Change of plan for the roll bar

 Once again locostbuilders to the rescue. I posted my pictures for the roll bar design and the feedback was, erm, not good. Some of the points were similar to what I was thinking, others I'd not really thought about.

MrWhippy was the most succinct in feedback;

1. Angle is far too steep. I wasn't sure about this, but it wasn't a good sign that 'the tube bender' only cuts roll bar fish mouths for up to 45 degrees. At what angle does a triangle no longer become a triangle? This MX5 roll bar is a perfect example;

Are those 'support bars' actually supporting? 

2. The brace is not mounted to a strong part of the chassis. This one I wasn't sure about, I think it was the picture angle. The brace comes down to the triangulation point of the chassis, as well as very close to the lateral support for the seats. That's five lengths of 25mm steel converging at one point. As long as I got the fixing right (welded 5mm plate for example) that would be more than enough. 

3. The brace is only supporting half the height of the hoop. Yep, very accurate. I'm introducing forces to the edge of a triangle rather than the corner. It needs to be brought all the way to the top. That would mean a tighter angle on the side bar though.

After a few back and forth discussions (thank you Russbost, Mr Whippy, adampage and adithorp) it is clear that whatever I do on the side I'll just be swapping one compromise for another. I wouldn't actually get the structure required to keep my head off the floor with the car upside down. 

adampage had an interesting idea though. Why not bring it down to the diff cage? Initially it was a no-go, that's where I have my fuel tank;

But as I looked at those photos I realised that I still need to build out a platform for the fuel tank. That strap is currently the only thing holding the tank on to about two inches of metal! Since I have to support the fuel tank already, why not simply build it out to either side to make platforms for rear braces. They could be triangulated to the diff cage (which is a substantial cube already) and I wouldn't lose any boot space because of it. 

The only thing I'd need to consider is getting the bars through the bodywork. I'd need to make sure that at least one end of the roll bar is as thin as it can get. There's a couple of options, both appear on this roll bar;

The first option is a sleeve with bolts through, the example can be seen top left and right and is a 'double bolt' arrangement. That would require finding some tube that fitted tightly inside the roll bar tube.

The second option is a double shear bracket with a bolt through, which is what is used on the diagonals in the image. I prefer that one to be honest. Demon Tweeks sell the OMP gudgeon pin and weldable roll bar connection, but I could also use steel plate. 

Hmm, looks like Race Spec Cages (AKA Ebat Tube Bender) have the sleeve option; https://www.ebay.co.uk/itm/331096168053. I'm not sure it would work though, to keep things tight the bar would not have any 'wiggle room' which would make it difficult to fit. 

Anyhow, first thing is back to the prototype stage to build out the platform and see if the rear stays would work around the fuel tank. 

Edit: A quick note on angles. Looking at the MSUK yearbook I can see that a minimum angle of 30 degrees is required. At least that gives me something to work to. It also gives some guidance on bracket types and there's a few options. I'm assuming if they're MSUK approved then they're good enough for me. Images K21 to K30 show the various brackets, starting on page 200 of the rule book.