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MG MGB Technical - Rollover rear cage for MG BGT

I am looking at fitting a rear rollover cage with a diagonal but no harness bar to our MG BGT. I intend to leave the harnesses attached where they are, at the base of the seat back as I want to be able to fold/unfold the rear seat as required. I am not required to have a rollover cage but would be more comfortable if one were fitted for the events we are doing. Couple of questions:

I suspect the seat back will foul the lower part of the diagonal? If this is the case how much would I need to reshape/trim the seatback to allow it fold/unfold?

We also use the inertias for transits and getting to events - can I mount the reels and use them once the roll bar is
fitted?

Thanks

Mike
Mike Dixon

Mike - what are the events you are involved with. I am guessing the risks might point you to race harnesses in place of seatbelts and abandoning the seat back unless the events involve a passenger.

Roger Walker

HERO and HRCR special tests plus we are starting to do some night stuff, also off to the Pyrenees on the back roads which can be quite interesting.

We have harnesses but inertia are much easier for the transits and getting to and from events. Not a show stopper.

We have to keep all seats fitted as part of the regs and to be honest we like it as an 'admin space' with ability to flatten it to get larger cases in to make most use of the baggage transfer facility on events

Mike Dixon

Mike--Could you do a rollbar with rear braces onto the top of the wheeltub and run your diagonals from the wheeltub up to the hoop ,well out of the way of the seatbacks and your head
Something like this pic but without the harness/cross bar--The diagonals could be positioned to allow the rear seat to fold down

William Revit

Which plane are you putting the diagonal in, main hoop or back stays? If the back stays, where are you contemplating mounting them to the body? The GT roof line provides several options for this and shifting the body mount as far back as possible might generate clearance for the seat back. It might also be a pain for access via the hatch of course.

Alternatively, can you dispense with the diagonal, or have a removable one? What regs do these events have for design/construction and do they require it? Having field tested a diagonal-less rollbar in my rally roadster back in the days before they were required I can verify that a main hoop made from 45 diameter x 3mm walls will withstand an 80mph inversion. So if the regs don't require one a bar of that style and dimension would make the rear much more useable.

In my case, lack of a diagonal let me swap between rally car and (one and only) family car quite easily. (see pic) In later years when the diagonal became mandatory for rallying I had to make a new cage, but still kept the old main hoop to swap in to keep the car more practical for road use and for the second generation (second pic)






Paul Walbran

Thanks, a removable diagonal would be my first choice but I will have to go bespoke if I wanted that. I think the removable bar is no longer allowed so no one makes them as standard.

I was planning on getting a Safety Devices one.

http://www.safetydevices.com/motorsport/products/roll-cage/MG+MGB+GT+no+sunroof-1965-1980-3-door-without-sunroof/353/1183/

Even to get one without a harness bar is a special order, requires me to state I am aware it may not meet regs and costs about 20% more than the standard one with the harness bar as it is a special order.

The diagonal I think fouls the seat, I just wondered by how much. I suspect it they could move it to get out of the way of the seat they would have done?
Mike Dixon

If they are anything like the rollcage builders here, they are used to fitting to a stripped out vehicle, and can't get their heads around why someone would want roll protection in a car that has anything other than the minimum trim required.
Paul Walbran

If you have racing harnesses and the locating points for the rear straps are located at the base of the seats they may well be UK motorsport legal but in the event of an accident the forward momentum of the person in the seat will act against the harness which will pivot on the rear locating point and transfer forward load to a downward load across your shoulders which will hurt. That is why there is recommended angle for the rear harness straps in relation to the person sitting in the seat. That is also the reason why harness bars are so desirable because they change the pivot point and move where the load is transferred to.

I would have thought a diagonal would always be legal and would be interested in any reference to a regulation which says it is not.

In the early days of roll over bars (just the rear) there would be a main hoop and back stays in British Club motorsport. However, any car racing in an international series abroad, or in the UK, where the event was run to international regs (FIA), the regulation was stricter and required the main hoop to be braced with a diagonal. The advantage of making that diagonal removable was that where it was not required (British Club event) it could be removed to save weight rather than suffer the penalty of having it in a field of British only events Club racers who didn't have it. At one time it was known as an FIA diagonal.
Daniel

This thread was discussed between 28/07/2023 and 11/08/2023

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