DIYSportsCar.com - Design

This section of the website has been almost completely re-written as I've drastically simplified what I originally proposed to do.

The design can be broken into two parts - Chassis Design (including suspension), and Styling.

Chassis Design

My original chassis designs had lots (and lots) of tubes and were desperately complex. The current one (which I'm actually building) is simpler but still more complicated than most other 'kit car' chassis. However, I need the chassis to pass a variety of strength tests for registration approval in Australia, and I believe the extra complexity will help me through that process as well as affording me some more safety in the vent of some moron in a LandCruiser running me down. ("Sorry mate, I didn't see you!")

Colin Chapman apparently described a chassis as "just a big bracket that holds everything together". That is to say, a chassis is purely a function of the jobs it is asked to do. For a small sports car like this, those jobs are fairly simple: carry the engine and passenger/s and keep those and the front and rear suspension mounts in the same position relative to each other. Supporting any bodywork is a secondary issue. As I drew version after version of my chassis though, I realised that the original Lotus 7 was almost all chassis and no bodywork and the Gecko as it currently stands was born.

Engine/Gearbox: Toyota's 1600cc DOHC 4A-GE engine is very well understood and cheap in my part of the world and, even in fully imported 20-valve form, is able to be complianced for emissions purposes in Australia. The 20-valve Japanese market Corolla engine comes, depending on version, attached to a 5 or 6-speed transverse gearbox with optional LSD. There's also a neat little jack-shaft from the offset diff which ensures equal length driveshafts. Mine is the so-called 'Silvertop' engine with 5-speed box and no LSD (yet).

Suspension - Front: The Corolla has, in common with many smaller FWD cars, a 4 x 100mm PCD. Although common in FWD, this pattern doesn't appear much in RWD cars, especially those with wishbone front suspension that could be donors for suspension uprights. After some research I found just two; the Mazda MX-5 (Miata in North America, Eunos Roadster in some other parts of the world), and the Holden Gemini (the Australian name for one of GM's first 'world cars', which was sold in most of the rest of the world as the Chevette). The MX-5 uprights are lovely but not cheap. The Gemini parts are light, cheap and readily available. The standard brakes may or may not be up to the job (road test will show one way or the other) but there's still an active tuning community in Australia and a variety of upgrades are available.

The actual layout of the front suspension is quite conventional - unequal length, non-parallel wishbones very similar to a book Locost. The springing will be a little less conventional

Suspension - Rear: This caused me the most work of any mechanical part of the car. I originally intended to reuse the Corolla strut suspension for simplicity and cost reasons. However, I convinced myself that the height of the struts was a styling problem plus I wasn't sure that I could provide a stiff enough chassis around the top of the engine to give the strut top the support it needed in all planes. From there I embarked on a number of versions of a De Dion layout, with four trailing arms and a Panhard rod (very much like a live axle Locost) or, in some versions, a Watts link, WOBlink or Mallock link for transverse location. But the more I looked at how to actually link the FWD Corolla uprights to a De Dion beam, the worse the option looked, with (relatively) complex and expensive engineering needed. And so I found myself back where I'd been two or more years previously, with the factory struts.

A turning point was the discovery, through a workmate, that sleeves to convert Macpherson struts to 'coil-over' are much less expensive than I had thought and that shortening a strut whilst retaining useful suspension travel was quite possible. Add to that re-reading old road tests of the original Toyota MR-2 and I was much more confident that I could make it work. The end result is a quite simple arrangement, very much in the 'Locost' spirit. The Corolla upright is used, unmodified except for a different brake caliper (Subaru 1800 front, which has a separate hand-brake mechanism). The Toyota strut-tube is modified by cutting off stock spring seat, shortening slightly, and welding on a retainer ring for a threaded sleeve to bear on.

Styling

Oh, the hours, nay MONTHS I have poured into various styling efforts :) If I could have even 1% of that time back I'd be happy. Alas, the time is spent but hopefully not wasted.

After expending a LOT of time on a full body design, which I got to a state I quite liked, I decided that the effort in actually constructing it would be just too much. So, drawing a leaf from the Locost book, I took the chassis outline and just wrapped it in flat panels. Add a fibreglass nosecone, scuttle, cycle guards and rear guards and a single curvature aluminium bonnet and that's it.

I drew many variants and they all had the same flaw – the tail was too ‘big’. Too high and ‘fat’ looking. Idly sketching one day I found a solution when I realised that only the engine area and strut tops needed to be full height and that lowering the surrounding panels made the whole thing look lower. This is the more complete sketch I drew that night (complete with the scuttle/windscreen too far back by 100mm or so). Click for a larger version.

And this is the subsequent 1:10 scale model which isn’t quite complete (no front guards, screen, rollbar etc) but shows the overall intent quite clearly. The real car will have slightly different proportions but that’s all. The black vents and trim line along the side are just drawn on the image too. Click for a bigger picture.

Comments are welcomed - email me: dominic @ diysportscar.org

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Last Updated 2004.01.17
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