Rollcage design question

[Notadoctor]

The initial message was deleted from this topic on 22 January 2015 at 20:48

[xRIEx]

I assume a bolted cage is for the versatility of being able to remove it if required, possibly at the expense of strength/rigidity.

IANAE but there looks to be a lack of triangulation on the sides.

[littleredrooster]

Back in the day when I was competing, bolted-in cages were OK for Clubman's cars but not national level stuff. I don't recall seeing one which was bolted together, however.

That wouldn't pass any kind of modern scrutineering at any level, I don't think; welded-in and all-welded construction is needed IIRC.

[etchacan]

Bolted in is better than no cage - as you say, it isn't going to be used in competition.

Design-wise, bolted sections are easier to install/remove, and don't require the same level of disassembly of the vehicle to construct.

[Al U]

Hi there. I've designed roll cages before. At the end of the day what is holding the tube members together is traditionally welds. In some rare cases they can be held together with bolts as yours is. This is no problem as long as the loading of the bolts has been taken into consideration during design i.e the tensile and shear loads they will experience has been accounted for both in terms of orientation and specification of the bolts so that when they are needed they won't fail.

I suspect the cage you have was produced in very low volume, so much so that the cost of designing and manufacturing a welding fixture wasn't feasible or the loading space meant a welded structure would not be possible to fit. I wouldn't be concerned, the most important part of the structure is the way it is mounted and roll cages are very commonly bolted in place rather than welded.

Edited by Al U on Thursday 4th December 20:03

Edited by Al U on Thursday 4th December 20:04

[Renovation]

The fit seems very good.

But the design of the bolted joints looks weak to me - I've had many bolted in cages and they were always U accepting a solid bar whereas that seems to simply be two flat sections.

It would be better than nothing - although I do know of several cases where people who have been in relatively minor shunts have been killed by their head hitting the solid rollbar - before you mention padding - would you let me hit you on the head with a padded baseball bat ?

[Oilchange]

Regarding bolts/welds I have an FIA/MSA certified 6 point roll cage in my car. It is welded in areas such as the roll hoop behind my head but all else is bolted to it. It's made from Chrome Molybdenum steel, the welds are pretty much perfect and it is absolutely solid. I have had scrutineers looking all over it, I wonder if it's because they haven't seen an Esprit with a cage fitted or simply because they are inspecting it very closely but it has never had a single issue in two years racing. Testament to Mr Robinson. Thank you Sir.
So as long as a reputable fabricator has his plate welded on it should be fine but if there is any doubt you could always take it to someone and they could inspect it/further bolster the bolts/welds to beef it up.

All the legal stuff is in the latest Blue Book, quite comprehensive.

Edited by Oilchange on Saturday 6th December 17:35

[Al U]

Personally, having had long look at the way the tube member that runs along the windscreen connects to the vertical member I wouldn't be too concerned. If you look at the position of the bar it is above the lower seal edge of the windscreen. Therefore in a frontal crash event, anything that contacts the horizontal member will have to first of come through the windscreen so in this instance it is adding additional protection(albeit in a very specific zone, but it has never been designed to assist with frontal impact) as it will be an additional barrier after the object/objects have breached the screen.

I think that if whatever came through the screen has enough force to bend the beam to the extent that it will contact a person inside the vehicle then I think you will have bigger things to worry about anyway, regardless of whether the cage is there or not. From a design point of view, the beam running across the windscreen will add significant support to the vertical beams if the vehicle does roll. If it was my vehicle I would keep it in but I would add some foam to the bars at the front of the vehicle where possible because in the event that you crash, it is very possible that your head or other body part could impact the bar especially if you haven't got a seatbelt on. The other poster said something about getting hit on the head with a foam covered baseball bat, but if you had the choice of being hit with a foam or non-foam covered baseball bat which one would you choose? Also slightly off topic, but in America it is illegal for a car to be sold from the manufacturer with a roll cage/hoop unless all exposed members have foam padding on them.

HTH.

Edited by Al U on Saturday 6th December 19:28

[V8RX7]

Al U said:

I think that if whatever came through the screen has enough force to bend the beam to the extent that it will contact a person inside the vehicle then I think you will have bigger things to worry about anyway

It tends to be a head hitting the bars to the side / rear of the head.

Not from the bar moving inwards but from the person moving around in a crash / roll - a seatbelt won't hold you like a harness does and a harness is a pain on a road car as you frequently can't see at junctions nor reach all the switches.

[archie456]

Oilchange said:

Testament to Mr Robinson. Thank you Sir.

If this is Andy Robinson Race Cars, then I all I can say is that Andy and his and his team are nothing short of magicians. I still can't figure out how they got the (fully welded) cage in my Ginetta, and it is just a work of art.

If you want to see how to make good use of a roll cage then look at his crash video on his website