Roll Cages
Motor racing is dangerous, we know this and even if we didn’t it says so on all the tickets.
This article is about reducing the consequences of that common racing occurrence, the crash. In particular it’s about adding a structure, a roll cage, to a car which should reduce the damage to the occupants if the car does crash. This is not just about race cars either, modern cars have all sorts of crash protection built in, not so a Pop or worse still a ‘glass 32 roadster. This is not so much about full-race, tube-framed cars where the roll cage is part of a more complicated structure but rather adding a cage to a modern monocoque car or traditional chassised car. Please note these are general comments, the MSA or FIA rule books are detailed and definitive if you plan to race. Also this article is mainly aimed to closed cars; the same principles apply to open cars but there are more compromises between strength and appearance. This stuff is also important for any car which has to go through the Single Vehicle Approval process because the section covering seat belts is hot on the strength of the anchorage points, again full specifications can be found in the Inspection Manual.
First thing to look at is the shape of the structure. The start point is a main loop, usually positioned just behind the driver, some inches above his head and some inches wider than his shoulders. For looks this loop should be as close to the body as possible, and in some cases to give access to the rear seats. This is the primary protection in the event of a roll and particularly important in an open car. This loop really needs support because it’s not very strong against forces along the length of the car, forces which will usually be present in a roll over situation because the car is travelling forward. It’s usual to have some bracing backward into the rear seat or boot area. This needs to be at a reasonable angle otherwise it will bend along with the main loop. Equally necessary is an angular brace from above the driver’s head to the bottom of the loop opposite. Many race regulations also require a horizontal bar about half way up the main loop to support the back of the driver’s seat. You now have the classic roll bar, roll over bar or four point cage, so called because you have four points of contact between the cage and the car’s basic structure. You can see an example in the A35 pictures. From here the usual development is forwards, typically tubes across the top of the door opening and down by the A-pillar and/or diagonal bars across the door opening. There is often a compromise to be made between strength and accessibility and sometimes removable or hinged bars are fitted in the door opening. Typically there will be horizontal tubes added above and below the screen to tie the sides together. This is now a 6 or 8 point cage depending who you ask. But it does genuinely enclose driver and passenger in a cage.
Beyond this further tubes can go forward through the bulkhead to pick up on suspension mounts or backwards from the main loop in an X shape. Although the cage is inherently strong its performance will depend on how well it’s integrated with the existing structure. All the "points" where the cage attaches should be strong points in the existing frame and you should look for these when choosing a layout. Typical points are suspension mounts, door sills near the A and B pillars, rear seat risers and of course the chassis on cars that have them. More often than not the chassis is not at the edge of the car under the sills in which case it may be necessary to add brackets to support the cage or additional diagonal tubes to transfer the loads to the chassis.
Although we’ve looked at the cage as a safety feature it can be used to stiffen the whole structure of the car by linking it to the body in more places than the mounting points. In fact this is such a good idea that it’s banned in some forms of racing. So you’ve designed the cage and found mounting points so all that’s left is to build it. There is no doubt that you should use cold drawn seamless (CDS) round tubing because weight for weight it’s much stronger than the alternative seamed tubing. This is not a situation to use exhaust pipe, scaffold pole or any other old tat you’ve got laying around, this could save your life. If you’re racing then minimum sizes and thicknesses will be specified in the MSA rules and there are some differences between circuit cars and drag cars. Typically you’re looking at 45 mm outside diameter (OD) and 3 mm thick walls for mild steel or 2.1mm for chrome moly. Some braces can be smaller but it is probably sensible to use these figures as a minimum. Once again the mounting points are important, without reinforcement the tubes will just push through typical sills and body panels and some kind of plate or bracket will be needed to adapt an existing suspension mounting point.
There are no simple rules here except the need to spread the load so look at the pictures for some good examples. Please note that these pictures were taken during construction so not all welds are finished. Once again the MSA rules have specifications particularly when the tubes just rest on the floor where no strong point is available. If it’s not possible to weld the cage in place then it can be bolted in, typically by sandwiching the existing panel between two 3mm plates and welding the cage to the top plate. Another critical concern during construction is where tubes join; it is essential the joint is as strong as possible which can be done by ensuring that the end of the joining tube is fish-mouthed to give the closest possible fit and by welding fully around the join. It requires some agility and planning to get to the joins on the top of the tubes near the roof but any gaps or failure to complete the welds will compromise the strength of the structure. Fish mouths can be made the hard way by filing, rather easier with a hole-saw of the right size or most easily with a mill.
Once again it is possible to use bolted joints but the essential need to carry the load between the tubes remains. Producing tubes which are tight to the body requires planning and some skill with a bender but a reasonable result is quite possible with patience. Some of the pictures show how not to do it, no proper fitting of tubes, poor and incomplete welding and spreader plates pop-riveted to the floor. Now it may be that this article has convinced you that a roll cage is a good thing but not something you want to do yourself. Of course there are many shops which will do this kind of work for you but you can use the principles to ensure the cage does what you want. Alternatively if you are buying a car with a cage already installed there are some points to look for Golden rules ;
Correct design – main loop, triangulation, bracing
Correct materials – CDS tubing, probably 45mm diameter depending on rules, thick wall
Correctly tied to original car – strong points, spreader plates
Correctly made – fish-mouthed tube, fully welded joints
Manufacturers plate and reference number
Should you bother with a roll cage? Well it’s worth remembering that manufacturers of modern cars spend a lot of time designing and testing their cars for crashworthiness because the volume and speed of traffic demands it. These concerns did not exist when Ford designed the Pop or Chevrolet the ’57 Bel Air, nor did these cars go anything like as fast then as we would like them to now. So to get the safety of a modern car as well as the performance get strength of a cage around you, you know it makes sense.