gaps that do not allign yet are close enough to do so. They seem expedient like on an, ahem, American car from the 1990s. I can think of less unsettling ways to divide the wing, bumper and bonnet.

  12. Greetings from America. Not to be a stickler, but that red Caddy is a 1974 model, not 1984. By 1984 Cadillac was through one session of downsizing and was about to embark on their first all FWD model line up. There had been ten years of development going on since the 1974 models. Typically during the 60’s and 1970’s American cars used flat topped, “squarish” front wheel openings. The 1966 Buick Riviera was an exception. Round wheel openings were considered to be sportier.

    I’ll throw my opinion in on the E Type. Because of it’s vaunted status it’s hard to criticize the design. It does have a problem with proportions, as the front overhang is definitely too long. I also feel that the body curves under too much from the belt line. to the rockers. I also much prefer the design of the Jaguar C type racer as opposed to the D type which always strikes me as a fish flopping on it’s side. Since the E type was based somewhat on he D Type, that explains a lot. I actually think that the XJS is a better proportioned design than the E type. I’ve got an ’89 convertible so we can skip any arguments about the buttresses!

    1. Good eyework. That was a typo. The sportiness of round versus the formality of square: I hadn´t precisely noted the distinction. I love this product semantics stuff.

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