Its chassis time!

Well the concept was simple enough - grab a heap of steel tubing and follow the plans in Ron Champion’s book to build a space-frame chassis for your Locost. The reality is that the process is not that simple and there are many decisions to be made and problems to solve before you have a useable chassis.

First step is to decide on the mechanical components that are to be used, especially the engine, as they determine a lot of the final design. For my donor vehicle I purchased a complete 2006 Mazda MX5 NC and then organised a selection of steel tubing. With that done I was then ready to begin my project.

First challenge was to rethink the chassis design because Ron Champion’s original chassis will not pass the torsional rigity testing required for road registration in Australia. With minimal additional bracing and triangulation my completed chassis had no problem passing the testing by an engineer.

There are many different ideas floating around on how to build the best Locost and I believe that no two cars are really identical. I obtained a great set of detailed plans from Formula Motor Sport in Queensland and incorporated many of their ideas into my chassis. Unfortunately they no longer sell their realistically priced Locost chassis plans due to the high cost of public liability and professional indemnity insurance.

The original Ron Champion “book” design is also a bit on the narrow side, therefore, like many other builders, I added extra width (130mm in my case) and also extended the length by 100mm in the cabin (for adjustable seats), 50mm in the footwells and 50mm in the engine bay plus 25mm higher sides from the scuttle forward and increased the size of the rear “boot” area. My Locost is very individual and designed to suit my needs.

I built a solid timber build table with steel legs to provide the level surface required for the space-frame construction, which also gave me a chance to practice my cutting and welding skills before launching into the serious construction stuff. The top was 20mm chipboard with 90mm x 30mm pine supports glued on the long edges and a steel tube frame underneath.

I haven't written a detailed build diary as the many photos below show how the chassis developed. I have included some notes with the photos where appropriate. So take a look at the photo album and follow my project as it grew into a home built, road registered sports car.

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First delivery of mild steel tubing cut into manageable lengths for storage.

The work table was the first item on the agenda, then I started building the sections of the space-frame and tacked them together. I took a different approach to the front rail design and carried the rails straight through. This required other alterations to the suspension mounting points but I beleive it's simpler and stronger. Others may not agree.

Top section of the engine bay. The chassis is only tack welded until it's completed, then the welding is progressively finished, taking care to keep heat and distortion to a minimum. It's amazing how much distortion can occur if you are not careful. I did manage to ruin one section and had to redo it because I rushed the welding, but that was very early in the build and I quickly learnt my lesson.

Timber blocks were screwed into the table to hold the tubes in place and reduce some of the movement and distortion during welding. The blocks also helped in lining up the top and bottom rail sections as they were built seperately.

Here, the vertical tubes at the front are temporary and just there to support the front of the frame until I get around to finishing off the front end and steering.

The angled rear tube required quite a bit of time and effort to ensure that it was accurately measured, cut and positioned.

The full MX5 rear subframe was reasonably easy to fit. Further bracing was added later.

A small amount was cut off the MX5 subframe ends to make it more compact. The open ends were closed off by welding steel plates in place. A crush tube has been welded into the square chassis tube for the mounting bolt and the section was later reinforced with a steel plate.

Some more excess subframe removed. Later plated to cover the open end.

Basic frame tacked together. Starting to look like a space-frame but still a long way to go.

My modified design for the top rails where I made them straight (parallel to centreline). Much later I extended the top rail straight to the firewall which helped brace the engine bay. In hindesight I should have run the tube all the way to the firewall at this stage.

Taken much later, this photo shows the final design of engine bay with extra bracing installed for tortional rigidity. Compare to previous and following photos for differences.

First trial fitting of engine and auto to determine position of tunnel and firewall etc. Table legs removed and timber sides extended to provide comfortable and level work surface. Table height 20mm higher than ride height to aid setting up wheels & suspension etc.

Fabricated engine mounts also strengthen the lower rails.

Checking ride height and wheel clearance etc. The NC model MX5 rear is about 55mm wider than the old NB rear, so I was able to widen the car by 130mm to an even 1200mm across under the seats.

Transmission tunnel, hand brake and auto shifter being set up. Only a few changes required to the original shifter and the tailshaft was not altered at all.

The shifter has been mounted and connected to the six speed tiptronic automatic. Diagonal bracing added to improve chassis rigidity.

MX5 handbrake fitted neatly after minor modification and utilises original cables.

MX5 master cylinder, power booster and pedal assembly in place. Mounts and bracing fabricated later but at least the location has been decided.

Extra bracing, strengthening and side intrusion protection added to seat & leg area. Will not stop a 4WD or tree but I will feel safer. I should have kept the extra 25mm height for the top rail all the way along. Should also have made the 25x50 side intrusion bar the top rail and not so low down.

More triangulation and bracing was added to the transmission tunnel to improve strength and help with ADR compliance for chassis torsional rigidity.

Mounting for rear coilover and roll bar hoop, fabricated and completed.

After some road miles the rear coilover top mounts were changed to Rod Ends to improve alignment with the lower mounts..

Fabricated rear lower coilover mount was located to the original MX5 strut lower mount position.

Cross bracing added to rear firewall and plating added to lower rear suspension mounting point.

Rollbar and supports completed. Some builders weld their rollbars in place but I bolted mine in to enable easy removal in case I want to install a more elaborate CAMS approved unit for track work at a later stage.

The rollbar was also used for the inertia reel seat belt upper mounts which were added later. The rollbar and supports are mounted in structurally strong positions and I believe it would be very safe and effective if needed.

The rear of the chassis was suitably strengthened as it will be supporting the fuel tank and spare wheel (to be mounted on the rear panel) which together will weigh in the vicinity of 80kg with a full tank.

Cross bracing adds to the torsional strength of the front end. The lower end of the cross brace fits directly behind the location for the lower wishbone mounting positions, thus adding even more strength to this high stress area.

A lot of effort went into bracing the front section of the chassis.

The front section of the upper and lower rails were shortened at a later stage, once the design for mounting the radiator and nose cone etc were determined.

Installing the engine and transmission was easy with a mobile engine hoist and adjustable balance bar. The engine was in and out many times during the build and it was important to ensure that it could be easily installed or removed as each new piece of the project was added.

Finally sitting on all four wheels and starting to look like a clubman.

At this stage the chassis is about 90% complete but still only tack welded together. Now it's time to strip it back to a bare frame and finish all the welding.

To further improve chassis torsional rigidity a brace was run from the firewall down to the lower outer rail, next to the engine mounts on each side. Two further upper diagonal braces were later fitted from the firewall to the top outer rails once the position of the engine and accessories were finalised.

Chassis stripped bare and ready to be fully welded. Up to this stage everything just tack welded to prevent movement due to heat from welding. Once everything in place it all worked together to reduce distortion as it's welded. However, caution still required to limit heat by only welding small sections at a time and moving to different parts of chassis as you go.

The frame completely welded. This was a very long process that took several days to finish. It's surprising just how many welds there are in what looks like a simple chassis, but if you consider that every joint has to be welded on four sides you can start to understand the size of the process. For Locost project I estimate I spent around $500 on gas for the MIG welder!

1mm mild steel bulkhead panels welded in place. Welding the panels stiffened the engine firewall and assisted with the torsional rigidity of the cabin and engine bay.

1mm mild steel reinforcing gusssets welded in place.

I used 1mm mild steel for the floor because I had the material and I felt it offered superior strength to aluminium. I also didn't like the idea of turning the bottom rails into swiss cheese by drilling mounting holes for an aluminium floor. However, if I ever built another Locost I would consider aluminium for the floor as I was disappointed with the amount of distortion that I had to rectify in the steel floor due to the welding process.

1mm mild steel reinforcing panels were installed at the front of the chassis under the steering rack and improved chassis strength.

The scuttle and dash mounting frame was welded in place to improve torsional rigidity of my longer than normal cabin area of the chassis. The MX5 instrument cluster was originally in the centre of the dash but after registration it was moved to behind the steering wheel.

Quite a bit of time was spent positioning and mounting the steering wheel and pedals to provide the ideal relationship with the driver. MX5 steering wheel, column and pedal assembly recieved minor mdifications and the steering column is height adjustable.

Once the chassis was completely welded the Locost was once again assembled and fabrication continued on all major components.

Looking rather shabby on its first test drive. But it went very well with only a few minor issues requiring changes.

Still missing a few components, but with trade plates fitted to make it legal, it was off for the first test drive.

Back home after its first test drive. A full dismantle commenced so the bare chassis could be prepared & painted. Friends could not believe that it was to be completely dismantled now that it had reached this stage.

Chassis stripped of all removable components and mounted on a rotisserie for ease of access to all surfaces. Ready to be taken back to bare metal in preparation for painting.

Once on the rotisserie it was a lot easier to get to all the tubes, welds and tight spaces. Sanding the chassis ready for paint took several days and was a very tedious task. However, the better the preparation the better the paint job.

Thoroughly sanded back to bare metal and ready for paint. Media blasting would have been quicker but a lot more costly. Maybe next time I will invest in media blasting and powder coating?

Painted in two pack over epoxy primer. The colour is Charcoal rather than black because the lighter colour makes early detection of stress cracks easier. Not that I expect any issues with chassis fatigue.

Not a lightweight racing chassis but a strong, robust unit that will provide many years of safe motoring pleasure.

During assembly further improvements were made to the engine bay. Two more bracing tubes were added to increase the tortional rigidity of the top outer rail. The engine bay is large enough to accomodate this improved design and they could have been included in the original construction but it was not clear at that stage whether the engine would clear these tubes.

The Locost was almost ready for registration. The final legal requirement was for the chassis to be tested for torsional rigidity and signed off by an automotive engineer. A test rig was borrowed from the Victorian Clubman Builders Group and the Locost set-up in my garage where the engineer performed the test. The required weight was added to the front beam, at the appropriate distance from the chassis centreline, and the resulting twist in the chassis was measured and the torsional rigidity graphed.

All four coilover shocks were removed and replaced with rigid sections of steel tube to keep the suspension at ride height. We only wanted the chassis to move not the suspension. The front beam was bolted to the hubs and a pivot point arranged under the centre of the chassis.

The rear beam was bolted to the hubs.

The rear beam was bolted to the floor to prevent it from lifting during the torsion test.

The test required six measuring points equally distributed along the chassis, including one at both the front axle and rear axle. Steel tubes were positioned on both sides of the car, one metre from the centreline.

Measurements were taken before and after the chassis had load applied and the engineer used this data to determine the result. It was very pleasing to note that it easily passed the legal requirement. The engineer stated it was amongst the best results for a Locost that he had tested.

Locost Clubman build

Its chassis time!

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