My Plan is to build a little mid-engine coupe IF I would gain a significant increass in stiffness.
As my little project has evolved, I have decided to have a full front windscreen and as its support frame vill be a part of the full spaceframe. And due to the fact that I planning to have a Rolloverbar.

If I would connect the rollbar and the windowframe would I gain stiffness?
This would mean some increased weight, but could there be some saving on the spaceframe?

By doing a coupe there would have to be some more work with the fact that it would have doors and it would need a proper heater/demister system.
One of the disadvantage would be that the centre of gravity will be higher.

One of the reasons for changing toward a coupe is that in Sweden the climate is so unfavourable for a roadster/convertible. I want to have fun in my car when its finished without getting a cold.


And to all of you, thank you for the inspiration. YOU got me stareted.

I have looked at this issue.

Cutting the roof frame including the windscreen surround and frame on top of the engine bay from the chassis of a stratos replica gave a reduction from about 6600 to 4000 ftlbs per degree of twist.

Cutting the roof of a typical steel monocoque chassis could lower the stiffness by as much as 80% or even more. Some mass produced convertables made by chopping the roof of monocoque saloon cars from the sixties through to the eighties had very low stiffnesses. Modern soft tops are much better as car makers have become much more aware of the need to ensure good handling and so resort to reinforcements in the floor pan and sills to limit the losses.

For something like a TVR it probably makes little difference as the main chassis structure is in the transmission tunnel and is consequently unaffected by what happens to the roof.

Incidently the "Stratos without a roof" is a fairly good place to start. A lot of small midengined spaceframes are very poor. This is mostly due to inadequate triangulation of the engine bay and rear suspension region. The Lotus 23 shows a simple but effective solution in the form of a Y brace over the engine bay top and a panelled in bulkhead across the chassis behind the engine. Most midengined spaceframe chassis would actually be much better if the same weight was put into a ladder frame.

A really good spaceframe could be developed from the Lotus 23 by adding a second bulkhead further back from the first and fully triangulating it to the first one. The mounts for rear wishbones could then be attached to the front and rear bulkheads thus removing the need for the Lotus 23 leading links and enabling conventional double wishbones to be used. The engine bay is then braced by two diagonals running from the front top corners of the engine bay (the back of the seats) and converging to join the top of the first bulkhead so that the positions of the joints roughly divide the width of the bulkhead into thirds. This will restrict the space available for the engine and will probably force a straight four design on you. The stiffness will be very good though, over 5000ftlbs if made of 1 inch 16 gauge round tube with a welded in 18 gauge floor.

Hope that's interesting!

I went through much the same anguish as you during my design. The thinking went something like this. I wanted a full cage for safety, and I think a roadster convertable with a full roll-cage is really ugly. So I went for a coupe. And yes I agree, CG rises, weight goes up, and dealing with doors sucks. But like I remind myself, I'm building a toy, not an F1 car. Plus it's nice to not worry about rain, sun, or getting constantly blown about by the wind.

Are you planning on having doors on your car?

It might be a better idea to have solid sides (like a seven does) and use slightly smaller tube to put additional diagonals in to increase the trinagulation and hence stiffness in the chassis. Take a look at www.junoracing.co.uk and look at the car page, also look at www.gpcmotorsport.com and car info.

These are manufacturers for the nationalsupersports sports prototype racing series. The cars use the gearbox and engine as structural members but take a look at the chassis deisgns. These cars complete with driver weigh 600kg's and are very stiff. they are open top two seater prototype cars; they use a double triangulated rollhoop and then weld panels in the sides to make them look more aesthetically pleasing. just a thought/example you might like to use from a proven application.

another option is a targa top style design, but without true triangulation won't add that much additional rigidty for the proportional weight increase.