I need to add a 4 point roll bar to my 85 car that is used for hillclimb. The car is still road registered but one of the tracks I run at require roll over protection if you are faster than a certain time, regardless of national rules not allowing roll bars for road registered classes (except convertibles)
I started looking at how people have fitted cages and roll bars to C4's (which is effectively from the day the C4 was born !)
Then I compared this to how our National governing body roll cage rules are set, and I realised the well-worn path is not going to work.
For the purposes of this thread, my process is now well underway, the car is with a race shop. I will post up some photos as it goes along. For info our governing body has a strict 10 degree lay back angle which presents problems getting a hoop under the targa bar.
For info here are a couple of pics of 2 different ways the roll bar has been achieved by others over the years.

this pic shows method 1, how the rear main hoop is attached to the side chassis rail (usually with angle steel welded as a base)
this method will cause serious fitment issues for my race seat and lose a fair bit of rear seat adjustment - my car is right hand drive so I have less room in the left side of the cab)


Second common method is to perch a short roll bar on top of the rearward chassis in the back section of the car.
This wouldn't be a best solution to get a roll hoop from one chassis base, across to the other chassis base behind the seats.

 

I don't see the problem with the second option.



This might be an option to keep seat movement.

 

Second option worked well for me. If the bar is installed on the steel shelf under the seat belt mounts, mid level, it’s easy to achieve less than 10 degrees from vertical and well under the B pillar halo

 

JoBy - that layback of the main hoop is way beyond 10 degrees; our governing body is strict on that so couldn't get away with it.

 

That setup is a good way around the issue, but again with our rules, they only allow that high up perch with road registered convertibles.
When I spoke to the cage builder he said it's not a good solution for my car and that putting the main hoop tubes on top of the chassis behind the seats will make the car stiffer.

 

To get on top of the chassis, rear of the seats, and maintain the 10 degree layback whilst getting under the targa, I will have to cut into the steel plate behind the seats.
Now I had no idea of what is behind that plate, I have never seen anyone cut into there before.
The whole rear tub under and behind the seats is steel chassis and steel plate and quite extensive, it appears to be quite over - engineered and very strong. It also houses the bracket for the forward 4 link of the rear suspension (didn't know that till I got in there !)

Here is the first cut into the steel plate. The opening needs to be a minimum size as the mounting plate has minimum dimensions.



Pic with a plastic pipe as a measuring tool. this is prior to removing the original seatbelt to be able to move the upright further outward.

 

the car is with Heritage Racecraft to get the roll bar installed.
first pic is the mounting plate being fitted. The plate that the mounting plate will be welded to, has been welded to the surrounding steel plates and bars.



And here is the base plate (left side) with everything welded up.

 

the main roll hoop is welded in, pic is the left side with the diagonal brace welded in.

 

Here is the 4 point complete, it has a diagonal across the main hoop, a straight bar on the backstays to tie in rear suspension locations and double as the harness mounting points. The shop has fitted threaded harness mounting points thru the rear bar. The cage has been painted and cover plates made for all 4 mounting points.
The shop has done a great job, I'm very happy with the outcome.



From the rear with the drivers seat and harness in place.