While there are many manufacturers and suppliers of roll bars, some of the bars offered on the market are of poor quality and construction and could be more of a danger than an asset in an accident. Here are a few points to observe when choosing a roll bar to assure you obtain the best quality available
First, the tubing should have a minimum outside diameter of 1.5″ x .120″ thickness in seamless mild steel for a Spitfire or 1.75″ x .120″ for a TR6. The mild steel should have the specification CDS or DOM. If you want to go absolutely first class, then a chrome molly alloy steel, SAE 4125, or SAE 4130. For the maximum protection, T45 should be used. For home constructed roll bars, mild steel is recommended due to the welding problems involved in fabricating chrome molly.
The hoop should be built from on continuous length of tubing and show no evidence of crimping at the bends, and have two braces of equal length attached as close as possible to the top of the hoop. Certainly, no more than 6″ below the top, and with an inclined angle of 30 degrees. The braces can be made of slightly smaller diameter than the main hoop, but certainly no smaller than 3 quarters of the diameter. A good quality bar usually has the braces the same diameter.
While most people choose street bars which will enable them to retain use of the convertible top, it should be kept in mind that for maximum protection, the bar should extend above the driver’s head by at least 2″. Usually, roll bars have mounting plates welded to the bottom of them. The mounting plates should be a minimum of 3/16″ thickness and 4″ x 4″ with equal size back-up plates and be attached at each point with four bolts at least 3/8″ in diameter and with a minimum grade of SAE 5. Welding should be of the heli-arc type showing a nice, clean continuous bead without any burn holes or cavities. Also, be sure when purchasing the roll bar, that the manufacturer states clearly that the seat travel and top operation are not effected if that is of importance.
Usually if a roll bar is purchased through one of the larger supply houses, the specifications can be obtained ahead of time and you will not be disappointed.
Finally, one word of warning. Stay away from low priced chromium plated roll hoops which are made of thin tubing and in the event of a serious accident or roll over, could result in the roll bar wrapping itself around your neck! Your life is not worth saving a few bucks!
If you are using you Spitfire for slalom and gymkhana type events, the organizers may require additional specifications and if you are contemplating constructing your own roll bar, then an excellent source of information is contained in the SCCA General Competition Rules; Appendix “Z”, from which some of the foregoing information was extracted.
The UK Motor Sports Association, the FIA associate for the UK, stipulates minimum safe dimensions for a simple roll cage as:
one hoop, braced to the rear wheel arches, the minimum is 48.3mm (1.9″) diameter x 2.6mm (.1″) thickness.
(MSA regulation Q:1.5.2)
Remember, this page is meant to general information only. Always double check with Sanctioning Bodies for the latest safety requirements. Remember, your safety should always be of paramount concern; never a place to save a few bucks!