Haynes Roadster – Gearbox Tunnel

Strength wise, I always thought the standard Locost gearbox tunnel was a neglected area. I’ve tried to add as much triangulation as I can.

Obviously, given the size of the Cosworth MT75 gearbox and the already limited interior space, there were quite a few design limitations. Luckily, the missus and me are both petite.

There are still a couple of small diameter, light weight tubes to be added, but essentially this design is now a ‘backbone chassis’. But, before I can add anything extra, I need seats, so I can place the gear lever and possibly the handbrake. The bell housing of the 4×4 MT75 will also require a few modifications (for which I have hopefully left enough space).

The gearbox is to be mated up to a Rover V8. The LT77 5 speed Rover gearbox had a hydraulic clutch and the Cosworth MT75 a cable one. Also the LT77 is a larger diameter bell-housing. Once the Engine is drawn and physically in place, I will try to extend this backbone concept through the engine bar to the front axle. At present the CAD drawing still shows the standard bracing for a 2lt pinto.

The Engine bay is currently the only weak spot in the structure. My FEA guru (Finite Element Analysis), told me this ages ago, hopefully once I do my mods, he’ll approve.

Haynes Roadster 3D CAD

Lotus Esprit S1The backbone chassis concept isn’t new, in fact the Tatra 11 of 1923 had one. However, the car or even manufacturer that sprang to my mind was Lotus. Just after I left school, I worked briefly in a garage where they had a Lotus Esprit in for major crash repairs.

Obviously my chassis has external rails, but the job of stopping flex in the central section has been given to the tunnel. Not only did I want to stop as much up and down flex as possible, but I also wanted to stop side to side flex and distortion.

My tunnel is asymmetrical, as the MT75 4×4 transfer box hangs out into the driver’s foot-well a little (or lot).

When designing a chassis it is important not to place tubes parallel to each other. Tubes laid out in rectangles or squares, simple aren’t strong. Sure, a perfectly rectangular chassis would be easy to saw and weld, but it would be very weak.

The top 2 tubes of the gearbox tunnel have obvious triangulation. In the standard design these tubes are parallel to each other

The single tube (shown in the CAD drawing) that runs from side to side along the base of the dash and above the knees will be replaced with another triangulated structure that will also mount the steering column.

Some people prefer to do everything in physical metal, building as they go. I prefer to do everything in CAD first. So far, this has really paid off. With only 4 whole days of ‘chassis time’ I have something that simply awaits brackets. I’ve not had to modify anything, every component has gone from vise to car without incident and I’ve not wasted any steel- Ah! ‘Smug Mode’

Locost Standard Chassis
Haynes Roadster Chassis

MT75 4x4 Haynes Roadster

I've had a trial fitment of the MT75 Ford Cosworth 4x4 gearbox and it was a little tighter than I hoped. The gear selector mechanism; which will be removed and replaced; fouled the tunnel but this was a known issue. More importantly, the gearbox's transfer casing is a little close ...

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